Abstract
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 3‐week intervals from January to June, using lower growth modal nonsmolting 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, five receptor genes (olfc13.1, olfc15.1, sorb, ora2, and asor1) and four olfactory‐specific genes (soig, ependymin, gst, and omp2) were differentially regulated between modal groups, suggesting altered olfactory function during smoltification. Several genes were differentially regulated in mature salmon compared with juveniles, suggesting that homing and odor recollection involve 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 presmolts 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.
Highlights
The anadromous salmonid life cycle begins in small freshwater streams, where eggs are hatched, and the juveniles stay stream‐ dwelling at the stage called parr
A suite of olfactory receptors and receptor‐specific proteins involved in functional aspects of olfaction and peripheral odor memorization was analyzed by qPCR
T3 is generally assumed to be the active form of thyroid hor‐ mone but it should be tested whether thyroxine (T4) has an effect since the olfactory epithelium has deiodinase activity and could po‐ tentially convert T4 into T3 locally in the tissue (Plate et al, 2002)
Summary
The anadromous salmonid life cycle begins in small freshwater streams, where eggs are hatched, and the juveniles stay stream‐ dwelling at the stage called parr. Based on a range of studies including field trials, behavioral experiments, elec‐ trophysiological and molecular analyses of olfactory epithelia, and associated neural tissue, there is good evidence that juvenile salmon somehow imprint on the chemistry of their native water (Bett & Hinch, 2016) This information is subsequently used for navigation and recognition upon return (Dittman, Quinn, & Nevitt, 1996; Quinn, 2005; Ueda, Yamamoto, & Hino, 2007). Olfactory marker proteins (OMP1 and OMP2) are expressed in a subpopulation of mature olfactory neurons in O. nerka (Kudo, Doi, Ueda, & Kaeriyama, 2009) Their role is unknown and their dynamics have not been reported during the PST. Additional experiments were done to investigate the ability of T3 to differentially regulate the ex‐ pression of olfactory genes by bolus injection into presmolts in vivo and by direct exposure of isolated olfactory rosettes to T3 ex vivo
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