Résumé
The life cycle of most marine bivalves is characterized by high fecundity, external fertilization, dispersal during a planktonic larval phase and a sessile adult phase (Fig. 1). From a successful pair cross resulting in millions of eggs (Widdows 1991), two adults on average (if the population stays constant) will participate to the next generation. However, the variance in reproductive success can be huge (Hedgecock 1994) and most of this variance is probably due to processes occurring before the spat stage. Considering a single reproductive event, genotype frequencies observed at the adult stage may be influenced by various parameters including parental fertility, larval fitness and the existence of assortative mating or segregation distortion. When adult populations are composed of several reproductive cohorts, temporal and spatial recruitment variation introduces even more complications. The importance of these processes is attested to by recurrent observations of departures from random-mating frequencies in allozyme surveys of samples at the post-metamorphic stage. Departure from Mendelian expectations are not uncommon in hatchery experiments (see Foltz 1986b, McGoldrick et al. 2000) and a general trend toward heterozygote deficiencies relative to Hardy-Weinberg equilibrium (HWE) is often observed in natural populations (reviews in Zouros and Foltz 1984, Blanc and Bonhomme 1986, Gaffney 1994, Raymond et al. 1997). Just as puzzling is the observation that multi-locus heterozygosity (MLH) is positively correlated with fitness-related traits (reviews in Mitton and Grant 1984, Zouros and Foltz 1987, David 1998). Small but significant spatial (Borsa et al. 1991), temporal (Hedgecock 1994) or spatio-temporal (David et al. 1997b) genetic structure at very fine-scale ("chaotic genetic patchiness" sensu
Johnson and Black 1982) contrasts with an apparent genetic homogeneity over long distances (review in Palumbi 1992). Finally, examples of environment-dependent genetic variation, such as the amino-peptidase allele frequency cline between estuarine habitats of Long Island Sound and oceanic coastal zones in the mussel Mytilus edulis (Koehn et al. 1980, Hilbish and Koehn 1985), imply selection starting from the beginning of the settled phase and maybe earlier.