MITOCHONDRIAL CONTROL REGION AND PROTEIN-CODING GENES SEQUENCE VARIATION AMONG PHENOTYPIC FORMS OF BROWN TROUT SALMO-TRUTTA FROM NORTHERN ITALY

The Po River basin of northern Italy is the home of distinctive and en demic morphological forms of brown trout Salmo trutta. We used PCR-dir ect sequencing and RFLP techniques to study variation in the mitochond rial control region of 225 trout in order to assess genetic relatednes s among 18 populations from that region. The distribution analysis of these genotypes among north Italian populations confirmed the phylogen etic differentiation of marbled trout Salmo trutta marmoratus populati ons and the postglacial origin of S. t. carpio. Extensive genetic hete rogeneity was observed among morphologically identical S. t. fario pop ulations. Introgression with domestic strains of Atlantic basin origin was detected in all forms. In order to assess the phylogenetic congru ence detected in coding and noncoding regions of the mitochondrial gen ome, we also analysed sequence variation in segments of the cytochrome b and ATPase subunit VI genes among representatives of all variants d etected in the analysis of the control region. Variation in protein co ding genes was only slightly less than that observed in the control re gion of the same individuals, both in terms of number of variants dete cted and of pairwise sequence divergence estimates among variants. Phy logenetic analysis based on protein coding genes sequences identified the same phylogenetic groupings defined by the control region analysis and also allowed a partial resolution of their phyletic relationships that was previously unresolved. However, coding and noncoding segment s differed substantially in the transition-transversion ratio (17:0 in coding segments vs. 17:6 in control region segments).