Tilapia are widely cultured worldwide, partly because they grow well in captivity, tolerate a wide range of environmental conditions and are sexually dimorphic, where males grow larger than females. As in other vertebrates, growth in tilapia is regulated by the growth hormone/ insulin like growth factor (GH/IGF) system. Once in circulation, GH can bind to its receptor (GHR) in target tissues, such as muscle, liver and gonads, resulting in a physiological response that, alone or through the mediation of IGFs, includes cell proliferation and differentiation, and protein synthesis. Moreover, environmental conditions, such as salinity, have been shown to directly modulate growth in tilapia. Less is known, however, on how the GH/IGF system may be differentially regulated between sexes at the tissue mRNA expression level and how salinity may modulate sexually dimorphic growth.
Our laboratory has developed a model for investigating effects of salinity and whole-organism GH by employing the Mozambique tilapia (Oreochromis mossambicus), a species that can thrive in either fresh water or seawater. In this model, we surgically remove the pituitary gland (Hypophysectomy - Hx) and replace GH and other pituitary hormones via intraperitoneal injections. With this approach, we examined whether the expression of ghr and igfs in muscle, liver and gonads were differentially affected between males and females in Hx fish and those injected with GH. We also compared the pituitary mRNA expression of gh in male and female Mozambique tilapia (Oreochromis mossambicus) acclimated to different salinity regimes.
Our results indicate that male and female fish differ in their sensitivities to GH by differentially expressing ghr and igfs in muscle, liver and gonads. These results suggest that the sexual size dimorphism in tilapia and its modulation by environmental salinity can at least be partially attributed to sex-specific differential regulation of the GH/IGF system.
[Supported in part by NIFA Hatch (#HAW02051-H), NOAA (#NA18OAR4170347), NIH (1R21DK111775-01) and NSF (IOS-1755016 and IOS- 1755131)]