Tributary and Acclimation Site Specific Patterns of Olfactory mRNA Expression in a Spring Chinook Salmon Population

Salmon are well known for their extraordinary homing migrations from oceanic feeding grounds back to their river of origin to spawn. These migrations are governed by olfactory discrimination of homestream odors that juvenile salmon learn (imprint to) prior to their seaward migrations. Our previous laboratory studies have suggested that one component of imprinting involves sensitization of the peripheral olfactory system to specific odorants and up-regulation of specific odorant receptor (OR) mRNAs. Therefore, we hypothesized that juvenile salmon exposed to tributary waters with distinct chemical constituents would display different patterns of OR expression. To test this hypothesis, we sampled two different cohorts (2005 and 2010) of age 1+ juvenile spring Chinook salmon reared as part of a supplementation program in the upper Yakima River, Washington. One-year-old juveniles were sampled at their central rearing hatchery (2005 only) just prior to transfer to acclimation/release sites and then again just prior to release as smolts from three different acclimation facilities (both cohorts). Using quantitative PCR, we assessed expression levels of several ORs representing distinct subfamilies of ORs expressed in the olfactory epithelium. We observed no effect of gender or genetic hatchery influence on olfactory mRNA expression but, after several weeks of rearing at different acclimation sites, smolts reared in different waters displayed distinct patterns of OR expression. Our findings were also corroborated in a separate but similar experiment sampling a single cohort of Sockeye salmon at the same age class reared in two distinct locations. These results are consistent with our hypothesis that distinct odors present in the waters of different tributaries may influence mRNA expression in the olfactory epithelium. These results may facilitate development of molecular tools for assessing imprinting and identifying critical chemical components in natural waters that are important for imprinting. Funded by the NWFSC.