Lack of specialisation in trophic morphology between genetically differentiated dwarf and normal forms of lake whitefish (Coregonus clupeaformis Mitchill) in Lac de l'Est, Quebec

We recently observed a bimodal distribution in size of sexually maturing lake whitefish Coregonus clupeaformis Mitchill) in Lac de L'Est, Quebec. The objective of this study was to test the hypothesis that these two spawning size classes represent genetically distinct ecotypes, potentially adapted in morphology for occupying different trophic niches. This was accomplished by quantifying the extent of genetic (mitochondrial DNA (mtDNA) and enzyme loci) and morphological differences. Significant differences in haplotype and allele frequency distributions confirmed the hypothesis that whitefish maturing at small (dwarf) and normal sizes are structured into two distinct gene pools. However, low F-st values at mtDNA and enzyme loci, coupled with the apparent lack of spatial and temporal spawning segregation, suggest that reproductive isolation is incomplete and that gene flow is still occurring between the two forms. Patterns of mtDNA diversity favoured the origin of genetically distinct dwarf and normal-size lake whitefish forms in Lac de l'Est through sympatric divergence. However, a lack of differences in morphological traits potentially related to trophic ecology did not support the hypothesis that the two forms represent ecotypes which are morphologically specialized in trophic niches. This study established that sympatric dwarf and normal-size spawning groups of lake whitefish in Lac de l'Est represent two genetically distinct populations despite the potential for gene flow between them. This, coupled with their low level of morphological diversification, suggests that they represent an early stage of population divergence, and therefore makes them of particular interest for understanding the gene-environment processes involved in the early steps of speciation events.