In order to thrive, ant colonies depend on everyone doing their part. For predatory ants, this means diligent scouts will seek out other nests and then direct hundreds of wild foragers to attack. They return with tongs grabbing dead young ants to feed the settlement. Clones are produced. The colony thrives.
But predatory ants are among about 50 species plagued by cheaters: parasitic ants that resemble queens. They greedily eat the colony’s food, but shy away from their own foraging duties and can only hatch more parasites instead of workers as they breed. How the false queens come about has long puzzled scientists.
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“It’s a real mystery how these things arrive,” said Ken Ross, an evolutionary geneticist at the University of Georgia.
A study published Tuesday in Current Biology offers a solution. A “supergene” that rapidly mutates between a single generation of predatory ants is likely responsible for the royal imposters. The discovery arose from a startling observation in a Rockefeller University lab, where Waring Trible and his graduate advisor Daniel Kronauer were studying colonies of predatory ants.
“These weird mutant queens just showed up,” said Trible, now at Harvard, who led the study. Isolated from the rest of the colony in a petri dish, it was clearly visible: some of the ants had wings.
It’s a common trait for queens of many species, but it was strange because predatory ants don’t usually have wings or queens.
“Seeing these winged females was very shocking, very impressive, right away,” Trible said. “I immediately thought it was something genetic.”
He set about sorting the 10,000 ant colony. His needle-in-a-haystack search found a total of 14 imposter queens, which he then had multiplied. Their offspring were always the winged parasites.
Trible and his colleagues devoted years to studying mutants and trying to discover their origins. Another geneticist, Sean McKenzie, assembled the entire genome of the regular ants, while Trible analyzed the genome of the mutants. By comparing the genomes, Trible was able to see where the regular and mutant ants differed.
In both species of ants, one chromosome carries a collection of genes that are all inherited together and can control important traits and functions known as a supergene. Normal ants have one copy of the supergene and another version that is mutated and recessive. But the cheat queens have two copies of the mutated supergene.
“Having a mutation like that in humans would probably kill you,” Trible said. “The chromosome is so degenerate. It’s a really evil mutation.”
But for these ants, what doesn’t kill them gives them wings.
It’s “overwhelmingly likely” that the supergene controls the growth of cheaters, but more focused research is needed to confirm the finding, Trible said.
For decades, the conventional wisdom was that complex traits or behaviors, such as parasitism or the number of queens in a colony, were determined by the combined influence of many genes, said Ross, who discovered and was the first supergene in ants. t involved in the new study. But scientists now know, he said, that “there can be many genes, but they tend to be trapped in supergenes.” That means complex features can depend on just a single factor, “which is just amazing,” he said.
While scientists have identified at least five other ant supergenes, this would be the first to control caste, or whether ants become workers, foragers, or queens; the others became attached to social behavior. Why ants evolve into adults of a particular caste is an open question. This supergene appears to have robbed the Parasites of their ability to be workers, allowing them to play a role not typically found in the Raider species.
“Isolating a caste-based mutant is a really powerful tool,” Trible said. Mutants, he said, can be an important window into evolution’s otherwise black boxes.
This is also the first time parasitic ants of the same species as their host have been observed. Although there are many parasitic ants, until now they have been a separate species from their host, perhaps slowly evolving from the original species into their parasite over time. The new study shows that a parasitic mutation can occur within a species and across a single generation, reversing previous hypotheses that such transitions take thousands of years or longer.
“It was a single step, but it wasn’t trivial,” Ross said.
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