Black Bears

[Matias]

Genetic architecture and evolution of color variation in American black bears

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Quote:Summary

Color variation is a frequent evolutionary substrate for camouflage in small mammals, but the underlying genetics and evolutionary forces that drive color variation in natural populations of large mammals are mostly unexplained. The American black bear, Ursus americanus (U. americanus), exhibits a range of colors including the cinnamon morph, which has a similar color to the brown bear, U. arctos, and is found at high frequency in the American southwest. Reflectance and chemical melanin measurements showed little distinction between U. arctos and cinnamon U. americanus individuals. We used a genome-wide association for hair color as a quantitative trait in 151 U. americanus individuals and identified a single major locus (p < 10−13). Additional genomic and functional studies identified a missense alteration (R153C) in Tyrosinase-related protein 1 (TYRP1) that likely affects binding of the zinc cofactor, impairs protein localization, and results in decreased pigment production. Population genetic analyses and demographic modeling indicated that the R153C variant arose 9.36 kya in a southwestern population where it likely provided a selective advantage, spreading both northwards and eastwards by gene flow. A different TYRP1 allele, R114C, contributes to the characteristic brown color of U. arctos but is not fixed across the range.

Quote:Two hypotheses about the selective forces acting upon the cinnamon morph of U. americanus have been proposed. First, lighter coloration aids in thermoregulation of bears in the hotter and/or drier climates of the southwest; second, the cinnamon morph is mimetic with U. arctos where these species are sympatric. We used BayEnv2 to test for correlation between R153C allele frequency and climate, geography, or the presence/absence of U. arctos prior to anthropogenic extirpation, but we identified no notable Bayes factors (Table S4). Although this approach cannot rigorously exclude thermoregulation or mimicry as a force driving selection, it is possible that an additional, as yet untested mechanism underlies weak selection such as crypsis in open canopy environments. For example, individuals color matched within their environment has been suggested to decrease infanticide and/or predation in the Giant Panda.

Implications for infraspecific taxonomy in American black bears:

Historically, brown coat color has been used as a defining phenotype for delimiting 4 of the 16 U. americanus subspecies: U. a. amblyceps (southwest/southern Rocky Mountains), U. a. cinnamomum (northern Rocky Mountains), U. a. luteolus (Louisiana), and U. a. machetes (western Mexico).  Further, the original description of U. luteolus from 1821 notes the cinnamon morph ranging from Virginia to Louisiana, where it is not presently found. Our results indicate that the geographic distribution of brown coats is being driven by recent and ongoing gene flow, suggesting that cinnamon morphs will increase in frequency within the eastern lineage in the future. We suggest that the young age of the causative variant and rapid spread via gene flow make this a poor character trait for infraspecific delimitation.

Conclusion:

Here, we show that the cinnamon morph of the American black bear shares phenotypic similarity with brown bears in their coat coloration, and we demonstrate that eumelanin dilution causes this similarity. We identified two independently evolved and species-specific alleles within TYRP1 that result in Arg to Cys residue changes that disrupt protein localization to the melanosome. Our functional assay shows that the U. arctos R114C change has a greater effect on pigment production in melan-b cells than the U. americanus R153C, yet R114C has not fixed within the species (Figure S2B), suggesting it may modify color without being the initial causative locus. By contrast, the U. americanus R153C is responsible for most color variation within the species. Neither variant appears detrimental to fitness in bears unlike in humans with OCA3; instead, R153C appears to be under weak positive selection. Crypsis as an adaptive mechanism has generally explained why prey species and ambush predators color match within their environments; here, we suggest crypsis as a broader adaptive mechanism for large-bodied species.

In mammals there are two types of melanin: pheomelanin is red or yellow and eumelanin is black or brown. Genetic variation in melanin biosynthesis is what causes the difference in hair, eye, and skin color. Reply