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Queen-of-the-Hill Gene?

Ants are social animals (try sharing your home with 100,000 in-laws) that live by a complex social code. Many house rules were thought to be flexible: When food is scarce, for instance, a colony with one queen might take on others. Not so, two researchers report in tomorrow's Proceedings of the National Academy of Sciences. A gene, they say, determines whether a colony has one matriarch or many.

Laurent Keller at the University of Lausanne in Switzerland and Kenneth Ross at the University of Georgia, Athens, were scanning the genes of various populations of fire ants (Solenopsis invicta) when they noticed something strange: Queen ants who had their own colonies carried two copies of the dominant variety of a particular gene--they were homozygous for the variety, that is--while those who shared a nest with other queens were heterozygous, carrying only one copy.

To see if the gene, Gp-9, was really a switch for social behavior, the researchers raised fire ants in captivity. In work published this summer (see ScienceNOW, 5 August), they relocated a homozygous queen from her nest to a nest with multiple queens. She was treated in a manner that would have made Henry VIII blush. "The heterozygous workers there cut her in pieces," says Keller, an evolutionary biologist. In their latest study Keller and Ross show that heterozygous queens transplanted to single-queen nests meet an identical fate. They also report that the gene prepares homozygous queens for the taxing task of mothering a whole hill: They weigh up to 50% more than heterozygous ones and produce eggs more rapidly, Keller says.

It's perplexing why evolution would favor a single genetic switch for something so complex, says Cornell University's H. Kern Reeve. At present, both colonies seem to be thriving. But given enough time, Reeve says, evolution should favor ants that can stretch their social structure to permit one or many queens. Because that hasn't happened yet, Reeve assumes the genetic switch is a relatively new mutation. If that's true, the dominant form of Gp-9 should strongly resemble the other, recessive variety. Keller says such tests are on the drawing board.