Impact of the reproduction dynamics on genetic variation in the European flat oyster Ostrea edulis

The European flat oyster (Ostrea edulis L.) is a marine bivalve whose natural geographical distribution ranges along the European Atlantic coast from Norway to Morocco, in addition to the Mediterranean and Black Sea. The latest results obtained on the genetic differentiation between these populations have led us to pursue studies at a finer scale, in order to estimate the effective number of breeders and the temporal dynamics of reproduction and, more specially, recruitment. Several experiments were performed to document (1) the variance in allele frequencies during a natural settlement period; (2) the paternal contribution to fertilization by analyzing larvae sampled at the brooding stage within individual females; (3) the variance of individual reproductive success within an experimental population. Firstly, 3 sets of collectors were successively deployed every 2 weeks and one set left during the whole recruitment period in 2001. In addition, adult oysters were sampled including 14 brooding females (ie, females presenting larvae in their paleal cavity). Their larvae were sampled and preserved in ethanol. Mitochondrial (12S fragment) and microsatellite (four loci) analyses were performed. Although the temporal cohorts did not exhibit any differentiation on the basis of the microsatellite markers, a slight but significant differentiation was observed with the mitochondrial marker. Moreover, our data on the genetic variability of single-female progenies show that females can be fertilized by a highly variable number of males, which can be, in some cases, very low. In such cases, a temporary low effective could lead to a level of inbreeding (even low) which would explain the correlation between growth and heterosis often observed. More investigations are needed to directly demonstrate that each male fertilizes several females. If males succeed in fertilizing several females, variability in environmental conditions should increase female variance in reproductive success more than male variance in reproductive success, thus reducing mitochondrial relative to nuclear effective size as has been observed in the study of the genetic structure of the populations along the species range.