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Several swallowtail butterflies mimic other poisonous butterflies. Here are different forms of the same species; in the center lies a female Papilio polytes that does not mimic other species. Photo by Matt Wood of the University of Chicago

A 2-million-year-old ‘supergene’ helps these butterflies mimic their poisonous cousins

When it comes to survival of the fittest, female swallowtail butterflies are masters of disguise. They’ve evolved a clever way of mimicking the wing colors, shapes and patterns of their poisonous cousins.

But how? A study published Tuesday traces this trait — called supergene mimicry — to a single ancestor who lived two million years ago.

“We are pretty amazed by the age of [this] supergene mimicry. It goes way back,” said Marcus Kronforst, an evolutionary biologist at the University of Chicago and coauthor of the report in Nature Communications. “It originated before all the species we are studying even existed, back before [they] evolved.”

Mimicry is a common defense mechanism that can involve blending into surroundings or scaring off predators. For example, nightjar birds knowingly hang out in areas that match their feathers, while the mimic octopus shapeshifts into the guise of a toxic sea snake and venomous lionfish.

For years, insect lovers have known that some swallowtail butterfly species — such as Pachliopta aristolochiae — make toxins to dissuade predators. Other swallowtails, such as Papilio polytes females, are masqueraders — they lack toxins, but mimic coloration and patterns of their poisonous kin to avoid getting munched on by birds, spiders or lizards.

In 2014, Kronforst and his colleagues determined that a supergene — called doublesex — was responsible. The gene experienced a chance mutation, whereby a section of the chromosome housing the doublesex gene had randomly flipped.

“We think natural selection is a dominating force in these butterflies, and in evolution in general. And mimicry is a classic example of natural selection in action,” Kronforst said. “But really big parts of this supergene history are influenced by chance.”

The Pachliopta aristolochiae and the Papilio polytes are two types of swallowtail butterflies--one toxic, the other harmless. P. polytes photo by Urs Achermann and P. aristolochiae photo by Zleng/via Flickr

The Pachliopta aristolochiae and the Papilio polytes are two types of swallowtail butterflies–one toxic, the other harmless. P. polytes photo by Urs Achermann and P. aristolochiae photo by Zleng/via Flickr

In their new study, the team traced the lineage of this mutation in five species and several variations of female swallowtails. As with many mutations, one would think that the butterflies with the out-of-order genes would soon die off. But Kronforst said natural selection maintained the flipped gene because it gave rise to the butterflies ability to mimic toxic butterflies.

The team found that the supergene in its current form originated in a single species two million years ago in Asia. It’s called supergene mimicry because this single origin filtered into several species of swallowtail butterflies.Since butterflies go through about eight generations a year, that’s millions and millions of generations to hone this trait, or in some cases, lose it if another mutation comes along. Today, the distribution of these mimics spans butterflies from central Asia to eastern Australia.

But not all Papilio polytes females participate in this copycat game. One would expect the butterflies who don’t mimic toxic species to be gobbled by predators, given they appear harmless. But a 2005 study in Japan found, somewhat puzzlingly, that the non-mimicking butterflies have longer lifespans, if they survive predation, relative to those who mimic.

“Those that do not mimic toxic butterflies are flying around telling [predators] they taste delicious,” Kronforst said.

This suggests while this swallowtail supergene mutation was advantageous in one sense, it may have also brought along hitchhiker traits that are detrimental and cause shorter lifespans in masquerading butterflies.

In future studies, Kronforst and his team aim to further pinpoint the origin of mimicry, and why these vulnerable, non-mimicking butterflies are still able to compete with their sisters that have the ability to mimic.

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