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Fire Ants Spurn Sex to Protect Genes, Study Says

Stefan Lovgren
for National Geographic News
June 29, 2005
 
Queens and males of an invasive ant species known as the little fire ant reproduce by cloning themselves, thereby keeping the gene pools of females and males separate, a new study reveals. Only sterile worker ants of the species are produced by sexual reproduction.

Clonal or asexual reproduction is not unique to little fire ants. Some lizard species, for example, produce female offspring clonally from adult females.

"What is unique about this ant is that not only are females produced clonally by females, but males are also produced clonally by males," said David Queller, an evolutionary biologist at Rice University in Houston, Texas, who was not invovled with the study.

The findings are reported in tomorrow's issue of the journal Nature.

Sexual Conflict

Sexual reproduction is the predominant force in the propagation of animals and many plants. However, it can lead to conflicts between the sexes. Characteristics that enhance the reproductive success of one sex can reduce the success of the other: Asexual females, for example, do not have to produce males to ensure future reproduction.

In most ants, females are typically produced by sexual reproduction, while males develop from unfertilized eggs.

But the small fire ant (Wasmannia auropunctata), which is considered an invasive pest in tropical habitats, is different, scientists found.

They have determined that queens and males each produce offspring with genes identical to their own, except when reproducing the sterile worker ants.

"In the evolutionary battle of opposing sexes, queens transfer all their genes to the reproductive females, and males thwart queens by eliminating the female genome during sexual brood development," said Denis Fournier at the University Libre de Bruxelles (Brussels) in Belgium. Fournier was the lead researcher of the study.

Counterattack

The ant's unusual reproductive strategy probably arose because queens sought to protect their own genes by clonal reproduction, using sexual reproduction only to produce workers.

"It's a selfish strategy initiated by females [in which] queens transmit 100 percent of their genome," Fournier said.

Instead of simply being cut out as evolutionary actors, however, male fire ants hit back. They also reproduce themselves clonally and pursue their own genetic lineage. Researchers suggest that male genomes are eliminating female genomes in fertilized eggs, making the eggs functional clones of the males.

"Males would normally die off, but in this system they were retained only so that diverse sexual workers could be produced," said Queller, who wrote an accompanying commentary for the Nature study. "This apparently gave males both the time and the means to evolve a counterattack—converting some of the workers into males."

Scientists hypothesize that genetic diversity in an ant colony is important for defending against parasites, as well as adapting to changes in environmental conditions.

"From an evolutionary point of view, this discovery concretely indicates that genetic variability is the major advantage of sexual reproduction and illustrates the extraordinary imagination of nature—or of the male ants—to counteract this female strategy," Fournier said.

While males and females remain affiliated by the mutual production of workers, the sexual conflict between the two could ultimately lead to each sex becoming its own species.

"I think if further work confirms that there is little or no gene exchange between males and females, we really might consider them to be separate species," Queller said. "It is odd that they are completely dependent upon each other. But then so are many other mutualistic pairs of species."

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