Phylogeographical lineages in brown trout (Salmo trutta):: investigating microgeographical differentiation between native populations from Northern Spain

1. The large microgeographical differentiation revealed by allozyme studies in brown trout (Salmo trutta) populations is one of the most striking features of this species. Additionally, allozymes showed great genetic differences between Atlantic and Mediterranean populations on a macrogeographical scale. 2. This study was carried out in order to assess whether the great differences observed between Atlantic and Mediterranean populations persisted where the two are geographically close (the 'microgeographical scale'). Sixteen populations of brown trout, S. trutta , were screened for genetic variation at 25 allozyme loci. The sampling sites, which occupied a relatively small geographical area, were distributed across Cantabrian (Atlantic) and Mediterranean drainages in Northern Spain. 3. The neighbour-joining tree, inferred from Nei's genetic distance, showed that brown trout populations clustered into two different groups. These groups corresponded to the Cantabrian and the Mediterranean groups of populations, although no clear geographical pattern emerged within each of the groups. This geographical pattern is basically caused by significant differences in the frequency distribution of the CK-A1 * locus, with a higher frequency of * 115 in Cantabrian samples (0.586 +/- 0.091) while allele * 100 was more frequent in Mediterranean samples (0.931 +/- 0.038). In addition, this study revealed alleles exclusive to the Mediterranean and Cantabrian populations, agreeing with previous findings. 4. Genetic differentiation between Cantabrian and Mediterranean regions (14.19%) was similar to that estimated in Spain at a larger scale (13%), showing that most of the differences between the regions can be observed even in a small geographical area.