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

Abstract

Coregonine fishes are notorious taxonomicproblems due to their extreme morphological andecological 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 arealso a devastated group, accounting for 10% ofthe fish species listed by the Committee on theStatus of Endangered Wildlife in Canada. Here,we complete the genetic characterization ofNorth American ciscoes with mitochondrial andmicrosatellites markers previously used toanalyse populations of C. artedi in orderto elucidate the evolutionary history andidentify appropriate conservation units. Ourresults revealed a complex evolutionary historymarked by postglacial reticulation eventscoupled with recent and independent evolutionof similar phenotypes (taxa). Genetic variationreflects geography rather than taxonomy, andconsequently, we recommend that a single taxon,C. artedi (sensu lato) be recognized.Local genetic differentiation is often coupledwith ecophenotypic diversification, and gillraker polymorphisms, depth-related habitatpreference and reproductive behaviour areconsidered as phenotypic traits with probableadaptive value contributing to the nicheexpansion of ciscoes. Ecomorphotypes of eachparticular locale thus represent a uniqueexpression of a diverse genetic pool stillundergoing divergence and sorting.Consequently, ciscoes from lakes with distinctecomorphotypes are recognized as ESUs, as wellas each of sympatric forms when they aregenetically differentiated. We recommend thatan ESU strategy focusing at a very local levelbe adopted for continental ciscoes as a validalternative to protect significant evolutionaryprocesses of divergence encountered inpolytypic species of newly colonized habitats.