Genetic divergence in natural populations of the Mediterranean brackish-water killifish Aphanius fasciatus

Samples of the brackish-water cyprinodontid fish Aphanius fasciatus from 11 Mediterranean coastal brackish-water habitats were examined for variation at 43 allozyme loci. Sixteen loci were polymorphic in at least 1 population. Estimates of genetic variability revealed low levels of polymorphism, with mean effective number of alleles per locus ranging from 1.02 (SE 0.01) to 1.10 (SE 0.04) and average expected heterozygosity values from 0.013 (SE 0.007) to 0.062 (SE 0.022). The mean Weir & Cockerham f = 0.060 (SE 0.019) confirmed the general concordance with Hardy-Weinberg equilibrium within populations assessed by exact tests. The presence of various private alleles and the mean value of the coancestry coefficient, theta = 0.507 (SE 0.078), indicated a marked genetic divergence among populations. Nei's genetic distance values were characteristic of populations within species, ranging from 0.001 (SE 0.000) to 0.098 (SE 0.044). Genetic affinities obtained by UPGMA cluster analysis were consistent with the geographical distribution of populations. The high degree of genetic divergence among A. fasciatus populations corresponds to the naturally fragmented distribution of the species and to restricted gene flow between populations, due to the limited dispersal potential of the species. Furthermore, genetic and geographical distances between populations are consistent with the prediction that the species is genetically structured by isolation by distance.