Terry Whitaker/Photo Researchers Inc.

Trimming down.
The lack of the ASIP gene may make gibbons slimmer than other primates.

Ever Seen a Fat Gibbon?

Could the absence of a single gene explain why gibbons are so much more slender than other primates? That's the conclusion of a new study, which, if correct, may someday help researchers home in on a genetic target for human obesity.

The obesity epidemic has sent researchers scurrying to find genes that might be implicated in what is now regarded as a major public health problem (Science, 4 June 2004, p. 1413). One candidate is agouti signaling protein (ASIP), which is highly expressed in human fat tissues and also plays a role in skin pigmentation. Many mammalian species have their own versions of ASIP, as do chickens and fish.

To trace the evolutionary history of the ASIP gene in primates, University of Tokyo biologists Kazuhiro Nakayama and Takafumi Ishida compared the gene's DNA sequence among several species. In addition to humans, the pair found the gene in chimps, gorillas, and several species of monkeys. The gene was very similar in all of these animals, but it was missing in the four species of gibbons they examined. If that's the case in the eight or so other species of gibbons, then ASIP must have been deleted during a genetic reshuffling that took place before the gibbons diverged from other apes on the evolutionary tree, probably around 25 million years ago, Nakayama and Ishida report in the April issue of Genome Research. They suggest that this genetic "knockout" helped the gibbons to adapt to life in the trees: The lightweight animals swing from branch to branch with a speed and agility that puts most other primates to shame.

John Fleagle, a primatologist at Stony Brook University in New York who has studied gibbons, finds the hypothesis plausible. "I have never seen a fat gibbon," he says. But some researchers think that the Japanese team may be going out on a limb. "It might be a stretch," says Angela van Daal, a molecular geneticist at Bond University in Gold Coast, Australia. "There are many other potential explanations for the smaller size of the gibbon that do not involve loss of the ASIP gene." Van Daal thinks it is more likely that the ASIP could help explain the coat colors of gibbons, many of which, especially the males, are black. She points out that mice genetically engineered to knock out the gene have entirely black coats.

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