ABSTRACT: Most Pacific salmonids undergo smoltification and transition from freshwater to saltwater. Saltwater acclimation requires salmonids to make various adjustments in color, shape, size, metabolism, catabolism, and osmotic and ion regulation. The molecular mechanisms underlying this transition are largely unknown. The present study acclimated coho salmon (Oncorhynchus kisutch) to four different salinities (<0.5, 8, 16, and 32 ppth) and assessed gene expression through microarray analysis of gill, liver and olfactory tissues. Gills are involved in osmotic regulation, liver plays a role in energetics, and olfactory tissues are involved in behavior. Between all salinity treatments, liver had the highest number of differentially expressed genes at 1,616. Gills had 1,074 differentially expressed genes and olfactory tissue had 924. The difference in the number of differentially expressed genes may be due to the higher responsiveness of liver to metabolic changes after salinity acclimation to provide energy to fuel other metabolic and osmoregulatory tissues like gills. Differentially expressed genes were tissue and salinity treatment dependent. There were no genes differentially expressed in all salinity treatments and all three tissues. Five genes were targeted for microarray confirmation by qPCR and included CCAAT/enhancer binding protein ? (CEBPB), calpain 1 (CAPN1), proto-oncogene, serine/threonine kinase (Pim1), aldolase B, fructose-bisphosphate (aldob), and complement component 3 (c3). qPCR expression profiles of these genes matched array outputs. Gene ontology term analysis revealed biological processes, molecular functions, and cellular components that were significant. Most terms were tissue dependent. For liver, oxygen binding and transport terms were highlighted, suggesting possible impacts on metabolism. For gills, muscle and cytoskeleton related terms were emphasized and for olfactory tissues, immune response related genes were accentuated. Interaction networks were examined in combination with GO terms and determined similarities between tissues for potential osmosenors and signal transduction cascades. Overall this study suggests that Pacific salmonids share many salinity acclimation molecular mechanisms with other species, with a few new genes identified, and that although the three tissues shared certain underlying mechanism, many of the differentially expressed genes were tissue-specific.
