The discovery of genetic variants that affect phenotypes is a rarity when studying natural populations. Even rarer is the opportunity to identify mechanisms responsible for the maintenance of genetic polymorphisms and resulting influence on fitness-related traits and eco-evolutionary dynamics. Wolf reintroduction to Yellowstone National Park over 20 years ago and subsequent monitoring has revealed one such fascinating case. Specifically, black coat color has been investigated in Yellowstone wolves and found to be due to a beta-defensin gene (CBD103, or the K-locus), with all black individuals carrying a 3-nucleotide deletion linked to this color phenotype, a mutation believed to have originated in domestic dogs of the Old World. Using Yellowstone pedigree data, the black allele was discovered to be dominant over the gray resulting in its presence in black-colored homozygous and heterozygous individuals, but absent in homozygous grays. Interestingly, the frequency of black and gray phenotypes has remained approximately equal over 20 years. These unexpected results set the stage for studies exploring the link between genetics, fitness, selection and maintenance of color polymorphisms. Integrating longitudinal data on reproduction, survival, mate choice, aggression, and disease with molecular techniques, a surprising story linking coat color genes to wolf ecology in the Yellowstone ecosystem is emerging.
Remarkably, it has been found that coat color genotypes are significantly linked to several fitness-related traits with population-level effects. First, differential rates of wolf survival and reproduction demonstrate significant black heterozygote fitness advantages. Second, the K-locus gene appears to be involved in immune system function through pleiotropic effects, with black wolves having greater survivorship following distemper outbreaks; currently, emerging cell line science is being used on black and gray wolf skin cultures grown from tissue biopsies to further test this hypothesis. Third, black wolves have lower baseline cortisol levels, and were less aggressive during territorial encounters. Fourth, there is strong mate choice between gray and black wolves, providing the first documented case of significant negative-assortative mating in mammals. Finally, Yellowstone wolves show strong selection for the black allele compared to other populations. Maintenance of this polymorphism is linked to both heterozygous advantage and mate choice. While research on Yellowstone wolves has been primarily designed to evaluate carnivores’ influence on biodiversity and ecosystem function, the story of a gene’s adaptive introgression into wild wolves and its influence on their ecology exemplifies how thoughtful, long-term ecological monitoring can lead to surprising scientific discoveries.