Coat colouration in domestic dogs… genetically speaking!

Dog colour patterns explained by modular promoters of ancient canid origin. Bannasch DL, Kaelin CB, Letko A, Loechel R, Hug P, Jagannathan V, Henkel J, Roosje P, Hytönen MK, Lohi H, Arumilli M; DoGA consortium, Minor KM, Mickelson JR, Drögemüller C, Barsh GS, Leeb T. Nat Ecol Evol. 2021 Oct;5(10):1415-1423. doi: 10.1038/s41559-021-01524-x.

OTHER VIDEOS YOU MIGHT LIKE:
• Taq polymerase: The workhorse of PCR. Neigh!!! -    • Taq polymerase: The workhorse of PCR....  
• Are broccoli, cabbage and cauliflower the same? The evolution of domesticated cruciferous vegetables -    • Are broccoli, cabbage and cauliflower...  
• The genetics of gravitropism in Arabidopsis thaliana root development -    • The genetics of gravitropism in Arabi...  

One of the things that makes domestic dogs unique from their canine ancestors is the diversity present in coat pigmentation patterns. Presumably this is due to a lack of selection for camouflage, but the genetic mechanisms that give rise to this diversity have remained elusive.

We know the two pigments involved in mammal colouration to be eumelanin, which expresses brown to black colour, and pheomelanin, which expresses cream to red colours. The base state in dogs is eumelanin production, due to the constant cycling of melanocyte-stimulating hormone (MSH) which stimulates eumelanin production in melanocytes, the cells that synthesise and release the pigments that colour individual canine hairs. The role of the antagonist of MSH, agouti signalling protein (ASIP) which stimulates pheomelanin production was soon recognised, although neither the exact mechanism by ASIP acted, nor its importance in pigmentation patterns were recognised until much later.

The focus of most studies in the genetics of pigmentation have focused on melanocytes, and specifically on a receptor on the cell surface called melanocortin 1 receptor; its role is to receive signals from signalling proteins such as MSH and ASIP that indicate what pigment should be manufactured. As the critical middleman, mutations in the MC1R genes can directly alter the pigment that gets produced. However, studies in this area explain individual anomalous coats, but fail to explain the sheer diversity of canine pigmentation.

Fortunately, a breakthrough came with the discovery of two promoters that control ASIP expression. What was significant about these two promoters was that they were in control of different regions of the body, with the ventral promoter (VP) controlling ASIP expression on the underside, while the hair cycle promoter (HCP) controlled expression across the back of the dog’s body. Furthermore, different alleles of this promoter were of different strength in expressing ASIP, leading to complex arrays of pigmentation patterns as different variants of these promoters express ASIP, and hence pheomelanin, in varying degrees of intensity in different regions of the canine coat.

Creator: Maxim Belford-Long

References:
Bannasch DL, Kaelin CB, Letko A, Loechel R, Hug P, Jagannathan V, Henkel J, Roosje P, Hytönen MK, Lohi H, Arumilli M; DoGA consortium, Minor KM, Mickelson JR, Drögemüller C, Barsh GS, Leeb T. Dog colour patterns explained by modular promoters of ancient canid origin. Nat Ecol Evol. 2021;5(10):1415-1423.
Dreger DL, Parker HG, Ostrander EA, Schmutz SM. Identification of a mutation that is associated with the saddle tan and black-and-tan phenotypes in Basset Hounds and Pembroke Welsh Corgis. J Hered. 2013;104(3):399-406.
Freedman AH, Wayne RK. Deciphering the origin of dogs: From fossils to genomes. Annu Rev Anim Biosci. 2017;5:281-307.
Kaelin CB, Barsh GS. Genetics of pigmentation in dogs and cats. Annu Rev Anim Biosci. 2013;1:125-156.
Newton JM, Wilkie AL, He L, Jordan SA, Metallinos DL, Holmes NG, Jackson IJ, Barsh GS. Melanocortin 1 receptor variation in the domestic dog. Mamm Genome. 2000;11(1):24-30.
Weich K, Affolter V, York D, Rebhun R, Grahn R, Kallenberg A, Bannasch D. Pigment intensity in dogs is associated with a copy number variant upstream of KITLG. Genes (Basel). 2020;11(1):75.
Wright S. Color inheritance in mammals. J Hered. 2017;8: 224–235.