EVOLUTION OF THE GENETIC-STRUCTURE OF BIVALVE COHORTS AT HYBRIDIZATION SITES OF THE MYTILUS-EDULIS MYTILUS-GALLOPROVINCIALIS COMPLEX

The genetic structure of populations of the Mytilus edulis (L.) -M. ga lloprovincialis (Lmk.) complex has been characterized by studying poly morphism by means of isoenzyme electrophoresis. Three known loci (MPI, EST-D, GPI) have been used to characterize samples taken from two hyb ridization sites on the French Atlantic coast. The purpose of this stu dy was to investigate the genotypical composition of a mussel brood du ring 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 stabl e in the syntopic zones over time, and that there can be a marked evol ution of genetic components, as demonstrated by the results from Groix Island between 1990 and 1991, where brood structure did not conform t o that of the parental population. Our study takes into account enviro nmental characteristics, gene flow, and the selection of dependent gen otypes responsible for the evolution of the observed biodiversity at h ybridization sites where bivalve culture is a contributory factor.