mtDNA

Piganeau G & Eyre-Walker A 2009 Evidence for variation in the effective population size of animal mitochondrial DNA. PLoS ONE 4:e4396.

  • it has recently been shown that levels of diversity in mitochondrial DNA are remarkably constant across animals of diverse census population sizes and ecologies
  • here 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
  • 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
  • the relative constancy of DNA diversity may be due to a negative correlation between the effective population size and the mutation rate per generation
  • neutral diversity might be held in check by the effects of adaptive substitutions, which purge diversity as they sweep through the population
  • if the rate of adaptive evolution is limited by the supply of mutations then the level of neutral diversity is a product of two conflicting processes
  • as the population size increases so neutral diversity tends to increase
  • at the same time the number of adaptive substitutions increases and this decreases diversity
  • Gillespie [5] has shown that these processes tend to cancel each other out to yield a constant level of neutral diversity across species with very different census population sizes
  • Bazin et al. [6] show, in support of this "genetic draft" hypothesis, that the neutrality index is significantly lower in invertebrates, which are likely to have higher census population sizes, than vertebrates
  • the neutrality index largely depends upon two factors
  • the proportion of substitutions that are adaptive, which reduces the neutrality index
  • the proportion of polymorphisms that are slightly deleterious, which increases the index
  • invertebrates have lower neutrality indices because they have a smaller proportion of slightly deleterious mutations, not because the rate of adaptive evolution is higher
  • the degree to which a genome is affected by genetic hitch-hiking depends on the number of adaptive substitutions per generation, not the proportion of substitutions that are adaptive
  • the NI may not be strongly correlated to the rate of genetic draft