A Genetic Basis for the Phenotypic Differentiation Between Lean and Siscowet Lake Trout

A variety of lake trout (Salvelinus namaycush) morphotypes were historically present in the Laurentian Great Lakes. A deepwater morphotype, the siscowet, is common in Lake Superior where they make up most of the lake trout biomass.  This fish is characterized by a short convex snout, high fat content in flesh and viscera, deep body, and a short, thick caudal peduncle. These and other metrics distinguish it from the other principal morphotype, the lean lake trout.  The siscowet is a deepwater form found at depths >80 meters, as opposed to lean lake trout that are more restricted to shallower water. Microsatellite analyses indicate that there are population genetic differences between lake trout morphotypes that suggest reproductive separation and perhaps a genetic basis for the differences in morphology and physiology.  However, since the morphotypes also inhabit different environments, it is unclear whether genetics or the environment is ultimately responsible for these differences. In a common garden rearing study, we have raised siscowet and lean lake trout from fertilized eggs under identical environmental conditions for over 4 years.  After one year, there were statistically significant differences in growth, morphometry (truss analysis) and lipid content between siscowets and leans raised communally under identical environmental conditions.  Cultured siscowets and leans continued to diverge in morphometry and lipid levels from year 1 to the present, and lipid levels are currently the most predictive measures of morphotype differences.  In addition to skeletal muscle lipid, circulating levels of lipid and glucose, as well as liver glycogen and lipid were also significantly different between leans and siscowets by year 3.  Siscowets had lower hepatic glycogen and blood glucose levels compared to leans but higher hepatic lipid levels.  Leans had higher circulating lipid levels. By year one, RNAseq analysis using Roche 454 pyrosequencing showed significant differences in the expression of a number of genes in the liver between morphotypes.  Interestingly, however, the expression of most of those genes were not significantly different by year 3.  The expression of several genes, including a complement-like C1q, did remain very different between morphotypes and the regulation of those differences is being investigated. The results of the rearing studies strongly suggest that morphological and physiological differences observed in wild leans and siscowets are genetically based and not a result of environmental plasticity.