A novel genetic study has revealed why chickens have yellow legs, demonstrating that though Charles Darwin was right about many things, his view on the origins of the chicken was not entirely correct. The study, published February 29 in the open-access journal PLoS Genetics, reveals the genetic basis for the appearance of yellow skin in billions of chickens raised worldwide.
Yellow-skinned chickens have a different version of a gene than their white-skinned cousins. Darwin believed that all chickens came from a wild species known as the red junglefowl. When the researchers looked for the yellow-skin gene in the red junglefowl, they only found the genetic variant that codes for white skin. More surprisingly, when they finally did find the yellow-skin version of the gene, it was present in a completely different wild species: the grey junglefowl.
Greger Larson, a Research Fellow at Uppsala University and at Durham University, UK said: "Darwin recognized the importance of studying domestic animals as a model of evolution and this insight has proved enormously influential. The ironic thing is that he believed that dogs were hybrids of several wild ancestors but that chickens only had one, and he was wrong on both counts."
Yellow coloring comes from pigments found in feed called carotenoids. The gene in question codes for an enzyme that degrades carotenoids into a colorless form. White-skinned chickens express this enzyme in skin. In contrast, yellow-skinned chickens do not express the enzyme in skin, allowing the carotenoids to accumulate and produce yellow coloring. Interestingly, the gene functions normally in other tissues and yellow-skinned chickens have no general defect in carotenoid metabolism.
"This is a beautiful demonstration of how important regulatory mutations are for evolutionary changes" said Leif Andersson, leader of the research team. "What we are interested in knowing now is why yellow skin in chickens is so ubiquitous. It could have been that yellow skin was perceived to be a marker of health or size or egg production, or it could just be that yellow skin was fun to look at. We're really not sure. Furthermore, the gene we have identified may be important for carotenoid-based pigmentation in other species like the pink color of the flamingo; the yellow legs of many birds of prey including eagles, falcons and hawks; the pink muscles of salmon, and even skin color in humans."
This project was funded by the European Molecular Biology Organization, the Swedish Foundation for Strategic Research, and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning and the Swedish Research Council.
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CITATION: Eriksson J, Larson G, Gunnarsson U, Bed'hom B, Tixier-Boichard M, et al. (2008) Identification of the Yellow Skin Gene Reveals a Hybrid Origin of the Domestic Chicken. PLoS Genet 4(2): e1000010. doi:10.1371/journal.pgen.1000010
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