The relative contribution of drift and selection to transcriptional divergence among Babine Lake tributary populations of juvenile rainbow trout

Fine-scale population structure has been widely described for salmonid populations using neutral genetic markers, but whether that structure reflects adaptive differences among the populations remains of interest to evolutionary biologists and conservation managers alike. The use of transcriptomics to quantify population differences in genetically controlled functional gene expression traits holds promise for investigating this divergence associated with possible local adaptation. We use custom microarrays to characterize population divergence in transcription at functionally relevant (metabolic and immune function) genes among tributary populations of rainbow trout from Babine Lake, BC and compare it to neutral divergence estimated from microsatellite markers. Transcriptional divergence (P-ST) was determined at resting state and in response to metabolic and immune challenges, two major sources of mortality and thus selective forces on juvenile salmonids. Results indicate that the majority of selected genes [56 genes (65%), 64 genes (63%) and 38 genes (78%) under control, temperature and immune challenges respectively] show transcriptional divergence (P-ST>F-ST) that is consistent with the action of divergent selection. Patterns of pairwise P-ST among populations are inconsistent with evolution by drift. In general, it appears that the magnitude and pattern of population divergence in transcription reflect the action of natural selection and identify selection on transcription as a mechanism for local adaptation. These results reinforce the need to conserve salmonids on a tributary basis and provide insight into genetic mechanisms that facilitate local adaptation.