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Lazy worker ants sprout queenlike wings and sponge off other workers

Among the predatory ants known as clonal raiders, worker ants called scouts track down the nests of other ant species, then recruit more workers to help steal that species’ young to be meals for their own colony (first video, below). But in at least one colony, those other workers have sprouted wings like queen ants and don’t budge from the comforts of their nest (second video, below). That’s because a genetic mutation has turned them into parasitic slackers that do little more than lay eggs and consume the food gathered by others, researchers reported last week in Current Biology. These newly discovered parasites may be an example of how mutations in “supergenes,” or clusters of genes that get inherited together, can kick evolution into warp speed.

Buck Trible
Buck Trible

“This paper provides the clearest evidence to date that a complex set of traits can emerge in a single mutational step,” says Jessica Purcell, an evolutionary biologist at the University of California, Riverside (UCR), who was not involved with the work. That such genetic changes could lead to rapid evolution had been proposed more than a decade ago, but there was no good evidence for the idea, adds Christian Rabeling, an evolutionary biologist at the University of Hohenheim who was also not involved with the new study. “Now they show that something like that can actually happen.”

Clonal raiders (Ooceraea biroi) are stocky ants about 1 millimeter long that originated in Bangladesh and have since become an invasive pest in China, India, and tropical and subtropical islands around the world. Whereas most ant species have a queen that lays eggs while workers gather food for her and the colony’s young, as well as themselves, clonal raiders lack queens. Instead, workers lay eggs that develop as more workers, clones of themselves. Clonal raiders rely heavily on a sense of smell to cooperate as they seek out other ant nests to raid, and scientists have found them to be useful subjects for studies of social insects.

In 2015, researchers at Rockefeller University noticed that a few workers in a clonal raider colony collected from Okinawa, Japan, had wings—just like queen ants typically do—for the first few days of their adulthood. Moreover, their young also developed wings, indicating the structures arose from a genetic mutation that could be passed on to the next generation. But the wings weren’t the only change caused by the mutation, and understanding its full impact proved to be “a great detective story,” says Alan Brelsford, a UCR evolutionary geneticist who was not involved with the work.

Over the next 7 years, entomologist Waring “Buck” Trible, the new study’s lead author, tested and observed the winged clonal raiders and found they laid more eggs than their nestmates and were much less likely to leave the nest to scout out or raid other ants’ nests. These aberrant ants are a perfect example of an ant parasite, Trible, now at Harvard University, and his colleagues reported in Current Biology.

Parasitic ants are not uncommon. About 400 species of ants live unobtrusively and unharmed inside the nests of other ants—typically of a different species—depending on the workers there to keep them and their young safe and well-fed. But most of these ants have been interlopers for millions of years and likely took many thousands of years to become such successful guests, making it hard to figure out the steps involved. In contrast, the transition in this single clonal raider colony only took one generation, making it much easier to study. “It provides an unprecedented view of the conditions very, very soon after a new mutation has appeared,” Purcell says.

Trible attributes this rapid transformation to mutations in a supergene that contains genes coding for several traits that determine what the workers look like and how they act. But because all the ants are clones, his team can’t do the usual experiment of breeding individuals with and without the mutation to confirm this DNA is the cause. However, Trible’s team found that most of the genetic differences between the ant parasites and its host relatives are in that supergene. “It shows that a change in a single region of the genome can shift where an ant sits on the spectrum from workerlike to queenlike,” Brelsford says.

Trible thinks many of today’s ant social parasites arose this way and that only through time did the parasite become a separate species from its host. But Rabeling is not so sure. Among ants, social parasitism has evolved about 91 times, he points out. “I would not assume that all social parasites evolved along the same mechanism.”

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