How Do Butterflies Mimic The Wings Of Toxic Species? Biologists Discover The Supergene

By Ben Wolford on March 5, 2014 1:18 PM EST

Down the center column, non-toxic butterflies display the markings of toxic ones. The butterflies to the left and right belong to the same species but do not bear the markings. A single gene determines which can mimic toxic butterflies and which cannot. (University of Chicago, Marcus Kronforst)

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Down the center column, non-toxic butterflies display the markings of toxic ones. The butterflies to the left and right belong to the same species but do not bear the markings. A single gene determines which can mimic toxic butterflies and which cannot. (University of Chicago, Marcus Kronforst)

Some nontoxic butterflies are masters of disguise, sprouting wings that appear just like the wings of toxic species. But other nontoxic butterflies have no such ability, living their lives as they are: vulnerable, nontoxic targets for hungry predators.

Evolutionary biologists have always been fascinated by this extraordinary trait, but they could never find the source of the power. They could never find the genes — collectively known as the "supergene" — that determined which butterflies could mimic their poisonous counterparts and which could not. On Wednesday, researchers at the University of Chicago announced they found the DNA responsible, but that it's not a cluster of genes at all. In fact, it's just a single one, discovered in an unlikely place.

"Conventional wisdom says that it should be multiple genes working together to control the whole wing pattern of a butterfly," said Marcus Kronforst, ecologist at the University of Chicago and senior author of the study, in a news release. Conventional wisdom had been a powerful force, with teams of scientists around the world racing to find the DNA sequences responsible for wing designs. Through subtle mutations that make them appear dangerous to predators, some butterflies and other species of insects have evolved these abilities known as mimicry. To discover the genes responsible would be to discover the source of one of nature's most brilliant designs: a genetic costume.

These findings potentially solve the riddle. Using genetic mapping, Kronforst and his colleagues theoretically mated butterflies of the same Asian species Papilio polytes with different wing patterns to produce 500 offspring. Through this process, they identified five possible genes involved in determining which butterflies are capable of mimicry and which aren't. Then they picked 30 — half mimetic, half non-mimetic — and sequenced their genomes, searching for correlations.

To their surprise, they found one in the gene called doublesex, a single gene responsible for determining sex. "This single gene that controls sexual differentiation has been co-opted to do a totally new job," Kronforst said. It kind of makes sense, given that only some females can mimic toxic species and no males can. Yet they had always assumed it was multiple genes working in tandem to create something so complex as wing design.

While they say they've identified the gene, they still have no idea how it works. "We've illustrated the genetic basis of female-limited mimicry in these butterflies," said study co-author Wei Zhang, postdoctoral fellow at the University of Chicago, in a statement. "But this is just the first step. How doublesex became involved in this process is still uncertain, and requires further study."

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