Mutation rate and genome reduction in endosymbiotic and free-living bacteria
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Genome reduction has been considered the hallmark of endosymbiotic bacteria, such as endocellular mutualists or obligatory pathogens until it was found exactly the same in several free-living bacteria. In endosymbiotic bacteria genome reduction is mainly attributed to degenerative processes due to small population size. These cannot affect the free-living bacteria with reduced genomes because they are known to have very large population sizes. It has been proposed that selection for simplification drove genome reduction in these free-living bacteria. For at least one of them (Prochlorococcus), genome reduction is associated with accelerated evolution and we suggest an alternative hypothesis based on increase in mutation rate as the primary cause of genome reduction in free-living bacteria.
KeywordsBacterial genomics Error threshold Genome size Molecular evolution Mutation–selection balance
We thank Siv Andersson, Vincent Daubin, and Eduardo Rocha and Pierre Alexis Gros for helpful comments on this manuscript. G.M. is a CNRS fellow, A.C. has a PhD fellowship from the French ministry of research and O.T. was funded by Agence Nationale de la Recherche grant (ANR-05JCJC0136-01).