Genetic differentiation in marine and estuarine natural populations of Crassostrea angulata

Abstract

Electrophoretic examination of 8 allozyme loci in 10 Spanish natural populations of Crassostrea angulata revealed a high level of genetic variability in terms of average allele number, polymorphism and mean heterozygosity. Significant departures of genotypic frequencies from the Hardy-Weinberg conditions, which could not be explained after pooling alleles, were displayed at some loci. All populations showed a deficit of heterozygotes, according to Selander's D [(Het.(observed) -Het.(expected))/Het.(expected)]; moreover, the mean D index across populations was negative for the Lap, Mdh-1, Mdh-2, Me-2 and Xdh genes, consistent with F(rr) estimates, which indicates deviations from Hardy-Weinberg equilibrium within the total population. In contrast, loci Me-1 and Pgi were at equilibrium and Pgm showed a heterozygous excess. Null alleles appeared in Lap and Mdh-1 loci, which could have been responsible for the disequilibrium, at least in Mdh-1, because the increase of frequencies of null alleles was also related to the increase in the deficit of heterozygotes. Although a high level of gene flow between populations was detected, heterogeneity in the allelic frequencies of Lap, Mdh-1, Mdh-2, Me-2, Pgm and Xdh genes was shown to occur. Mean F(ST) (F-statistic indicating deviations from Hardy-Weinberg equilibrium among populations) across loci was 0.031, meaning a slight genetic differentiation; however, no relationships were shown to exist between genetic distances, geographic distances and their location (estuarine or marine sites).