MICROSATELLITE POLYMORPHISM AND POPULATION SUBDIVISION IN NATURAL-POPULATIONS OF EUROPEAN SEA BASS DICENTRARCHUS-LABRAX (LINNAEUS, 1758)

Polymorphism of microsatellite markers was used to study the genetic v ariability and structure in natural populations of European sea bass D icentrarchus labrax. The data consisted of six microsatellite loci ana lysed for 172 individuals from three samples collected in the Golfe-du -Lion (France) and one sample collected in the Golfo-de-Valencia (Spai n). Our goals were (i) to assess the level of genetic variability as r evealed by these markers, (ii) to estimate the genetic differentiation among natural populations within a restricted area, and (iii) to eval uate how microsatellite loci fit the predictions of the two most widel y used mutation models (the infinite allele model and the stepwise mut ation model). As expected, our results indicate that the genetic polym orphism is very high when compared with previously used genetic marker s, the mean expected heterozygosity per locus ranging between 0.69 and 0.93. We also found that all loci but one fitted the infinite allele model better. Using this model as a lower limit, we could extrapolate from the observed diversity effective population sizes on the order of 35 000 individuals. Our results also suggest that there may be a slig ht genetic differentiation between the two gulfs (F-ST = 0.007, P < 0. 05), indicating that the corresponding populations are likely to be dy namically independent. This finding for a species with high dispersal abilities, if confirmed, has important bearings on fish-stock assessme nt.