Microgeographic allozyme differentiation in the hybrid zone of Mytilus galloprovincialis Lmk and M-edulis L on the continental European coast

The European Mytilus galloprovincialis Lmk. and M. edulis L. coexist and hybridize in different proportions in extended areas of the British and Atlantic French coasts. M. galloprovincialis typical allozymes seem to predominate in wave exposed areas, at high levels of attachment and in larger mussels in the British hybrid zone. Mussel samples from exposed and sheltered areas, 200 m apart, and from high and low levels of attachment were collected from a location of the French hybrid zone in 1988-92. Pure M. galloprovincialis and M. edulis populations were also taken as controls. Diagnostic enzyme loci for both Mytilus (EST-D*, LAP-1*, MPI*, ODH*) and AP-1*, LAP-2* and PGM* loci were studied. The frequencies of the M. galloprovincialis typical alleles were significantly greater in exposed populations than in sheltered samples (e.g. 0.729 to 0.803 vs 0.192 to 0.581 for EST-D*90), and at high level of attachment than at low level for the sheltered area (e.g. 0.581 vs 0.192 for EST-D*90). Putative M. galloprovincialis was more abundant on the exposed coast (0.591 and 0.702) than on the sheltered shore, where it predominated at the high shore but not at the low shore location (0.371 vs 0.045). Significantly positive correlations between shell length and typical M. galloprovincialis compound allele frequencies were found only for populations from exposed areas. Relationships between the Mytilus genetic differentiation and ecological factors are discussed.