A recessive coat color dilution in Dexter cattle attributed to a missense mutation in SLC45A2

Three colors of Dexter cattle are currently recognized: black, red, and dun. In Dexters, dun is determined by a recessive genotype of TYRP1 (b/b) that dilutes an otherwise black animal (MC1R genotype ED/-); this variant does not impact red cattle. A subset of Dexters with dilute coat colors described as dark dun/chocolate (CD) or light dun/cream (CL) were identified. Although phenotypically similar to dun, they did not have the expected TYRP1 b/b genotype. Given relationships among the reported individuals, we hypothesized that a novel recessive genotype is causative of CD on a black background and CL on a red background. Whole-genome sequence was generated from four dilute Dexters (three CD and one CL), and one black calf of a CD dam. None of the cattle sequenced had the TYRP1 b/b genotype. The comparison of variants in the five Dexter cattle to those from 226 non-Dexter control cattle resulted in the identification of a missense variant in SLC45A2 (NC_037347.1: g.39790189A>C; XM_002696386.6: c.398A>C) that fit the proposed hypothesis. Sanger sequencing of additional Dexter cattle (n = 227) demonstrated complete segregation of the recessive genotype with the CD and CL phenotypes. The mutation, predicted to result in a substitution of glutamine with proline (XP_002696432.2: p.Gln133Pro) in a transmembrane helix was classified as deleterious by SIFT. Further supporting its implication, SLC45A2 is responsible for coat color dilutions and oculocutaneous albinism type IV in multiple species. Testing for the SLC45A2 variant can be a valuable resource for Dexter breeders interested in coat color.