Melanocortin receptor variants with phenotypic effects in horse, pig, and chicken

The melanocortin system is of considerable interest in domestic animals because their energy metabolism and pigmentation have been under strong selection. This article reviews our work on MC1R variants in horse, pig, and chicken, as well as a study on MC4R polymorphism in the pig. The chestnut coat color in horses is caused by an MC1R missense mutation (S83F). In the pig, we have described seven MC1R alleles controlling four different coat color phenotypes (wild type, dominant black, black spotting, and recessive red). The most interesting allele is the one causing black spotting because it carries two causative mutations, a frameshift and a missense mutation. The frameshift mutation is somatically unstable, and the black spots reflect somatic reversion events restoring the reading frame. Classic genetics have established eight alleles at the Extended black locus in chicken, which is assumed to correspond to the Extension locus in mammals. We have analyzed the co-segregation of alleles at MC1R and Extended black using a red jungle fowl x White Leghorn intercross and provide compelling evidence that these loci are identical. A previous study indicated that a missense mutation (D298N) in pig MC4R has an effect on fatness, growth, and feed intake. We could not confirm this association using an intercross between the wild boar and Large White domestic pigs, but it is possible that our F-2 generation was too small to detect the rather modest effect reported for this polymorphism.