Smolting is an important development stage of salmonid, and an energy trade-off occurs between osmotic regulation and growth during smolting in rainbow trout (Oncorhynchus mykiss). Growth hormone releasing hormone, somatostatin, growth hormone and insulin-like growth factor (GHRH-SST-GH-IGF) axis exhibit pleiotropic effects in regulating growth and osmotic adaptation. Due to salmonid specific genome duplication, increased paralogs are identified in the ghrh-sst-gh-igf axis, however, their physiology in modulating osmoregulation has yet to be investigated. In this study, seven sst genes (sst1a, sst1b, sst2, sst3a, sst3b, sst5, sst6) were identified in trout. We further investigated the ghrh-sst-gh-igf axis of diploid and triploid trout in response to seawater challenge. Kidney sst (sst1b, sst2, sst5) and sstr (sstr1b1, sstr5a, sstr5b) expressions were changed (more than 2-fold increase (except for sstr5a with 1.99-fold increase) or less than 0.5-fold decrease) due to osmoregulation, suggesting a pleiotropic physiology of SSTs in modulating growth and smoltification. Triploid trout showed significantly down-regulated brain sstr1b1 and igfbp2a1 (p < 0.05), while diploid trout showed up-regulated brain igfbp1a1 (~2.61-fold, p = 0.057) and igfbp2a subtypes (~1.38-fold, p < 0.05), suggesting triploid trout exhibited a better acclimation to the seawater environment. The triploid trout showed up-regulated kidney igfbp5a subtypes (~6.62 and 7.25-fold, p = 0.099 and 0.078) and significantly down-regulated igfbp5b2 (~0.37-fold, p < 0.05), showing a conserved physiology of teleost IGFBP5a in regulating osmoregulation. The IGFBP6 subtypes are involved in energy and nutritional regulation. Distinctive igfbp6 subtypes patterns (p < 0.05) potentially indicated trout triggered energy redistribution in brain and kidney during osmoregulatory regulation. In conclusion, we showed that the GHRH-SST-GH-IGF axis exhibited pleiotropic effects in regulating growth and osmoregulatory regulation during trout smolting, which might provide new insights into seawater aquaculture of salmonid species.

Chemokines are cytokines with chemoattractant capacities that exert their physiological functions through the binding of chemokine receptors. Thus, chemokine and receptor complexes exert important roles in regulating development and homeostasis during routine immune surveillance and inflammation. Compared to mammals, the physiology and structure of chemokine receptors in fish have not been systematically studied. Furthermore, the salmonid-specific whole genome duplication has significantly increased the number of functional paralogs of chemokine receptors. In this context, in the current study, trout exhibited 17 cxcr genes, including 12 newly identified and 5 previously identified receptors. Interestingly, gene expression of brain cxcr1 and cxcr4, kidney cxcr3 and cxcr4, and spleen cxcr3, cxcr4, and cxcr5 subtypes were altered by bacterial infection, whereas brain cxcr1, kidney cxcr1 and cxcr7, and liver cxcr2, cxcr3, and cxcr4 subtypes were changed in response to environmental changes. Based on protein structures predicted by ColabFold, the conserved amino acids in binding pockets between trout CXCR4.1 subtypes and human CXCR4 were also analyzed. Our study is valuable from a comparative point of view, providing new insights into the identification and physiology of salmonid chemokine receptors.