Evolutionary genetics of the melanocortin-1 receptor in vertebrates

The molecular genetic basis of adaptive change in phenotype is a major outstanding issue in evolutionary biology. Evolutionary change in coat and plumage color is a promising system for making progress in this field. Most notably, recent work on the molecular genetic basis of hair and feather color has identified several genes which are candidates for involvement in evolutionary color change in mammals and birds. We have investigated the evolution of one of these candidate genes, the melanocortin-1 receptor (MC1R) gene, in relation to changes in melanin distribution among a wide variety of primate species, and in bananaquits (Coereba flaveola), which are a classic case of melanic plumage polymorphism in birds. In primates, a role of the MC1R coding region in coat color evolution can be ruled out in several cases in which closely related species have drastically different distributions of eumelanin and/or pheomelanin. However, reconstruction of MC1R sequences over primate evolution shows the presence of mutations at important functional sites in several lineages. Most notably, the lion tamarins (Leontopithecus) show a striking pattern of MC1R evolution, including deletions and several nonconservative amino acid changes. In the bananaquit, an E92K substitution in the MC1R is strongly associated with melanism, and this is likely to be the causative mutation. Reconstruction of the evolution of bananaquit MC1R alleles shows that melanism is a derived trait in this species. These results confirm the utility of a candidate gene approach to color evolution in vertebrates and open the way for extensive future research.