Abstract
Water temperature and dietary protein play an important role in influencing the expression of somatotropic gene. Based on central composite rotatable experimental design and response surface method, the interacting effects of temperature (20–34°C) and dietary protein level (25–50%) on expression of somatotropic gene and its interrelationship with specific growth rates (SGR) and feed efficiency (FE) of juvenile GIFT tilapia were studied under laboratory conditions. Results showed that the linear, quadratic and interacting effects of temperature and dietary protein on both pituitary growth hormone (GH) and hepatic growth hormone receptor 1 (GHR1) mRNA levels were statistically significant (P<0.05). The linear and interacting effects of temperature and dietary protein on hepatic insulin-like growth factor I (IGF-I) mRNA levels were highly significant (P<0.01); but the quadratic effect of temperature was nonsignificant (P>0.05). The regression equations of pituitary GH, hepatic GHR1 and IGF-I mRNA towards the two factors of interest were established, with the coefficients of determination being 0.958, 0.938 and 0.978, respectively (P<0.01). Pituitary GH, hepatic GHR1 and IGF-I showed positive relationships with SGR and FE. These results suggest that the expression of somatotropic gene be affected by different combination of temperature and dietary protein level as they regulate growth and FE.
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