M locus – Merle Dog Coat Color
Merle dog coat color is a coat pattern in domestic dogs, specific for patches of diluted pigment mixed together with normal melanin. Merle is classified as a standard coloration by the American Kennel Club for several breeds, such as Shetland Sheepdog, Australian Shepherd, Cardigan Welsh Corgi and Dachshund. Merle coat color in the Dachshund is referred to as dapple. In the Great Dane, merle dog coat color is not desirable, but the popular harlequin pattern is a result of the interaction of the M locus (merle) and a individual, seprated H locus (harlequin). Other than named, Merle coat color can appear in Catahoula Leopard Dog, Norwegian Dunker, Border Collie, Hungarian Mudi, Collie (Rough and Smooth), Shetland Sheepdog, Beauceron, Bergamasco, Pyrenean Shepherd, Chihuahua, American Staffordshire Terrier, American Cocker Spaniel and Pomeranian. Although attractive coat color, genes piebald and merle are associated with deafness, blindness and other health problems in dogs.
Melanocytes cells that produce melanin and can be found in the bottom layer of the skin’s epidermis, in the eye, the inner ear, bones and heart. In the dog coat color, there are two basic types of pigments: eumelanin and phaemelanin. Eumelanin is the black pigment and phaeomelanin is the red pigment. Both of these pigments can vary in their intensity and color shade. Merle or M locus affects the eumelanin. Phaeomelanin will not be affected and will appear as normal. It dilutes different parts of the coat, which will change into a lighter color. Only the patches of original color will be present. These patches can vary in size and location on the body. The diluted parts of merle are also variable, from very pale to a dark grey. The merle coat usually darkens as the dog ages.
Dog homozygous for merle gene is known as double merle (genotype MM). Double merle has been recognized as sublethal and is associated with many abnormalities in the reproductive, cardiac and skeletal system. Many of these abnormalities share similarities with Waardenburg syndrome (WS), an autosomal dominant auditory-pigmentation disorder in humans.
Cryptic merle, also known as hidden merle, appears as only very slight merle coloration, or most often, is not recognizable at all. Cryptic merle dogs show no serious health problems that appear to be frequent among regular merle dogs. Cryptic merle dogs carry also merle gene, but it is a shorter version. It is advised to breed cryptic merle dogs only with non-merle dogs, due to health risks connected with merle gene.
In both heterozygous ( genotype Mm) and homozygous (genotype MM) merle dogs eze and ear abnormalities have been observed, which included deafness, increased intraocular pressure, bad reflection ability (ametropia) and coloboma ( a hole in one of structures of the eye). Absence of pigment affects also hairs in the inner ear, causing complete deafness. A research proved a deafness prevalence of 54.6% in double merles and 36.8% in single merles.
Merle dog coat color is caused by a gene PMEL17, also known as SILV (silver). It is localized on canine chromosome 10. On 3′ end of SINE insertion, oligo(dA)-rich tails have been identified. In some dogs, deletions within the oligo(dA)-rich tails have been recognized. Such deletions allow normal pigmentation. Due to severe health problems, it is not advised to breed two merle dogs. Due to possibility of possessing the merle gene without its display in the phenotype, genetic testing is advised in order to prevent getting double merle (MM) cubs and risking development of severe health problems associated with this genotype.
Leigh Anne Clark, Jacquelyn M. Wahl, Christine A. Rees, and Keith E. Murphy (2006): Retrotransposon insertion in SILV is responsible for merle patterning of the domestic dog. PNAS 103 (5) 1376-1381, 2006,doi:10.1073/pnas.0506940103