SPECIES-SPECIFIC SEGREGATION OF GENDER-ASSOCIATED MITOCHONDRIAL-DNA TYPES IN AN AREA WHERE 2 MUSSEL SPECIES (MYTILUS-EDULIS AND M-TROSSULUS) HYBRIDIZE

In each of the mussel species Mytilus edulis and M. trossulus there ex ist two types of mtDNA, the F type transmitted through females and the M type transmitted through males. Because the two species produce fer tile hybrids in nature, F and M types of one may introgress into the o ther. We present the results from a sun;ey of a population in which ex tensive hybridization occurs between these two species. Among specimen s classified as ''pure'' M. edulis or ''pure'' M. trossulus on the bas is of allozyme analysis, we observed no animal that carried the F or t he M mitotype of the other species. In most animals of mixed nuclear b ackground, an individual's mtDNA came from the species that contribute d the majority of the individual's nuclear genes. Most importantly, th e two mtDNA types in post-F-1 male hybrids were of the same species or igin. We interpret this to mean that there are intrinsic barriers to t he exchange of mtDNA between these two species. Because such barriers were not noted in other hybridizing species pairs (many being even les s interfertile than M. edulis and M. trossulus), their presence in Myt ilus could be another feature of the unusual mtDNA system in this genu s.