While many wild animal species are relatively uniformly coloured, a wide variety of coat colours are found in domestic animals. Shift from natural selection towards criterias that are based on human preferences is most likely to account for the observed increase in coat colour variation. This makes domestic animals unique for studying gene function and gene regulation with regards to loci affecting pigmentation. Following an initial evolutionary discussion, this review will focus on two aspects of mammalian pigmentation; regulation of pigment synthesis and distribution of pigment producing cells. Molecular interaction between the G-protein coupled receptor MC1-R (melanocyte stimulating hormone receptor) and the agouti protein is the main regulatory system known to control the synthesis of eumelanin (brown or black pigment) versus phaeomelanin (red or yellow pigment). For both genes, mutations that explain phenotypical variants are characterised in several species. This includes numerous dominant-acti ng mutations of the MC1-R gene a constitutively activated receptor, and subsequently synthesis of black pigment. Additionally, it has been shown that the agouti gene, which is known to antagonise the MC1-R, is able to modify the expression of a wild-type MC1-R, as well as a semi-dominant variant of this receptor. Whereas agouti and extension often cause pigment switches, the distribution of pigment depends on proliferation and migration of neural crest derived melanocytes. Several genes, including the tyrosine kinase c-kit receptor and its ligand steel factor, regulate these processes. Functional mutations within the c-kit gene or variants of the ligand have been identified, and both loci are documented to influence the level of spotting.
Keywords: Molecular Genetics, Pigmentation, Domestic Animals, Regulation of pigment synthesis, G-protein coupled receptor MC1-R, Melanocyte stimulating hormone receptor, MC1-R gene, Agouti protein, Coat colour evolution, Transmembrane MC1-R
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