Abstract
BackgroundIt has recently been shown that levels of diversity in mitochondrial DNA are remarkably constant across animals of diverse census population sizes and ecologies, which has led to the suggestion that the effective population of mitochondrial DNA may be relatively constant.ResultsHere we present several lines of evidence that suggest, to the contrary, that the effective population size of mtDNA does vary, and that the variation can be substantial. First, we show that levels of mitochondrial and nuclear diversity are correlated within all groups of animals we surveyed. Second, we show that the effectiveness of selection on non-synonymous mutations, as measured by the ratio of the numbers of non-synonymous and synonymous polymorphisms, is negatively correlated to levels of mitochondrial diversity. Finally, we estimate the effective population size of mitochondrial DNA in selected mammalian groups and show that it varies by at least an order of magnitude.ConclusionsWe conclude that there is variation in the effective population size of mitochondria. Furthermore we suggest that the relative constancy of DNA diversity may be due to a negative correlation between the effective population size and the mutation rate per generation.
Highlights
Two observations puzzled early workers in the field of molecular evolution
The fact that allozyme diversity varies remarkably little across species is surprising, under the neutral theory of molecular evolution, because under this theory, levels of diversity are expected to be proportional to the effective population size of the organism
Bazin et al [6] showed, for the first time, that levels of sequence diversity, in mitochondrial DNA, are remarkably constant across species that apparently have very different census population sizes; for example they showed that several groups of animals including mammals and molluscs, have very similar levels of mitochondrial DNA sequence diversity
Summary
Two observations puzzled early workers in the field of molecular evolution. Bazin et al [6] showed, for the first time, that levels of sequence diversity, in mitochondrial DNA, are remarkably constant across species that apparently have very different census population sizes; for example they showed that several groups of animals including mammals and molluscs, have very similar levels of mitochondrial DNA sequence diversity. They showed that there is no apparent difference in the diversity levels of marine and freshwater fish, and marine and terrestrial molluscs - we might expect marine organisms to have much larger population sizes than non-marine organisms. It has recently been shown that levels of diversity in mitochondrial DNA are remarkably constant across animals of diverse census population sizes and ecologies, which has led to the suggestion that the effective population of mitochondrial DNA may be relatively constant
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