Species flock in the North American Great Lakes: Molecular ecology of LakeNipigon Ciscoes (Teleostei : Coregonidae : Coregonus)

Studies on north temperate fish species indicate that new habitat availabil ity following the last ice sheet retreat has promoted ecological speciation in postglacial lakes. Extensive ecophenotypic polymorphisms observed among the North American Great Lakes ciscoes suggest that this fish group has ra diated through trophic adaptation and reproductive isolation. This study ai ms at relating the ecomorphological and genetic polymorphisms expressed by the Lake Nipigon ciscoes to evaluate the likelihood of an intralacustrine d ivergence driven by the exploitation of alternative resources. Morphologica l variation and trophic and spatial niches are characterized and contrasted among 203 individuals. Genetic variation at six microsatellite loci is als o analyzed to appraise the extent of genetic differentiation among these mo rphotypes. Ecomorphological data confirm the existence of four distinct mor photypes dis-playing various levels of trophic and depth niche overlap and specialization. However, ecological and morphological variations were not c oupled as expected, suggesting that trophic morphology is not always predic tive of ecology. Although extensive genetic variability was observed, littl e genetic differentiation was found among morphotypes, with only one morph being slightly but significantly differentiated. Contrasting patterns of mo rphological, ecological, and genetic polymorphisms did not support the hypo thesis of ecological speciation: the most ecologically different forms were morphologically most similar, while the only genetically differentiated mo rph was the least ecologically specialized. The low levels of genetic diffe rentiation and the congruence between theta and phi estimates altogether su ggest a recent (most likely postglacial) process of divergence and/or high gene flow among morphs A, C, and D, whereas higher phi estimates for compar ison involving morph B suggest that this morph may be derived from another colonizing lineage exchanging little genes with the other morphs. Patterns of ecophenotypic and genetic diversity are also compatible with a more comp lex evolutionary history involving hybridization and introgression.