Génétique des populations de bivalves en milieu lagunaire: la palourde dans l'étang de Thau (Méditerranée)

The genetic structuration of populations of the palourde, Ruditapes decussatus, a bivalve species of the Mediterranean coastal lagoons, was studied at the enzymatic gene level, in relation to their ecology and their demography. A genetic comparison of local populations of the Languedocian lagoons suggested that they constitute a metapopulation. In the Thau lagoon, genetic heterogeneity could be related to demographic features. Genetic differences between cohorts appeared to account for significant heterozygote deficiencies, although not fully explaining them. For this, a Wahlund effect could be rejected. The hypothesis of selection affecting pre-recruits appeared most likely and it was confirmed by preliminary results of experiments conducted on larvae. It was shown that mean heterozygosity increases in a cohort monitored over one generation. This also occurs during sudden environmental stress (malaïgue). Selective interactions between loci also could be detected. Thus, consistent observations account for the existence of a selective control of the distribution of multilocus, as weIl as monolocus genotypes, at both the larval and post-settled stages, between individuals within a cohort, and between cohorts within the population.