Color Me Blind: Animal Pigmentation and Coat Pattern Genetics
Why does your dog have spotted fur? Why is the coat black at the knees, brown over the eyebrows? During embryonic development, pigment-producing cells called melanocytes proliferate across the body unevenly and paint the fur in a unique way. The behavior of melanocytes, where they go and which pigments they produce, is controlled by only a few genes.
Seemingly minute changes in a single section of a gene can alter an animal’s outward appearance. The genes MC1R and ASIP are antagonists; their relative levels of expression dictate what color melanocytes produce. When one of these genes is downregulated, it produces drastic visual changes on the body. Two flycatcher species on the Solomon Islands illustrate this phenomenon. The chestnut-bellied flycatcher and the melanic flycatcher have discernible feather colors, linked to a single protein substitution on the MC1R gene. The presence of a clay or black colored belly in these birds is perfectly associated with a subtle molecular switch. Populations of deer mice in Nebraska evolved camouflaged hair to hide from birds of prey. A mutation at the ‘Agouti’ locus of the ASIP gene alters the width of the subapical pheomelanic band within each hair follicle. This delicate anatomical change controls the brightness of fur color across the entire body, and kickstarts the formation of new species.
Domestic dogs in particular display an eclectic range of coat colors. The link between domestic behaviors and coat color is a point of contention, because of findings from a Russian experiment conducted in the 1960s. Following the persecution of geneticists by Soviet thought-leader Trofim Lysenko, Dmitry Belyaev stabled hundreds of Russian Wild Red Foxes in Siberia and bred them to exhibit certain behaviors. Through artificial selection, his team produced groups of docile and aggressive foxes. New physical traits arose in the tame group, including spotted fur and floppy ears. It turns out that friendly behavior may not be the root cause of these new traits. In 2013, research by Linderholm et al demonstrated that there is no explicit genetic link between domestic behaviors and novel coat colors, even though the two are correlated.
Although domestic behavior doesn’t produce a spotted coat, the genes that regulate color effect other anatomical traits. Melanocytes control fur brightness but are also linked to inner ear development. As an embryo, these melanocytes migrate asymmetrically and express varying amounts of proteins depending on their role in development. The cells that construct the inner ear may produce a secondary effect of darker coat color in that part of the body. This phenomenon produces a clumped fur arrangement on a Doberman or Border Collie.
Color-producing genes control an animal’s physiology as well. Mutations in the ASIP gene (studied in the deer mice above) produce a breed of yellow mice that are more susceptible to obesity and cancer. Coat color appears to be a superficial trait but instead unearths deeper findings on sensory development and disease prevention.