Dukes, Juliet Patricia; (2002) Odorant receptor genes and their expression in migratory Atlantic salmon (Salmo salar, L.). Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Being anadromous, the Atlantic salmon (Salmo salar, L.), spends part of its life cycle in fresh water and part of it at sea. They undertake return migrations of up to 4000km, negotiating complex oceanic environments, dendritic river systems and making numerous choices at river junctions to return to specific natal sites. As the fish near coastal waters, olfaction has been shown to be pivotal in the selection of the appropriate estuary and identification of the natal stream. Juvenile salmon are thought to imprint on biotic and/or abiotic environmental odours around the time of parr-smolt transformation (PST), and retain this information at least partly within the olfactory sensory neurons. These olfactory cues are then exploited with remarkable precision by adult migrants returning to the natal stream to spawn. Variation in olfactory receptors (OR) and pheromone receptors (or vomeronasal receptors: VNRs) expressed by these sensory neurons may therefore play a vital role in the maintenance of the structure of salmon populations, enabling numerous reproductively isolated communities to exist within one catchment area. Here, the isolation and characterisation of both OR and VNR genes from S.salar has facilitated further elucidation of the olfactory changes associated with parr-smolt transformation. Both sets of primary receptors have representatives expressed in male germ cells as well as olfactory tissue. Real-time quantitative RT-PCR has revealed that a group of OR genes (SORB) is expressed at a higher level during the early stages of PST. One group of VNR genes (SVRA) however, shows a peak of expression later in PST. There were also expression differences observed between families of fish. Only one family showed a significant increase in expression of SORB and SVRA, the other family presumably using other receptor types not included in this study. Molecular evidence therefore indicates that there is more than one incidence of specific-olfactory sensitivity involved in the smelting process. The stimulation of expression of two independent groups of chemosensory receptors indicates that both odours and semiochemicals play a role in olfactory imprinting. The odorant receptors involved in olfactory imprinting appear to vary between families of fish which suggests interfamilial differences in odour stimuli.

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