On the roles of selection, mutation and drift in the evolution of mitochondrial DNA diversity in British Mytilus edulis (Mytilidae; Mollusca) populations

Species of the marine mussel genus Mytilus possess two distinct mtDNA genomes. The F genome is inherited maternally, the M genome is inherited paternally. This mode of inheritance provides a unique opportunity for studying evolutionary forces and phylogeny within the genus, because patterns of variation at the two genomes can be cross compared. Previous studies of evolutionary forces acting on Mytilus mtDNA have been carried out by analysing sequence data from relatively small numbers of individuals, often comparing different species. In the present study we use an RELP approach to study population variation in a single species Mytilus edulis from five localities in Britain. These localities lie within an area in which previous allozyme studies had failed to detect significant geographic differentiation. In the present study, significant mtDNA differentiation is observed for both genomes both for haplotype frequencies and for nucleotide divergence. Nucleotide diversity within and divergence between populations is greater for the M genome consistent with a higher mutation rate and/or lower purifying selection for this genome. Application of the Ewens-Watterson test provides evidence for excess ‘homozygosity' much greater in magnitude for the F genome consistent with the stronger action of purifying selection on this genome. The distribution of pairwise nucleotide divergence values within populations was compared with theoretical distributions obtained by computer simulation. Much better fit of the empirical results is observed to a purifying selection model than to an equilibrium neutral or population expansion model. The results are in agreement with those of previous studies of DNA sequences, adopting different tests, in providing evidence for the action of purifying selection on both genomes but of greater intensity on the F genome. It is concluded that the results are explained well by the nearly neutral theory of evolution giving important roles to selection, drift and mutation as causes of the observed patterns of variation.