Phylogenetic relationships and introgression patterns between incipient parapatric species of Italian brown trout (Salmo trutta L. complex)

Authors


  • This work is a part of Elisabetta Giuffra's PhD thesis on the geographical variation at protein loci and mtDNA in the S. trutta complex from Northern Italy. Reni Guyomard is a population geneticist at the Laboratory of Fish Genetics of INRA. This laboratory is involved in research programs on conservation, management and selective breeding of freshwater fish species. Gilberto Forneris is ichthyologist at the Dipartimento di Produzioni Animali, Epidemiologia ed Ecologia of the University of TuMo and is responsible for programs of conservation of natural population of freshwater fish in the Piemonte Province.

Fax: (33) 1 34 65 23 90. Tel.: (33) 1 34 65 23 94. E-mail: guyomard@jouy.inra.fr

Abstract

Genetic variation at 47 protein loci was investigated in 16 wild brown trout populations from the Po basin and three major domesticated stocks used for stocking this area. Twenty-four loci were polymorphic and large frequency differences were found at 15 of them. The most significant allozyme variations were congruent with the mtDNA sequence polymorphism previously observed in the same samples. We confirmed the occurrence of two parapatric incipient species, Salmo marmoratus 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 introgression was detected between adjacent samples of the two forms. Divergence levels at both mtDNA and nuclear loci suggested that the differentiation between S. marmoratus and S. trutta fario started between 3 and 1 million years before present. Variation at protein loci and mtDNA supported the hypothesis that the third species found in this area, S. carpio (an endemic population of the lake Garda) was issued from a recent hybridization of the two first species. Finally, we showed that three of the major Italian fish-farm strains originated from the Atlantic side and displayed substantial genetic differences with the natural populations of the Po 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 diversity present in this area should be urgently applied.

Ancillary