An analysis of the effects of mitochondrial DNA polymorphisms on sperm motility and the evolution of a male associated sperm protein, lysin, in marine mussels (Mytilus sp.)

Some families of bivalves have an unusual mode of mitochondrial DNA (mtDNA) transmission called doubly uniparental inheritance (DUI), characterised by the presence of two gender-associated mtDNA genomes that are transmitted either through males (M-type mtDNA) or females (F-type mtDNA). In Mytilid mussels, an F type occasionally invades the male route of inheritance, referred to as a "role reversal" event, and is thereafter transmitted as a new M-type. Phylogenetic studies have demonstrated that the new or "recently masculinized" M-types may eventually replace the older or "standard" M-types over time. To investigate whether this replacement process could be due to an advantage in sperm swimming behaviour, differences in various motility parameters between Mytilus edulis sperm with either the standard or recently masculinized M-type were analysed. Sperm with recently masculinized M-type mtDNA showed a significant increase in curvilinear velocity and average path velocity when compared to sperm with standard M-type, which could explain the evolutionary replacement of standard M-types by recently masculinized M-types in Mytilid mussels. To investigate whether there is support for the hypothesis that masculinization events may be associated with population divergence leading to speciation, a male associated sperm protein, lysin, was examined in male M. edulis and male M. trossulus mussels containing recently masculinized M-type or standard M-type mtDNA. The lysin protein was chosen because rapid divergence of lysin driven by positive Darwinian selection has been observed in other marine invertebrate species. However, no clustering of lysin alleles, specifically the exon 5 region of the gene, corresponding to mitotype was observed so the hypothesis of incipient speciation within M. edulis was rejected.