Synchronous reproduction may facilitate introgression in a hybrid mussel (Mytilus) population

Mussel populations on the Irish Atlantic coast comprise an interbreeding mixture of the blue mussel, Mytilus edulis (L.) and the Mediterranean mussel, Mytilus galloprovincialis (Lmk.). The occurrence of hybrid genotypes varies between sites but can be as high 80%. This study compares the reproductive cycle of M. edulis, M. galloprovincialis and their hybrids to determine if the extensive hybridisation observed at Irish Atlantic coast sites is linked to spawning synchrony between the two taxa. Mussels (40-45 mm size class) were collected monthly from a sheltered shore in Galway Bay from January to December 2005. Two major spawning events (March-June and September-October) were observed and gametogenesis took place throughout the year. The spawning cycles of the three taxa were largely overlapping. Small differences were observed in the timing of peak spawning which occurred in March and October in M. galloprovincialis and in May-June and September in M. edulis. Spawning of hybrid individuals was intermediate between the parental genotypes. Fecundity was slightly higher in M. galloprovincialis females compared to the other taxa (up to 30% difference, P < 0.05). This advantage, which is not shared by males, may be offset by high numbers of hybrid genotypes releasing gametes during peak spawning of M. galloprovincialis. There was no evidence of the increased mortality in hybrid males; sex ratios did not deviate from the 1:1 ratio. The results show that in this region of the hybrid zone the timing of reproduction does not present a barrier to gene flow between M. edulis and M. galloprovincialis. However, there are small differences in the timing of peak spawning between the taxa thus preventing the complete introgression of the genotypes. Hybrids outnumber the parental genotypes, undergo complete gametogenesis and show no evidence of depressed fitness (i.e. hybrids are reproductively successful) suggesting a high degree of introgression.