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  • br Contributions br Acknowledgements This research was

    2021-11-29


    Contributions
    Acknowledgements This research was supported by Natural Sciences and Engineering Research Council (NSERC) Discovery grant No. 341538 and Ontario Graduate Scholarships to HM and VS.
    Introduction Sexual size dimorphism describes size difference between male and female individuals of the same species and has been observed in many farmed fish species, including rainbow trout (Oncorhynchus mykiss) (Bye and Lincoln 1986), tilapia (Oreochromis niloticus) (Beardmore et al. 2001) and yellow catfish (Pelteobagrus fulvidraco) (Gui, J.F. and Zhu, Z.Y., 2012, Mei, J. and Gui, J.F., 2015). In teleosts, somatic growth is regulated by neuroendocrine hormones in the growth hormone (GH)/insulin-like growth factor (IGF) axis (Furukuma, S., et al., 2008, Li, W. and Lin, H., 2010). However, the exact molecular mechanism for sexual size dimorphism is still unclear. Ghrelin, a peptide hormone was initially discovered in rat stomach as an endogenous ligand to GHSR (Kojima et al. 1999). Besides high Cathepsin Inhibitor 1 in stomach, it is also expressed in other tissues including hypothalamus and pituitary (Kojima, M., et al., 1999, Kaiya, H., et al., 2008). Several studies have identified and characterized ghrelin (Unniappan, S., et al., 2002, Kaiya, H., et al., 2003a, Kaiya, H., et al., 2003b, Murashita, K., et al., 2009) and GHSR gene (Chan, C.B. and Cheng, C.H., 2004, Kaiya, H., et al., 2009, Kaiya, H., et al., 2010) in various fish species. Ghrelin is the critical circulatory hormone known to promote food intake and body weight gain through central and peripheral modes of action (Tschop, M., et al., 2000, Nakazato, M., et al., 2001). In mammals, ghrelin has an important role in appetite stimulation, because its level increases in fasting and decreases immediately after food intake (Tschop et al. 2000). However, the functions of fish ghrelin vary from species to species. For instance it acts as an orexigenic hormone in goldfish (Unniappan, S., et al., 2002, Matsuda, K., et al., 2006, Miura, T., et al., 2009) and probably in sea bass (Terova et al. 2008) and zebrafish (Amole and Unniappan 2009), while as an anorexigenic hormone in rainbow trout (Jonsson et al. 2010) and probably in burbot (Nieminen, P., et al., 2003, Jonsson, E., et al., 2007). Yellow catfish is an important freshwater fish species in China with an annual yield of 334 thousand tons during 2014 (China Bureau of Fisheries, 2015). Recently, YY super-male yellow catfish has been created by sex manipulation technique assisted by sex-linked markers, since male yellow catfish grow faster than females (Wang, D., et al., 2009, Dan, C., et al., 2013). Comprehensive transcriptome analysis revealed substantial genetic information for yellow catfish (Jing, J., et al., 2014, Zhang, J., et al., 2014, Chen, X., et al., 2015, Wu, J., et al., 2015). Previously we determined that sexually different expression of GH/IGF signaling may play an important role in the sexual size dimorphism in yellow catfish (Ma et al. 2015). As a potent stimulator of pituitary GH release, ghrelin and its functional receptor GHSR regulate food intake (Skibicka, K.P., et al., 2011, Tinoco, A.B., et al., 2014). Therefore, we cloned and characterized ghrelin and GHSR gene from yellow catfish. Further, we investigated the sex biased expression of ghrelin and GHSR under normal condition, feeding and fasting condition and 17a-methyltestosterone (MT) treatment and has provided a clue to reveal the sexual size dimorphism in yellow catfish.
    Materials and methods
    Result
    Discussion Sex differences in growth performance have been demonstrated in many fish species including yellow catfish. Our previous study demonstrated that sex differences in the expression of GH/IGF signaling genes might be involved in the sexual size dimorphism in yellow catfish (Ma et al. 2015). However, there is no report about the correlation of ghrelin, a stimulator of pituitary GH release with sexual size dimorphism. In the present study, we characterized the sequence, protein structure and expression pattern of ghrelin and its receptor, GHSR genes. The predicted domain structures and functional sites of ghrelin and GHSR genes are highly conserved among fish species. Similar to mammals (Gualillo et al. 2003), ghrelin was mainly expressed in stomach, gut, hypothalamus and pituitary of yellow catfish (Fig. 4a), thus offered an endocrine link among organs like stomach, gut, hypothalamus and pituitary, and may be involved in the regulation of growth and energy balance (Tschop, M., et al., 2000, Inui, A., 2001).