Molecular population genetics of the male and female mtDNA molecules in the California sea mussel, Mytilus californianus

The presence of two sex-associated mitochondrial genomes in marine mussels provides a unique opportunity to investigate the dynamics of mtDNA evolution without complications inherent in interspecific comparisons. Here, we assess the relative importance of selection, mutation, and differential constraint in shaping the patterns of polymorphism within and divergence between the male (M) and female (F) mitochondrial genomes of the California sea mussel, Mytilus californianus. Partial sequences were obtained from homologous regions of four genes (nad2, cox1, atp6, and nad5) totaling 2307 bp in length. The M and F mtDNA molecules of M. californianus exhibited extensive levels of nucleotide polymorphism and were more highly diverged than observed in other Mytilids (overall TAMURA-NEI distances > 40%). Consistent with previous studies, the M molecule had significantly higher levels of silent and replacement polymorphism relative to F. Both genomes possessed large numbers of singleton and low frequency mutations that gave rise to significantly negative TAJIMA's D values. Mutation rate scalars estimated for silent and replacement mutations were elevated in the M genome but were not sufficient to account for its higher level of polymorphism. McDONALD-KREITMAN tests were highly significant at all loci due to excess numbers of fixed replacement mutations between molecules. Strong purifying selection was evident in both genomes in keeping the majority of replacement mutations at low population frequencies but appeared to be slightly relaxed in M. Our results suggest that a reduction in selective constraint acting on the M genome remains the best explanation for its greater levels of polymorphism and faster rate of evolution.

Key Words: Mytilus californianus, doubly uniparental inheritance, mitochondrial DNA, mutation, purifying selection

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