Coexistence and origin of trophic ecotypes of pygmy whitefish, P rosopium coulterii , in a south‐western A laskan lake

• Published in: Journal of evolutionary biology 2012-12, Vol.25 (12), p.2432-2448
• Main Authors: Gowell, C. P., Quinn, T. P., Taylor, E. B.
• Format: Article
• Language: English
• ISSN: 1010-061X; 1420-9101
• DOI: 10.1111/jeb.12011

Ecologically, morphologically and genetically distinct populations within single taxa often coexist in postglacial lakes and have provided important model systems with which to investigate ecological and evolutionary processes such as niche partitioning and ecological speciation. Within the Salmonidae, these species complexes have been well studied, particularly within the C oregonus clupeaformis – C . laveratus (lake and E uropean whitefish, respectively) group, but the phenomenon has been less well documented in the other whitefish genera, P rosopium and S tenodus . Here, we examined the morphology, feeding biology and genetic structure of three putative forms of the pygmy whitefish, P rosopium coulterii (Eigenmann & Eigenmann, 1892), first reported from C hignik L ake, south‐western A laska, over 40 years ago. Field collections and morphological analyses resolved a shallow water ( 15 gill rakers) form. The two low gill raker count forms fed almost exclusively on benthic invertebrates (mostly chironomids), while the deepwater, high gill raker count form fed almost exclusively on zooplankton; differences in diet were also reflected in differences both in δ 13 C and δ 15 N stable isotopes. All three forms were characterized by the same major mitochondrial DNA clade that has been associated with persistence in, and postglacial dispersal from, a B eringian glacial refugium. Analysis of variation at nine microsatellite DNA loci indicated low, but significant differentiation among forms, especially between the two low gill raker count forms and the high gill raker count form. The extent of differentiation along phenotypic (considerable) and genetic (subtle) axes among the C hignik L ake forms is similar to that found among distinct taxa of P rosopium found in pre‐glacial B ear L ake ( U tah– I daho, USA ) which is probably at least ten times older than Chignik Lake. Our analyses illustrate the potential for the postglacial differentiation in traits subject to divergent natural selection across variable environments.