Contrasting levels of sympatric divergence within lacustrine Arctic charr Salvelinus alpinus forms flock: high differentiation between size forms, low differentiation between seasonal races

The formation of reproductive isolation barriers is fundamental to sympatric speciation. In teleost fishes, an important mechanism for the reduction of gene flow between emerging species and intraspecific forms is the divergence in reproductive strategies, in particular in spawning time. Large time difference between spawning periods of sympatric forms typically provides strong pre-zygotic isolation and rapid genetic and morphological divergence between them. However, our results indicate that it may not always be the case. We investigated genetic divergence between recently discovered autumn-spawning and spring-spawning subpopulations of the small planktivorous Arctic charr form in one of the Transbaikalian lakes using 14 microsatellite loci and found that it was very low (Fst = 0.0028) as compared with their differentiation from a sympatric large piscivorous form (Fst = 0.284–0.301). Charr of these subpopulations manifested subtle differences in morphology and life history and had similar diets. We consider that low genetic, phenotypic and ecological differentiation between autumn and spring spawners of the small form is the result of their incomplete reproductive isolation due to the transformation between them (change of spawning time) in successive generations.