Human populations that have high starch diets have an increase in the number of copies of a gene whose product breaks down starch, reports a study to be published online in Nature Genetics. Although copy number variation has attracted a lot of recent attention, this is one of the first documented examples of positive selection on gene copy number in humans.
Starch has become a prominent component of the human diet. It is metabolized in part by salivary amylase, and the gene encoding it, AMY1, shows extensive variation in copy number. George Perry and colleagues estimated AMY1 copy number in 50 European Americans and showed that the levels of salivary amylase protein are positively correlated with gene copy number. They went on to show that individuals from three populations with high-starch diets tend to have more copies of AMY1 than individuals from populations with low-starch diets. Finally, the authors compared the extent of variation across the genome between two Asian populations -- Japanese (high-starch diet) and Yakut pastoralists (low-starch diet) -- and found that variation at AMY1 exceeds that of more than 97% of the other sites in the genome that were assessed.
The authors conclude from this that natural selection favored increased AMY1 copy number in at least some populations with high-starch diets. Interestingly, humans have significantly more copies of AMY1 than chimpanzees, which ingest relatively little starch. Increased AMY1 expression would probably improve the digestion of starchy foods, and possibly maintain energy absorption in the face of intestinal disease.
Nathaniel Dominy (University of California, Santa Cruz, CA, USA)
Abstract available online.
(C) Nature Genetics press release.
Message posted by: Trevor M. D'Souza
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