Gene Expression in the Brains of Two Lines of Chicken Divergently Selected for High and Low Body Weight

Artificial divergent selection of chickens for high and low body weight at 8 weeks of age has produced two lines: the high (HWS) and low (LWS) body weight chicken lines. In addition to the difference in body weight, the lines show extreme differences in feeding behaviour and body composition. The aim of this study was to uncover the genetic and molecular factors that contribute to and determine these differences, especially regarding body energy regulation and appetite. In papers I and II, genome-wide gene expression in a brain sample containing hypothalamus and in dissected hypothalamus was analysed using DNA microarray and qRT-PCR. We found that levels of differential expression were generally moderate, which was consistent with the idea that polygenic factors were involved in the establishment of the chicken lines. Genes associated with neural plasticity, lipid metabolism and body energy regulation were differentially expressed. This result indicated that the neural systems regulating feeding behaviour and body weight were altered in the chicken lines. However, genes that were involved in the central melanocortin system were not systematically differentially expressed. Interestingly, the biggest differences in expression between the lines found in endogenous retrovirus sequences of the ALV subgroup E. Thus, in paper III, we characterized the number of integrations, the expression of ALVE retroviral elements and their effects on body weight. A significant correlation between low body weight and high ALVE expression was observed in female F9 birds from an HWS x LWS advanced intercross line. This implied that ev-loci contributing to increased ALVE expression levels were genetically linked to loci influencing the low body weight of the pullets. In paper IV, the carnitine palmitoyltransferase-1b gene (CPT1B), which was highly differentially expressed in the hypothalami, was investigated. We mapped chicken CPT1B to the distal tip of chromosome 1p. The levels of CPT1B mRNA in the HWS line were higher in the hypothalamus and lower in muscle than in the LWS line. This pattern of differential expression indicates that this gene could contribute to the remarkable phenotypic differences between HWS and LWS chickens. However, comparison with quantitative trait loci data showed that the expression of CPT1B is a trans effect, rather than a direct causative locus. In conclusion, the data suggested that the long-term selection for body weight resulted in differential gene expression in the brains of the selected chicken lines. These results may have relevance for the poultry industry and will also contribute to increasing knowledge about human diseases such as obesity and anorexia