Differential expression of olfactory genes in Atlantic salmon (Salmo salar) during the parr-smolt transformation

The anadromous salmon life-cycle includes two migratory events, downstream smolt migration and adult homing migration, during which they must navigate with high precision. During homing migration, olfactory cues are used for navigation in coastal and freshwater areas, and studies have suggested that the parr-smolt transformation has a sensitive period for imprinting. Accordingly, we hypothesized that there would be significant changes in gene expression in the olfactory epithelium specifically related to smoltification and sampled olfactory rosettes from hatchery-reared upper growth modal juvenile Atlantic salmon at three-week intervals from January to June, using lower growth modal non-smolting siblings as controls. A suite of olfactory receptors and receptor-specific proteins involved in functional aspects of olfaction and peripheral odor memorization was analyzed by qPCR. Gene expression in juveniles was compared with mature adult salmon of the same genetic strain caught in the river Gudenaa. All mRNAs displayed significant variation over time in both modal groups. Furthermore, 5 receptor genes (olfc13.1, olfc15.1, sorb, ora2, asor1) and four olfactory-specific genes (soig, ependymin, gst, omp2) were differentially regulated between modal groups, suggesting altered olfactory function during smoltification. Several genes were differentially regulated in mature salmon compared to juveniles, suggesting that homing and odor recollection involves a different set of genes than during imprinting. Thyroid hormone receptors thrα and thrβ mRNAs were elevated during smolting suggesting increased sensitivity to thyroid hormones. Treatment of pre-smolts with triiodothyronine in vivo and ex vivo had, however, only subtle effects on the investigated olfactory targets, questioning the hypothesis that thyroid hormones directly regulate gene expression in the olfactory epithelium.