| Summary: | It has been hypothesized that salmonids use olfactory cues to return to their natal rivers and streams. However, the key components of the molecular pathway involved in imprinting and homing are still unknown. If odorants are involved in salmon homing migration then olfactory receptors should play a critical role in the dissipation of information from the environment to the fish. To understand the molecular basis for imprinting and homing in Atlantic salmon (Salmo salar), it is important to identify and characterize the olfactory receptors in the Atlantic salmon genome. Aquatic chemical cues are received through the salmon nares and into the nasal cavity that contains a single olfactory organ, olfactory rosette. The olfactory rosette contains sensory neurons, which are thought to express only one olfactory receptor. In this study, three major superfamilies of fish olfactory receptors (MOR, ora and OlfC) were examined. To identify the olfactory genes in Atlantic salmon several genomic and bioinformatic techniques were used. First, an Atlantic salmon bacterial artificial chromosome (BAC) library was screened with probes designed from previously identified fish olfactory receptor sequences. Then a selected number of hybridization positive BACs containing olfactory receptors were shotgun cloned and sequenced. From these BAC sequences, two ora genes and 55 OlfC genes were identified in Atlantic salmon. The second technique used to identify olfactory receptors in Atlantic salmon was a bioinformatic approach that involved screening a 3-fold Atlantic salmon genome sequence for olfactory receptors. Using this approach, 24 MOR and the remaining five ora genes were identified, as well as another 24 partial genes or pseudogenes. As a first step to understand how olfactory receptors are involved in imprinting and homing, a suite of olfactory receptors were selected to examine the expression profiles of these genes across different life stages and life histories of wild Atlantic salmon from Newfoundland, Canada. Seven differentially expressed OlfC genes were identified in juvenile anadromous salmon compared to returning adult salmon. From this research, I hypothesize that OlfC genes may play an important role in the imprinting of home stream water olfactory cues in anadromous Atlantic salmon.
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