Ants manage incest without inbreeding

Unorthodox family structure may have helped insect spread

An odd reproductive biology lets longhorn crazy ants mate with their siblings without inbreeding — and it also turns out to be useful for world domination.

That power has probably helped Paratrechina longicornis become one of the most widespread invasive ants in the tropics, says evolutionary biologist Morgan Pearcy of the Free University of Brussels. The tiny ants with long antennae, nicknamed crazy ants because they dash along erratically instead of following foraging trails, now occupy so much of the tropics that scientists haven’t figured out where they originated.

In lab tests, queens produce some daughters that are clones of themselves and that will also become queens. The queen’s sons — very oddly — turn out to be genetically identical to the queen’s mate. Thus a queen’s son can mate with her daughter in a pairing that’s genetically equivalent to a pairing of nonsiblings. The next generation thus does not suffer the loss of genetic diversity that comes from brother-sister inbreeding.

There is still some normal sexual gene shuffling among longhorn crazy ants, however. When the queen produces daughters that will grow up to be workers, they turn out to carry the usual, sexually blended mix of mom and dad’s genes, Pearcy and his colleagues report in a paper to be posted online the week of January 31 in the Proceedings of the Royal Society B.

The novelty of the report is in linking the biology to invasive success, comments Jürgen Heinze of the University of Regensburg in Germany. Such double-clone reproduction has been detected only twice before; the first case known was in the little fire ant, not the species bedeviling the southern United States but an invader all the same.

Longhorn crazy ants don’t bite or sting people, but they forage far and fast and can track pathogens from trash to table. “They are in the food; they are in the cabinets. When you order a pizza, they can be in the pizza,” Pearcy says. Hospitals in particular dread them because they have become a major source of infection spread.

He and his colleagues worked with ants collected in Bangkok and brought back to the lab to reproduce in single-queen colonies. Outdoors, colonies typically have multiple queens, sometimes hundreds, so that just checking genetic markers in a multiple-queen colony without knowing which eggs came from which mother doesn’t reveal the unusual clonal descent of the reproductive females and males.

Uncovering the system was “a complete accident,” says coauthor Mike Goodisman of Georgia Tech in Atlanta. What was supposed to be a routine genetic analysis as part of another project turned out to have weird results.

Just how that clonal reproduction works, especially on the male side, remains to be explored. Pearcy says that when the news of dual-cloning broke for the little fire ant in 2005, researchers suspected that dad’s genes would kick out the mom’s in fertilized eggs. Now, however, he says he’s leaning toward the hypothesis that some trait of the queens lets them produce “empty eggs” with none of the queen’s DNA in them at all. So when sperm reaches them, they become clones of dad.

CRAZY INVADERS Longhorn crazy ants (shown with plump white mealybugs that ants tend as a source of sugary secretions) have rare reproductive biology that lets queens and males pass along their genes clonally. Alton N. Sparks Jr./University of Georgia,

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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