Genetic Background of the Polish Primitive Horse (Konik) Coat Color Variation-New Insight into Dun Dilution Phenotypic Effect

Only the blue dun coat color, produced by the action of the dun allele on the background of a black base coat, is officially permitted in the Polish primitive horse (PPH, Konik) breed, yet the population is not visually homogenous and various coat color shades occur. Herein, the molecular background of PPH coat color was studied based on genotyping of known causative variants in equine coat color-related genes (ASIP, MC1R, TBX3, SLC36A1, SLC45A2, PMEL17, and RALY). Additionally, screening for the new polymorphisms was conducted for the ASIP gene coding sequence and the TBX3 1.6-kb insert (associated with the dun dilution). We did not observe the champagne, silver, or cream color dilution variants in the PPH breed. A significant association (P < 0.01) was recorded for the genotype in TBX3 gene 1.6 kb in/del and the degree of dun coat dilution, demonstrating that the dominant action of the dun mutation is not fully penetrant. In addition to the effect of the 1.6 kb in/del zygosity, variants within the TBX3 insert were significantly associated with PPH coat color variability (P < 0.01), suggesting the presence of an additional allele at this locus. Finally, we identified a high frequency (35%) of genetically bay dun-colored PPH individuals that are officially recorded as blue (black base coat) duns. We propose that the difficulty in distinguishing these 2 phenotypes visually is due to an independent locus upstream of the ASIP gene, which was recently described as darkening the typical bay pigmentation shade.