Supergene controls butterfly mimicry

A very special "transformer" butterfly species relies on just one supergene to mimic seven different toxic butterfly species (video)

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Pictured are Heliconius numata (top) and its co-mimic Melinaea mneme (bottom) (French Guiana).

Image: Mathieu Chouteau (one time use with this story). DOI: 10.1038/nature10341.

The colorful Amazonian butterfly, Heliconius numata, has seven different wing colour patterns, each of which mimics the wing patterns of seven different species in the genusMelinaea. The reason for this mimicry is obvious: biological warfare. Melinaea caterpillars eat plants in the deadly nightshade family and sequester their host plant's poisons in their tissues. These accumulated poisons make Melinaea butterflies toxic to birds, who learn to associate specific wing patterns with a sickening meal. By adopting the same warning patterns as their toxic Melinaea butterfly neighbors, H. numata also benefit.

But H. numata are also toxic to birds, although they rely on a different poison: their caterpillars feed on toxic plants in the passion flower family. This evolutionary phenomenon where two or more harmful species -- such as H. numata and Melinaea -- mimic each other's warning signals is known as Müllerian mimicry.

Melinaea (left) and Heliconius numata mimetic forms. 

Image: Mathieu Joron (one time use with this story). DOI: 10.1038/nature10341.

But how does one species show so many distinct wing colour patterns? Mathieu Joron, an ecologist at the Muséum National d'Histoire Naturelle, Paris, led a team of French and British scientists who think they've found the secret.

The team found that variations in H. numata's wing patterns are controlled by a "supergene". A supergene is a small region on a chromosome comprised of a group of neighboring genes that are locked together into a functionally-related unit. A supergene preserves favourable gene combinations in the population, whilst preventing non-mimetic gene combinations from arising. Although inherited as a unit, individual genes within the supergene tend to act independently of each other to regulate different elements of H. numata's wing pattern.

"We were blown away by what we found," said Dr Joron in a press release. "These butterflies are the 'transformers' of the insect world. But instead of being able to turn from a car into a robot with the flick of switch, a single genetic switch allows these insects to morph into several different mimetic forms -- it is amazing and the stuff of science fiction. Now we are starting to understand how this switch can have such a pervasive effect."

Supergenes are not uncommon: they regulate a wide range of natural variation, from the shape of primrose flowers to the colour and pattern of snail shells.

"This supergene region not only allows insects to mimic each other, as inHeliconius, but also to mimic the soot blackened background of the industrial revolution -- it's a gene that really packs an evolutionary punch," remarked co-author Richard ffrench-Constant, a biosciences professor at the University of Exeter in the UK.

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