Do differences in the activities of carbohydrate metabolism enzymes between Lake Whitefish ecotypes match predictions from transcriptomic studies?

Transcriptomic studies are facilitating the search for the molecular bases of adaptation in natural populations, but the impact of these differences in mRNA content on animal physiology are often unknown. One way to determine if molecular changes have the potential to influence animal physiology and performance is to test for correlated changes at higher levels of biological organization, including enzyme activity. Here, we measure the activities of carbohydrate metabolism enzymes to test if previously documented genetic and transcriptomic variation between 'dwarf' and 'normal' Lake Whitefish ecotypes are associated with corresponding changes in enzyme activity (measured as maximal rate, Vmax) in liver and skeletal muscle. We use laboratory-reared fish from the same populations as prior transcriptomic studies and find that white muscle mRNA content is a good predictor of glycolytic and glycogen metabolism enzyme activity, and dwarf whitefish have evolved higher activities than normal whitefish. However, the differences in hepatic mRNA content found between ecotypes in prior studies are not associated with comparable changes in enzyme activity. For example, dwarf whitefish have lower enzyme activities, but higher transcript abundances for two glycolytic enzymes compared to normal whitefish. Overall, we find that transcriptomic studies successfully highlight evolutionary variation in enzyme activities, but not always in the direction predicted, indicating that a variety of tissue-specific regulatory mechanisms contributed to the evolution of energy metabolism in Lake Whitefish.