Background: Classification of species within the genus Salmo is still a matter of discussion due to their high level of diversity and to the low power of resolution of mitochondrial (mt)DNA-based phylogeny analyses that have been traditionally used in evolutionary studies of the genus. We apply a new marker system based on nuclear (n)DNA loci to present a novel view of the phylogeny of Salmo representatives and we compare it with the mtDNA-based phylogeny.
Methods: Twenty-two nDNA loci were sequenced for 76 individuals of the brown trout complex: Salmo trutta (Danubian, Atlantic, Adriatic, Mediterranean and Duero mtDNA lineages), Salmo marmoratus (marble trout), Salmo obtusirostris (softmouth trout), and Salmo ohridanus (Ohrid belvica or belushka). Sequences were phylogenetically analyzed using maximum-likelihood and Bayesian Inference methods. The divergence time of the major clades was estimated using the program BEAST.
Results: The existence of five genetic units i.e. S. salar, S. ohridanus, S. obtusirostris, S. marmoratus and the S. trutta complex, including its major phylogenetic lineages was confirmed. Contrary to previous observations, S. obtusirostris was found to be sister to the S. trutta complex and the S. marmoratus clade rather than to the S. ohridanus clade. Reticulate evolution of S. obtusirostris was confirmed and a time for its pre-glacial origin suggested. S. marmoratus was found to be a separate species as S. trutta and S. obtusirostris. Relationships among lineages within the S. trutta complex were weakly supported and remain largely unresolved.
Conclusions: Nuclear DNA-based results showed a fairly good match with the phylogeny of Salmo inferred from mtDNA analyses. The comparison of nDNA and mtDNA data revealed at least four cases of mitochondrial-nuclear DNA discordance observed that were all confined to the Adriatic basin of the Western Balkans. Together with the well-known extensive morphological and genetic variability of Balkan trouts, this observation highlights an interesting and variegated evolutionary history of Salmo in this area.