Given the diversity in the genetic repertoire of mimivirus, its evolutionary history is an area of particular interest. While the origin of the virus continues to perplex scientists, several hypotheses have been proposed and supported.
- Attempting to explain why the mimivirus genome contains many translation-related genes, one hypothesis suggests that mimivirus evolved from a more complex cellular life form with a more complete ancestral translation apparatus that was gradually lost through the process of genome reduction. Several phylogenetic analyses support the notion that mimivirus genes could not have been acquired independently and are therefore derived from a single ancestor.
- Another hypothesis that accounts for the complexity of the mimivirus genome (and that is not in direct opposition to the first hypothesis) is that DNA viruses may have a common ancestor that originated before the individualization of cellular organisms from the three domains of life. If this is true, the mimivirus lineage may have diverged very early on in evolution, which would explain the presence of eukaryotic and prokaryotic features in the viral genome.
- Contrary to the previous hypotheses, it has been suggested that mimivirus evolved through piecemeal accumulation of genes on top of an ancestral NCLDV core. According to this theory, the virus acquired genes from eukaryotic hosts and endosymbiotic bacteria, and grew through extensive gene duplication. Eugene Koonin, a proponent of this hypothesis, argues that since the NCLDV core genes are not present outside of eukaryotic viruses, it is unlikely that the NCLDVs predate eukaryotic divergence. He also points out that viruses are capable of evolving quickly and therefore do not need to be of ancient origin.
All of these arguments have been supported with scientific evidence, and further understanding will likely require both greater knowledge of the mimivirus genome and isolation of other members of the Mimiviridae family or other large dsDNA viruses.
Raoult et al (2004)