Gene interactions with Agouti Signaling Protein produce complex pigmentation phenotypes in the domestic dog

Abstract

Much of what is understood about canine coat color genetics focuses on single gene inheritance. However, within the traditional color patterns, there is noticeable variation, suggesting modification of known coat color alleles. Alleles of the Agouti Signaling Protein (ASIP) gene control the temporal and spatial expression of red-based phaeomelanin and black-based eumelanin pigments. While up to six ASIP alleles have been predicted by previous studies, only two alleles, ay fawn and a recessive black, have been identified as polymorphisms within ASIP. Through sequence analysis, a SINE insertion in intron 1 of ASIP was detected that segregates with the black-and-tan, saddle tan and recessive black phenotypes. Together with the diagnostic tests for the ay and a alleles, detection of the SINE allows for complete genotyping of the four common ASIP alleles: ay, aw, at, a. Salukis and Afghan Hounds exhibit a phenotype known as grizzle or domino. This phenotype is similar to the black-and-tan phenotype, also present in both breeds, though the phaeomelanin points extend further up the limbs, onto the ventral surface, and form a widow’s peak on the face. Sequencing of the Melanocortin 1 Receptor (MC1R) gene identified a G78V polymorphism that segregates with the grizzle/domino phenotype. Further genotype analysis showed that expression of the grizzle/domino pattern requires a genotype interaction involving ASIP, MC1R, and Canine Beta-Defensin 103 (DEFB103). The saddle tan phenotype is present in scent hounds, terriers, and herding breeds, and is characterized by phaeomelanin pigment on the limbs, often extending to the midline of the back, and encompassing the entire head. Genome wide association study analysis, fine mapping, and sequence analysis identified a 16 bp tandem duplication in intron 5 of the hnRNP-Associated with Lethal Yellow (RALY) gene that segregates with the black-and-tan phenotype, versus saddle tan, in Basset Hounds and Pembroke Welsh Corgis. In breeds that never have the saddle tan phenotype, but frequently have the black-and-tan phenotype, the RALY duplication does not segregate with black-and-tan. This, together with further genotype analysis, suggests a gene interaction of ASIP, MC1R, DEFB103, RALY, and an additional modifier gene is required for expression of saddle tan or black-and-tan.