From a colony of wild Cape honeybees, the researchers sequenced the genomes of a number of individuals, half of whom were the parasites’ clones and half of whom were not. “We compared these two groups,” says Denise Aumer of the Martin-Luther-Universität Halle-Wittenberg, in Germany, a biologist and a lead author of the paper. “We found only a single locus in the whole genome where they differed significantly.” After decades of mystery, seeing this difference was striking, says Eckart Stolle, Aumer’s colleague and co–lead author: “It was super cool to find a strong signal like this, because you wouldn’t necessarily expect it.”

At that place, they found a one-letter difference between the bees’ genetic codes. Looking closer at the gene, the researchers determined that it codes for a little-studied protein lodged in the membrane of cells, which may be involved in trafficking substances in and out. They also discovered that for bees to switch into this parasitic mode, they must carry a certain version of a second gene. On its own, this gene is innocent of any wrongdoing, unless it winds up in a bee with the one-letter change. Other factors in the bee’s environment, like a weakening of the queen’s hormonal control or the changed bee’s arrival in a fresh host colony, must also align. But with these two genes, a bee is capable of the switch.