Enzyme reduction explains lazy flies.

Couch potatoes exist even among fruit flies, and scientists now know the genetic reason--at least for the insect version of this laziness. In populations of fruit flies gathered from the wild, about 70 percent are rovers, and the remainder are sitters, according to Marla B. Sokolowski of York University in Toronto and her colleagues. When there is no food around, both groups range far away for a meal, but after eating, rovers head out to forage for their next meal while sitters linger where they have just eaten.

Sokolowski and her group had previously determined that this subtle difference in feeding behavior is inherited, and they traced the gene responsible to the fruit fly's chromosome 2. In the Aug. 8 Science, the scientists now identify it as dg2, a gene encoding three similar enzymes belonging to the class known as protein kinases.

The crucial piece of evidence implicating dg2 was the discovery that rovers make slightly more of dg2's enzymes than sitters do, the researchers note. Moreover, a sitter turns into a rover when researchers artificially increase the fly's production of the enzymes.

The enzymes encoded by dg2 seem to help transmit signals inside cells, but little is known about the molecules with which they interact, says study coauthor Ralph J. Greenspan of the Neurosciences Institute in San Diego, who has explored how other protein kinases sway behavior. Whether a human counterpart of dg2 subtly regulates people's eating patterns remains an open, and provocative, question, he adds.