The genetic basis of melanism in the gray squirrel (Sciurus carolinensis)

The black squirrel is a melanic variant of the gray squirrel (Sciurus carolinensis). We found 3 coat color variants in the gray squirrel: the wild-type gray, a jet-black, and a brown–black phenotype. These 3 morphs are due to varying distributions of eumelanin and phaeomelanin pigment in hairs. The melanocortin 1 receptor (MC1R) plays a central role in regulating eumelanin and phaeomelanin production. We sequenced the MC1R gene for all 3 coat color phenotypes and found a 24 base-pair deletion. The gray phenotype was homozygous for the wild-type allele E+, the jet-black phenotype was homozygous for the MC1R-Δ24 allele EB, and the brown–black phenotype was heterozygous for the E+ and EB alleles. We conclude that melanism in gray squirrels is associated with the MC1R-Δ24 EB allele at amino acid positions 87–94 and that this allele is incompletely dominant to the wild-type allele. We predict that the MC1R-Δ24 EB allele encodes a constitutively active or hyperactive receptor.