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TOKYO- Every breed of domestic chicken that ever lived can be traced to a single subspecies of red junglefowl native to Thailand, according to mitochondrial DNA evidence discovered by Japanese researchers.

The researchers compared divergences in DNA sequences and RFLP (restriction fragment length polymorphism) typing of mitochondrial DNA from a selection of wild and domestic breeds of fowl. One hundred and nineteen birds representing 26 domestic breeds, 30 green junglefowl, and 14 subspecies of red junglefowl were studied.

Using these techniques, the research team was able to eliminate all but a single subspecies of red junglefowl (Gallus gallus gallus) native to Thailand as the ancestor to all subsequent breeds of the domesticated chicken.

Because the domestication of the chicken is a relatively recent event in human history, studies of nuclear genes would not provide much useful data because of their low mutation rate. The mitochondrial genome, on the other hand, has a high and constant mutation rate, as it is impervious to generation time differences between species. Thus, the mitochondrial genome proved useful in tracking down the original domestication event, notes Dr. Susumu Ohno of the Beckman Research Institute.

Biologists have pondered the origins of the domestic chicken for many decades. Paleontologists first fixed the original date of chicken domestication some 4,000 years ago at a site in Pakistan. However, subsequent discoveries of chicken bones at Neolithic sites at the mouth of the Yellow River in China push the date back to about 7,500 years ago. However, the red junglefowl was not native to that arid region of China, suggesting an older heritage in a more tropical area.

The new findings by the Japanese researchers suggest that domestication took place more than 8,000 years ago in what is now Thailand and Vietnam, the region in which this red junglefowl is found today. Moreover, this data indicates that the chicken is a notable exception to the general rule that the domestication of a species results in the extinction of its wild ancestor, the researchers note.

The complete study can be reviewed in the Proceedings of the National Academy of Science, v.91, pp. 12505-12509, 12/20/94, Fumihito et al.

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