PHYLOGENETIC-RELATIONSHIPS AND INTROGRESSION PATTERNS BETWEEN INCIPIENT PARAPATRIC SPECIES OF ITALIAN BROWN TROUT (SALMO-TRUTTA L COMPLEX)

Genetic variation at 47 protein loci was investigated in 16 wild brown trout populations from the P (o) over cap basin and three major domes ticated stocks used for stocking this area. Twenty-four loci were poly morphic and large frequency differences were found at 15 of them. The most significant allozyme variations were congruent with the mtDNA seq uence polymorphism previously observed in the same samples. We confirm ed the occurrence of two parapatric incipient species, Salmo marmoratu s and S. trutta fario, previously identified by morphological traits. These two species were fixed or nearly fixed for alternate alleles at eight loci (Nei's standard genetic distance = 0.16-0.18), but introgre ssion was detected between adjacent samples of the two forms. Divergen ce levels at both mtDNA and nuclear loci suggested that the differenti ation between S. marmoratus and S. trutta fario started between 3 and 1 million years before present. Variation at protein loci and mtDNA su pported the hypothesis that the third species found in this area, S. c arpio (an endemic population of the lake Garda) was issued from a rece nt hybridization of the two first species. Finally, we showed that thr ee of the major Italian fish-farm strains originated from the Atlantic side and displayed substantial genetic differences with the natural p opulations of the P (o) over cap basin. Most of these populations were contaminated by stocking with introgression rate ranging from 0 to 70 % and measures of protection and restoration of the rich genetic diver sity present in this area should be urgently applied.