Divergent Immunity and Energetic Programs in the Gills of Migratory and Resident Oncorhynchus mykiss

Divergent life history strategies occur in steelhead or rainbow trout Oncorhynchus mykiss, and many populations produce both migrants (anadromous, move to the ocean after rearing) and residents (remain in fresh water). Mechanisms leading to each type are only partially understood; while the general tendency of a population is heritable, individual tendency may be plastic, influenced by local environment. Steelhead hatchery programs aim to mitigate losses in wild stocks by producing trout that will migrate to the ocean and not compete with wild trout for limited freshwater resources. Here we compare gill transcriptomes of hatchery-released fish either at the release site (residents) or five river kilometers downstream while still in full fresh water (migrants). Additionally, we evaluate the expression of these differentially expressed genes in gills of migrant-like and undifferentiated fish while still in the hatchery (pre-release). Results confirm the gradual process of smoltification, and the functions important to migrants (energetics, gill remodelling and ion transport capacity). Furthermore, immunity-related transcripts such as antiviral or pathogen recognition receptors follow different profiles in the two phenotypes. The best smoltification marker candidate, protein s100a4 was highly correlated with Na+, K+ ATPase (NKA) activity and smolt-like morphology in pre- and postrelease trout gills.