Genetic structure of brown trout, Salmo trutta L., at the southern limit of the distribution range of the anadromous form

Genetic variation at 33 protein loci was investigated in 41 wild brown trou t populations from four river basins in Galicia (northwest Spain) to analys e the amount and distribution of genetic diversity in a marginal area, loca ted in the distribution limit of the anadromous form of this species. The g enetic diversity detected within populations (H between 0 and 6%) lies with in the range quoted for this species in previous reports. The Mino, the mos t southern river basin analysed, showed a significantly lower genetic diver sity and the highest genetic differentiation among the river basins studied . The hierarchical gene diversity analysis showed high population different iation in a restricted area (G(ST) = 27%), mostly due to differences among populations within basins (G(SC) = 22%). The reduction of Gs, observed when the isolated samples were excluded from the analysis (G(ST) = 17%) showed the importance of habitat fragmentation on the heterogeneity defected. Gene flow among populations was comparatively evaluated by three indirect metho ds, which in general revealed low figures of absolute number of migrants pe r generation, slightly higher than 1. The gene flow among basins reflected a positive relationship with geographical distance. This trend was confirme d by the significant correlation observed between geographical and genetic distances, including all population pairs, which suggests a component of is olation by distance in brown trout genetic structure. Nevertheless, the non significant intrabasin correlation demonstrates the complexity of genetic r elationships among populations in this species. The model of genetic struct ure in brown trout is discussed in the light of the results obtained.