Molecular and evolutionary history of melanism in North American gray wolves

Abstract

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.

Fig. 1

Distribution of melanism and K locus genotypes in North American gray wolves. (A) Location and coat color phenotype of Canadian samples used here and as described (4). (B) Age-related graying and the associated difficulty of inferring genotype from phenotype in gray animals. Each pair of photos shows the same individual at different ages (10 months and 10 years) and documents an increasingly gray appearance at 10 years, reflecting the dilution of eumelanin in the K^B/k^y individual (left pair of images) and dilution of both eumelanin and pheomelanin in the k^y/k^y individual (right pair of images). [Images courtesy of Monty Sloan, Wolf Park, Battle Ground, Indiana] (C) Co-segregation of K^B and black coat color in a three-generation pedigree from the Leopold pack in Yellowstone National Park (17). ΔG indicates the dominant K^B allele, whereas + indicates the wild-type allele, k^y.

Fig. 2

Polymorphism and haplotype structure of the K locus in North American gray wolves [(A) to (E), 1 K^B/K^B, 20 K^B/k^y, and 26 k^y/k^y] and domestic dogs [(F), 6 K^B/K^B and 6 k^y/k^y]. (A) Polymorphism (θ_W, ±SD) as a function of distance from CBD103. (B) Wolf haplotype structure was inferred on the basis of 36 SNPs; each row represents a K^B- or k^y- bearing chromosome; blue and yellow squares represent the major and minor alleles, respectively; and the gray squares represent missing data. Red and black arrows indicate examples of haplotypes likely to represent historical recombination between K^B- and k^y-bearing chromosomes at the 5′ and 3′ ends of the locus, respectively. (C) Pairwise LD values (expressed as D') for all wolf chromosomes; the red outline indicates a core region (as in Fig. 3) unlikely to have undergone historical recombination. (D) Haplotype bifurcation diagrams for K^B- or k^y-bearing chromosomes, in which the central dark blue dot represents CBD103, branches represent haplotype divergence, and the thickness of the lines is proportional to the number of chromosomes. (E and F) EHH for K^B- or k^y-bearing chromosomes in wolves (E) and dogs (F) as a function of distance from CBD103^ΔG23.

Fig. 3

Evolutionary relationships and history of the K locus in canids. (A) K^B haplotype structure in wolflike canids based on genotypes defined by 52 SNPs. Each row represents a K^B-bearing haplotype found in coyotes (C), dogs (D), or wolves (W) listed with their respective frequencies on the right and colored as in Fig. 2B. (B) Inferred genealogical relationships of the core region (Fig. 2C) haplo-types (with bootstrap values from 500 replicates shown next to branches).Each branch represents 1 of 18 different haplotypes, with the number of chromosomes for each haplotype indicated underneath according to species. (C) TMRCA estimates for indicated chromosome subsets calculated according to a molecular clock (22) and expressed as a fraction of the divergence time for all wolflike canids. Individual points represent sets of chromosome segments whose relative TMRCA increases as a function of distance from CBD103, presumably due to ancient hybridization and recombination. (D) Timeline scenario for K locus evolution in dogs and wolves, in which ancestral k^y chromosomes are indicated in orange, derivative K^B chromosomes in gray, and recombinant chromosomes as an orange-gray checkered pattern. The k^y-to-K^B mutation may have overlapped or even predated domestication, but the introgression of K^B into North American gray wolves is more recent.