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The genetic structure of populations of the Mytilus edulis (L.)-M. galloprovincialis (Lmk.) complex has been characterized by studying polymorphism by means of isoenzyme electrophoresis. Three known loci (MPI, EST-D, GPI) have been used to characterize samples taken from two hybridization sites on the French Atlantic coast. The purpose of this study was to investigate the genotypical composition of a mussel brood during two consecutive years (1990 and 1991) in relation to the genetic structure of parental populations analyzed in 1989. The 1989 parental samples showed a strong hybridization at the two sites, and there was a correspondingly high degree of evolution in the 1990 and 1991 broods towards M. edulis. This indicates that genetic structure is not stable in the syntopic zones over time, and that there can be a marked evolution of genetic components, as demonstrated by the results from Groix Island between 1990 and 1991, where brood structure did not conform to that of the parental population. Our study takes into account environmental characteristics, gene flow, and the selection of dependent genotypes responsible for the evolution of the observed biodiversity at hybridization sites where bivalve culture is a contributory factor.

A genetic study carried out on nine natural mussel populations on the French Atlantic coast from 1989–1990 revealed interdigitation between typicallyMytilus edulis and typicallyM. galloprovincialis populations and intermediate populations. The allele components of the populations followed aM. edulis/M. galloprovincialis gradient which does not correspond to a geographical gradient. Strong hybridization was evident in samples with intermediate allele frequencies. The respective importance of gene flow and selection is discussed in the light of the results and the evironmental features of the sampling zone.