MOLECULAR BASIS FOR COAT COLOUR VARIATION IN THE PIG

Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences, Uppsala Biomedical Centre, Box 597, S-751 24 Uppsala, Sweden.

We have investigated the molecular basis for coat colour variation in pigs in an intercross between the European Wild Boar and Large White domestic pigs and in various pig breeds. We have identified the gene corresponding to two major coat colour loci in the pig, The Dominant White (I) and the Extension (E) loci. We have found that Dominant White is located on pig chromosome 8 and corresponds to KIT encoding the mast/stem cell growth factor receptor (Johansson Moller et al. 1996, Mammalian Genome 7:822-830; Marklund et al. 1998, Genome Research 8:826-833). Three alleles have been identified, i for wild type colour, Ip for the Patch phenotype and I for Dominant white. Ip is associated with a gene duplication including the entire coding sequence of KIT. The fully dominant allele I is due to two mutations, the duplication and a splice mutation in one of the two copies. The splice mutation leads to skipping of exon 17 and expression of a truncated form of the receptor. The result provides an explanation for the drastic effect on pigmentation combined with full viability in the homozygous condition. We have recently shown that the E locus is equivalent to the locus for melanocortin receptor 1 (MC1R) on chromosome 6 (Kijas et al. 1998, Genetics 150:1177-1185). Five different allelic variants have been characterised at the molecular level, E(+) for wild type coat colour, E(D1) and E(D2) for dominant black, E(p) for black spotting and e for recessive red. I/KIT and E/MC1R are apparently the two major loci controlling coat colour variation within and between pig breeds but additional (not yet characterised) loci contribute to the variation.