Geographical patterns of variability at allozyme loci in the European oyster Ostrea edulis

The variability of 14 enzyme-coding genes has been analysed in samples from 19 populations of the oyster Ostrea edulis L., collected along the Atlantic and Mediterranean coasts of Europe. We found an abundance of clines, which appeared at 8 loci, including the most polymorphic (AP-2 ^*, ARK ^*, EST-4 ^*, MDH-2 ^*, ME-1 ^*, 6PGH ^*, PGI ^* and PGM ^*). Another 6 loci (ALDH ^*, EST-3 ^*, EST-5 ^*, IDH-2 ^*, MDH-1 ^*, ME-2 ^*) exhibited V-shaped patterns of gene-frequency variation, with clines at one or both sides of the Straits of Gibraltar. The observation of coincident clines at many loci can be explained by a model of secondary intergradation. The geographical location of the midpoints of the clines and V-shaped patterns suggests the existence of two ancient Atlantic and Mediterranean oyster stocks which became differentiated in allopatry and subsequently merged. Clines observed along Atlantic and/or Mediterranean coasts at the loci with V-shaped patterns must have arisen independently. The large heterogeneity observed in the levels of gene differentiation (G _ST ) across loci (G _ST ranged from 0.008 to 0.290) and important differences in estimates of gene flow obtained by different methods suggest that the populations of O. edulis are not in genetic equilibrium. Lack of population equilibrium can be due to natural selection and/or restrictions to gene flow. The average among-population variability was higher than in other oyster species that do not show incubatory habits, and represented 8.8% of the total heterozygosity. Levels of intrapopulation variability were lowest in populations from the North Atlantic, suggesting low population sizes in that area.