Rosenberg lab at Stanford University

Abstracts of Rosenberg lab publications

[2014] [2013] [2012] [2011] [2010] [2009] [2008] [2007] [2006] [2005 and earlier]


[122] A Goldberg, P Verdu, NA Rosenberg (2014) Autosomal admixture levels are informative about sex bias in admixed populations. Genetics 198: 1209-1229. [PDF]

Sex-biased admixture has been observed in a wide variety of admixed populations. Genetic variation in sex chromosomes and functions of quantities computed from sex chromosomes and autosomes have often been examined to infer patterns of sex-biased admixture, typically using statistical approaches that do not mechanistically model the complexity of a sex-specific history of admixture. Here, expanding on a model of Verdu and Rosenberg (2011) that did not include sex specificity, we develop a model that mechanistically examines sex-specific admixture histories. Under the model, multiple source populations contribute to an admixed population, potentially with their male and female contributions varying over time. In an admixed population descended from two source groups, we derive the moments of the distribution of the autosomal admixture fraction from a specific source population as a function of sex-specific introgression parameters and time. Considering admixture processes that are constant in time, we demonstrate that surprisingly, although the mean autosomal admixture fraction from a specific source population does not reveal a sex bias in the admixture history, the variance of autosomal admixture is informative about sex bias. Specifically, the long-term variance decreases as the sex bias from a contributing source population increases. This result can be viewed as analogous to the reduction in effective population size for populations with an unequal number of breeding males and females. Our approach suggests that it may be possible to use the effect of sex-biased admixture on autosomal DNA to assist with methods for inference of the history of complex sex-biased admixture processes.


[121] MD Edge, NA Rosenberg (2014) Upper bounds on FST in terms of the frequency of the most frequent allele and total homozygosity: the case of a specified number of alleles. Theoretical Population Biology 97: 20-34. [PDF]

FST is one of the most frequently-used indices of genetic differentiation among groups. Though FST takes values between 0 and 1, authors going back to Wright have noted that under many circumstances, FST is constrained to be less than 1. Recently, we showed that at a genetic locus with an unspecified number of alleles, FST for two subpopulations is strictly bounded from above by functions of both the frequency of the most frequent allele (M) and the homozygosity of the total population (HT). In the two-subpopulation case, FST can equal one only when the frequency of the most frequent allele and the total homozygosity are 1/2. Here, we extend this work by deriving strict bounds on FST for two subpopulations when the number of alleles at the locus is specified to be I. We show that restricting to I alleles produces the same upper bound on FST over much of the allowable domain for M and HT, and we derive more restrictive bounds in the windows M ∈ [1/I,1/(I-1)) and HT ∈ [1/I,I/(I2-1)). These results extend our understanding of the behavior of FST in relation to other population-genetic statistics.


[120] P Verdu, TJ Pemberton, R Laurent, BM Kemp, A Gonzalez-Oliver, C Gorodesky, CE Hughes, MR Shattuck, B Petzelt, J Mitchell, H Harry, T William, R Worl, JS Cybulski, NA Rosenberg, RS Malhi (2014) Patterns of admixture and population structure in native populations of northwest North America. PLoS Genetics 10: e1004530. [PDF] [Supplement]

The initial contact of European populations with indigenous populations of the Americas produced diverse admixture processes across North, Central, and South America. Recent studies have examined the genetic structure of indigenous populations of Latin America and the Caribbean and their admixed descendants, reporting on the genomic impact of the history of admixture with colonizing populations of European and African ancestry. However, relatively little genomic research has been conducted on admixture in indigenous North American populations. In this study, we analyze genomic data at 475,109 single-nucleotide polymorphisms sampled in indigenous peoples of the Pacific Northwest in British Columbia and Southeast Alaska, populations with a well-documented history of contact with European and Asian traders, fishermen, and contract laborers. We find that the indigenous populations of the Pacific Northwest have higher gene diversity than Latin American indigenous populations. Among the Pacific Northwest populations, interior groups provide more evidence for East Asian admixture, whereas coastal groups have higher levels of European admixture. In contrast with many Latin American indigenous populations, the variance of admixture is high in each of the Pacific Northwest indigenous populations, as expected for recent and ongoing admixture processes. The results reveal some similarities but notable differences between admixture patterns in the Pacific Northwest and those in Latin America, contributing to a more detailed understanding of the genomic consequences of European colonization events throughout the Americas.


[119] TJ Pemberton, NA Rosenberg (2014) Population-genetic influences on genomic estimates of the inbreeding coefficient: a global perspective. Human Heredity 77: 37-48. [PDF] [Supplementary Figure 1] [Supplementary Figure 2] [Supplementary Table 1] [Supplementary Table 2] [Supplementary Table 3]

Background/Aims: Culturally driven marital practices provide a key instance of an interaction between social and genetic processes in shaping patterns of human genetic variation, producing, for example, increased identity by descent through consanguineous marriage. A commonly used measure to quantify identity by descent in an individual is the inbreeding coefficient, a quantity that reflects not only consanguinity, but also other aspects of kinship in the population to which the individual belongs. Here, in populations worldwide, we examine the relationship between genomic estimates of the inbreeding coefficient and population patterns in genetic variation. Methods: Using genotypes at 645 microsatellites, we compare inbreeding coefficients from 5,043 individuals representing 237 populations worldwide to demographic consanguinity frequency estimates available for 26 populations as well as to other quantities that can illuminate population-genetic influences on inbreeding coefficients. Results: We observe higher inbreeding coefficient estimates in populations and geographic regions with known high levels of consanguinity or genetic isolation and in populations with an increased effect of genetic drift and decreased genetic diversity with increasing distance from Africa. For the small number of populations with specific consanguinity estimates, we find a correlation between inbreeding coefficients and consanguinity frequency (r=0.349, p=0.040). Conclusions: The results emphasize the importance of both consanguinity and population-genetic factors in influencing variation in inbreeding coefficients, and they provide insight into factors useful for assessing the effect of consanguinity on genomic patterns in different populations.


[118] CV Than, NA Rosenberg (2014) Mean deep coalescence cost under exchangeable probability distributions. Discrete Applied Mathematics 174: 11-26. [PDF]

We derive formulas for mean deep coalescence cost, for either a fixed species tree or a fixed gene tree, under probability distributions that satisfy the exchangeability property. We than apply the formulas to study mean deep coalescence cost under two commonly used exchangeable models - the uniform and Yule models. We find that mean deep coalescence cost, for either a fixed species tree or a fixed gene tree, tends to be larger for unbalanced trees than for balanced trees. These results provide a better understanding of the deep coalescence cost, as well as allow for the development of new species tree inference criteria.


[117] M DeGiorgio, J Syring, AJ Eckert, AI Liston, R Cronn, DB Neale, NA Rosenberg (2014) An empirical evaluation of two-stage species tree inference strategies using a multilocus dataset from North American pines. BMC Evolutionary Biology 14: 67. [PDF] [Supplementary File 1 (.xlsx, accession numbers)] [Supplementary File 2 (.pdf, supplementary analyses)] [Supplementary File 3 (.zip, data)]

BACKGROUND. As it becomes increasingly possible to obtain DNA sequences of orthologous genes from diverse sets of taxa, species trees are frequently being inferred from multilocus data. However, the behavior of many methods for performing this inference has remained largely unexplored. Some methods have been proven to be consistent given certain evolutionary models, whereas others rely on criteria that, although appropriate for many parameter values, have peculiar zones of the parameter space in which they fail to converge on the correct estimate as data sets increase in size.
RESULTS. Here, using North American pines, we empirically evaluate the behavior of 24 strategies for species tree inference using three alternative outgroups (72 strategies total). The data consist of 120 individuals sampled in eight ingroup species from subsection Strobus and three outgroup species from subsection Gerardianae, spanning ~47 kilobases of sequence at 121 loci. Each "strategy" for inferring species trees consists of three features: a species tree construction method, a gene tree inference method, and a choice of outgroup. We use multivariate analysis techniques such as principal components analysis and hierarchical clustering to identify tree characteristics that are robustly observed across strategies, as well as to identify groups of strategies that produce trees with similar features. We find that strategies that construct species trees using only topological information cluster together and that strategies that use additional non-topological information (e.g., branch lengths) also cluster together. Strategies that utilize more than one individual within a species to infer gene trees tend to produce estimates of species trees that contain clades present in trees estimated by other strategies. Strategies that use the minimize-deep-coalescences criterion to construct species trees tend to produce species tree estimates that contain clades that are not present in trees estimated by the Concatenation, RTC, SMRT, STAR, and STEAC methods, and that in general are more balanced than those inferred by these other strategies.
CONCLUSIONS. When constructing a species tree from a multilocus set of sequences, our observations provide a basis for interpreting differences in species tree estimates obtained via different approaches that have a two-stage structure in common, one step for gene tree estimation and a second step for species tree estimation. The methods explored here employ a number of distinct features of the data, and our analysis suggests that recovery of the same results from multiple methods that tend to differ in their patterns of inference can be a valuable tool for obtaining reliable estimates.


[116] EM Jewett, NA Rosenberg (2014) Theory and applications of a deterministic approximation to the coalescent model. Theoretical Population Biology 93: 14-29. [PDF]

Under the coalescent model, the random number nt of lineages ancestral to a sample is nearly deterministic as a function of time when nt is moderate to large in value, and it is well approximated by its expectation E[nt]. In turn, this expectation is well approximated by simple deterministic functions that are easy to compute. Such deterministic functions have been applied to estimate allele age, effective population size, and genetic diversity, and they have been used to study properties of models of infectious disease dynamics. Although a number of simple approximations of E[nt] have been derived and applied to problems of population-genetic inference, the theoretical accuracy of the resulting approximate formulas and the inferences obtained using these approximations is not known, and the range of problems to which they can be applied is not well understood. Here, we demonstrate general procedures by which the approximation nt ≈ E[nt] can be used to reduce the computational complexity of coalescent formulas, and we show that the resulting approximations converge to their true values under simple assumptions. Such approximations provide alternatives to exact formulas that are computationally intractable or numerically unstable when the number of sampled lineages is moderate or large. We also extend an existing class of approximations of E[nt] to the case of multiple populations of time-varying size with migration among them. Our results facilitate the use of the deterministic approximation nt ≈ E[nt] for deriving functionally simple, computationally efficient, and numerically stable approximations of coalescent formulas under complicated demographic scenarios.


[115] NA Rosenberg (2014) Core elements of a TPB paper. Theoretical Population Biology 92: 118-119 [PDF] (editorial)

(No abstract)


[114] DM Behar*, M Metspalu*, Y Baran, NM Kopelman, B Yunusbayev, A Gladstein, S Tzur, H Sahakyan, A Bahmanimehr, L Yepiskoposyan, K Tambets, EK Khusnutdinova, A Kushniarevich, O Balanovsky, E Balanovsky, L Kovacevic, D Marjanovic, E Mihailov, A Kouvatsi, C Triantaphyllidis, RJ King, O Semino, A Torroni, MF Hammer, E Metspalu, K Skorecki, S Rosset, E Halperin, R Villems, NA Rosenberg (2013) No evidence from genome-wide data of a Khazar origin for the Ashkenazi Jews. Human Biology 85: 859-900. [PDF]

The origin and history of the Ashkenazi Jewish population have long been of great interest, and advances in high-throughput genetic analysis have recently provided a new approach for investigating these topics. We and others have argued on the basis of genome-wide data that the Ashkenazi Jewish population derives its ancestry from a combination of sources tracing to both Europe and the Middle East. It has been claimed, however, through a reanalysis of some of our data, that a large part of the ancestry of the Ashkenazi population originates with the Khazars, a Turkic-speaking group that lived to the north of the Caucasus region ~1,000 years ago. Because the Khazar population has left no obvious modern descendants that could enable a clear test for a contribution to Ashkenazi Jewish ancestry, the Khazar hypothesis has been difficult to examine using genetics. Furthermore, because only limited genetic data have been available from the Caucasus region, and because these data have been concentrated in populations that are genetically close to populations from the Middle East, the attribution of any signal of Ashkenazi-Caucasus genetic similarity to Khazar ancestry rather than shared ancestral Middle Eastern ancestry has been problematic. Here, through integration of genotypes on newly collected samples with data from several of our past studies, we have assembled the largest data set available to date for assessment of Ashkenazi Jewish genetic origins. This data set contains genome-wide single-nucleotide polymorphisms in 1,774 samples from 106 Jewish and non- Jewish populations that span the possible regions of potential Ashkenazi ancestry: Europe, the Middle East, and the region historically associated with the Khazar Khaganate. The data set includes 261 samples from 15 populations from the Caucasus region and the region directly to its north, samples that have not previously been included alongside Ashkenazi Jewish samples in genomic studies. Employing a variety of standard techniques for the analysis of populationgenetic structure, we find that Ashkenazi Jews share the greatest genetic ancestry with other Jewish populations, and among non-Jewish populations, with groups from Europe and the Middle East. No particular similarity of Ashkenazi Jews with populations from the Caucasus is evident, particularly with the populations that most closely represent the Khazar region. Thus, analysis of Ashkenazi Jews together with a large sample from the region of the Khazar Khaganate corroborates the earlier results that Ashkenazi Jews derive their ancestry primarily from populations of the Middle East and Europe, that they possess considerable shared ancestry with other Jewish populations, and that there is no indication of a significant genetic contribution either from within or from north of the Caucasus region.


[113] PJ Oefner, G Hölzl, P Shen, I Shpirer, D Gefel, T Lavi, E Woolf, J Cohen, C Cinnioglu, PA Underhill, NA Rosenberg, J Hochrein, JM Granka, J Hillel, MW Feldman (2013) Genetics and the history of the Samaritans: Y-chromosomal microsatellites and genetic affinity between Samaritans and Cohanim. Human Biology 85: 825-857. [PDF]

The Samaritans are a group of some 750 indigenous Middle Eastern people, about half of whom live in Holon, a suburb of Tel Aviv, and the other half near Nablus. The Samaritan population is believed to have numbered more than a million in late Roman times, but less than 150 in 1917. The ancestry of the Samaritans has been subject to controversy from late Biblical times to the present. In this study, liquid chromatographyelectrospray ionization quadrupole ion trap mass spectrometry was used to allelotype 13 Y-chromosomal and 15 autosomal microsatellites in a sample of 12 Samaritans chosen to have as low a level of relationship as possible, and 461 Jews and non-Jews. Estimation of genetic distances between the Samaritans and seven Jewish and three non-Jewish populations from Israel, as well as populations from Africa, Pakistan, Turkey, and Europe, revealed that the Samaritans were closely related to Cohanim. This result supports the position of the Samaritans that they are descendants from the tribes of Israel dating to before the Assyrian exile in 722-720 BCE. In concordance with previously published singlenucleotide polymorphism haplotypes, each Samaritan family, with the exception of the Samaritan Cohen lineage, was observed to carry a distinctive Y-chromosome short tandem repeat haplotype that was not more than one mutation removed from the six-marker Cohen modal haplotype.


[112] NA Rosenberg, SP Weitzman (2013) From generation to generation: the genetics of Jewish populations. Human Biology 85: 817-823. [PDF]

(No abstract)


[111] NA Rosenberg (2013) Coalescent histories for caterpillar-like families. IEEE/ACM Transactions on Computational Biology and Bioinformatics 10: 1253-1262. [PDF]

A coalescent history is an assignment of branches of a gene tree to branches of a species tree on which coalescences in the gene tree occur. The number of coalescent histories for a pair consisting of a labeled gene tree topology and a labeled species tree topology is important in gene tree probability computations, and more generally, in studying evolutionary possibilities for gene trees on species trees. Defining the Tr-caterpillar-like family as a sequence of n-taxon trees constructed by replacing the r-taxon subtree of n-taxon caterpillars by a specific r-taxon labeled topology Tr, we examine the number of coalescent histories for caterpillar-like families with matching gene tree and species tree labeled topologies. For each Tr with size r ≤ 8, we compute the number of coalescent histories for n-taxon trees in the Tr-caterpillar-like family. Next, as n → ∞, we find that the limiting ratio of the numbers of coalescent histories for the Tr family and caterpillars themselves is correlated with the number of labeled histories for Tr. The results support a view that large numbers of coalescent histories occur when a tree has both a relatively balanced subtree and a high tree depth, contributing to deeper understanding of the combinatorics of gene trees and species trees.


[110] MD Edge, P Gorroochurn, NA Rosenberg (2013) Windfalls and pitfalls: applications of population genetics to the search for disease genes. Evolution, Medicine, and Public Health 2013: 254-272. [Full-text at journal website] [PDF]

Association mapping can be viewed as an application of population genetics and evolutionary biology to the problem of identifying genes causally connected to phenotypes. However, some population-genetic principles important to the design and analysis of association studies have not been widely understood or have even been generally misunderstood. Some of these principles underlie techniques that can aid in the discovery of genetic variants that influence phenotypes ('windfalls'), whereas others can interfere with study design or interpretation of results ('pitfalls'). Here, considering examples involving genetic variant discovery, linkage disequilibrium, power to detect associations, population stratification and genotype imputation, we address misunderstandings in the application of population genetics to association studies, and we illuminate how some surprising results in association contexts can be easily explained when considered from evolutionary and population-genetic perspectives. Through our examples, we argue that population-genetic thinking — which takes a theoretical view of the evolutionary forces that guide the emergence and propagation of genetic variants — substantially informs the design and interpretation of genetic association studies. In particular, population-genetic thinking sheds light on genetic confounding, on the relationships between association signals of typed markers and causal variants, and on the advantages and disadvantages of particular strategies for measuring genetic variation in association studies.


[109] NA Rosenberg (2013) Discordance of species trees with their most likely gene trees: a unifying principle. Molecular Biology and Evolution 30: 2709-2713. [Abstract] [Full-text at journal website] [PDF]

A labeled gene tree topology that disagrees with a labeled species tree topology is said to be anomalous if it is more probable under a coalescent model for gene lineage evolution than the labeled gene tree topology that matches the species tree. It has previously been shown that as a consequence of short internal branches of the species tree, for every labeled species tree topology with five or more taxa, and for asymmetric four-taxon species tree topologies, an assignment of species tree branch lengths can be made which gives rise to anomalous gene trees (AGTs). Here, I offer an alternative characterization of this result — a labeled species tree topology produces AGTs if and only if it contains two consecutive internal branches in an ancestor-descendant relationship — and I provide a proof that follows from the change in perspective. The reformulation and alternative proof of the existence result for AGTs provide the insight that it is not merely short internal branches that generate AGTs, but instead, short internal branches that are arranged consecutively.


[108] P Zhang, X Zhan, NA Rosenberg, S Zöllner (2013) Genotype imputation reference panel selection using maximal phylogenetic diversity. Genetics 195: 319-330. [PDF]

The recent dramatic cost reduction of next-generation sequencing technology enables investigators to assess most variants in the human genome to identify risk variants for complex diseases. However, sequencing large samples remains very expensive. For a study sample with existing genotype data, such as array data from genome-wide association studies, a cost-effective approach is to sequence a subset of the study sample and then to impute the rest of the study sample, using the sequenced subset as a reference panel. The use of such an internal reference panel identifies population-specific variants and avoids the problem of a substantial mismatch in ancestry background between the study population and the reference population. To efficiently select an internal panel, we introduce an idea of phylogenetic diversity from mathematical phylogenetics and comparative genomics. We propose the "most diverse reference panel", defined as the subset with the maximal "phylogenetic diversity", thereby incorporating individuals that span a diverse range of genotypes within the sample. Using data both from simulations and from the 1000 Genomes Project, we show that the most diverse reference panel can substantially improve the imputation accuracy compared to randomly selected reference panels, especially for the imputation of rare variants. The improvement in imputation accuracy holds across different marker densities, reference panel sizes, and lengths for the imputed segments. We thus propose a novel strategy for planning sequencing studies on samples with existing genotype data.


[107] NA Rosenberg, TJ Pemberton, JZ Li, JW Belmont (2013) Runs of homozygosity and parental relatedness. Genetics in Medicine 15: 753-754. [PDF]

(No abstract)


[106] L Huang, EO Buzbas, NA Rosenberg (2013) Genotype imputation in a coalescent model with infinitely-many-sites mutation. Theoretical Population Biology 87: 62-74. [PDF]

Empirical studies have identified population-genetic factors as important determinants of the properties of genotype-imputation accuracy in imputation-based disease association studies. Here, we develop a simple coalescent model of three sequences that we use to explore the theoretical basis for the influence of these factors on genotype-imputation accuracy, under the assumption of infinitely-many-sites mutation. Employing a demographic model in which two populations diverged at a given time in the past, we derive the approximate expectation and variance of imputation accuracy in a study sequence sampled from one of the two populations, choosing between two reference sequences, one sampled from the same population as the study sequence and the other sampled from the other population. We show that, under this model, imputation accuracy — as measured by the proportion of polymorphic sites that are imputed correctly in the study sequence — increases in expectation with the mutation rate, the proportion of the markers in a chromosomal region that are genotyped, and the time to divergence between the study and reference populations. Each of these effects derives largely from an increase in information available for determining the reference sequence that is genetically most similar to the sequence targeted for imputation. We analyze as a function of divergence time the expected gain in imputation accuracy in the target using a reference sequence from the same population as the target rather than from the other population. Together with a growing body of empirical investigations of genotype imputation in diverse human populations, our modeling framework lays a foundation for extending imputation techniques to novel populations that have not yet been extensively examined.


[105] ZA Szpiech, J Xu, TJ Pemberton, W Peng, S Zöllner, NA Rosenberg, JZ Li (2013) Long runs of homozygosity are enriched for deleterious variation. American Journal of Human Genetics 93: 90-102. [PDF] [Supplement] [Data]

Exome sequencing offers the potential to study the population-genomic variables that underlie patterns of deleterious variation. Runs of homozygosity (ROH) are long stretches of consecutive homozygous genotypes probably reflecting segments shared identically by descent as the result of processes such as consanguinity, population size reduction, and natural selection. The relationship between ROH and patterns of predicted deleterious variation can provide insight into the way in which these processes contribute to the maintenance of deleterious variants. Here, we use exome sequencing to examine ROH in relation to the distribution of deleterious variation in 27 individuals of varying levels of apparent inbreeding from 6 human populations. A significantly greater fraction of all genome-wide predicted damaging homozygotes fall in ROH than would be expected from the corresponding fraction of nondamaging homozygotes in ROH (p<0.001). This pattern is strongest for long ROH (p<0.05). ROH, and especially long ROH, harbor disproportionately more deleterious homozygotes than would be expected on the basis of the total ROH coverage of the genome and the genomic distribution of nondamaging homozygotes. The results accord with a hypothesis that recent inbreeding, which generates long ROH, enables rare deleterious variants to exist in homozygous form. Thus, just as inbreeding can elevate the occurrence of rare recessive diseases that represent homozygotes for strongly deleterious mutations, inbreeding magnifies the occurrence of mildly deleterious variants as well.


[104] TJ Pemberton, M DeGiorgio, NA Rosenberg (2013) Population structure in a comprehensive data set on human microsatellite variation. G3: Genes, Genomes, Genetics 3: 891-907. [Full-text at journal website] [PDF] [Supplement] [Data (.zip)]

Over the past two decades, microsatellite genotypes have provided the data for landmark studies of human population-genetic variation. However, the various microsatellite data sets have been prepared with different procedures and sets of markers, so that it has been difficult to synthesize available data for a comprehensive analysis. Here, we combine eight human population-genetic data sets at the 645 microsatellite loci they share in common, accounting for procedural differences in the production of the different data sets, to assemble a single data set containing 5795 individuals from 267 worldwide populations. We perform a systematic analysis of genetic relatedness, detecting 240 intra-population and 92 inter-population pairs of previously unidentified close relatives and proposing standardized subsets of unrelated individuals for use in future studies. We then augment the human data with a data set of 84 chimpanzees at the 246 loci they share in common with the human samples. Multidimensional scaling and neighbor-joining analyses of these data sets offer new insights into the structure of human populations and enable a comparison of genetic variation patterns in chimpanzees with those in humans. Our combined data sets are the largest of their kind reported to date and provide a resource for use in human population-genetic studies.


[103] CV Than, NA Rosenberg (2013) Mathematical properties of the deep coalescence cost. IEEE/ACM Transactions on Computational Biology and Bioinformatics 10: 61-72. [PDF]

In the minimizing-deep-coalescences (MDC) approach for species tree inference, a tree that has the minimal deep coalescence cost for reconciling a collection of gene trees is taken as an estimate of the species tree topology. The MDC method possesses the desirable Pareto property, and in practice it is quite accurate and computationally efficient. Here, in order to better understand the MDC method, we investigate some properties of the deep coalescence cost. We prove that the unit neighborhood of either a rooted species tree or a rooted gene tree under the deep coalescence cost is exactly the same as the tree's unit neighborhood under the rooted nearest-neighbor interchange (NNI) distance. Next, for a fixed species tree, we obtain the maximum deep coalescence cost across all gene trees as well as the number of gene trees that achieve the maximum cost. We also study corresponding problems for a fixed gene tree.


[102] M Jakobsson, MD Edge, NA Rosenberg (2013) The relationship between FST and the frequency of the most frequent allele. Genetics 193: 515-528. [PDF]

FST is frequently used as a summary of genetic differentiation among groups. It has been suggested that FST depends on the allele frequencies at a locus, as it exhibits a variety of peculiar properties related to genetic diversity: higher values for biallelic single-nucleotide polymorphisms (SNPs) than for multiallelic microsatellites, low values among high-diversity populations viewed as substantially distinct, and low values for populations that differ primarily in their profiles of rare alleles. A full mathematical understanding of the dependence of FST on allele frequencies, however, has been elusive. Here, we examine the relationship between FST and the frequency of the most frequent allele, demonstrating that the range of values that FST can take is restricted considerably by the allele-frequency distribution. For a two-population model, we derive strict bounds on FST as a function of the frequency M of the allele with highest mean frequency between the pair of populations. Using these bounds, we show that for a value of M chosen uniformly between 0 and 1 at a multiallelic locus whose number of alleles is left unspecified, the mean maximum FST is ~0.3585. Further, FST is restricted to values much less than 1 when M is low or high, and the contribution to the maximum FST made by the most frequent allele is on average ~0.4485. Using bounds on homozygosity that we have previously derived as functions of M, we describe strict bounds on FST in terms of the homozygosity of the total population, finding that the mean maximum FST given this homozygosity is 1 - ln 2 ≈ 0.3069. Our results provide a conceptual basis for understanding the dependence of FST on allele frequencies and genetic diversity and for interpreting the roles of these quantities in computations of FST from population-genetic data. Further, our analysis suggests that many unusual observations of FST, including the relatively low FST values in high-diversity human populations from Africa and the relatively low estimates of FST for microsatellites compared to SNPs, can be understood not as biological phenomena associated with different groups of populations or classes of markers but rather as consequences of the intrinsic mathematical dependence of FST on the properties of allele-frequency distributions.


[101] NA Rosenberg (2013) Editorial. Theoretical Population Biology 83: A2-A3. [PDF] (editorial)

(No abstract)


[100] M DeGiorgio, NA Rosenberg (2013) Geographic sampling scheme as a determinant of the major axis of genetic variation in principal components analysis. Molecular Biology and Evolution 30: 480-488. [PDF]

Principal component (PC) maps, which plot the values of a given PC estimated on the basis of allele frequency variation at the geographic sampling locations of a set of populations, are often used to investigate the properties of past range expansions. Some studies have argued that in a range expansion, the axis of greatest variation (i.e., the first PC) is parallel to the axis of expansion. In contrast, others have identified a pattern in which the axis of greatest variation is perpendicular to the axis of expansion. Here, we seek to understand this difference in outcomes by investigating the effect of the geographic sampling scheme on the direction of the axis of greatest variation under a two-dimensional range expansion model. From datasets simulated using each of two different schemes for the geographic sampling of populations under the model, we create PC maps for the first PC. We find that depending on the geographic sampling scheme, the axis of greatest variation can be either parallel or perpendicular to the axis of expansion. We provide an explanation for this result in terms of intra- and interpopulation coalescence times.


[99] NM Kopelman, L Stone, O Gascuel, NA Rosenberg (2013) The behavior of admixed populations in neighbor-joining inference of population trees. Pacific Symposium on Biocomputing 18: 263-274. [PDF]

Neighbor-joining is one of the most widely used methods for constructing evolutionary trees. This approach from phylogenetics is often employed in population genetics, where distance matrices obtained from allele frequencies are used to produce a representation of population relationships in the form of a tree. In phylogenetics, the utility of neighbor-joining derives partly from a result that for a class of distance matrices including those that are additive or tree-like — generated by summing weights over the edges connecting pairs of taxa in a tree to obtain pairwise distances — application of neighbor-joining recovers exactly the underlying tree. For populations within a species, however, migration and admixture can produce distance matrices that reflect more complex processes than those obtained from the bifurcating trees typical in the multispecies context. Admixed populations — populations descended from recent mixture of groups that have long been separated — have been observed to be located centrally in inferred neighbor-joining trees, with short external branches incident to the path connecting their source populations. Here, using a simple model, we explore mathematically the behavior of an admixed population under neighbor-joining. We show that with an additive distance matrix, a population admixed among two source populations necessarily lies on the path between the sources. Relaxing the additivity requirement, we examine the smallest nontrivial case — four populations, one of which is admixed between two of the other three — showing that the two source populations never merge with each other before one of them merges with the admixed population. Furthermore, the distance on the constructed tree between the admixed population and either source population is always smaller than the distance between the source populations, and the external branch for the admixed population is always incident to the path connecting the sources. We define three properties that hold for four taxa and that we hypothesize are satisfied under more general conditions: antecedence of clustering, intermediacy of distances, and intermediacy of path lengths. Our findings can inform interpretations of neighbor-joining trees with admixed groups, and they provide an explanation for patterns observed in trees of human populations.


[98] LK Nakhleh, NA Rosenberg, T Warnow (2013) Phylogenomics and population genomics: models, algorithms, and analytical tools. Pacific Symposium on Biocomputing 18: 247-249. [PDF] (session introduction)

(No abstract)


[97] JH Degnan, NA Rosenberg, T Stadler (2012) A characterization of the set of species trees that produce anomalous ranked gene trees. IEEE/ACM Transactions on Computational Biology and Bioinformatics 9: 1558-1568. [PDF]

Ranked gene trees, which consider both the gene tree topology and the sequence in which gene lineages separate, can potentially provide a new source of information for use in modeling genealogies and performing inference of species trees. Recently, we have calculated the probability distribution of ranked gene trees under the standard multispecies coalescent model for the evolution of gene lineages along the branches of a fixed species tree, demonstrating the existence of anomalous ranked gene trees (ARGTs), in which a ranked gene tree that does not match the ranked species tree can have greater probability under the model than the matching ranked gene tree. Here, we fully characterize the set of unranked species tree topologies that give rise to ARGTs, showing that this set contains all species tree topologies with five or more taxa, with the exceptions of caterpillars and pseudocaterpillars. The results have implications for the use of ranked gene trees in phylogenetic inference.


[96] EO Buzbas (2012) On the article titled "Estimating species trees using approximate Bayesian computation" (Fan and Kubatko, Molecular Phylogenetics and Evolution 59:354-363). Molecular Phylogenetics and Evolution 65: 1014-1016 (2012). [PDF]

(No abstract)


[95] C Wang, KB Schroeder, NA Rosenberg (2012) A maximum-likelihood method to correct for allelic dropout in microsatellite data with no replicate genotypes. Genetics 192: 651-669. [Full-text at journal website] [PDF] [Supplement] [Software]

Allelic dropout is a commonly observed source pf missing data in microsatellite genotypes, in which one or both allelic copies at a locus fail to be amplified by the polymerase chain reaction. Especially for samples with poor DNA quality, this problem causes a downward bias in estimates of observed heterozygosity and an upward bias in estimates of inbreeding, owing to mistaken classifications of heterozygotes as homozygotes when one of the two copies drops out. One general approach for avoiding allelic dropout involves repeated genotyping of homozygous loci to minimize the effects of experimental error. Existing computational alternatives often require replicate genotyping as well. These approaches, however, are costly and are suitable only when enough DNA is available for repeated genotyping. In this study, we propose a maximum-likelihood approach together with an expectation-maximization algorithm to jointly estimate allelic dropout rates and allele frequencies when only one set of nonreplicated genotypes is available. Our method considers estimates of allelic dropout caused by both sample-specific factors and locus-specific factors, and it allows for deviation from Hardy-Weinberg equilibrium owing to inbreeding. Using the estimated parameters, we correct the bias in the estimation of observed heterozygosity through the use of multiple imputations of alleles in cases where dropout might have occurred. With simulated data, we show that our method can (1) effectively reproduce patterns of missing data and heterozygosity observed in real data; (2) correctly estimate model parameters, including sample-specific dropout rates, locus-specific dropout rates, and the inbreeding coefficient; and (3) successfully correct the downward bias in estimating the observed heterozygosity. We find that our method is fairly robust to violations of model assumptions caused by population structure and by genotyping errors from sources other than allelic dropout. Because the data sets imputed under our model can be investigated in additional subsequent analyses, our method will be useful for preparing data for applications in diverse contexts in population genetics and molecular ecology.


[94] C Wang, S Zöllner, NA Rosenberg (2012) A quantitative comparison of the similarity between genes and geography in worldwide human populations. PLoS Genetics 8: e1002886. [Full-text at journal website] [PDF] [Supplement]

Multivariate statistical techniques such as principal components analysis (PCA) and multidimensional scaling (MDS) have been widely used to summarize the structure of human genetic variation, often in easily visualized two-dimensional maps. Many recent studies have reported similarity between geographic maps of population locations and MDS or PCA maps of genetic variation inferred from single-nucleotide polymorphisms (SNPs). However, this similarity has been evident primarily in a qualitative sense; and, because different multivariate techniques and marker sets have been used in different studies, it has not been possible to formally compare genetic variation datasets in terms of their levels of similarity with geography. In this study, using genome-wide SNP data from 128 populations worldwide, we perform a systematic analysis to quantitatively evaluate the similarity of genes and geography in different geographic regions. For each of a series of regions, we apply a Procrustes analysis approach to find an optimal transformation that maximizes the similarity between PCA maps of genetic variation and geographic maps of population locations. We consider examples in Europe, Sub-Saharan Africa, Asia, East Asia, and Central/South Asia, as well as in a worldwide sample, finding that significant similarity between genes and geography exists in general at different geographic levels. The similarity is highest in our examples for Asia and, once highly distinctive populations have been removed, Sub-Saharan Africa. Our results provide a quantitative assessment of the geographic structure of human genetic variation worldwide, supporting the view that geography plays a strong role in giving rise to human population structure.


[93] TJ Pemberton, F-Y Li, EK Hanson, NU Mehta, S Choi, J Ballantyne, JW Belmont, NA Rosenberg, C Tyler-Smith, PI Patel (2012) Impact of restricted marital practices on genetic variation in an endogamous Gujarati group. American Journal of Physical Anthropology 149: 92-103. [PDF] [Supplement (.docx)] [Data]

Recent studies have examined the influence on patterns of human genetic variation of a variety of cultural practices. In India, centuries-old marriage customs have introduced extensive social structuring into the contemporary population, potentially with significant consequences for genetic variation. Social stratification in India is evident as social classes that are defined by endogamous groups known as castes. Within a caste, there exist endogamous groups known as gols (marriage circles), each of which comprises a small number of exogamous gotra (lineages). Thus, while consanguinity is strictly avoided and some randomness in mate selection occurs within the gol, gene flow is limited with groups outside the gol. Gujarati Patels practice this form of "exogamic endogamy." We have analyzed genetic variation in one such group of Gujarati Patels, the Chha Gaam Patels (CGP), who comprise individuals from six villages. Population structure analysis of 1,200 autosomal loci offers support for the existence of distinctive multilocus genotypes in the CGP with respect to both non-Gujaratis and other Gujaratis, and indicates that CGP individuals are genetically very similar. Analysis of Y-chromosomal and mitochondrial haplotypes provides support for both patrilocal and patrilineal practices within the gol, and a low-level of female gene flow into the gol. Our study illustrates how the practice of gol endogamy has introduced fine-scale genetic structure into the population of India, and contributes more generally to an understanding of the way in which marriage practices affect patterns of genetic variation.


[92] EM Jewett*, M Zawistowski*, NA Rosenberg, S Zöllner (2012) A coalescent model for genotype imputation. Genetics 191: 1239-1255. [PDF]

The potential for imputed genotypes to enhance an analysis of genetic data depends largely on the accuracy of imputation, which in turn depends on properties of the reference panel of template haplotypes used to perform the imputation. To provide a basis for exploring how properties of the reference panel affect imputation accuracy theoretically rather than with computationally intensive imputation experiments, we introduce a coalescent model that considers imputation accuracy in terms of population-genetic parameters. Our model allows us to investigate sampling designs in the frequently occurring scenario in which imputation targets and templates are sampled from different populations. In particular, we derive expressions for expected imputation accuracy as a function of reference panel size and divergence time between the reference and target populations. We find that a modestly sized "internal" reference panel from the same population as a target haplotype yields, on average, greater imputation accuracy than a larger "external" panel from a different population, even if the divergence time between the two populations is small. The improvement in accuracy for the internal panel increases with increasing divergence time between the target and reference populations. Thus, in humans, our model predicts that imputation accuracy can be improved by generating small population-specific custom reference panels to augment existing collections such as those of the HapMap or 1000 Genomes Projects. Our approach can be extended to understand additional factors that affect imputation accuracy in complex population-genetic settings, and the results can ultimately facilitate improvements in imputation study designs.


[91] TJ Pemberton, D Absher, MW Feldman, RM Myers, NA Rosenberg, JZ Li (2012) Genomic patterns of homozygosity in worldwide human populations. American Journal of Human Genetics 91: 275-292. [PDF] [Main Supplement] [Supplementary Table 2 (.zip)] [Supplementary Table 3 (.zip)] [Supplementary Table 4 (.zip)] [Supplementary Table 5 (.zip)]

Genome-wide patterns of homozygosity runs and their variation across individuals provide a valuable and often untapped resource for studying human genetic diversity and evolutionary history. Using genotype data at 577,489 autosomal SNPs, we employed a likelihood-based approach to identify runs of homozygosity (ROH) in 1,839 individuals representing 64 worldwide populations, classifying them by length into classes — short, intermediate, long — with a model-based clustering algorithm. For each class, the number and total length of ROH per individual show considerable variation across individuals and populations. The total lengths of short and intermediate ROH per individual increase with the distance of a population from East Africa, in agreement with similar patterns previously observed for locus-wise homozygosity and linkage disequilibrium. By contrast, total lengths of long ROH show large interindividual variations that probably reflect recent inbreeding patterns, with higher values occurring more often in populations with known high frequencies of consanguineous unions. Across the genome, distributions of ROH are not uniform, and they have distinctive continental patterns. ROH frequencies across the genome are correlated with local genomic variables such as recombination rate, as well as with signals of recent positive selection. In addition, long ROH are more frequent in genomic regions harboring genes associated with autosomal-dominant diseases than in regions not implicated in Mendelian diseases. These results provide insight into the way in which homozygosity patterns are produced, and they generate baseline homozygosity patterns that can be used to aid homozygosity mapping of genes associated with recessive diseases.


[90] D Bryant, R Bouckaert, J Felsenstein, NA Rosenberg, A RoyChoudhury (2012) Inferring species trees directly from biallelic genetic markers: bypassing gene trees in a full coalescent analysis. Molecular Biology and Evolution 29: 1917-1932. [PDF] [Supplement]

The multispecies coalescent provides an elegant theoretical framework for estimating species trees and species demographics from genetic markers. However, practical applications of the multispecies coalescent model are limited by the need to integrate or sample over all gene trees possible for each genetic marker. Here we describe a polynomial-time algorithm that computes the likelihood of a species tree directly from the markers under a finite-sites model of mutation effectively integrating over all possible gene trees. The method applies to independent (unlinked) biallelic markers such as well-spaced single nucleotide polymorphisms, and we have implemented it in SNAPP, a Markov chain Monte Carlo sampler for inferring species trees, divergence dates, and population sizes. We report results from simulation experiments and from an analysis of 1997 amplified fragment length polymorphism loci in 69 individuals sampled from six species of Ourisia (New Zealand native foxglove).


[89] LJ Helmkamp, EM Jewett, NA Rosenberg (2012) Improvements to a class of distance matrix methods for inferring species trees from gene trees. Journal of Computational Biology 19: 632-649. [PDF]

Among the methods currently available for inferring species trees from gene trees, the GLASS method of Mossel and Roch (2010), the Shallowest Divergence (SD) method of Maddison and Knowles (2006), the STEAC method of Liu et al. (2009), and a related method that we call Minimum Average Coalescence (MAC) are computationally efficient and provide branch length estimates. Further, GLASS and STEAC have been shown to be consistent estimators of tree topology under a multispecies coalescent model. However, divergence time estimates obtained with these methods are all systematically biased under the model because the pairwise interspecific gene divergence times on which they rely must be more ancient than the species divergence time. Jewett and Rosenberg (2012) derived an expression for the bias of GLASS and used it to propose an improved method that they termed iGLASS. Here, we derive the biases of SD, STEAC, and MAC, and we propose improved analogues of these methods that we call iSD, iSTEAC, and iMAC. We conduct simulations to compare the performance of these methods with their original counterparts and with GLASS and iGLASS, finding that each of them decreases the bias and mean squared error of pairwise divergence time estimates. The new methods can therefore contribute to improvements in the estimation of species trees from information on gene trees.


[88] EM Jewett, NA Rosenberg (2012) iGLASS: an improvement to the GLASS method for estimating species trees from gene trees. Journal of Computational Biology 19: 293-315. [PDF]

Several methods have been designed to infer species trees from gene trees while taking into account gene tree/species tree discordance. Although some of these methods provide consistent species tree topology estimates under a standard model, most either do not estimate branch lengths or are computationally slow. An exception, the GLASS method of Mossel and Roch, is consistent for the species tree topology, estimates branch lengths, and is computationally fast. However, GLASS systematically overestimates divergence times, leading to biased estimates of species tree branch lengths. By assuming a multispecies coalescent model in which multiple lineages are sampled from each of two taxa at L independent loci, we derive the distribution of the waiting time until the first interspecific coalescence occurs between the two taxa, considering all loci and measuring from the divergence time. We then use the mean of this distribution to derive a correction to the GLASS estimator of pairwise divergence times. We show that our improved estimator, which we call iGLASS, consistently estimates the divergence time between a pair of taxa as the number of loci approaches infinity, and that it is an unbiased estimator of divergence times when one lineage is sampled per taxon. We also show that many commonly used clustering methods can be combined with the iGLASS estimator of pairwise divergence times to produce a consistent estimator of the species tree topology. Through simulations, we show that iGLASS can greatly reduce the bias and mean squared error in obtaining estimates of divergence times in a species tree.


[87] SB Reddy, NA Rosenberg (2012) Refining the relationship between homozygosity and the frequency of the most frequent allele. Journal of Mathematical Biology 64: 87-108. [PDF]

Recent work has established that for an arbitrary genetic locus with its number of alleles unspecified, the homozygosity of the locus confines the frequency of the most frequent allele within a narrow range, and vice versa. Here we extend beyond this limiting case by investigating the relationship between homozygosity and the frequency of the most frequent allele when the number of alleles at the locus is treated as known. Given the homozygosity of a locus with at most K alleles, we find that by taking into account the value of K, the width of the allowed range for the frequency of the most frequent allele decreases from 2/3-π2/18 ≈ 0.1184 to 1/3-1/(3K)-{K/[3(K-1)]} ∑ k=2,3,...,K 1/k2. We further show that properties of the relationship between homozygosity and the frequency of the most frequent allele in the unspecified-K case can be obtained from the specified-K case by taking limits as K → ∞. The results contribute to a greater understanding of the mathematical properties of fundamental statistics employed in population-genetic analysis.


[86] FA San Lucas, NA Rosenberg, P Scheet (2012) Haploscope: a tool for the graphical display of haplotype structure in populations. Genetic Epidemiology 35: 17-21. [PDF]

Patterns of linkage disequilibrium are often depicted pictorially by using tools that rely on visualizations of raw data or pairwise correlations among individual markers. Such approaches can fail to highlight some of the more interesting and complex features of haplotype structure. To enable natural visual comparisons of haplotype structure across subgroups of a population (e.g. isolated subpopulations or cases and controls), we propose an alternative visualization that provides a novel graphical representation of haplotype frequencies. We introduce Haploscope, a tool for visualizing the haplotype cluster frequencies that are produced by statistical models for population haplotype variation. We demonstrate the utility of our technique by examining haplotypes around the LCT gene, an example of recent positive selection, in samples from the Human Genome Diversity Panel. Haploscope, which has flexible options for annotation and inspection of haplotypes, is available for download at http://scheet.org/software.


[85] JH Degnan, NA Rosenberg, T Stadler (2012) The probability distribution of ranked gene trees on a species tree. Mathematical Biosciences 235: 45-55. [PDF]

The properties of random gene tree topologies have recently been studied under a coalescent model that treats a species tree as a fixed parameter. Here we develop the analogous theory for random ranked gene tree topologies, in which both the topology and the sequence of coalescences for a random gene tree are considered. We derive the probability distribution of ranked gene tree topologies conditional on a fixed species tree. We then show that similar to the unranked case, ranked gene trees that do not match either the ranking or the topology of the species tree can have greater probability than the matching ranked gene tree.


[84] NA Rosenberg (2011) A population-genetic perspective on the similarities and differnces among worldwide human populations. Human Biology 83: 659-684. [PDF]

Recent studies have produced a variety of advances in the investigation of genetic similarities and differences among human populations. Here, I pose a series of questions about human population-genetic similarities and differences, and I then answer these questions by numerical computation with a single shared population-genetic data set. The collection of answers obtained provides an introductory perspective for understanding key results on the features of worldwide human genetic variation.


[83] L Huang*, M Jakobsson*, TJ Pemberton, M Ibrahim, T Nyambo, S Omar, JK Pritchard, SA Tishkoff, NA Rosenberg (2011) Haplotype variation and genotype imputation in African populations. Genetic Epidemiology 35: 766-780. [PDF] [Supplement] [Data]

Sub-Saharan Africa has been identified as the part of the world with the greatest human genetic diversity. This high level of diversity causes difficulties for genome-wide association (GWA) studies in African populations — for example, by reducing the accuracy of genotype imputation in African populations compared to non-African populations. Here, we investigate haplotype variation and imputation in Africa, using 253 unrelated individuals from 15 Sub-Saharan African populations. We identify the populations that provide the greatest potential for serving as reference panels for imputing genotypes in the remaining groups. Considering reference panels comprising samples of recent African descent in Phase 3 of the HapMap Project, we identify mixtures of reference groups that produce the maximal imputation accuracy in each of the sampled populations. We find that optimal HapMap mixtures and maximal imputation accuracies identified in detailed tests of imputation procedures can instead be predicted by using simple summary statistics that measure relationships between the pattern of genetic variation in a target population and the patterns in potential reference panels. Our results provide an empirical basis for facilitating the selection of reference panels in GWA studies of diverse human populations, especially those of African ancestry.


[82] P Verdu, NA Rosenberg (2011) A general mechanistic model for admixture histories of hybrid populations. Genetics 189: 1413-1426. [PDF] [Supplementary Text] [Supplementary Figure 1] [Supplementary Table 1]

Admixed populations have been used for inferring migrations, detecting natural selection, and finding disease genes. These applications often use a simple statistical model of admixture rather than a modeling perspective that incorporates a more realistic history of the admixture process. Here, we develop a general model of admixture that mechanistically accounts for complex historical admixture processes. We consider two source populations contributing to the ancestry of a hybrid population, potentially with variable contributions across generations. For a random individual in the hybrid population at a given point in time, we study the fraction of genetic admixture originating from a specific one of the source populations by computing its moments as functions of time and of introgression parameters. We show that very different admixture processes can produce identical mean admixture proportions, but that such processes produce different values for the variance of the admixture proportion. When introgression parameters from each source population are constant over time, the long-term limit of the expectation of the admixture proportion depends only on the ratio of the introgression parameters. The variance of admixture decreases quickly over time after the source populations stop contributing to the hybrid population, but remains substantial when the contributions are ongoing. Our approach will facilitate the understanding of admixture mechanisms, illustrating how the moments of the distribution of admixture proportions can be informative about the historical admixture processes contributing to the genetic diversity of hybrid populations.


[81] S Ramachandran, NA Rosenberg (2011) A test of the influence of continental axes of orientation on patterns of human gene flow. American Journal of Physical Anthropology 146: 515-529. [PDF] [Supplementary Figure 1] [Supplementary Figure 2] [Supplementary Tables]

The geographic distribution of genetic variation reflects trends in past population migrations and can be used to make inferences about these migrations. It has been proposed that the east-west orientation of the Eurasian landmass facilitated the rapid spread of ancient technological innovations across Eurasia, while the north-south orientation of the Americas led to a slower diffusion of technology there. If the diffusion of technology was accompanied by gene flow, then this hypothesis predicts that genetic differentation in the Americas along lines of longitude will be greater than that in Eurasia along lines of latitude. We use 678 microsatellite loci from 68 indigenous populations in Eurasia and the Americas to investigate the spatial axes that underlie population-genetic variation. We find that genetic differentiation increases more rapidly along lines of longitude in the Americas than along lines of latitude in Eurasia. Distance along lines of latitude explains a sizeable portion of genetic distance in Eurasia, whereas distance along lines of longitude does not explain a large proportion of Eurasian genetic variation. Genetic differentiation in the Amercias occurs along both latitudinal and longitudinal axes and has a greater magnitude than corresponding differentiation in Eurasia, even when adjusting for the lower level of genetic variation in the American populations. These results support the view that continental orientation has influenced migration patterns and has played an important role in determining both the structure of human genetic variation and the distribution and spread of cultural traits.


[80] M DeGiorgio, JH Degnan, NA Rosenberg (2011) Coalescence-time distributions in a serial founder model of human evolutionary history. Genetics 189: 579-593. [PDF]

Simulation studies have demonstrated that a variety of patterns in worldwide genetic variation are compatible with the trends predicted by a serial founder model, in which populations expand outward from an initial source via a process in which new populations contain only subsets of the genetic diversity present in their parental populations. Here, we provide analytical results for key quantities under the serial founder model, deriving distributions of coalescence times for pairs of lineages sampled either from the same population or from different populations. We use these distributions to obtain expectations for coalescence times and for homozygosity and heterozygosity values. A predicted approximate linear decline in expected heterozygosity with increasing distance from the source population reproduces a pattern that has been observed both in human genetic data and in simulations. Our formulas predict that populations close to the source location have lower between-population gene identity than populations far from the source, also mirroring results obtained from data and simulations. We show that different models that produce similar declining patterns in heterozygosity generate quite distinct patterns in coalescence-time distributions and gene identity measures, thereby providing a basis for distinguishing these models. We interpret the theoretical results in relation to their implications for human population genetics.


[79] SM Boca, NA Rosenberg (2011) Mathematical properties of Fst between admixed populations and their parental source populations. Theoretical Population Biology 80: 208-216. [PDF]

We consider the properties of the Fst measure of genetic divergence between an admixed population and its parental source populations. Among all possible populations admixed among an arbitrary set of parental populations, we show that the value of Fst between an admixed population and a specific source population is maximized when the admixed population is simply the most distant of the other source populations. For the case with only two parental populations, as a function of the admixture fraction, we further demonstrate that this Fst value is monotonic and convex, so that Fst is informative about the admixture fraction. We illustrate our results using example human population-genetic data, showing how they provide a framework in which to interpret the features of Fst in admixed populations.


[78] ZA Szpiech, NA Rosenberg (2011) On the size distribution of private microsatellite alleles. Theoretical Population Biology 80: 100-113. [PDF]

Private microsatellite alleles tend to be found in the tails rather than in the interior of the allele size distribution. To explain this phenomenon, we have investigated the size distribution of private alleles in a coalescent model of two populations, assuming the symmetric stepwise mutation model as the mode of microsatellite mutation. For the case in which four alleles are sampled, two from each population, we condition on the configuration in which three distinct allele sizes are present, one of which is common to both populations, one of which is private to one population, and the third of which is private to the other population. Conditional on this configuration, we calculate the probability that the two private alleles occupy the two tails of the size distribution. This probability, which increases as a function of mutation rate and divergence time between the two populations, is seen to be greater than the value that would be predicted if there was no relationship between privacy and location in the allele size distribution. In accordance with the prediction of the model, we find that in pairs of human populations, the frequency with which private microsatellite alleles occur in the tails of the allele size distribution increases as a function of genetic differentiation between populations.


[77] Z Yang, M Rosenthal, NA Rosenberg, S Talarico, L Zhang, C Marrs, VO Thomsen, T Lillebaek, AB Andersen (2011) How dormant is Mycobacterium tuberculosis during latency? A study integrating genomics and molecular epidemiology. Infection, Genetics and Evolution 11: 1164-1167. [PDF]

Mycobacterium tuberculosis may survive for decades in the human body in a state termed latent tuberculosis infection (LTBI). We investigated the occurrence during LTBI of insertion/deletion events in a selected set of mononucleotide simple sequence repeats, DNA sequence changes in four M. tuberculosis genes, and large sequence variations in 4750 M. tuberculosis open reading frames. We studied 13 paired M. tuberculosis clinical isolates, with each pair representing a reactivation of LTBI more than three decades after primary infection. Absence of sequence variations between paired isolates in nearly all investigated loci suggests a low likelihood of bacterial replication during LTBI.


[76] EO Buzbas, P Joyce, NA Rosenberg (2011) Inference on the strength of balancing selection for epistatically interacting loci. Theoretical Population Biology 79: 102-113. [PDF]

Existing inference methods for estimating the strength of balancing selection in multi-locus genotypes rely on the assumption that there are no epistatic interactions between loci. Complex systems in which balancing selection is prevalent, such as sets of human immune system genes, are known to contain components that interact epistatically. Therefore, current methods may not produce reliable inference on the strength of selection at these loci. In this paper, we address this problem by presenting statistical methods that can account for epistatic interactions in making inference about balancing selection. A theoretical result due to Fearnhead (2006) is used to build a multi-locus Wright-Fisher model of balancing selection, allowing for epistatic interactions among loci. Antagonistic and synergistic types of interactions are examined. The joint posterior distribution of the selection and mutation parameters is sampled by Markov chain Monte Carlo methods, and the plausibility of models is assessed via Bayes factors. As a component of the inference process, an algorithm to generate multi-locus allele frequencies under balancing selection models with epistasis is also presented. Recent evidence on interactions among a set of human immune system genes is introduced as a motivating biological system for the epistatic model, and data on these genes are used to demonstrate the methods.


[75] CV Than, NA Rosenberg (2011) Consistency properties of species tree inference by minimizing deep coalescences. Journal of Computational Biology 18: 1-15. [PDF]

Methods for inferring species trees from sets of gene trees need to account for the possibility of discordance among the gene trees. Assuming that discordance is caused by incomplete lineage sorting, species tree estimates can be obtained by finding those species trees that minimize the number of "deep" coalescence events required for a given collection of gene trees. Efficient algorithms now exist for applying the minimizing-deep-coalescence (MDC) criterion, and simulation experiments have demonstrated its promising performance. However, it has also been noted from simulation results that the MDC criterion is not always guaranteed to infer the correct species tree estimate. In this article, we investigate the consistency of the MDC criterion. Using the multipscies coalescent model, we show that there are indeed anomaly zones for the MDC criterion for asymmetric four-taxon species tree topologies, and for all species tree topologies with five or more taxa.


[74] M DeGiorgio*, I Jankovic*, NA Rosenberg (2010) Unbiased estimation of gene diversity in samples containing related individuals: exact variance and arbitrary ploidy. Genetics 186: 1367-1387. [PDF]

Gene diversity, a commonly used measure of genetic variation, evaluates the proportion of heterozygous individuals expected at a locus in a population, under the assumption of Hardy-Weinberg equilibrium. When using the standard estimator of gene diversity, the inclusion of related or inbred individuals in a sample produces a downward bias. Here, we extend a recently developed estimator shown to be unbiased in a diploid autosomal sample that includes known related or inbred individuals to the general case of arbitrary ploidy. We derive an exact formula for the variance of the new estimator, \tilde{H}, and present an approximation to facilitate evaluation of the variance when each individual is related to at most one other individual in a sample. When examining samples from the human X chromosome, which represent a mixture of haploid and diploid individuals, we find that \tilde{H} performs favorably compared to the standard estimator, both in theoretical computations of mean squared error and in data analysis. We thus propose that \tilde{H} is a useful tool in characterizing gene diversity in samples of arbitrary ploidy that contain related or inbred individuals.


[73] TJ Pemberton, C Wang, JZ Li, NA Rosenberg (2010) Inference of unexpected genetic relatedness among individuals in HapMap Phase III. American Journal of Human Genetics 87: 457-464. [PDF] [Supplement]

The International Haplotype Map Project (HapMap) has provided an essential database for studies of human population genetics and genome-wide association. Phases I and II of the HapMap project generated genotype data across 3 million SNP loci in 270 individuals representing four populations. Phase III provides dense genotype data on 1.5 million SNPs, generated by Illumina and Affymetrix platforms in a larger set of individuals. Release 3 of phase III of the HapMap contains 1397 individuals from 11 populations, including 250 of the original 270 phase I and phase II individuals and 1147 additional individuals. Although some known relationships among the phase III individuals have been described in the data release, the genotype data that are currently available provide an opportunity to empirically ascertain previously unknown relationships. We performed a systematic analysis of genetic relatedness and were able not only to confirm the reported relationships, but also to detect numerous additional, previously unidentified pairs of close relatives in the HapMap sample. The inferred relative pairs make it possible to propose standardized subsets of unrelated individuals for use in future studies in which relatedness needs to be clearly defined.


[72] E Borràs*, M Pineda*, I Blanco, EM Jewett, F Wang, A Teulé, T Caldés, M Urioste, C Martínez-Bouzas, J Brunet, J Balmaña, A Torres, T Ramón y Cajal, J Sanz, L Pérez-Cabornero, S Castellví-Bel, A Alonso, A Lanas, S González, V Moreno, SB Gruber, NA Rosenberg, B Mukherjee, C Lázaro, G Capellá (2010) MLH1 founder mutations with moderate penetrance in Spanish Lynch syndrome families. Cancer Research 70: 7379-7391. [PDF] [Supplementary Figure 1] [Supplementary Table 1] [Supplementary Table 2] [Supplementary Methods]

The variants c.306+5G>A and c.1865T>A (p.Leu622His) of the DNA repair gene MLH1 occur frequently in Spanish Lynch syndrome families. To understand their ancestral history and clinical effect, we performed functional assays and a penetrance analysis and studied their genetic and geographic origins. Detailed family histories were taken from 29 carrier families. Functional analysis included in silico and in vitro assays at the RNA and protein levels. Penetrance was calculated using a modified segregation analysis adjusted for ascertainment. Founder effects were evaluated by haplotype analysis. The identified MLH1 c.306+5G>A and c.1865T>A (p.Leu622His) variants are absent in control populations and segregate with the disease. Tumors from carriers of both variants show microsatellite instability and loss of expression of the MLH1 protein. The c.306+5G>A variant is a pathogenic mutation affecting mRNA processing. The c.1865T>A (p.Leu622His) variant causes defects in MLH1 expression and stability. For both mutations, the estimated penetrance is moderate (age-cumulative colorectal cancer risk by age 70 of 20.1% and 14.1% for c.306+5G>A and of 6.8% and 7.3% for c.1865T>A in men and women carriers, respectively) in the lower range of variability estimated for other pathogenic Spanish MLH1 mutations. A common haplotype was associated with each of the identified mutations, confirming their founder origin. The ages of c.306+5G>A and c.1865T>A mutations were estimated to be 53 to 122 and 12 to 22 generations, respectively. Our results confirm the pathogenicity, moderate penetrance, and founder origin of the MLH1 c.306+5G>A and c.1865T>A mutations. These findings have important implications for genetic counseling and molecular diagnosis of Lynch syndrome.


[71] NA Rosenberg (2010) Review of First Peoples in a New World: Colonizing Ice Age America by DJ Meltzer. Quarterly Review of Biology 85: 380-381.

(No abstract)


[70] I Jankovic, BM vonHoldt, NA Rosenberg (2010) Heterozygosity of the Yellowstone wolves. Molecular Ecology 19: 3246-3249. [PDF]

(No abstract)


[69] NA Rosenberg, L Huang*, EM Jewett*, ZA Szpiech*, I Jankovic*, M Boehnke (2010) Genome-wide association studies in diverse populations. Nature Reviews Genetics 11: 356-366. [PDF]

Genome-wide association (GWA) studies have identified a large number of SNPs associated with disease phenotypes. As most GWA studies have been performed in populations of European descent, this Review examines the issues involved in extending the consideration of GWA studies to diverse worldwide populations. Although challenges exist with issues such as imputation, admixture and replication, investigation of a greater diversity of populations could make substantial contributions to the goal of mapping the genetic determinants of complex diseases for the human population as a whole.


[68] NA Rosenberg, JH Degnan (2010) Coalescent histories for discordant gene trees and species trees. Theoretical Population Biology 77: 145-151. [PDF]

Given a gene tree and a species tree, a coalescent history is a list of the branches of the species tree on which coalescences in the gene tree take place. Each pair consisting of a gene tree topology and a species tree topology has some number of possible coalescent histories. Here we show that, for each n &ge 7, there exist a species tree topology S and a gene tree topology G &ne S, both with n leaves, for which the number of coalescent histories exceeds the corresponding number of coalescent histories when the species tree topology is S and the gene tree topology is also S. This result has the interpretation that the gene tree topology G discordant with the species tree topology S can be produced by the evolutionary process in more ways than can the gene tree topology that matches the species tree topology, providing further insight into the surprising combinatorial properties of gene trees that arise from their joint consideration with species trees.


[67] M DeGiorgio, JH Degnan (2010) Fast and consistent estimation of species trees using supermatrix rooted triples. Molecular Biology and Evolution 27: 552-569. [PDF]

Concatenated sequence alignments are often used to infer species-level relationships. Previous studies have shown that analysis of concatenated data using maximum likelihood (ML) can produce misleading results when loci have differing gene tree topologies due to incomplete lineage sorting. Here, we develop a polynomial time method that utilizes the modified mincut supertree algorithm to construct an estimated species tree from inferred rooted triples of concatenated alignments. We term this method SuperMatrix Rooted Triple (SMRT) and use the notation SMRT-ML when rooted triples are inferred by ML. We use simulations to investigate the performance of SMRT-ML under Jukes-Cantor and general time-reversible substitution models for four- and five-taxon species trees and also apply the method to an empirical data set of yeast genes. We find that SMRT-ML converges to the correct species tree in many cases in which ML on the full concatenated data set fails to do so. SMRT-ML can be conservative in that its output tree is often partially unresolved for problematic clades. We show analytically that when the species tree is clocklike and mutations occur under the Cavender-Farris-Neyman substitution model, as the number of genes increases, SMRT-ML is increasingly likely to infer the correct species tree even when the most likely gene tree does not match the species tree. SMRT-ML is therefore a computationally efficient and statistically consistent estimator of the species tree when gene trees are distributed according to the multispecies coalescent model.


[66] C Wang, ZA Szpiech, JH Degnan, M Jakobsson, TJ Pemberton, JA Hardy, AB Singleton, NA Rosenberg (2010) Comparing spatial maps of human population-genetic variation using Procrustes analysis. Statistical Applications in Genetics and Molecular Biology 9: 13. [PDF]

Recent applications of principal components analysis (PCA) and multidimensional scaling (MDS) in human population genetics have found that "statistical maps" based on the genotypes in population-genetic samples often resemble geographic maps of the underlying sampling locations. To provide formal tests of these qualitative observations, we describe a Procrustes analysis approach for quantitatively assessing the similarity of population-genetic and geographic maps. We confirm in two scenarios, one using single-nucleotide polymorphism (SNP) data from Europe and one using SNP data worldwide, that a measurably high level of concordance exists between statistical maps of population-genetic variation and geographic maps of sampling locations. Two other examples illustrate the versatility of the Procrustes approach in population-genetic applications, verifying the concordance of SNP analyses using PCA and MDS, and showing that statistical maps of worldwide copy-number variants (CNVs) accord with statistical maps of SNP variation, especially when CNV analysis is limited to samples with the highest-quality data. As statistical maps with PCA and MDS have become increasingly common for use in summarizing population relationships, our examples highlight the potential of Procrustes-based quantitative comparisons for interpreting the results in these maps.


[65] JT Mosher, TJ Pemberton, K Harter, C Wang, EO Buzbas, P Dvorak, C Simon, SJ Morrison, NA Rosenberg (2010) Lack of population diversity in commonly used human embryonic stem-cell lines. New England Journal of Medicine 362: 183-185. [PDF] [Supplement]

(No abstract)


[64] TJ Pemberton, CI Sandefur, M Jakobsson, NA Rosenberg (2009) Sequence determinants of human microsatellite variability. BMC Genomics 10: 612. [Full text at journal website] [PDF] [Supplementary table 1 (XLS)] [Supplementary table 2 (XLS)] [Supplementary tables 3-6 (PDF)] [Data]

BACKGROUND. Microsatellite loci are frequently used in genomic studies of DNA sequence repeats and in population studies of genetic variability. To investigate the effect of sequence properties of microsatellites on their level of variability we have analyzed genotypes at 627 microsatellite loci in 1,048 worldwide individuals from the HGDP-CEPH cell line panel together with the DNA sequences of these microsatellites in the human RefSeq database.
RESULTS. Calibrating PCR fragment lengths in individual genotypes by using the RefSeq sequence enabled us to infer repeat number in the HGDP-CEPH dataset and to calculate the mean number of repeats (as opposed to the mean PCR fragment length), under the assumption that differences in PCR fragment length reflect differences in the numbers of repeats in the embedded repeat sequences. We find the mean and maximum numbers of repeats across individuals to be positively correlated with heterozygosity. The size and composition of the repeat unit of a microsatellite are also important factors in predicting heterozygosity, with tetra-nucleotide repeat units high in G/C content leading to higher heterozygosity. Finally, we find that microsatellites containing more separate sets of repeated motifs generally have higher heterozygosity.
CONCLUSIONS. These results suggest that sequence properties of microsatellites have a significant impact in determining the features of human microsatellite variability.


[63] NM Kopelman, L Stone, C Wang, D Gefel, MW Feldman, J Hillel, NA Rosenberg (2009) Genomic microsatellites identify shared Jewish ancestry intermediate between Middle Eastern and European populations. BMC Genetics 10: 80. [Full text at journal website] [PDF]

BACKGROUND. Genetic studies have often produced conflicting results on the question of whether distant Jewish populations in different geographic locations share greater genetic similarity to each other or instead, to nearby non-Jewish populations. We perform a genome-wide population-genetic study of Jewish populations, analyzing 678 autosomal microsatellite loci in 78 individuals from four Jewish groups together with similar data on 321 individuals from 12 non-Jewish Middle Eastern and European populations.
RESULTS. We find that the Jewish populations show a high level of genetic similarity to each other, clustering together in several types of analysis of population structure. Further, Bayesian clustering, neighbor-joining trees, and multidimensional scaling place the Jewish populations as intermediate between the non-Jewish Middle Eastern and European populations.
CONCLUSION. These results support the view that the Jewish populations largely share a common Middle Eastern ancestry and that over their history they have undergone varying degrees of admixture with non-Jewish populations of European descent.


[62] L Huang, C Wang, NA Rosenberg (2009) The relationship between imputation error and statistical power in genetic association studies in diverse populations. American Journal of Human Genetics 85: 692-698. [PDF] [Supplement]

Genotype-imputation methods provide an essential technique for high-resolution genome-wide association (GWA) studies with millions of single-nucleotide polymorphisms. For optimal design and interpretation of imputation-based GWA studies, it is important to understand the connection between imputation error and power to detect associations at imputed markers. Here, using a 2x3 chi-square test, we describe a relationship between genotype-imputation error rates and the sample-size inflation required for achieving statistical power at an imputed marker equal to that obtained if genotypes at the marker were known with certainty. Surprisingly, typical imputation error rates (~2%-6%) lead to a large increase in the required sample size (~10%-60%) and in some African populations whose genotypes are particularly difficult to impute, the required sample-size increase is as high as ~30%-150%). In most populations, each 1% increase in imputation error leads to an increase of ~5%-13% in the sample size required for maintaining power. These results imply that in GWA sample-size calculations investigators will need to account for a potentially considerable loss of power from even low levels of imputation error and that development of additional genomic resources that decrease imputation error will translate into substantial reduction in the sample sizes needed for imputation-based detection of the variants that underlie complex human diseases.


[61] M DeGiorgio, M Jakobsson, NA Rosenberg (2009) Explaining worldwide patterns of human genetic variation using a coalescent-based serial founder model of migration outward from Africa. Proceedings of the National Academy of Sciences USA 106: 16057-16062. [PDF] [Supplement]

Studies of worldwide human variation have discovered three trends in summary statistics as a function of increasing geographic distance from East Africa: a decrease in heterozygosity, an increase in linkage disequilibrium (LD), and a decrease in the slope of the ancestral allele frequency spectrum. Forward simulations of unlinked loci have shown that the decline in heterozygosity can be described by a serial founder model, in which populations migrate outward from Africa through a process where each of a series of populations is formed from a subset of the previous population in the outward expansion. Here, we extend this approach by developing a retrospective coalescent-based serial founder model that incorporates linked loci. Our model both recovers the observed decline in heterozygosity with increasing distance from Africa and produces the patterns observed in LD and the ancestral allele frequency spectrum. Surprisingly, although migration between neighboring populations and limited admixture between modern and archaic humans can be accommodated in the model while continuing to explain the three trends, a competing model in which a wave of outward modern human migration expands into a series of preexisting archaic populations produces nearly opposite patterns to those observed in the data. We conclude by developing a simpler model to illustrate that the feature that permits the serial founder model but not the archaic persistence model to explain the three trends observed with increasing distance from Africa is its incorporation of a cumulative effect of genetic drift as humans colonized the world.


[60] NA Rosenberg, JM VanLiere (2009) Replication of genetic associations as pseudoreplication due to shared genealogy. Genetic Epidemiology 33: 479-487. [PDF]

The genotypes of individuals in replicate genetic association studies have some level of correlation due to shared descent in the complete pedigree of all living humans. As a result of this genealogical sharing, replicate studies that search for genotype-phenotype associations using linkage disequilibrium between marker loci and disease-susceptibility loci can be considered as pseudoreplicates rather than true replicates. We examine the size of the pseudoreplication effect in association studies simulated from evolutionary models of the history of a population, evaluating the excess probability that both of a pair of studies detect a disease association compared to the probability expected under the assumption that the two studies are independent. Each of nine combinations of a demographic model and a penetrance model leads to a detectable pseudoreplication effect, suggesting that the degree of support that can be attributed to a replicated genetic association result is less than that which can be attributed to a replicated result in a context of true independence.


[59] JH Degnan, M DeGiorgio, D Bryant, NA Rosenberg (2009) Properties of consensus methods for inferring species trees from gene trees. Systematic Biology 58: 35-54. [PDF]

Consensus methods provide a useful strategy for summarizing information from a collection of gene trees. An important application of consensus methods is to combine gene trees to estimate a species tree. To investigate the theoretical properties of consensus trees that would be obtained from large numbers of loci evolving according to a basic evolutionary model, we construct consensus trees from rooted gene trees that occur in proportion to gene-tree probabilities derived from coalescent theory. We consider majority-rule, rooted triple (R*), and greedy consensus trees obtained from known, rooted gene trees, both in the asymptotic case as numbers of gene trees approach infinity and for finite numbers of genes. Our results show that for some combinations of species-tree branch lengths, increasing the number of independent loci can make the rooted majority-rule consensus tree more likely to be at least partially unresolved. However, the probability that the R* consensus tree has the species-tree topology approaches 1 as the number of gene trees approaches infinity. Although the greedy consensus algorithm can be the quickest to converge on the correct species-tree topology when increasing the number of gene trees, it can also be positively misleading. The majority-rule consensus tree is not a misleading estimator of the species-tree topology, and the R* consensus tree is a statistically consistent estimator of the species-tree topology. Our results therefore suggest a method for using multiple loci to infer the species-tree topology, even when it is discordant with the most likely gene tree.


[58] JH Degnan, NA Rosenberg (2009) Gene tree discordance, phylogenetic inference and the multispecies coalescent. Trends in Ecology and Evolution 24: 332-340. [PDF] [Supplement]

The field of phylogenetics is entering a new era in which trees of historical relationships between species are increasingly inferred from multilocus and genomic data. A major challenge for incorporating such large amounts of data into inference of species trees is that conflicting genealogical histories often exist in different genes throughout the genome. Recent advances in genealogical modeling suggest that resolving close species relationships is not quite as simple as applying more data to the problem. Here we discuss the complexities of genealogical discordance and review the issues that new methods for multilocus species tree inference will need to address to account successfully for naturally occurring genomic variability in evolutionary histories.


[57] KB Schroeder, M Jakobsson, MH Crawford, TG Schurr, SM Boca, DF Conrad, RY Tito, LP Osipova, LA Tarskaia, SI Zhadanov, JD Wall, JK Pritchard, RS Malhi, DG Smith, NA Rosenberg (2009) Haplotypic background of a private allele at high frequency in the Americas. Molecular Biology and Evolution 26: 995-1016. [PDF] [Supplement]

Recently, the observation of a high-frequency private allele, the 9-repeat allele at microsatellite D9S1120, in all sampled Native American and Western Beringian populations has been interpreted as evidence that all modern Native Americans descend primarily from a single founding population. However, this inference assumed that all copies of the 9-repeat allele were identical by descent and that the geographic distribution of this allele had not been influenced by natural selection. To investigate whether these assumptions are satisfied, we genotyped 34 single nucleotide polymorphisms across ~500 kilobases (kb) around D9S1120 in 21 Native American and Western Beringian populations and 54 other worldwide populations. All chromosomes with the 9-repeat allele share the same haplotypic background in the vicinity of D9S1120, suggesting that all sampled copies of the 9-repeat allele are identical by descent. Ninety-one percent of these chromosomes share the same 76.26 kb haplotype, which we call the "American Modal Haplotype" (AMH). Three observations lead us to conclude that the high frequency and widespread distribution of the 9-repeat allele are unlikely to be the result of positive selection: 1) aside from its association with the 9-repeat allele, the AMH does not have a high frequency in the Americas, 2) the AMH is not unusually long for its frequency compared with other haplotypes in the Americas, and 3) in Latin American mestizo populations, the proportion of Native American ancestry at D9S1120 is not unusual compared with that observed at other genomewide microsatellites. Using a new method for estimating the time to the most recent common ancestor (MRCA) of all sampled copies of an allele on the basis of an estimate of the length of the genealogy descended from the MRCA, we calculate the mean time to the MRCA of the 9-repeat allele to be between 7,325 and 39,900 years, depending on the demographic model used. The results support the hypothesis that all modern Native Americans and Western Beringians trace a large portion of their ancestry to a single founding population that may have been isolated from other Asian populations prior to expanding into the Americas.


[56] M DeGiorgio, NA Rosenberg (2009) An unbiased estimator of gene diversity in samples containing related individuals. Molecular Biology and Evolution 26: 501-512. [PDF]

Gene diversity is sometimes estimated from samples that contain inbred or related individuals. If inbred or related individuals are included in a sample, then the standard estimator for gene diversity produces a downward bias caused by an inflation of the variance of estimated allele frequencies. We develop an unbiased estimator for gene diversity that relies on kinship coefficients for pairs of individuals with known relationship and that reduces to the standard estimator when all individuals are noninbred and unrelated. Applying our estimator to data simulated based on allele frequencies observed for microsatellite loci in human populations, we find that the new estimator performs favorably compared with the standard estimator in terms of bias and similarly in terms of mean squared error. For human population-genetic data, we find that a close linear relationship previously seen between gene diversity and distance from East Africa is preserved when adjusting for the inclusion of close relatives.


[55] L Huang, Y Li, AB Singleton, JA Hardy, G Abecasis, NA Rosenberg, P Scheet (2009) Genotype-imputation accuracy across worldwide human populations. American Journal of Human Genetics 84: 235-250. [PDF] [Supplement]

A current approach to mapping complex-disease-susceptibility loci in genome-wide association (GWA) studies involves leveraging the information in a reference database of dense genotype data. By modeling the patterns of linkage disequilibrium in a reference panel, genotypes not directly measured in the study samples can be imputed and tested for disease association. This imputation strategy has been successful for GWA studies in populations well represented by existing reference panels. We used genotypes at 513,008 autosomal single-nucleotide polymorphism (SNP) loci in 443 unrelated individuals from 29 worldwide populations to evaluate the "portability" of the HapMap reference panels for imputation in studies of diverse populations. When a single HapMap panel was leveraged for imputation of randomly masked genotypes, European populations had the highest imputation accuracy, followed by populations from East Asia, Central and South Asia, the Americas, Oceania, the Middle East, and Africa. For each population, we identified "optimal" mixtures of reference panels that maximized imputation accuracy, and we found that in most populations, mixtures including individuals from at least two HapMap panels produced the highest imputation accuracy. From a separate survey of additional SNPs typed in the same samples, we evaluated imputation accuracy in the scenario in which all genotypes at a given SNP position were unobserved and were imputed on the basis of data from a commercial "SNP chip," again finding that most populations benefited from the use of combinations of two or more HapMap reference panels. Our results can serve as a guide for selecting appropriate reference panels for imputation-based GWA analysis in diverse populations.


[54] S Ramachandran, NA Rosenberg, MW Feldman, J Wakeley (2008) Population differentiation and migration: coalescence times in a two-sex island model for autosomal and X-linked loci. Theoretical Population Biology 74: 291-301. [PDF]

Evolutionists have debated whether population-genetic parameters, such as effective population size and migration rate, differ between males and females. In humans, most analyses of this problem have focused on the Y chromosome and the mitochondrial genome, while the X chromosome has largely been omitted from the discussion. Past studies have compared FST values for the Y chromosome and mitochondrion under a model with migration rates that differ between the sexes but with equal male and female population sizes. In this study we investigate rates of coalescence for X-linked and autosomal lineages in an island model with different population sizes and migration rates for males and females, obtaining the mean time to coalescence for pairs of lineages from the same deme and for pairs of lineages from different demes. We apply our results to microsatellite data from the Human Genome Diversity Panel, and we examine the male and female migration rates implied by observed FST values.


[53] ZA Szpiech, M Jakobsson, NA Rosenberg (2008) ADZE: a rarefaction approach for counting alleles private to combinations of populations. Bioinformatics 24: 2498-2504 [Full text at journal website] [PDF] [Software]

Motivation: Analysis of the distribution of alleles across populations is a useful tool for examining population diversity and relationships. However, sample sizes often differ across populations, sometimes making it difficult to assess allelic distributions across groups.
Results: We introduce a generalized rarefaction approach for counting alleles private to combinations of populations. Our method evaluates the number of alleles found in each of a set of populations but absent in all remaining populations, considering equal-sized subsamples from each population. Applying this method to a worldwide human microsatellite dataset, we observe a high number of alleles private to the combination of African and Oceanian populations. This result supports the possibility of a migration out of Africa into Oceania separate from the migrations responsible for the majority of the ancestry of the modern populations of Asia, and it highlights the utility of our approach to sample size correction in evaluating hypotheses about population history.
Availability: We have implemented our method in the computer program ADZE, which is available for download at http://rosenberglab.bioinformatics.med.umich.edu/adze.html.
Contact: szpiechz@umich.edu


[52] NA Rosenberg, M Jakobsson (2008) The relationship between homozygosity and the frequency of the most frequent allele. Genetics 179: 2027-2036. [PDF]

Homozygosity is a commonly used summary of allele-frequency distributions at polymorphic loci. Because high-frequency alleles contribute disproportionately to the homozygosity of a locus, it often occurs that most homozygotes are homozygous for the most frequent allele. To assess the relationship between homozygosity and the highest allele frequency at a locus, for a given homozygosity value, we determine the lower and upper bounds on the frequency of the most frequent allele. These bounds suggest tight constraints on the frequency of the most frequent allele as a function of homozygosity, differing by at most 1/4 and having an average difference of 2/3 - π2 / 18 &cong 0.1184. The close connection between homozygosity and the frequency of the most frequent allele — which we illustrate using allele frequencies from human populations — has the consequence that when one of these two quantities is known, considerable information is available about the other quantity. This relationship also explains the similar performance of statistical tests of population-genetic models that rely on homozygosity and those that rely on the frequency of the most frequent allele, and it provides a basis for understanding the utility of extended homozygosity statistics in identifying haplotypes that have been elevated to high frequency as a result of positive selection.


[51] JM VanLiere, NA Rosenberg (2008) Mathematical properties of the r2 measure of linkage disequilibrium. Theoretical Population Biology 74: 130-137. [PDF]

Statistics for linkage disequilibrium (LD), the non-random association of alleles at two loci, depend on the frequencies of the alleles at the loci under consideration. Here, we examine the r2 measure of LD and its mathematical relationship to allele frequencies, quantifying the constraints on its maximum value. Assuming independent uniform distributions for the allele frequencies of two biallelic loci, we find that the mean maximum value of r2 is ~0.43051, and that r2 can exceed a threshold of 4/5 in only ~14.232% of the allele frequency space. If one locus is assumed to have known allele frequencies — the situation in an association study in which LD between a known marker locus and an unknown trait locus is of interest — we find that the mean maximum value of r2 is greatest when the known locus has a minor allele frequency of ~0.30131. We find that in 1/4 of the space of allowed values of minor allele frequencies and haplotype frequencies at a pair of loci, the unconstrained maximum r2 allowing for the possibility of recombination between the loci exceeds the constrained maximum assuming that no recombination has occurred. Finally, we use r2max to examine the connection between r2 and the D' measure of linkage disequilibrium, finding that r2/r2max = D'2 for ~72.683% of the space of allowed values of (pa,pb,pab). Our results concerning the properties of r2 have the potential to inform the interpretation of unusual LD behavior and to assist in the design of LD-based association-mapping studies.


[50] TJ Pemberton*, M Jakobsson*, DF Conrad, G Coop, JD Wall, JK Pritchard, PI Patel, NA Rosenberg (2008) Using population mixtures to optimize the utility of genomic databases: linkage disequilibrium and association study design in India. Annals of Human Genetics 72: 535-546. [PDF] [Data]

When performing association studies in populations that have not been the focus of large-scale investigations of haplotype variation, it is often helpful to rely on genomic databases in other populations for study design and analysis — such as in the selection of tag SNPs and in the imputation of missing genotypes. One way of improving the use of these databases is to rely on a mixture of database samples that is similar to the population of interest, rather than using the single most similar database sample. We demonstrate the effectiveness of the mixture approach in the application of African, European, and East Asian HapMap samples for tag SNP selection in populations from India, a genetically intermediate region underrepresented in genomic studies of haplotype variation.


[49] O François, MGB Blum, M Jakobsson, NA Rosenberg (2008) Demographic history of European populations of Arabidopsis thaliana. PLoS Genetics 4: e1000075. [Full text at journal website] [PDF] [Supplement]

The model plant species Arabidopsis thaliana is successful at colonizing land that has recently undergone human-mediated disturbance. To investigate the prehistoric spread of A. thaliana, we applied approximate Bayesian computation and explicit spatial modeling to 76 European accessions sequenced at 876 nuclear loci. We find evidence that a major migration wave occurred from east to west, affecting most of the sampled individuals. The longitudinal gradient appears to result from the plant having spread in Europe from the east ~10,000 years ago, with a rate of westward spread of ~0.9 km/year. This wave-of-advance model is consistent with a natural colonization from an eastern glacial refugium that overwhelmed ancient western lineages. However, the speed and time frame of the model also suggest that the migration of A. thaliana into Europe may have accompanied the spread of agriculture during the Neolithic transition.


[48] JM Macpherson, J Gonzalez, DM Witten, JC Davis, NA Rosenberg, AE Hirsh, DA Petrov (2008) Nonadaptive explanations for signatures of partial selective sweeps in Drosophila. Molecular Biology and Evolution 25: 1025-1042. [PDF]

A beneficial mutation that has nearly but not yet fixed in a population produces a characteristic haplotype configuration, called a partial selective sweep. Whether nonadaptive processes might generate similar haplotype configurations has not been extensively explored. Here, we consider 5 population genetic data sets taken from regions flanking high-frequency transposable elements in North American strains of Drosophila melanogaster, each of which appears to be consistent with the expectations of a partial selective sweep. We use coalescent simulations to explore whether incorporation of the species' demographic history, purifying selection against the element, or suppression of recombination caused by the element could generate putatively adaptive haplotype configurations. Whereas most of the data sets would be rejected as nonneutral under the standard neutral null model, only the data set for which there is strong external evidence in support of an adaptive transposition appears to be nonneutral under the more complex null model and in particular when demography is taken into account. High-frequency, derived mutations from a recently bottlenecked population, such as we study here, are of great interest to evolutionary genetics in the context of scans for adaptive events; we discuss the broader implications of our findings in this context.


[47] NA Rosenberg, R Tao (2008) Discordance of species trees with their most likely gene trees: the case of five taxa. Systematic Biology 57: 131-140. [Full text at journal website] [PDF] [Supplement]

Under a coalescent model for within-species evolution, gene trees may differ from species trees to such an extent that the gene tree topology most likely to evolve along the branches of a species tree can disagree with the species tree topology. Gene tree topologies that are more likely to be produced than the topology that matches that of the species tree are termed anomalous, and the region of branch-length space that gives rise to anomalous gene trees (AGTs) is the anomaly zone. We examine the occurrence of anomalous gene trees for the case of five taxa, the smallest number of taxa for which every species tree topology has a nonempty anomaly zone. Considering all sets of branch lengths that give rise to anomalous gene trees, the largest value possible for the smallest branch length in the species tree is greater in the five-taxon case (0.1934 coalescent time units) than in the previously studied case of four taxa (0.1568). The five-taxon case demonstrates the existence of three phenomena that do not occur in the four-taxon case. First, anomalous gene trees can have the same unlabeled topology as the species tree. Second, the anomaly zone does not necessarily enclose a ball centered at the origin in branch-length space, in which all branches are short. Third, as a branch length increases, it is possible for the number of AGTs to increase rather than decrease or remain constant. These results, which help to describe how the properties of anomalous gene trees increase in complexity as the number of taxa increases, will be useful in formulating strategies for evading the problem of anomalous gene trees during species tree inference from multilocus data.


[46] M Jakobsson*, SW Scholz*, P Scheet*, JR Gibbs, JM VanLiere, H-C Fung, ZA Szpiech, JH Degnan, K Wang, R Guerreiro, JM Bras, JC Schymick, DG Hernandez, BJ Traynor, J Simon-Sanchez, M Matarin, A Britton, J van de Leemput, I Rafferty, M Bucan, HM Cann, JA Hardy, NA Rosenberg, AB Singleton (2008) Genotype, haplotype and copy-number variation in worldwide human populations. Nature 451: 998-1003. [PDF] [Supplement]

Genome-wide patterns of variation across individuals provide a powerful source of data for uncovering the history of migration, range expansion, and adaptation of the human species. However, high-resolution surveys of variation in genotype, haplotype and copy number have generally focused on a small number of population groups. Here we report the analysis of high-quality genotypes at 525,910 single-nucleotide polymorphisms (SNPs) and 396 copy-number-variable loci in a worldwide sample of 29 populations. Analysis of SNP genotypes yields strongly supported fine-scale inferences about population structure. Increasing linkage disequilibrium is observed with increasing geographic distance from Africa, as expected under a serial founder effect for the out-of-Africa spread of human populations. New approaches for haplotype analysis produce inferences about population structure that complement results based on unphased SNPs. Despite a difference from SNPs in the frequency spectrum of the copy-number variants (CNVs) detected — including a comparatively large number of CNVs in previously unexamined populations from Oceania and the Americas — the global distribution of CNVs largely accords with population structure analyses for SNP data sets of similar size. Our results produce new inferences about inter-population variation, support the utility of CNVs in human population-genetic research, and serve as a genomic resource for human-genetic studies in diverse worldwide populations.


[45] K Zhang, NA Rosenberg (2007) On the genealogy of a duplicated microsatellite. Genetics 177: 2109-2122. [PDF]

When a microsatellite locus is duplicated in a diploid organism, a single pair of PCR primers may amplify as many as four distinct alleles. To study the evolution of a duplicated microsatellite, we consider a coalescent model with symmetric stepwise mutation. Conditional on the time of duplication and a mutation rate, both in a model of completely unlinked loci and in a model of completely linked loci, we compute the probabilities for a sampled diploid individual to amplify one, two, three, or four distinct alleles with one pair of microsatellite PCR primers. These probabilities are then studied to examine the nature of their dependence on the duplication time and the mutation rate. The mutation rate is observed to have a stronger effect than the duplication time on the four probabilities, and the unlinked and linked cases are seen to behave similarly. Our results can be useful for helping to interpret genetic variation at microsatellite loci in species with a very recent history of gene and genome duplication.


[44] S Wang*, CM Lewis Jr*, M Jakobsson*, S Ramachandran, N Ray, G Bedoya, W Rojas, MV Parra, JA Molina, C Gallo, G Mazzotti, G Poletti, K Hill, AM Hurtado, D Labuda, W Klitz, R Barrantes, MC Bortolini, FM Salzano, ML Petzl-Erler, LT Tsuneto, E Llop, F Rothhammer, L Excoffier, MW Feldman, NA Rosenberg, A Ruiz-Linares (2007) Genetic variation and population structure in Native Americans. PLoS Genetics 3: 2049-2067. [Full text at journal website] [PDF] [Supplement] [Data] [Readme for datafile]

We examined genetic diversity and population structure in the American landmass using 678 autosomal microsatellite markers genotyped in 422 individuals representing 24 Native American populations sampled from North, Central, and South America. These data were analyzed jointly with similar data available in 54 other indigenous populations worldwide, including an additional five Native American groups. The Native American populations have lower genetic diversity and greater differentiation than populations from other continental regions. We observe gradients both of decreasing genetic diversity as a function of geographic distance from the Bering Strait and of decreasing genetic similarity to Siberians — signals of the southward dispersal of human populations from the northwestern tip of the Americas. We also observe evidence of: (1) a higher level of diversity and lower level of population structure in western South America compared to eastern South America, (2) a relative lack of differentiation between Mesoamerican and Andean populations, (3) a scenario in which coastal routes were easier for migrating peoples to traverse in comparison with inland routes, and (4) a partial agreement on a local scale between genetic similarity and the linguistic classification of populations. These findings offer new insights into the process of population dispersal and differentiation during the peopling of the Americas.


[43] M Jakobsson, NA Rosenberg (2007) CLUMPP: a cluster matching and permutation program for dealing with label switching and multimodality in analysis of population structure. Bioinformatics 23: 1801-1806. [Full text at journal website] [PDF] [Software]

Motivation: Clustering of individuals into populations on the basis of multilocus genotypes is informative in a variety of settings. In population-genetic clustering algorithms, such as BAPS, STRUCTURE and TESS, individual multilocus genotypes are partitioned over a set of clusters, often using unsupervised approaches that involve stochastic simulation. As a result, replicate cluster analyses of the same data may produce several distinct solutions for estimated cluster membership coefficients, even though the same initial conditions were used. Major differences among clustering solutions have two main sources: (1) `label switching' of clusters across replicates, caused by the arbitrary way in which clusters in an unsupervised analysis are labeled, and (2) `genuine multimodality,' truly distinct solutions across replicates.
Results: To facilitate the interpretation of population-genetic clustering results, we describe three algorithms for aligning multiple replicate analyses of the same data set. We have implemented these algorithms in the computer program CLUMPP (CLUster Matching and Permutation Program). We illustrate the use of CLUMPP by aligning the cluster membership coefficients from 100 replicate cluster analyses of 600 chickens from 20 different breeds.
Availability: CLUMPP is freely available at http://rosenberglab.bioinformatics.med.umich.edu/clumpp.html
Contact: Mattias Jakobsson


[42] MGB Blum, NA Rosenberg (2007) Estimating the number of ancestral lineages using a maximum likelihood method based on rejection sampling. Genetics 176: 1741-1757. [PDF]

Estimating the number of ancestral lineages of a sample of DNA sequences at time t in the past can be viewed as a variation on the problem of estimating the time to the most recent common ancestor. To estimate the number of ancestral lineages, we develop a maximum-likelihood approach that takes advantage of a prior model of population demography, in addition to the molecular data summarized by the pattern of polymorphic sites. The method relies on a rejection sampling algorithm that is introduced for simulating conditional coalescent trees given a fixed number of ancestral lineages at time t. Computer simulations show that the number of ancestral lineages can be estimated accurately, provided that the number of mutations that occurred since time t is sufficiently large. The method is applied to 986 present-day human sequences located in hypervariable region 1 of the mitochondrion to estimate the number of ancestral lineages of modern humans at the time of potential admixture with the Neanderthal population. Our estimates support a view that the proportion of the modern population consisting of Neanderthal contributions must be relatively small, less than ~5%, if the admixture happened as recently as 30,000 years ago.


[41] NA Rosenberg (2007) Counting coalescent histories. Journal of Computational Biology 14: 360-377. [PDF]

Given a species tree and a gene tree, a valid coalescent history is a list of the branches of the species tree on which coalescences in the gene tree take place. I develop a recursion for the number of valid coalescent histories that exist for an arbitrary gene tree/species tree pair, when one gene lineage is studied per species. The result is obtained by defining a concept of m-extended coalescent histories, enumerating and counting these histories, and taking the special case of m=1. As a sum over valid coalescent histories appears in a formula for the probability that a random gene tree evolving along the branches of a fixed species tree has a specified labeled topology, the enumeration of valid coalescent histories can considerably reduce the effort required for evaluating this formula.


[40] NA Rosenberg, MGB Blum (2007) Sampling properties of homozygosity-based statistics for linkage disequilibrium. Mathematical Biosciences 208: 33-47. [PDF]

Homozygosity-based statistics such as Ohta's identity-in-state (IIS) excess offer the potential to measure linkage disequilibrium for multiallelic loci in small samples. However, previous observations have suggested that for independent loci, in small samples these statistics might produce values that more frequently lie on one side rather than on the other side of zero. Here we investigate the sampling properties of the IIS excess. We find that for any pair of independent polymorphic loci, as sample size n approaches infinity, the sampling distribution of the IIS excess approaches a normal distribution. For large samples, the IIS excess tends towards symmetry around zero, and the probabilities of positive and of negative IIS excess both approach 1/2. Surprisingly, however, we also find that for sufficiently large n, independent loci can be chosen so that the probability of a sample having positive IIS excess is arbitrarily close to either 0 or 1. The results are applied to interpretation of data from human populations, and we conclude that before employing homozygosity-based statistics to measure LD in a particular sample, especially for loci with either very small or very large homozygosities, it is useful to verify that loci with the observed homozygosity values are not likely to produce a large bias in IIS excess in samples of the given size.


[39] M Jakobsson, NA Rosenberg (2007) The probability distribution under a population divergence model of the number of genetic founding lineages of a population or species. Theoretical Population Biology 71: 502-523. [PDF]

The composition of genetic variation in a population or species is shaped by the number of events that led to the founding of the group. We consider a neutral coalescent model of two populations, where a derived population is founded as an offshoot of an ancestral population. For a given locus, using both recursive and nonrecursive approaches, we compute the probability distribution of the number of genetic founding lineages that have given rise to the derived population. This number of genetic founding lineages is defined as the number of ancestral individuals that contributed at the locus to the present-day derived population, and is formulated in terms of interspecific coalescence events. The effects of sample size and divergence time on the probability distribution of the number of founding lineages are studied in detail. For 99.99% of the loci in the derived population to each have one founding lineage, the two populations must be separated for >=9.9N generations. However, only ~0.87N generations must pass since divergence for 99.99% of the loci to have <6 founding lineages. Our results are useful as a prior expectation on the number of founding lineages in scenarios that involve the evolution of one population from the splitting of an ancestral group, such as in the colonization of islands, the formation of polyploid species, and the domestication of crops and livestock from wild ancestors.


[38] L David, NA Rosenberg, U Lavi, MW Feldman, J Hillel (2007). Genetic diversity and population structure inferred from the partially duplicated genome of domesticated carp, Cyprinus carpio L. Genetics Selection Evolution 39: 319-340. [PDF]

Genetic relationships among eight populations of domesticated carp (Cyprinus carpio L.), a species with a partially duplicated genome, were studied using 12 microsatellites and 505 AFLP bands. The populations included three aquacultured carp strains and five ornamental carp (koi) variants. Grass carp (Ctenopharyngodon idella) was used as an outgroup. AFLP-based gene diversity varied from 5% (grass carp) to 32% (koi) and reflected the reasonably well understood histories and breeding practices of the populations. A large fraction of the molecular variance was due to differences between aquacultured and ornamental carps. Further analyses based on microsatellite data, including cluster analysis and neighbor-joining trees, supported the genetic distinctiveness of aquacultured and ornamental carps, despite the recent divergence of the two groups. In contrast to what was observed for AFLP-based diversity, the frequency of heterozygotes based on microsatellites was comparable among all populations. This discrepancy can potentially be explained by duplication of some loci in Cyprinus carpio L., and a model that shows how duplication can increase heterozygosity estimates for microsatellites but not for AFLP loci is discussed. Our analyses in carp can help in understanding the consequences of genotyping duplicated loci and in interpreting discrepancies between dominant and co-dominant markers in species with recent genome duplication.


[37] KB Schroeder, TG Schurr, JC Long, NA Rosenberg, MH Crawford, LA Tarskaia, LP Osipova, SI Zhadanov, DG Smith (2007). A private allele ubiquitous in the Americas. Biology Letters 3: 218-223. [PDF] [Supplement]

The three-wave migration hypothesis of Greenberg et al. has permeated the genetic literature on the peopling of the Americas. Greenberg et al. proposed that Na-Dene, Aleut-Eskimo and Amerind are language phyla which represent separate migrations from Asia to the Americas. We show that a unique allele at autosomal microsatellite locus D9S1120 is present in all sampled North and South American populations, including the Na-Dene and Aleut-Eskimo, and in related Western Beringian groups, at an average frequency of 31.7%. This allele was not observed in any sampled putative Asian source populations or in other worldwide populations. Neither selection nor admixture explains the distribution of this regionally specific marker. The simplest explanation for the ubiquity of this allele across the Americas is that the same founding population contributed a large fraction of ancestry to all modern Native American populations.


[36] NA Rosenberg (2007) Statistical tests for taxonomic distinctiveness from observations of monophyly. Evolution 61: 317-323. [PDF]

The observation of monophyly for a specified set of genealogical lineages is often used to place the lineages into a distinctive taxonomic entity. However, it is sometimes possible that monophyly of the lineages can occur by chance as an outcome of the random branching of lineages within a single taxon. Thus, especially for small samples, an observation of monophyly for a set of lineages — even if strongly supported statistically — does not necessarily indicate that the lineages are from a distinctive group. Here I develop a test of the null hypothesis that monophyly is a chance outcome of random branching. I also compute the sample size required so that the probability of chance occurrence of monophyly of a specified set of lineages lies below a prescribed tolerance. Under the null model of random branching, the probability that monophyly of the lineages in an index group occurs by chance is substantial if the sample is highly asymmetric, that is, if only a few of the sampled lineages are from the index group, or if only a few lineages are external to the group. If sample sizes are similar inside and outside the group of interest, however, chance occurrence of monophyly can be rejected at stringent significance levels (P < 10^{-5}) even for quite small samples (~20 total lineages). For a fixed total sample size, rejection of the null hypothesis of random branching in a single taxon occurs at the most stringent level if samples of nearly equal size inside and outside the index group — with a slightly greater size within the index group — are used. Similar results apply, with smaller sample sizes needed, when reciprocal monophyly of two groups, rather than monophyly of a single group, is of interest. The results suggest minimal sample sizes required for inferences to be made about taxonomic distinctiveness from observations of monophyly.


[35] NA Rosenberg, S Mahajan, C Gonzalez-Quevedo, MGB Blum, L Nino-Rosales, V Ninis, P Das, M Hegde, L Molinari, G Zapata, JL Weber, JW Belmont, PI Patel (2006) Low levels of genetic divergence across geographically and linguistically diverse populations from India. PLoS Genetics 2: 2052-2061. [Full-text at journal website] [PDF] [Supplementary Tables 1-3 (DOC)] [Supplementary Tables 1-3 (PDF)]

Ongoing modernization in India has elevated the prevalence of many complex genetic diseases associated with a western lifestyle and diet to near-epidemic proportions. However, although India comprises more than one sixth of the world's human population, it has largely been omitted from genomic surveys that provide the backdrop for association studies of genetic disease. Here, by genotyping India-born individuals sampled in the United States, we carry out an extensive study of Indian genetic variation. We analyze 1,200 genome-wide polymorphisms in 432 individuals from 15 Indian populations. We find that populations from India, and populations from South Asia more generally, constitute one of the major human subgroups with increased similarity of genetic ancestry. However, only a relatively small amount of genetic differentiation exists among the Indian populations. Although caution is warranted due to the fact that United States-sampled Indian populations do not represent a random sample from India, these results suggest that the frequencies of many genetic variants are distinctive in India compared to other parts of the world and that the effects of population heterogeneity on the production of false positives in association studies may be smaller in Indians (and particularly in Indian-Americans) than might be expected for such a geographically and linguistically diverse subset of the human population.


[34] DF Conrad*, M Jakobsson*, G Coop*, X Wen, JD Wall, NA Rosenberg, JK Pritchard (2006) A worldwide survey of haplotype variation and linkage disequilibrium in the human genome. Nature Genetics 38: 1251-1260. [PDF] [Supplement (methods, note, and figures] [Supplementary Table 1] [Data]

Recent genomic surveys have produced high-resolution haplotype information, but only in a small number of human populations. We report haplotype structure across 12 Mb of DNA sequence in 927 individuals representing 52 populations. The geographic distribution of haplotypes reflects human history, with a loss of haplotype diversity as distance increases from Africa. Although the extent of linkage disequilibrium (LD) varies markedly across populations, considerable sharing of haplotype structure exists, and inferred recombination hotspot locations generally match across groups. The four samples in the International HapMap Project contain the majority of common haplotypes found in most populations: averaging across populations, 83% of common 20-kb haplotypes in a population are also common in the most similar HapMap sample. Consequently, although the portability of tag SNPs based on the HapMap is reduced in low-LD Africans, the HapMap will be helpful for the design of genome-wide association mapping studies in nearly all human populations.


[33] NA Rosenberg (2006) Standardized subsets of the HGDP-CEPH Human Genome Diversity Cell Line Panel, accounting for atypical and duplicated samples and pairs of close relatives. Annals of Human Genetics 70: 841-847. [PDF] [Supplement] [Data] [Spreadsheet with recommended subsets (txt format)] [Spreadsheet with recommended subsets (xls format)]

The HGDP-CEPH Human Genome Diversity Cell Line Panel is a widely-used resource for studies of human genetic variation. Here, pairs of close relatives that have been included in the panel are identified. Together with information on atypical and duplicated samples, the inferred relative pairs suggest standardized subsets of the panel for use in future population-genetic studies.


[32] NA Rosenberg (2006) Gene genealogies. Chapter 12 in CW Fox and JB Wolf, eds. Evolutionary Genetics: Concepts and Case Studies. Oxford: Oxford University Press, pp. 173-189. [PDF of final version]

(No abstract)


[31] NA Rosenberg, M Nordborg (2006) A general population-genetic model for the production by population structure of spurious genotype-phenotype associations in discrete, admixed or spatially distributed populations. Genetics 173: 1665-1678. [PDF]

In linkage disequilibrium mapping of genetic variants causally associated with phenotypes, spurious associations can potentially be generated by any of a variety of types of population structure. However, mathematical theory of the production of spurious associations has largely been restricted to population structure models that involve the sampling of individuals from a collection of discrete subpopulations. Here, we introduce a general model of spurious association in structured populations, appropriate whether the population structure involves discrete groups, admixture among such groups, or continuous variation across space. Under the assumptions of the model, we find that a single common principle — applicable to both the discrete and admixed settings as well as to spatial populations — gives a necessary and sufficient condition for the occurrence of spurious associations. Using a mathematical connection between the discrete and admixed cases, we show that in admixed populations, spurious associations are less severe than in corresponding mixtures of discrete subpopulations, especially when the variance of admixture across individuals is small. This observation, together with the results of simulations that examine the relative influences of various model parameters, has important implications for the design and analysis of genetic association studies in structured populations.


[30] JH Degnan, NA Rosenberg (2006) Discordance of species trees with their most likely gene trees. PLoS Genetics 2: 762-768. [Full-text at journal website] [PDF]

Because of the stochastic way in which lineages sort during speciation, gene trees may differ in topology from each other and from species trees. Surprisingly, assuming that genetic lineages follow a coalescent model of within-species evolution, we find that for any species tree topology with five or more species, there exist branch lengths for which gene tree discordance is so common that the most likely gene tree topology to evolve along the branches of a species tree differs from the species phylogeny. This counterintuitive result implies that in combining data on multiple loci, the straightforward procedure of using the most frequently observed gene tree topology as an estimate of the species tree topology can be asymptotically guaranteed to produce an incorrect estimate. We conclude with suggestions that can aid in overcoming this new obstacle to accurate genomic inference of species phylogenies.


[29] NA Rosenberg (2006) The mean and variance of the numbers of r-pronged nodes and r-caterpillars in Yule-generated genealogical trees. Annals of Combinatorics 10: 129-146. [PDF]

The Yule model is a frequently-used evolutionary model that can be utilized to generate random genealogical trees. Under this model, using a backwards counting method differing from the approach previously employed by Heard (Evolution 46: 1818-1826), for a genealogical tree of n lineages, the mean number of nodes with exactly r descendants is computed (2 &le r &le n-1). The variance of the number of r-pronged nodes is also obtained, as are the mean and variance of the number of r-caterpillars. These results generalize computations of McKenzie and Steel for the case of r=2 (Math. Biosci. 164: 81-92, 2000). For a given n, the two means are largest at r=2, equaling 2n/3 for n &ge 5. However, for n &ge 9, the variances are largest at r=3, equaling 23n/420 for n &ge 7. As n &rarr &infin, the fraction of internal nodes that are r-caterpillars for some r approaches (e2-5)/4 &asymp 0.59726.


[28] NA Rosenberg, S Mahajan, S Ramachandran, C Zhao, JK Pritchard, MW Feldman (2005) Clines, clusters, and the effect of study design on the inference of human population structure. PLoS Genetics 1: 660-671. [Full-text at journal website] [PDF] [Data]

Previously, we observed that without using prior information about individual sampling locations, a clustering algorithm applied to multilocus genotypes from worldwide human populations produced genetic clusters largely coincident with major geographic regions. It has been argued, however, that the degree of clustering is diminished by use of samples with greater uniformity in geographic distribution, and that the clusters we identified were a consequence of uneven sampling along genetic clines. Expanding our earlier dataset from 377 to 993 markers, we systematically examine the influence of several study design variables — sample size, number of loci, number of clusters, assumptions about correlations in allele frequencies across populations, and the geographic dispersion of the sample — on the ``clusteredness'' of individuals. With all other variables held constant, geographic dispersion is seen to have comparatively little effect on the degree of clustering. Examination of the relationship between genetic and geographic distance supports a view in which the clusters arise not as an artifact of the sampling scheme, but from small discontinuous jumps in genetic distance for most population pairs on opposite sides of geographic barriers, in comparison with genetic distance for pairs on the same side. Thus, analysis of the 993-locus dataset corroborates our earlier results: if enough markers are used with a sufficiently large worldwide sample, individuals can be partitioned into genetic clusters that match major geographic subdivisions of the globe, with some individuals from intermediate geographic locations having mixed membership in the clusters that correspond to neighboring regions.


[27] NA Rosenberg (2005) Algorithms for selecting informative marker panels for population assignment. Journal of Computational Biology 12: 1183-1201. [PDF]

Given a set of potential source populations, genotypes of an individual of unknown origin at a collection of markers can be used to predict the correct source population of the individual. For improved efficiency, informative markers can be chosen from a larger set of markers to maximize the accuracy of this prediction. However, selecting the loci that are individually most informative does not necessarily produce the optimal panel. Here, using genotypes from eight species — carp, cat, chicken, dog, fly, grayling, human, and maize — this univariate accumulation procedure is compared to new multivariate "greedy" and "maximin" algorithms for choosing marker panels. The procedures generally suggest similar panels, although the greedy method often recommends inclusion of loci that are not chosen by the other algorithms. In seven of the eight species, when applied to five or more markers, all methods achieve at least 94% assignment accuracy on simulated individuals, with one species — dog — producing this level of accuracy with only three markers, and the eighth species — human — requiring ~13-16 markers. The new algorithms produce substantial improvements over use of randomly selected markers; where differences among the methods are noticeable, the greedy algorithm leads to slightly higher probabilities of correct assignment. Although none of the approaches necessarily chooses the panel with optimal performance, the algorithms all likely select panels with performance near enough to the maximum that they all are suitable for practical use.


[26] S Ramachandran, O Deshpande, CC Roseman, NA Rosenberg, MW Feldman, LL Cavalli-Sforza (2005) Support from the relationship of genetic and geographic distance in human populations for a serial founder effect originating in Africa. Proceedings of the National Academy of Sciences USA 102: 15942-15947. [PDF] [Supplementary Figure 6] [Supplementary Table 2] [Supplementary text] [Data]

Equilibrium models of isolation by distance predict an increase in genetic differentiation with geographic distance. Here we find a linear relationship between genetic and geographic distance in a worldwide sample of human populations, with major deviations from the fitted line explicable by admixture or extreme isolation. A close relationship is shown to exist between the correlation of geographic distance and genetic differentiation (as measured by Fst) and the geographic pattern of heterozygosity across populations. Considering a worldwide set of geographic locations as possible sources of the human expansion, we find that heterozygosities in the globally distributed populations of the data set are best explained by an expansion originating in Africa and that no geographic origin outside of Africa accounts as well for the observed patterns of genetic diversity. Although the relationship between Fst and geographic distance has been interpreted in the past as the result of an equilibrium model of drift and dispersal, simulation shows that the geographic pattern of heterozygosities in this data set is consistent with a model of a serial founder effect starting at a single origin. Given this serial-founder scenario, the relationship between genetic and geographic distance allows us to derive bounds for the effects of drift and natural selection on human genetic variation.


[25] NA Rosenberg (2005) A sharp minimum on the mean number of steps taken in adaptive walks. Journal of Theoretical Biology 237: 17-22. [PDF]

It was recently conjectured by H.A. Orr [2003. A minimum on the mean number of steps taken in adaptive walks. J. Theor. Biol. 220, 241-247] that from a random initial point on a random fitness landscape of alphabetic sequences with one-mutation adjacency, chosen from a larger class of landscapes, no adaptive algorithm can arrive at a local optimum in fewer than on average e-1 steps. Here, using an example in which the mean number of steps to a local optimum equals (A-1)/A, where A is the number of distinct "letters" in the "alphabet" from which sequences are constructed, it is shown that as originally stated, the conjecture does not hold. It is also demonstrated that (A-1)/A is a sharp minimum on the mean number of steps taken in adaptive walks on fitness landscapes of alphabetic sequences with one-mutation adjacency. As the example that achieves the new lower bound has properties that are not often considered as potential attributes for fitness landscapes — non-identically distributed fitnesses and negative fitness correlations for adjacent points — a weaker set of conditions characteristic of more commonly studied fitness landscapes is proposed under which the lower bound on the mean length of adaptive walks is conjectured to equal e-1.


[24] M Nordborg, TT Hu, Y Ishino, J Jhaveri, C Toomajian, H Zheng, E Bakker, P Calabrese, J Gladstone, R Goyal, M Jakobsson, S Kim, Y Morozov, B Padhukasahasram, V Plagnol, NA Rosenberg, C Shah, JD Wall, J Wang, K Zhao, T Kalbfleisch, V Schulz, M Kreitman, J Bergelson (2005) The pattern of polymorphism in Arabidopsis thaliana. PLoS Biology 3: 1289-1299. [Full-text at journal website] [PDF]

We resequenced 876 short fragments in a sample of 96 individuals of Arabidopsis thaliana that included stock center accessions as well as a hierarchical sample from natural populations. Although A. thaliana is a selfing weed, the pattern of polymorphism in general agrees with what is expected for a widely distributed, sexually reproducing species. Linkage disequilibrium decays rapidly, within 50 kb. Variation is shared worldwide, although population structure and isolation by distance are evident. The data fail to fit standard neutral models in several ways. There is a genome-wide excess of rare alleles, at least partially due to selection. There is too much variation between genomic regions in the level of polymorphism. The local level of polymorphism is negatively correlated with gene density and positively correlated with segmental duplications. Because the data do not fit theoretical null distributions, attempts to infer natural selection from polymorphism data will require genome-wide surveys of polymorphism in order to identify anomalous regions. Despite this, our data support the utility of A. thaliana as a model for evolutionary functional genomics.


[23] H Innan, K Zhang, P Marjoram, S Tavaré, NA Rosenberg (2005) Statistical tests of the coalescent model based on the haplotype frequency distribution and the number of segregating sites. Genetics 169: 1763-1777. [PDF] [Software]

Several tests of neutral evolution employ the observed number of segregating sites and properties of the haplotype frequency distribution as summary statistics and use simulations to obtain rejection probabilities. Here we develop a "haplotype configuration test" of neutrality (HCT) based on the full haplotype frequency distribution. To enable exact computation of rejection probabilities for small samples, we derive a recursion under the standard coalescent model for the joint distribution of the haplotype frequencies and the number of segregating sites. For larger samples, we consider simulation-based approaches. The utility of the HCT is demonstrated in simulations of alternative models and in application to data from Drosophila melanogaster.


[22] NA Rosenberg, PP Calabrese (2004) Polyploid and multilocus extensions of the Wahlund inequality. Theoretical Population Biology 66: 381-391. [PDF]

Wahlund's inequality informally states that if a structured and an unstructured population have the same allele frequencies at a locus, the structued population contains more homozygotes. We show that this inequality holds generally for ploidy level P, that is, the structured population has more P-polyhomozygotes. Further, for M randomly chosen loci (M greater than or equal to 2), the structured population is also expected to contain more M-multihomozygotes than an unstructured population with the same single-locus homozygosities. The extended inequalities suggest multilocus identity coefficients analogous to FST. Using microsatellite genotypes from human populations, we demonstrate that the multilocus Wahlund inequality can explain a positive bias in "identity-in-state excess."


[21] MM Tanaka, NA Rosenberg, PM Small (2004) The control of copy number of IS6110 in Mycobacterium tuberculosis. Molecular Biology and Evolution 21: 2195-2201. [PDF]

Insertion sequence (IS) elements are bacterial genes that are able to transpose to different locations in the genome. These elements are often used in molecular epidemiology as genetic markers that track the spread of pathogens. Transposable elements have frequently been described as "selfish DNA" because they facilitate their own transposition, causing damage when they insert into coding regions, while contributing little if anything to the bacterial host. According to this hypothesis, the expansion of copy number of insertion sequences is opposed by negative selection against high copy numbers. From an alternative point of view, we might expect IS elements to intrinsically regulate transposition within cells, thereby limiting damage to their bacterial host. Here, we report evidence that the copy number of IS6110 in Mycobacterium tuberculosis is controlled by selection against the element. We first construct 12 different models of marker change resulting from a combination of possible transposition functions and selective regimes. We then compute the Akaike Information Criterion for each model to identify the models that best explain data consisting of serial isolates of M. tuberculosis genotyped with IS6110. We find that the best performing models all include selection against the accumulation of copies. Specifically, our analysis points to the interaction of separate copies of the element causing lethal effects. We discuss the implications of these findings for genome evolution and molecular epidemiology.


[20] NA Rosenberg (2004) Review of Probability Models for DNA Sequence Evolution by R Durrett. Journal of the American Statistical Association 99: 560-561. [PDF]

(No abstract)


[19] S Ramachandran, NA Rosenberg, LA Zhivotovsky, MW Feldman (2004) Robustness of the inference of human population structure: a comparison of X-chromosomal and autosomal microsatellites. Human Genomics 1: 87-97 (2004). [PDF]

In this paper, data on 20 X-chromosomal microsatellite polymorphisms from the HGDP-CEPH cell line panel are used to infer human population structure. Inferences from these data are compared to those obtained from autosomal microsatellites. Some of the major features of the structure seen with 377 autosomal markers are generally visible with the X-linked markers, although the latter provide less resolution. Differences between the X-chromosomal and autosomal results can be explained without requiring major differences in demographic parameters between males and females. The dependence of the partitioning on the number of individuals sampled from each region and on the number of markers used is discussed.


[18] NA Rosenberg (2004) Distruct: a program for the graphical display of population structure. Molecular Ecology Notes 4: 137-138. [PDF] [Software]

In analysis of multilocus genotypes from structured populations, individual coefficients of membership in subpopulations are often estimated using programs such as structure. Distruct provides a general method for visualizing these estimated membership coefficients. Subpopulations are represented as colours, and individuals are depicted as bars partitioned into coloured segments that correspond to membership coefficients in the subgroups. Distruct, available at http://www.cmb.usc.edu/~noahr/distruct.html, can also be used to display subpopulation assignment probabilities when individuals are assumed to have ancestry in only one group.


[17] NA Rosenberg, LM Li, R Ward, JK Pritchard (2003) Informativeness of genetic markers for inference of ancestry. American Journal of Human Genetics 73: 1402-1422. [PDF] [Supplement] [Microsatellite data] [SNP data] [SNP data readme] [Solution to Problem 11039 required in appendix of paper (American Mathematical Monthly 112: 572-573, 2005)] [Software]

Inference of individual ancestry is useful in various applications, such as admixture mapping and structured-association mapping. Using information-theoretic principles, we introduce a general measure, the informativeness for assignment (In), applicable to any number of potential source populations, for determining the amount of information that multiallelic markers provide about individual ancestry. In a worldwide human microsatellite data set, we identify markers of highest informativeness for inference of regional ancestry and for inference of population ancestry within regions; these markers, which are listed in online-only tables in our article, can be useful both in testing for and in controlling the influence of ancestry on case-control genetic association studies. Markers that are informative in one collection of source populations are generally informative in others. Informativeness of random dinucleotides, the most informative class of microsatellites, is five to eight times that of random single-nucleotide polymorphisms (SNPs), but 2%-12% of SNPs have higher informativeness than the median for dinucleotides. Our results can aid in decisions about the type, quantity, and specific choice of markers for use in studies of ancestry.


[16] NA Rosenberg, AE Hirsh (2003) On the use of star-shaped genealogies in inference of coalescence times. Genetics 164: 1677-1682. [PDF]

Genealogies from rapidly growing populations have approximate "star" shapes. We study the degree to which this approximation holds in the context of estimating the time to the most recent common ancestor (TMRCA) of a set of lineages. In an exponential growth scenario, we find that unless the product of population size (N) and growth rate (r) is at least 105, the "pairwise comparison estimator" of TMRCA that derives from the star genealogy assumption has bias of 10-50%. Thus, the estimator is appropriate only for large populations that have grown very rapidly. The "tree-length estimator" of TMRCA is more biased than the pairwise comparison estimator, having low bias only for extremely large values of Nr.


[15] NA Rosenberg (2003) The shapes of neutral gene genealogies in two species: probabilities of monophyly, paraphyly, and polyphyly in a coalescent model. Evolution 57: 1465-1477. [PDF]

The genealogies of samples of orthologous regions from multiple species can be classified by their shapes. Using a neutral coalescent model of two species, I give exact probabilities of each of four possible genealogical shapes — reciprocal monophyly, two types of paraphyly, and polyphyly. After the divergence that forms two species, each of which has population size N, polyphyly is the most likely genealogical shape for the lineages of the two species. At ~1.300N generations after divergence, paraphyly becomes most likely, and reciprocal monophyly becomes most likely at ~1.665N generations. For a given species, the time at which 99% of its loci acquire monophyletic genealogies is ~5.298N generations, assuming all loci in its sister species are monophyletic. The probability that all lineages of two species are reciprocally monophyletic given that a sample from the two species has a reciprocally monophyletic genealogy increases rapidly with sample size, as does the probability that the most recent common ancestor (MRCA) for a sample is also the MRCA for all lineages from the two species. The results have potential applications for the testing of evolutionary hypotheses.


[14] NA Rosenberg, JK Pritchard, JL Weber, HM Cann, KK Kidd, LA Zhivotovsky, MW Feldman (2003) Response to comment on "Genetic structure of human populations." Science 300: 1877. [PDF] [Data]

Our higher within-group variance component estimate in relation to comparable past studies is due to our use of allelic indicator variables, inclusion of tetranucleotide loci, and analysis of a sample that contained proportionately fewer geographically well-separated populations. The 83.4% estimate of Excoffier and Hamilton employs a subset of groups that are nearly maximally differentiated within regions, and it can therefore be regarded as a lower bound.


[13] NA Rosenberg, AG Tsolaki, MM Tanaka (2003) Estimating change rates of genetic markers using serial samples: applications to the transposon IS6110 in Mycobacterium tuberculosis. Theoretical Population Biology 63: 347-363. [PDF]

In infectious disease epidemiology, it is useful to know how quickly genetic markers of pathogenic agents evolve while inside hosts. We propose a modular framework with which these genotype change rates can be estimated. The estimation scheme requires a model of the underlying process of genetic change, a detection scheme that filters this process into observable quantities, and a monitoring scheme that describes the timing of observations. We study a linear "birth-shift-death" model for change in transposable element genotypes, obtaining maximum-likelihood estimators for various detection and monitoring schemes. The method is applied to serial genotypes of the transposon IS6110 in Mycobacterium tuberculosis. The estimated birth rate of 0.0161 (events per copy of the transposon per year) and death rate of 0.0108 are both significantly larger than the estimated shift rate of 0.0018. The sum of these estimates, which corresponds to a "half-life" of 2.4 years for a typical strain that has 10 copies of the element, substantially exceeds a previous estimate of 0.0135 total changes per copy per year. We consider experimental design issues that enable the precision of estimates to be improved. We also discuss extensions to other markers and implications for molecular epidemiology.


[12] LA Zhivotovsky, NA Rosenberg, MW Feldman (2003) Features of evolution and expansion of modern humans, inferred from genomewide microsatellite markers. American Journal of Human Genetics 72: 1171-1186. [PDF] [Data]

We study data on variation in 52 worldwide populations at 377 autosomal short tandem repeat loci, to infer a demographic history of human populations. Variation at di-, tri-, and tetranucleotide repeat loci is distributed differently, although each class of markers exhibits a decrease of within-population genetic variation in the following order: sub-Saharan Africa, Eurasia, East Asia, Oceania, and America. There is a similar decrease in the frequency of private alleles. With multidimensional scaling, populations belonging to the same major geographic region cluster together, and some regions permit a finer resolution of populations. When a stepwise mutation model is used, a population tree based on TD estimates of divergence time suggests that the branches leading to the present sub-Saharan African populations of hunter-gatherers were the first to diverge from a common ancestral population (~71-142 thousand years ago). The branches corresponding to sub-Saharan African farming populations and those that left Africa diverge next, with subsequent splits of branches for Eurasia, Oceania, East Asia, and America. African hunter-gatherer populations and populations of Oceania and America exhibit no statistically significant signature of growth. The features of population subdivision and growth are discussed in the context of the ancient expansion of modern humans.


[11] NA Rosenberg, JK Pritchard, JL Weber, HM Cann, KK Kidd, LA Zhivotovsky, MW Feldman (2002) Genetic structure of human populations. Science 298: 2381-2385. [Full Text at Science website] [PDF] [Supplement] [Data in Excel] [Data in structure and NEXUS formats] [Software for drawing figures] [Español]

We studied human population structure using genotypes at 377 autosomal microsatellite loci in 1056 individuals from 52 populations. Within-population differences among individuals account for 93 to 95% of genetic variation; differences among major groups constitute only 3 to 5%. Nevertheless, without using prior information about the origins of individuals, we identified six main genetic clusters, five of which correspond to major geographic regions, and subclusters that often correspond to individual populations. General agreement of genetic and predefined populations suggests that self-reported ancestry can facilitate assessments of epidemiological risks but does not obviate the need to use genetic information in genetic association studies.


[10] NA Rosenberg, D Nettle (2002) Joining forces to uncover human evolutionary history. Trends in Ecology and Evolution 17: 301-302. (report on Genes, Peoples, and Languages conference) [PDF]

(No abstract)


[9] NA Rosenberg, M Nordborg (2002) Genealogical trees, coalescent theory, and the analysis of genetic polymorphisms. Nature Reviews Genetics 3: 380-390. [PDF] [article at NRG website (includes "bullet point" summary)]

Improvements in genotyping technologies have led to the increased use of genetic polymorphism for inference about population phenomena, such as migration and selection. Such inference presents a challenge, because polymorphism data reflect a unique, complex, non-repeatable evolutionary history. Traditional analysis methods do not take this into account. A stochastic process known as "the coalescent" presents a coherent statistical framework for analysis of genetic polymorphisms.


[8] NA Rosenberg (2002) The probability of topological concordance of gene trees and species trees. Theoretical Population Biology 61: 225-247. [PDF]

The concordance of gene trees and species trees is reconsidered in detail, allowing for samples of arbitrary size to be taken from the species. A sense of concordance for gene tree and species tree topologies is clarified, such that if the "collapsed gene tree" produced by a gene tree has the same topology as the species tree, the gene tree is said to be topologically concordant with the species tree. The term speciodendric is introduced to refer to genes whose trees are topologically concordant with species trees. For a given three-species topology, probabilities of each of the three possible collapsed gene tree topologies are given, as are probabilities of monophyletic concordance and concordance in the sense of N. Takahata (1989), Genetics 122, 957-966. Increasing the sample size is found to increase the probability of topological concordance, but a limit exists on how much the topological concordance probability can be increased. Suggested sample sizes beyond which this probability can be increased only minimally are given. The results are discussed in terms of implications for molecular studies of phylogenetics and speciation.


[7] NA Rosenberg, MW Feldman (2002) The relationship between coalescence times and population divergence times. Chapter 9 in M Slatkin and M Veuille, eds. Modern Developments in Theoretical Population Genetics. Oxford: Oxford University Press, pp. 130-164. [PDF of final version]

The divergence time of two populations is the amount of time that has elapsed since the populations arose from an ancestral group, while the coalescence time of a set of copies of a gene is the amount of time that has elapsed since the most recent common ancestor of the gene copies lived. We briefly review the methods that have been used to infer divergence times and coalescence times from genetic data. We then consider the relationship between divergence times and coalescence times in a population genetic model that includes divergence followed by migration between two descendant populations, paying particular attention to the fact that migration can cause coalescence to occur more recently than divergence. Insights gained from the model and its special cases are applied to four examples: the divergences of humans and chimpanzees, modern humans and Neanderthals, Africans and non-Africans, and Native Americans and Asians. For each example, we discuss the connection between hypothesized divergence times and estimated coalescence times.


[6] NA Rosenberg, T Burke, MW Feldman, P Friedlin, MAM Groenen, J Hillel, A Mäki-Tanila, M Tixier-Boichard, A Vignal, K Wimmers, S Weigend (2001) Empirical evaluation of genetic clustering methods using multilocus genotypes from 20 chicken breeds. Genetics 159: 699-713. [PDF] [Data] [Photo]

We tested the utility of genetic cluster analysis in ascertaining population structure of a large data set for which population structure was previously known. Each of 600 individuals representing 20 distinct chicken breeds was genotyped for 27 microsatellite loci, and individual multilocus genotypes were used to infer genetic clusters. Individuals from each breed were inferred to belong mostly to the same cluster. The clustering success rate, measuring the fraction of individuals that were properly inferred to belong to their correct breeds, was consistently ~98%. When markers of highest expected heterozygosity were used, genotypes that included at least 8-10 highly variable markers from among the 27 markers genotyped also achieved >95% clustering success. When 12-15 highly variable markers and only 15-20 of the 30 individuals per breed were used, clustering success was at least 90%. We suggest that in species for which population structure is of interest, databases of multilocus genotypes at highly variable markers should be compiled. These genotypes could then be used as training samples for genetic cluster analysis and to facilitate assignments of individuals of unknown origin to populations. The clustering algorithm has potential applications in defining the within-species genetic units useful in problems of conservation.


[5] MM Tanaka, NA Rosenberg (2001) Optimal estimation of transposition rates of insertion sequences for molecular epidemiology. Statistics in Medicine 20: 2409-2420. [PDF]

Outbreaks of infectious disease can be confirmed by identifying clusters of DNA fingerprints among bacterial isolates from infected individuals. This procedure makes assumptions about the underlying properties of the genetic marker used for fingerprinting. In particular, it requires that each fingerprint changes sufficiently slowly within an individual that isolates from separate individuals infected by the same strain will exhibit similar or identical fingerprints. We propose a model for the probability that an individual's fingerprint will change over a given period of time. We use this model together with published data in order to estimate the fingerprint change rate for IS6110 in human tuberculosis, obtaining a value of 0.0139 changes per copy per year. Although we focus on insertion sequences (IS), our method applies to other fingerprinting techniques such as pulsed-field gel electrophoresis (PFGE). We suggest sampling intervals that produce the least error in estimates of the fingerprint change rate, as well as sample sizes that achieve specified levels of error in the estimate.


[4] NA Rosenberg, E Woolf, JK Pritchard, T Schaap, D Gefel, I Shpirer, U Lavi, B Bonné-Tamir, J Hillel, MW Feldman (2001) Distinctive genetic signatures in the Libyan Jews. Proceedings of the National Academy of Sciences, USA 98: 858-863. [PDF] [Data]

Unlinked autosomal microsatellites in six Jewish and two non-Jewish populations were genotyped, and the relationships among these populations were explored. Based on considerations of clustering, pairwise population differentiation, and genetic distance, we found that the Libyan Jewish group retains genetic signatures distinguishable from those of the other populations, in agreement with some historical records on the relative isolation of this community. Our methods also identified evidence of some similarity between Ethiopian and Yemenite Jews, reflecting possible migration in the Red Sea region. We suggest that high-resolution statistical methods that use individual multilocus genotypes may make it practical to distinguish related populations of extremely recent common ancestry.


[3] L Jin, ML Baskett, LL Cavalli-Sforza, LA Zhivotovsky, MW Feldman, NA Rosenberg (2000) Microsatellite evolution in modern humans: a comparison of two data sets from the same populations. Annals of Human Genetics 64: 117-134. [PDF] [Data]

We genotyped 64 dinucleotide microsatellite repeats in individuals from populations that represent all inhabited continents. Microsatellite summary statistics are reported for these data, as well as for a data set that includes 28 out of 30 loci studied by Bowcock (1994) in the same individuals. For both data sets, diversity statistics such as heterozygosity, number of alleles per locus, and number of private alleles per locus produced the highest values in Africans, intermediate values in Europeans and Asians, and low values in Americans. Evolutionary trees of populations based on genetic distances separated groups from different continents. Corresponding trees were topologically similar for the two data sets, with the exception that the (&delta&mu)2 genetic distance reliably distinguished groups from different continents for the larger data set, but not for the smaller one. Consistent with our results from diversity statistics and from evolutionary trees, population growth statistics Sk and &beta, which seem particularly useful for indicating recent and ancient population size changes, confirm a model of human evolution in which human populations expand in size and through space following the departure of a small group from Africa.


[2] JK Pritchard, M Stephens, NA Rosenberg, P Donnelly (2000) Association mapping in structured populations. American Journal of Human Genetics 67: 170-181. [PDF]

The use in association studies of the forthcoming dense genome-wide collection of SNPs has been heralded as a potential breakthrough in studying the genetic basis of common complex disorders. A serious problem with association mapping is that population structure can lead to spurious associations between a candidate marker and a phenotype. One common solution has been to abandon case-control studies in favour of family-based tests of association, such as the TDT, but this comes at a considerable cost in the need to collect DNA from close relatives of affected individuals. In this article we describe a novel, statistically valid, method for case-control association studies in structured populations. Our method uses a set of unlinked genetic markers to infer details of population structure, and estimate the ancestry of sampled individuals, before using this information to test for associations within subpopulations. It provides power comparable with the TDT in many settings, and may substantially outperform it if there are conflicting associations in different subpopulations.


[1] JK Pritchard, NA Rosenberg (1999) Use of unlinked genetic markers to detect population stratification in association studies. American Journal of Human Genetics 65: 220-228. [PDF]

We examine the issue of population stratification in association mapping studies. In case-control studies of association, population subdivision or recent admixture of populations can lead to spurious associations between a phenotype and unlinked candidate loci. Using a model of sampling from a structured population, we show that if population stratification exists, it can be detected using unlinked marker loci. We show that the case-control study design using unrelated control individuals is a valid approach for association mapping, provided that marker loci unlinked to the candidate locus are included in the study in order to test for stratification. We suggest guidelines for how many unlinked marker loci should be used.