Decreased expression of the satiety signal receptor CCKAR is responsible for increased growth and body weight during the domestication of chickens

Animal domestication has resulted in changes in growth and size. It has been suggested this may have involved selection for differences in appetite. Divergent growth between chickens selected for egg laying or meat production is one such example. The neurons expressing AGRP and POMC in the basal hypothalamus are important components of appetite regulation as are the satiety feedback pathways which carry information from the intestine including CCK and its receptor CCKAR (CCK1 receptor). Using 16 generations of a cross between fast and a relatively slow growing strain of chicken has identified a region on chromosome 4, downstream of the CCKAR gene, responsible for up to a 19% difference in body weight at 12 weeks of age. Animals possessing the high growth haplotype at the locus have lower expression of mRNA and immunoreactive CCKAR in the brain, intestine and exocrine organs which is correlated with increased levels of orexigenic AGRP in the hypothalamus. Animals with the high growth haplotype are resistant to the anorectic effect of exogenously administered CCK suggesting their satiety set point has been altered. Comparison with traditional breeds shows the high growth haplotype has been present in the founders of modern meat type strains and may have been selected early in domestication. This is the first dissection of the physiological consequences of a genetic loci for a quantitative trait which alters appetite and gives us an insight into the domestication of animals. This will allow elucidation of how differences in appetite occur in birds but also mammals.