From whence the humble chicken? Gallus gallus is a domesticated chicken-like bird (thus, the name “chicken”) that originates in southeast Asia. Ever since Darwin we’ve known that the chicken originated in southeast Asia, although the exact details of which one or more of several possible jungle fowls is the primal form has been debated. The idea that more than one wild species contributed to the early chicken has been on the table for a long time, though perhaps not as long as the chickens themselves have been on the table.Notice the yellow legs on this chicken. If you pluck out the feathers, you’ll notice that the skin is yellow as well. But if you go find, say, a crow, and pluck its feathers, it will be grayish in color. Or maybe black, I don’t know, it’s been a while since I’ve defeathered a crow. The point is, that some birds are yellow, some are not.There is a gene that is expressed in certain tissues that produces an enzyme that cleaves the carotenoid molecules that provide the yellow color. If there is no functional copy of this gene (if the individual is homozygotic for the broken version) then this cleaving does not happen, and you get a yellow bird (depending on other factors we shall ignore).In short, new research confirms as previously thought that the red jungle fowl (Gallus gallus) is ancestral to the modern chicken, as Darwin suspected. But this research also suggests that another bird, the grey jungle fowl (Gallus sonneratii) also contributed to the chicken’s genome, providing the yellow color we see on this chicken’s legs.The research, reported in PLoS Genetics, gives us two results. One is the first characterization of the process of pigmentation mentioned above, and the second is a new family tree for this bird.
Many bird species possess yellow skin and legs whereas other species have white or black skin color. Yellow or white skin is due to the presence or absence of carotenoids. The genetic basis underlying this diversity is unknown. Domestic chickens with yellow skin are homozygous for a recessive allele, and white skinned chickens carry the dominant allele. As a result, chickens represent an ideal model for analyzing genetic mechanism responsible for skin color variation. In this study we demonstrate that yellow skin is caused by regulatory mutation(s) that inhibit expression of the beta-carotene dioxygenase 2 (BCDO2) enzyme in skin, but not in other tissues. Because BCDO2 cleaves colorful carotenoids into colorless apocarotenoids, a reduction in expression of this gene produces yellow skin. This study also provides the first conclusive evidence of a hybrid origin of the domestic chicken. It has been generally assumed that the red junglefowl is the sole ancestor of the domestic chicken. A phylogenetic analysis, however, demonstrates that though the white skin allele originates from the red junglefowl, the yellow skin allele originates from a different species, most likely the grey junglefowl. This result significantly advances our understanding of chicken domestication.
Here is the phylogenetic tree that the authors of this paper present:Click here for a much larger image (84kb)You will read in press reports that “Darwin got it wrong” when it comes to chickens. Let’s have a look at what he said and see how wrong he was. Darwin addressed the two major theories of his time. One is a multiregional theory, much like the now discredited version of human evolution, where each kind of chicken was domesticated from a different wild form. The other is that all descended from one ancestor, Gallus gallus bankiva, also known as Gallus bankiva.Darwin uses chickens in a big way in developing his ideas about evolution. Chickens were perhaps as important as pigeons for examining breed characteristics. Therefore, he wrote quite a bit about chickens. In the end, he favored the single origin hypothesis, but he also describes the primordial species of his choosing … the red jungle fowl … as much more diverse in character than it is generally characterized today…
… Gallus bankiva, has a much wider geographical range than the three previous species; … This species varies considerably in the wild state. Mr. Blyth informs me that the specimens, both male and female, brought from near the Himalaya, are rather paler coloured than those from other parts of India; whilst those from the Malay peninsula and Java are brighter coloured than the Indian birds. I have seen specimens from these countries, and the difference of tint in the hackles was conspicuous. The Malayan hens were a shade redder on the breast and neck than the Indian hens. The Malayan males generally had a red ear-lappet, instead of a white one as in India; but Mr. Blyth has seen one Indian specimen without the white ear-lappet. The legs are leaden blue in the Indian, whereas they show some tendency to be yellowish in the Malayan and Javan specimens. In the former Mr. Blyth finds the tarsus remarkably variable in length. According to Temminck20 the Timor specimens differ as a local race from that of Java. These several wild varieties have not as yet been ranked as distinct species; if they should, as is not unlikely, be hereafter thus ranked, the circumstance would be quite immaterial as far as the parentage and differences of our domestic breeds are concerned. The wild G. bankiva agrees most closely with the blackbreasted red Game-breed, in colouring and in all other respects, except in being smaller, and in the tail being carried more horizontally. But the manner in which the tail is carried is highly variable in many of our breeds,…(Darwin 1868:233)
What we see here (my emphasis added) is evidence that skin color varied across different populations of this species.The study at hand asserts:
On the basis of observed character differences and cross-breeding experiments, Darwin concluded that domestic chickens were derived solely from the red junglefowl, though this was later challenged by Hutt , who stated that as many as four different species of junglefowls may have contributed to chicken domestication. Molecular studies of mtDNA and retroviral insertions have supported Darwin’s view. A study that analyzed both repeat nuclear elements and mitochondrial sequences found evidence that grey and Ceylon junglefowls may hybridize with domestic chickens, but did not provide evidence that these two species have contributed to chicken domestication. To date, no studies have compared gene sequences associated with a specific phenotype found in domestic chickens across numerous wild junglefowls and domestic breeds….We searched for the causal mutation … This analysis revealed a surprisingly high sequence diversity between the two groups (0.81%), well above the genome average for chicken (~0.5%)  and approaching the sequence divergence between chimpanzee and human (1.2%). We therefore included three other species of junglefowls in the sequence comparison: grey (G. sonneratii), Ceylon (G. lafayetii), and green (G. varius) junglefowls. This step was also motivated by the fact that grey and Ceylon junglefowls have red or yellowish legs which implies deposition of carotenoids and a Y/Y genotype…In contrast, mtDNA sequences from the same samples showed the expected pattern in which domestic chickens cluster with red junglefowl within a clade well separated from other junglefowls
The grey and red jungle fowl have, at present, disjunct ranges, but that may be a product of recent ecological changes, including human alterations of habitats. Also, in the early days of chicken domestication, there is no reason to suspect that a single origin would be followed by immediate isolation from wild forms, and in fact, all the available evidence including that reported here suggests the contrary.I think the truth of the matter is that Darwin did not really get the origin of the chicken wrong … he had it substantially right. Rather, Darwin had a better idea of variation in the wild forms than we may appreciate today, and he leaned a bit more towards a simpler history at the start than we tend to today. That’s not bad considering that all of the modern theory about origins of domesticated forms post dates, and often derives from, Darwin.In other words, Newton understood gravity, so today we can design an airplane. But if Newton designed and airplane that did not fly, would that mean that he got gravity wrong?I think not.
(More on Darwin here)Darwin, C. R. 1868. The variation of animals and plants under domestication. London: John Murray. First edition, first issue. Volume 1.Eriksson, J., Larson, G., Gunnarsson, U., Bed’hom, B., Tixier-Boichard, M., StrÃ?Â¶mstedt, L., Wright, D., Jungerius, A., Vereijken, A., Randi, E., Jensen, P., Andersson, L., Georges, M. (2008). Identification of the Yellow Skin Gene Reveals a Hybrid Origin of the Domestic Chicken. PLoS Genetics, 4(2), e1000010. DOI: 10.1371/journal.pgen.1000010