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 genom es. The F genome is inherited maternally the M genome is inherited paternal ly. This mode of inheritance provides a unique opportunity for studying evo lutionary forces and phylogeny within the genus, because patterns of variat ion at the two genomes carl be cross compared. Previous studies of evolutio nary forces acting on Mytilus mtDNA have been carried out by analysing sequ ence data from relatively small numbers of individuals, often comparing dif ferent species. In the present study we use an RELP approach to study popul ation variation in a single species. Mytilus edulis from five localities in Britain. These localities lie within an area in which previous allozyme st udies had failed to detect significant geographic differentiation. In the p resent study, significant mtDNA differentiation is observed for both genome s both for haplotype frequencies and for nucleotide divergence. Nucleotide diversity within and divergence between populations is greater for the M ge nome consistent with a higher mutation rate and/or lower purifying selectio n for this genome. Application of the Ewens Watterson test provides evidenc e for excess 'homozygosity' much greater in magnitude for the F genome cons istent with the stronger action of purifying selection on this genome. The distribution of pairwise nucleotide divergence values within populations Wa s compared with theoretical distributions obtained by computer simulation. Much better Fit of the empirical results is observed to a purifying selecti on model than to an equilibrium neutral or population expansion model. The results are in agreement xith those of previous studies of DNA sequences, a dopting different tests, in providing evidence for the action of purifying selection on both genomes but of greater intensity on the F genome. It is c oncluded thar the results are explained well by the nearly neutral theory o f evolution giving important roles to selection, drift and mutation as caus es of the observed patterns of variation. (C) 1999 The Linnean Society of L ondon.