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Patterns and processes of salmonid diversification in Europe's oldest body of freshwater, Lake Ohrid

Final Activity Report Summary - SALMOHRID (Patterns and processes of salmonid diversification in Europe's oldest body of freshwater, Lake Ohrid)

Fishes of the family Salmonidae (e.g. trout & Salmon) are extremely important for the European community in both economic and ecological terms. Despite their importance and popularity, modern researchers have all but neglected the rich diversity of salmonid fishes in the Balkan region, especially habitats within the Adriatic drainage. The Adriatic drainage may have served as an evolutionary centre for the genus Salmo (to which Atlantic salmon and brown trout belong) and also contains one of the most unique bodies of freshwater in Europe, Lake Ohrid. Lake Ohrid is Europe's oldest lake, believed to have been formed as early as the Tertiary period, and is thus characterised by a highly unique flora and fauna. The primary goal of this project was to characterise the diversity of Salmo fishes in Lake Ohrid and its surrounding aquatic environments. This characterisation was to be mainly based on genetic approaches, but some phenotypic (i.e. morphological) data was also analysed. Samples were collected from different Adriatic river systems and screened for variation using a variety of different genetic markers, including so-called microsatellites and genes from both the mtDNA and nuclear genomes. These data were then analysed using an array of statistical approaches that allow one to draw conclusions about the relationships among populations within species, their historical demography, and the times of isolation or divergence both for populations within species, and between different species. This allows an understanding of both the present diversity of Salmo fishes in the region, and especially Lake Ohrid, as well as a view of how this diversity evolved in geographic and demographic terms.

The main results of this analysis revealed that Lake Ohrid endemic taxon Salmo ohridanus (local name 'Belvica') is a highly unique and divergent member of the genus Salmo. Based on substitution rate differences in mtDNA genes, we attempted to estimate the age of this taxon. Our analysis shows that Belvica, together with a closely related species (the softmouth trout, endemic to a few Adriatic river systems), first arose from a common ancestor of brown trout Salmo trutta, over 4 MYA. This age overlaps with minimum age estimates of the formation of Lake Ohrid, and implies that the formation of this great water body may have played a fundamental role in the evolutionary history of genus Salmo. In contrast to the antiquity of Belvica, Lake Ohrid brown trout Salmo letnica were shown to be more recent colonisers of Lake Ohrid, and to have evolved from within a lineage of brown trout. Furthermore, no objective data could be found supporting the presence of multiple forms of Lake Ohrid brown trout, despite the fact that trout are found reproducing in the lake in different places, and at different times of the year. Nevertheless, based on genetic analyses the endemic Ohrid trout represents a monophyletic lineage, isolated from other Adriatic basin populations. We also could find no evidence that this fish has suffered loss of genetic variation based on the overfishing that has presumably taken place over the last decades. In the interests of protecting the unique biodiversity of this ancient ecosystem, we recommend retaining the name Salmo letnica for the endemic Ohrid trout, despite the fact that it has evolved from within the species complex of brown trout (S. trutta).