Synergy between MC1R and ASIP for coat color in horses (Equus caballus)

Abstract

Through domestication and human selection, horses have acquired various coat colors, including seven phenotypes: Black, brown, dark bay, bay, chestnut, white, and gray. Here we determined the genotypes for melanocortin-1 receptor (MC1R) and agouti signaling protein (ASIP) in 709 horses from 15 breeds. We found that the E E E E genotype frequency at MC1R decreased from dark to light colors (black = 64.5%, brown = 67.5%, dark bay = 47.0%, bay = 16.5%, and chestnut = 0.0%), whereas the A A A A genotype frequency at ASIP increased as coat color lightened (black = 0.0%, brown = 22.9%, dark bay = 69.2%, and bay = 83.0%). When combined genotypes at MC1R and ASIP were examined, different advantage genotype combinations were found for each color: Black E E E E-A a A a (64.5%), brown E E E E-A A A a (47.0%), dark bay E E E E-A A A A, and E E E e-A A A A (36.2% and 33.0%, totally 69.2%), bay E E E e-A A A A (69.6%), and chestnut E e E e-A A A A (62.6%). The Ï ‡ 2 test showed that the phenotypes of horse coat colors were significantly related with the genotypes of MC1R and ASIP (p < 0.001). Furthermore, in contrast to a previous study where A a A a was only found in black, chestnut, and gray horses, we also found this allele in brown, dark bay, bay, and white horses. These results indicated that MC1R and ASIP may synergistically affect the levels of melanin in equine coat colors and that additional genes are likely involved in regulating coat colors, especially for white and gray colors. Our research provides new data for further studies on the synergetic actions of MC1R and ASIP in coat color of horses.