Allopatric and sympatric diversification within roach (Rutilus rutilus) of large prealpine lakes

The possibility for intraspecific diversification, has until now rarely been evaluated for most of the other northern freshwater fish species. Here, we assess the potential for intraspecific differentiation between and within lake populations of roach (Rutilus rutilus) – a widespread and abundant cyprinid species - in lakes in which salmonids have evolved endemic adaptive radiations. Based on more than 3,000 polymorphic RADseq markers, we detected low but significant genetic differentiation between the roach populations of the two ultraoligotrophic lakes and between these and the other lakes. This, together with differentiation in head morphology and stable isotope signatures, suggests evolutionary and ecological differentiation among roach populations within some of the lakes used in this study. Next, we tested for intralacustrine diversification within the roach population of Lake Brienz, the most pristine lake surveyed in this study. We found significant phenotypic evidence for intralacustrine ecological differentiation between roach caught over a muddy substrate and those caught over rocks, but evidence for intralacustrine genetic differentiation is at best subtle. Overall, our findings suggest roach can differentiate between ecologically distinct lakes, but the extent of intralacustrine ecological differentiation is weak, which contrasts with the strong differentiation among endemic species of whitefish in the same lake.