PHYLOGENETIC EVIDENCE FOR ROLE-REVERSALS OF GENDER-ASSOCIATED MITOCHONDRIAL-DNA IN MYTILUS (BIVALVIA, MYTILIDAE)

Distinct gender-associated mitochondrial DNA (mtDNA) lineages (i.e., l ineages which are transmitted either through males or through females) have been demonstrated in two families of bivalves, the Mytilidae (ma rine mussels) and the Unionidae (freshwater mussels), which have been separated for more than 400 Myr. The mode of transmission of these M ( for male-transmitted) and F (for female-transmitted) molecules has bee n referred to as doubly uniparental inheritance (DUI), in contrast to standard maternal inheritance (SMI), which is the norm in animals. A p revious study suggested that at least three distinct origins of DUI ar e required to explain the phylogenetic pattern of M and F lineages in freshwater and marine mussels. Here we present phylogenetic evidence b ased on partial sequences of the cytochrome c oxidase subunit I gene a nd the 16S RNA gene that indicates that DUI is a dynamic phenomenon. S pecifically, we demonstrate that F lineages in three species of Mytilu s mussels, M. edulis, M, trossulus, and M. californianus, have spawned separate lineages which are now associated only with males. This proc ess is referred to as ''masculinization'' of F mtDNA. By extension, we propose that DUI may be a primitive bivalve character and that period ic masculinization events combined with extinction of previously exist ing M types effectively reset the time of divergence between conspecif ic gender-associated mtDNA lineages.