Salmonid Reproductive Migration and Effects on Sexual Maturation

Hayashida, Kazufumi; Fukaya, Kosuke; Palstra, Arjan P.; Ueda, Hiroshi

doi:10.1007/978-3-642-31049-2_1

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“…sGnRH is localized to the olfactory bulb-terminal nerve as well as to the POA [ 12 ] and, due to the way the dissection of the brain was performed in this study, sgnrh expression can be expected in both the forebrain (including the olfactory bulb and the telencephalon) and the post brain (including the POA) samples. In earlier studies by Yamada et al [ 30 ] and by Makino et al [ 6 ], sGnRH levels first showed a peak in the olfactory bulb from the coastal sea to the mouth of Ishikari River and then in the telencephalon at the branch point between the Ishikari River and the Chitose River that leads to the spawning grounds (reviewed by Hayashida et al [ 31 ]). In this study, chum sgnrh expression was detected in forebrain and post brain of fish that had already migrated further upstream the Chitose River to the pre-spawning ground ( Fig 1 ).…”

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confidence: 99%

The Olfactory Transcriptome and Progression of Sexual Maturation in Homing Chum Salmon Oncorhynchus keta

et al. 2015

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Reproductive homing migration of salmonids requires accurate interaction between the reception of external olfactory cues for navigation to the spawning grounds and the regulation of sexual maturation processes. This study aimed at providing insights into the hypothesized functional link between olfactory sensing of the spawning ground and final sexual maturation. We have therefore assessed the presence and expression levels of olfactory genes by RNA sequencing (RNAseq) of the olfactory rosettes in homing chum salmon Oncorhynchus keta Walbaum from the coastal sea to 75 km upstream the rivers at the pre-spawning ground. The progression of sexual maturation along the brain-pituitary-gonadal axis was assessed through determination of plasma steroid levels by time-resolved fluoroimmunoassays (TR-FIA), pituitary gonadotropin subunit expression and salmon gonadotropin-releasing hormone (sgnrh) expression in the brain by quantitative real-time PCR. RNAseq revealed the expression of 75 known and 27 unknown salmonid olfactory genes of which 13 genes were differentially expressed between fish from the pre-spawning area and from the coastal area, suggesting an important role of these genes in homing. A clear progression towards final maturation was characterised by higher plasma 17α,20β-dihydroxy-4-pregnen-3-one (DHP) levels, increased pituitary luteinizing hormone β subunit (lhβ) expression and sgnrh expression in the post brain, and lower plasma testosterone (T) and 17β-estradiol (E2) levels. Olfactomedins and ependymin are candidates among the differentially expressed genes that may connect olfactory reception to the expression of sgnrh to regulate final maturation.

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