McRobie, H., Thomas, A. & Kelly, J., 2009. The genetic basis of melanism in the gray squirrel (Sciurus carolinensis). The Journal of heredity, 100(6), pp. 709–14

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-Delta24 allele E(B), and the brown-black phenotype was heterozygous for the E(+) and E(B) alleles. We conclude that melanism in gray squirrels is associated with the MC1R-Delta24 E(B) allele at amino acid positions 87-94 and that this allele is incompletely dominant to the wild-type allele. We predict that the MC1R-Delta24 E(B) allele encodes a constitutively active or hyperactive receptor.

McRobie, HR., Moncrief, ND. & Mundy, NI., 2019. Multiple origins of melanism in two species of North American tree squirrel (Sciurus). BMC evolutionary biology, 19(1), pp. 140

While our understanding of the genetic basis of convergent evolution has improved there are still many uncertainties. Here we investigate the repeated evolution of dark colouration (melanism) in eastern fox squirrels (Sciurus niger; hereafter "fox squirrels") and eastern gray squirrels (S. carolinensis; hereafter "gray squirrels").