GEOGRAPHICAL PATTERNS OF VARIABILITY AT ALLOZYME LOCI IN THE EUROPEANOYSTER 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 t he Atlantic and Mediterranean coasts of Europe. We found an abundance of clines, which appeared at 8 loci, including the most polymorphic (A P-2, ARK*, EST-4*, MDH-2*, ME-1*, 6PGH*, PGI* and PGM*). Another 6 lo ci (ALDH EST-3*, EST-5*, IDH-2*, MDH-1*, ME-2*) exhibited V-shaped pa tterns of gene-frequency variation, with dines at one or both sides of the Straits of Gibraltar. The observation of coincident dines at many loci can be explained by a model of secondary intergradation. The geo graphical location of the midpoints of the dines and V-shaped patterns suggests the existence of two ancient Atlantic and Mediterranean oyst er stocks which became differentiated in allopatry and subsequently me rged. Clines observed along Atlantic and/or Mediterranean coasts at th e loci with V-shaped patterns must have arisen independently. The larg e heterogeneity observed in the levels of gene differentiation (G(ST)) across loci (G(ST) ranged from 0.008 to 0.290) and important differen ces in estimates of gene flow obtained by different methods suggest th at the populations of O. edulis are not in genetic equilibrium. Lack o f population equilibrium can be due to natural selection and/or restri ctions to gene flow. The average among-population variability was high er than in other oyster species that do not show incubatory habits, an d represented 8.8% of the total heterozygosity. Levels of intrapopulat ion variability were lowest in populations from the North Atlantic, su ggesting low population sizes in that area.