Identification of a premature stop codon in the melanocyte-stimulating hormone receptor gene (MC1R) in Labrador and Golden retrievers with yellow coat colour

We have examined whether black/yellow coat colour in Labrador retrievers is controlled by allelic variants at the extension locus. As the gene encodin g the melanocyte-stimulating hormone receptor (MC1R) has been shown to corr espond to the extension locus in several species, we have determined the ge nomic MC1R sequence in Labrador retrievers with black and with yellow coat colour. Using primers based on the fox (Vulpes vulpes) MC1R sequence we ini tially isolated and sequenced the innerpart of the canine MC1R. By means of inverse PCR we succeeded in the characterization of both flanking regions of the MC1R gene (Genbank: AF064455). Comparison of the complete MC1R seque nces of a yellow and a black Labrador retriever revealed a single C-->T mut ation at nucleotide position 916 in the yellow dog. This transition changed the codon for arginine at position 305 into a stop codon, resulting in the elimination of the evolutionary strongly conserved 10 carboxyterminal amin o acid residues. With an allele-specific-oligonucleotide (ASO) test it was shown that the mutation cosegregated with the recessively inherited yellow coat colour in the Labrador retriever. Golden retrievers also appeared to b e homozygous for the mutation. Seventeen other breeds were all negative for the mutation. Since the Labrador and Golden retriever are closely related, we suggest a common founder for the yellow coat colour in Labrador and Gol den retrievers.