To thrive, ant colonies rely on everyone pulling their weight. For raider ants, this means diligent scouts track down other nests, then direct hundreds of savage foragers to attack. They return with pincers gripping dead young ants to feed the settlement. The colony thrives. But raider ants are among about 50 species plagued by impostors: parasitic ants that resemble queens. They greedily eat the colony’s food, but shirk their own foraging duties, and can only hatch more parasites instead of workers when they reproduce. How the fake queens emerge has long puzzled scientists. “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 mutates rapidly, between a single generation of raider ants, is likely responsible for the royal impostors. The discovery arose from a surprising observation in a lab at Rockefeller University, where e Waring Trible and his graduate advisor, Daniel Kronauer, studied colonies of raider ants. “These weird, mutant queens just showed up,” said Dr. Trible, now at Harvard, who led the study. Isolated from the rest of the colony in a petri dish, it was plain to see: Several of the ants had wings. It’s a typical trait for queens in many species, but it was odd because raider ants don’t normally have wings, or queens.”Seeing these winged females was very shocking, very striking, right away,” Dr. Trible said. “I immediately thought it was something genetic.” He set about sorting through the 10,000-ant colony. His needle-in-a-haystack search found a total of 14 impostor queens, which he then let reproduce. Their progeny were always the winged parasites. Dr. Trible and his colleagues devoted years to studying the mutants and trying to figure out their origin. Another geneticist, Sean McKenzie, compiled the regular ants’ who , while Dr. Trible analyzed the mutants’ genome. Comparing the gen omes let Dr. Trible see where the regular and mutant ants differed. In both types 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. Regular ants have one copy of the supergene and another version that is mutated and recessive. But the impostor queens have two copies of the mutated supergene. “Such a mutation in humans would likely kill you,” Dr. Trible said. “The chromosome is so degenerated. It’s a really nasty mutation.” But for these ants, what doesn’t kill them makes them grow wings. It is “overwhelmingly likely” that the supergene controls the growth of impostor queens, but more targeted research is needed to confirm the finding, Dr. Trible said. For decades, the common thinking was that complex traits or behaviors, such as parasitism or the number of queens in a colony has, would be determined by the combined influence of many genes, said Dr. Ross, who discovered the first superge ne in ants and wasn’t involved in the new study. But scientists now know, he said, that “there may be lots of genes, but they tend to be locked up in supergenes.” That means complex traits 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 have been tied to social behavior. Why ants develop into adults of a certain caste is an open question. This supergene appears to have removed the parasites’ ability to be workers and allows them to play a role not typically found in the raider species.”Isolating a caste-based Mutants are a really powerful tool,” Dr. Trible said. Mutants, he said, can be an important window into the otherwise black boxes of evolution. This is also the first time parasitic ants of the same species as their host have been observed. While there are many parasitic ants, until now they have been a separate species from their host, perhaps evolving slowly over time from the original species to become their parasite. The new study shows that a parasitic mutation can occur within a species and across a single generation, upending previous hypotheses that take such transitions of years or longer. “It was a single step, but it wasn’t trivial,” Dr. Ross said.