Molecular and Evolutionary History of Melanism in North American Gray Wolves
Referenced in Coyote Trapping School podcast #64.
Morphological diversity within closely related species is an essential aspect of evolution and adaptation. Mutations in the Melanocortin 1 receptor (Mc1r) gene contribute to pigmentary diversity in natural populations of fish, birds, and many mammals. However, melanism in the gray wolf, Canis lupus, is caused by a different melanocortin pathway component, the K locus, that encodes a beta-defensin protein that acts as an alternative ligand for Mc1r. We show that the melanistic K locus mutation in North American wolves derives from past hybridization with domestic dogs, has risen to high frequency in forested habitats, and exhibits a molecular signature of positive selection. The same mutation also causes melanism in the coyote, Canis latrans, and in Italian gray wolves, and hence our results demonstrate how traits selected in domesticated species can influence the morphological diversity of their wild relatives.
The correspondence between coat color and habitat is often attributed to natural selection, but rarely is supporting evidence provided at the molecular level. In North American gray wolves, coat color frequencies differ between wolves of forested and open habitats throughout western North America (1), including Denali National Park (2) and the Kenai Peninsula in Alaska (3), and much of the Canadian Arctic (4, 5). These differences are especially dramatic between wolves of the high tundra that are migratory and follow barren-ground caribou to their breeding areas, and wolves that are year-round residents in the neighboring boreal forest and hunt nonmigratory prey. Dark-colored wolves are extremely rare in the tundra but increase in frequency along a southwest cline toward forested areas (Fig. 1A). The potential selective value of dark versus light coat color has been suggested to include concealment during predation and/or indirect effects due to pleiotropy, but remains unresolved because the underlying gene(s) have not been identified (5–7).