Effect of genotype-nutrition interaction on growth and somatotropic gene expression in the chicken

The neuroendocrine system integrates genotype with external factors such as nutrition to regulate animal growth. To investigate the role of somatotropic axis in the interaction of genotype and nutrition, two series of experiments were conducted using broiler and layer chickens as a model. In the first experiment, both strains of chickens were raised on their respective standard diets and the mRNA expressions of somatotropic genes were investigated on day 5 (D5), D21, and D42 after hatching as composites of genotype and nutrition. The hypothalamic somatostatin (SS) and pituitary growth hormone (GH) mRNA expression as well as the plasma GH levels were higher in layer chickens while the opposite was true for hepatic GH receptor (GHR) mRNA. Regulation of GHR mRNA expression was found to be tissue-specific. Hepatic GHR mRNA content increased with age whereas in the muscle, the peak levels of expression were observed at D5 with significantly higher abundance ratio in the layer. To evaluate genotype-diet interaction on growth and the patterns of gene expression of both layer and broiler chickens, layers were fed broiler diet and vice versa from D1 to 42 in the second experiment. The D42 body weight of layer chickens increased by 35% when fed with broiler diet, whereas that of broiler chickens decreased by 51% when fed with layer food. The diet exchange completely reversed the patterns of hypothalamic SS and pituitary GH mRNA expression and the strain differences vanished when the comparison was made on the same diet basis. The hepatic GHR mRNA decreased by 46.1% in broilers fed with layer food, but increased by 45.6% in the layer fed with broiler diet. The strain differences were diminished but did not completely disappear on the same diet basis for hepatic GH receptor mRNA. In contrast, however, the muscle GHR mRNA expression was not affected by diet exchange and thus, was more genotype-specific. The results suggest that genes of the somatotropic axis respond to nutrition differently at the level of transcription.