The Selfish Genes of Fire Ant Assassins

The idea of selfish genes, which stick around even if they do no obvious good for the individual carrying them, has some new evidence to back it up. A particularly vicious set of such genes has been found in red fire ants, which apparently kill members of their nest to ensure that one form of these genes is passed on. The finding could have a payoff, too: Isolating the proteins coded by these genes might lead to an effective pesticide against the stinging pests, which torment livestock and people in the southern United States.

The unusual genes appear to fit the profile of a so-called green-beard gene, in which a distinctive trait (for example, a hypothetical green beard) prompts the gene's carriers to help each other, thereby increasing the odds that the gene will be passed to the next generation. Real-life examples, however, have been hard to find, and some geneticists have doubted their existence.

While studying the genetics of fire ants in Georgia, evolutionary ecologists Laurent Keller of the University of Lausanne, Switzerland, and Kenneth Ross of the University of Georgia, Athens, found that in ant colonies with several queens, all the queens were heterozygous--carrying two different versions of a gene--for a group of genes called Gp-9. The researchers next found that workers and queens carrying two copies of the recessive Gp-9 genes died soon after hatching. The other extreme was lethal as well: Queens with two copies of the dominant Gp-9 genes were attacked and killed by worker ants before they could lay eggs.

Keller and Ross found that it is mostly heterozygous workers, bearing one copy each of the dominant and the recessive genes, that kill the dominant-only queens--thus ensuring that the recessive genes are passed on to the next generation. Dominant-only workers are not attacked, but neither do they lay eggs. It's a mystery what the Gp-9 proteins do. Ants with various Gp-9 combos may be flagged by a distinctive odor--when the scientists rubbed workers on the abdomen and thorax of a dominant-only queen, the workers were attacked too.

Although the green-beard hypothesis had "generally been dismissed" as too complicated to be probable, says evolutionary biologist David Haig of Harvard University, the fire ant research suggests the concept is "perhaps simpler than we thought." Keller says he hopes the finding might be put to practical use: If the scientists could isolate the signal that provokes the attack, he says, it might be used to inspire devastating civil wars in ant colonies.