NONNEUTRAL EVOLUTION AND DIFFERENTIAL MUTATION-RATE OF GENDER-ASSOCIATED MITOCHONDRIAL-DNA LINEAGES IN THE MARINE MUSSEL MYTILUS

Mussels have two types of mitochondrial DNA (mtDNA). The hi type is tr ansmitted paternally, and the F type is transmitted maternally. To tes t hypotheses of the molecular evolution of both mtDNA genomes, 50 nucl eotide sequences were obtained for 396 bp of the COIII gene of Europea n populations of Mytilus edulis and the Atlantic and Mediterranean for ms of ii. galloprovincialis. Analysis based on the proportion of synon ymous and nonsynonymous substitutions indicate that mtDNA is evolving in a non-neutral and complex fashion. Previous studies on American mus sels demonstrated that the F genome experiences a higher purifying sel ection and that the M genome evolves faster. Here we show that these p atterns also hold in European populations. However, in contrast to Ame rican populations, where an excess of replacement substitution between F and hi lineages has been reported, a significant excess of replacem ent polymorphism within mtDNA lineages is observed in European populat ions of M, galloprovincialis. European populations also show an excess of replacement polymorphism within the F but not within the M genome with respect to American ill. trossulus, as well as a consistent patte rn of excess of rare variants in both F and M genomes. These results a re consistent with a nearly neutral model of molecular evolution and a recent relaxation of selective constraints on European mtDNA. Levels of diversity are significantly higher for the M than F genome, and the M genome also accumulates synonymous and nonsynonymous substitutions at a higher rate, in contrast with earlier reports where no difference for the synonymous rate was observed. It is suggested that a subtle b alance between relaxed selection and a higher mutation rate explains t he faster evolutionary rate of the M lineage.