How a butterfly survivor changes its spots

By mimicking the wing patterns of other butterflies that predators find distasteful, the Amazonian (right) boosts its chances of staying off the menu.

Until now, however, scientists couldn't explain how it changed its wing pattern and colouring to fit in with its less palatable neighbours.

According to research published in the journal Nature today, there are three distinct gene arrangements for the Amazonian butterfly, Heliconius numata. Which arrangement a population has depends on the appearance of the local unpalatable butterflies. And on the birds that eat them.

"In a way, both species are sharing the costs in educating the birds not to eat either species," said Melbourne University's Siu Fai (Ronald) Lee, a member of the international research team.

This explains why the Amazonian butterfly's wing pattern in French Guiana differs so noticeably from its counterparts in Peru, 2900kilometres away. While they are same species with the same DNA, both look more like the local butterflies that birds have learned to avoid eating.

Known as Mullerian mimicry, predators learn to avoid an organism with certain markings and will shun all similar-looking species. "Each locality has an optimal wing pattern," Dr Lee said. "And in each place the Heliconius numata tries to mimic the most successful form in that locality."

The scientists managed to pinpoint the region of a chromosome responsible for changing wing patterns by analysing the butterfly's genetic traits. They also located the "supergene" that allows the insects to morph into several different forms over generations.

"This supergene has evolved from one version to another simply by flipping the DNA," Dr Lee said, explaining that it was much like shuffling a pack of cards. “This is a fascinating adaptive strategy and the genetic solution is surprisingly elegant. The results show that small chromosomal changes can preserve successful gene combinations and help the species to adapt,” he said.