Genetic Evidence for Reproductive Isolation and Multiple Origins of Sympatric Trophic Ecotypes of Whitefish (coregonus).

We assessed variation in mitochondrial DNA (mtDNA) by restriction fragment length polymorphism (RFLP) analysis and in nuclear genes by allozyme analysis among sympatric pairs of limnetic and benthic ecotypes of whitefish (Coregonus) coexisting in three lakes of southern Yukon to address three evolutionary questions regarding their origins. Are sympatric low and high gill-raker count ecotypes genetically differentiated? Are they issued from monophyletic or polyphyletic evolutionary events? If they are polyphyletic in origins, did they originate from multiple allopatric speciation events or intralacustrine radiation? Our results corroborated previous genetic and ecological studies of these ecotypes, indicating that they represent genetically distinct reproductive units, and therefore refuting the hypothesis of phenotypic polymorphism within a single population. However, the amount of gene flow between ecotypes varied among lakes, correlating with the extent of morphological differentiation and the potential for premating reproductive isolation. The results indicated a polyphyletic origin of ecotypes whereby each of them have been expressed independently more than once. In the two lakes of Squanga Creek drainage, the existence of sympatric pairs was best explained by the secondary contact of two monophyletic whitefish groups that evolved in allopatry during the last glaciation events. In Dezadeash L. of Alsek R. drainage, our results could not verify either sympatric or allopatric (or microallopatric) origin of ecotypes. Regardless of the mode of speciation involved in their origins, these sympatric whitefish populations provided further evidence that Pleistocene glaciation events created conditions favoring rapid divergence and phenotypic differentiation among northern freshwater fishes.