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'Asexual' Ants May Be Having Sex After All

Even ants can't completely swear off sex. A new survey of a Latin American species famed for being one of the world's few asexual ants has uncovered a surprising find: secret nooky. And that could explain why the insect has managed to survive for so long.

The species in question, Mycocepurus smithii, is a fungus-harvesting ant that, like a farmer, sows fungi for food. Researchers didn't suspect anything unusual about it until a 2005 study reported that colonies of M. smithii in Puerto Rico seemed to be missing something important: males. Two 2009 studies discovered similar societies made up entirely of female workers and one or more queens. These matriarchs were reproductively mature, but their spermatheca, chambers that store sperm postmating, remained bone dry, says Christian Rabeling, an evolutionary biologist at Harvard University and a co-author of one of the 2009 studies. "When I did the first dissections, ... I suspected that I made a mistake," he says.

Rabeling's surprise was understandable. With males a no-show, queens would have to reproduce asexually, somehow turning eggs into larvae without fertilization from sperm. Of the more than 10,000 known species of ants, researchers have identified only a handful that could boast similar skills. And most of those seemed to mix sexual and asexual reproduction.

Celibate animal species are rare for a reason, Rabeling says. Because asexual populations can't mix and match genes through mating, they often lack the genetic diversity to respond to unexpected challenges like disease.

In the new study, Rabeling and his colleagues looked wider, sampling genetic patterns in 234 M. smithii colonies from 39 different populations throughout Latin America. Like the Puerto Rico colonies, most of these ants lived like nuns in a convent. In 35 of the populations, every ant appeared to be an identical clone of its mother.

But in the other four populations Rabeling got a surprise, he says. More like a sexual revolution. In these groups, which live along the Amazon River and one of its tributaries in Brazil, worker ants toted an eclectic mix of genes, much like you'd see in any sexual society. He and colleagues again dissected queens, and this time they found sperm, a sure sign that these few matriarchs had been breaking their vows.

Such indecorous affairs are a big testament to the power of sex, Rabeling says. "Without the exchange of genetic material, no [species] survives over the long term." Without at least some gene swapping, most animals will go extinct over a few million years or less, he suspects. The Amazonian populations, because they sit at the center of the ant's range, seem to be assembly lines for making one thing: diverse genetic codes. These lines churn out unique queens, which then spread out to colonize other areas, ensuring that the species as a whole is genetically rejuvenated, the team reports online today in the Proceedings of the National Academy of Sciences.

It's not shocking to see sly hookups holding together a largely asexual species, says Julien Foucaud, an evolutionary biologist at CNRS in Gif-sur-Yvette. But he cautions against overlooking the perks of asexuality, too. Sex adds new genes to flailing communities, but it can also break apart sets of genes that work well together. "Sometimes it's bad to be sexual," he says. Foucaud suspects that as scientists begin to look more closely at known ant species, they'll find a lot more chastity belts than expected.

Scientists are finding not just more partially sexless ant species but an enormous array of reproductive behaviors, says Jürgen Heinze, an evolutionary biologist at the University of Regensburg in Germany: "This is really a boom of different strategies." Rabeling's study isn't the first to spot islands of sexual ants hidden among celibate queens, but it is the most complete, Heinze says.

Rabeling admits that much of the ins and outs of how the sexual colonies give the species life support still aren't clear. Researchers, for instance, still haven't found a single M. smithii male in the sexual islands, making these ants, perhaps, true Amazons. Like the female-ruled tribe of mythology, the sexual queens are probably using, and then losing, the studs. "They don't do anything in the society," he says. "After the mating, ... they are physically kicked out of the nests."