Meiosis and ascospore development in the pseudohomothallic species Neurospora tetrasperma. (N.B. Raju)
Dodge (1927) described the cytological basis for pseudohomothallism in the four-spored species N. tetrasperma. Early ascus development is nearly identical to that in the eight-spored species N. crassa. The first-division spindle is usually aligned longitudinally in the ascus, and the mating types almost always segregate at this division. The two interphase I nuclei (mat A and mat a) reposition themselves to form two parallel and overlapping spindles at the second division. This alignment of spindles brings two nuclei of opposite mating type into each end of the ascus. At the postmeiotic mitosis (third division), the two pairs of nuclei again form pairs of overlapping, parallel spindles, but this time they are aligned across the ascus. Following this mitosis, mat A and mat a nuclei associate in pairs, and all four pairs of nuclei realign along the length of the ascus and cut out four binucleate, heterokaryotic ascospores (Raju 1992; Raju and Perkins 1994). Another mitosis in the young ascospores makes them four-nucleate. In up to 5% of asci, one or more heterokaryotic, large ascospores are replaced by pairs of small, homokaryotic ascospores.
In N. tetrasperma, any centromere-linked gene (e.g. mating type, Spore killer) that segregates its alleles at the first-division of meiosis results in ascospores that are heterokaryotic for the segregating alleles. In contrast, ascus program is quite different in several other four-spored pseudohomothallic ascomycetes, where mating-type genes segregate at the second-division and the two spindles are aligned in tandem (Raju and Perkins, 1994).
Chromosome behavior during ascus development in N. tetrasperma is illustrated here with one schematic diagram and 39 photomicrographs in Figs. 1-40.
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