Invading Ants Benefit from Shared Genes

Scientists may have discovered the secret behind the success of Argentine ants, one of America's most invasive species. The six-legged creatures actually benefit from low genetic diversity.

Far-flung Argentine ant hives share remarkably similar genetic profiles, according to researchers at the University of California, San Diego. This relatedness allows them to treat distant ants like kin. As a result, they don't engage in the usual turf wars that consume neighboring hives, and can focus on the task at hand: in this case, taking over vast swaths of the American Southwest.

The researchers report their findings in the current issue of Proceedings of the National Academy of Sciences.

Argentine ants arrived in the United States around 1891. Scientists believe they stowed away on a boat from South America and jumped ship once they reached New Orleans. Since then, they have been spreading inexorably westward. Along the way, they have attacked crops and pushed out native ant species.

In California, the ants inspire more calls to the exterminator than any other pest.

Back in their native South America, though, the ants are little more than garden variety bugs — bothersome, but not a major concern.

Hoping to discover what makes the transplanted ants so successful, the researchers traveled down to Argentina to compare the genetics and behavior of the northern and southern groups.

As it turns out, the local ants proved significantly more genetically diverse than those in California. They are also a lot less friendly to outsiders. An ant can smell how genetically related it is to other ants, and will treat unknowns accordingly. Given that genetic profiles vary over relatively short ranges in Argentina, there are lots of confrontations.

"It was surprising to go to Argentina," said to Ted Case, an author on the study. "Down there, you have ants in a one kilometer area fighting. Here in California, we have nests that don't show aggression from the border of Mexico to Northern California."

This lack of aggression over such large ranges allows transplanted ants to form "supercolonies," easily overwhelming disorganized native groups, according to Neil Tsutsui, lead author of the study. "They can find food quicker and can recruit a large force to protect it," he said.

Argentine ants have their brave stow-away forebears to thank for these cooperative colonies, the researchers said. Because all invading ants descended from a small group, they share such similar genes.

According to David Queller of Rice University, "These 'genetic bottlenecks' may aid the invasions of other colonial species."

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