Genetic Studies of Pigmentation in Chicken

• Domestic animals have been selected by humans for thousands of years, which have drastically altered their genetic constitution and phenotypes. In this thesis, several of the most important genes causing pigmentation differences between the wild red junglefowl (Gallus gallus) and domestic chickens have been identified. Pigmentation phenotypes are easily scored, and the genes underlying these phenotypes are valuable models to study gene function and gene interaction. Dominant white colour is widespread among domestic chickens. The Dominant white allele specifically inhibits the expression of black (eumelanin) pigment and we identified several insertion/deletion mutations in the PMEL17 gene causing the different phenotypes controlled by this locus. The Silver allele on the other hand inhibits the expression of red (pheomelanin) colour and is a genetic variant of the SLC45A2 gene. Silver is the first pheomelanin-specific mutation(s) reported for this gene. An 8 kb deletion, including a conserved enhancer element, 14 kb upstream of the transcription factor SOX10 is causing the Dark brown phenotype. This phenotype restricts the expression of eumelanin and enhances red pheomelanin in specific parts of the plumage. These three gene identifications have extended the knowledge about genes affecting melanocyte function. Carotenoid-based pigmentation is of utmost importance in birds and other animals. The yellow skin allele in chicken allows deposition of carotenoids in skin and explains why most domestic chickens have yellow legs. We demonstrated that the yellow skin phenotype is caused by a tissue specific regulatory mutation in the gene for the enzyme beta-caroten dioxygenase 2 (BCDO2). This was the first identification of a specific gene underlying carotenoid-based pigmentation. Interestingly, the yellow skin haplotype was shown to originate from the grey junglefowl (Gallus sonneratii) and not the red junglefowl as expected, thus presenting the first conclusive evidence for a hybrid origin of the domestic chicken.  

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