Genetic differentiation between populations of gilthead seabream (Sparusaurata) along the Tunisian coast

The genetic variation at 21 protein loci was investigated in six natural gilthead seabream (Sparus aurata L., 1758) populations from the Tunisian coast to analyse the amount and distribution of genetic diversity of this species. In the same way, one reared population has been studied. A strong differentiation was detected (D: 0.023-0.083, pairwise FS,: -- 0.0085--0.2654) between our samples, mainly attributable to the existence of two divergent groups of populations: a southern group located in the Eastern Mediterranean basin and a northern group located in the Western Mediterranean basin. Gene flow between the two groups is probably restricted, which reveals the complexity of the genetic relationships among populations living in the two basins. Ibis divergence was mainly a consequence of the unequal distribution of some alleles and a cline of frequencies in other ones, which were correlated with substantial differences in genetic diversity among regions. The comparison of the genetic diversity detected within populations (H between 0.0556 plus or minus 0.0246 and 0.1857 plus or minus 0.0648) to the range quoted for this species in previous reports revealed that the southem populations presented a similar values to those recorded in Mediterranean and Atlantic populations, whereas the northern populations showed a higher values.Original Abstract: La variabilite genetique des populations naturelles de daurade royale (Sparus aurata L., 1758) de plusieurs localites a ete apprehendee par une analyse allozymique de 21 loci. Les taux d'heterozygotie et de polymorphisme et le nombre moyen d'alleles par locus, qui donnent une premiere estimation de la variabilite genetique, sont comparables a ceux qui ont ete trouves chez d'autres populations de la Mediterranee. Ces valeurs mettent en evidence une structuration geographique de la variabilite genetique. Cette structuration est le resultat d'inversions des frequences alleliques ou de l'apparition de nouveaux alleles a certains loci. L' arbre phylogenetique etabli a partir de la matrice des distances genetiques permet de distinguer deux groupes largement differencies: le premier contenant les populations du Nord et le deuxieme contenant les autres populations qui sont a leur tour subdivisees en deux sous--groupes organises selon leur distribution geographique. Cette differenciation serait le resultat de l'influence d'environnements differents ou ces populations segregent.