Reticulate evolution and phenotypic diversity in North American ciscoes, Coregonus ssp (Teleostei : Salmonidae): implications for the conservation of an evolutionary legacy

Coregonine fishes are notorious taxonomic problems due to their extreme morphological and ecological variation. In North America, diversity is particularly baffling among ciscoes, and both morphological and phylogenetic analyses have resulted in major polytomy among the 8 taxa of the Coregonus artedi species complex. Ciscoes are also a devastated group, accounting for 10% of the fish species listed by the Committee on the Status of Endangered Wildlife in Canada. Here, we complete the genetic characterization of North American ciscoes with mitochondrial and microsatellites markers previously used to analyse populations of C. artedi in order to elucidate the evolutionary history and identify appropriate conservation units. Our results revealed a complex evolutionary history marked by postglacial reticulation events coupled with recent and independent evolution of similar phenotypes (taxa). Genetic variation reflects geography rather than taxonomy, and consequently, we recommend that a single taxon, C. artedi (sensu lato) be recognized. Local genetic differentiation is often coupled with ecophenotypic diversification, and gill raker polymorphisms, depth-related habitat preference and reproductive behaviour are considered as phenotypic traits with probable adaptive value contributing to the niche expansion of ciscoes. Ecomorphotypes of each particular locale thus represent a unique expression of a diverse genetic pool still undergoing divergence and sorting. Consequently, ciscoes from lakes with distinct ecomorphotypes are recognized as ESUs, as well as each of sympatric forms when they are genetically differentiated. We recommend that an ESU strategy focusing at a very local level be adopted for continental ciscoes as a valid alternative to protect significant evolutionary processes of divergence encountered in polytypic species of newly colonized habitats.