The Postman butterfly, a brightly colored favorite of collectors and scientists since its discovery in the Victorian era, tastes bad -- very bad. Predators who have bitten into one shy away from future contact because of the foul aftertaste. The butterflies have taken advantage of this trait by developing distinctive black and red wing coloration that quickly warns predators to stay away.
An international collaboration of scientists has now sequenced the genome of the Postman butterfly -- more formally known as Heliconius melpomene -- and shown that this unusual coloring has been passed among related species by hybridization, a crossbreeding among species that is rarely found in the wild because it usually makes the offspring less likely to survive. The coloring is then retained in the other species by a phenomenon known as introgression, in which the hybrid breeds with a parent species, transferring the crucial genes.
Heliconius, with 43 species and hundreds of races, is a widely studied insect because it is very useful for studying adaptive radiation and development of new species. Caterpillars of H. melpomene feast on passion fruit vines in the Peruvian Amazon.
To learn more about the species, a collaboration of more than 70 researchers at nine institutions around the world decided to sequence its genome -- the long coil of DNA that serves as the blueprint for the butterfly. The growing availability of low-cost sequencing techniques made it possible for the group, called the Heliconius Genome Consortium, to fund the research by diverting small amounts of money from their other grants. Their results were published Thursday in the journal Nature.
The team found that the distinctive wing patterns were produced by sets of genes that were shared among the three different species of Heliconius they studied. “What we discovered is that one butterfly species can gain its protective color pattern genes ready-made from a different species by hybridizing with it -- a much faster process than having to evolve one’s color patterns from scratch,” said co-author Kanchon Dasmahapatra of University College London.
“This project really changes how we think about adaptation in general,” said co-author Marcus Kronforst of Harvard University. “Evolutionary biologists often wonder whether different species use the same genes to generate similar traits, like the mimetic wing patterns of Heliconius butterflies. This study shows us that sometimes different species not only use the same genes, but the same exact stretches of DNA, which they pass around by hybridization.”
Heliconius species, which are active mostly by day, were known to have excellent visual perception to help them chase food, while other butterflies that are active at night are thought to have better taste and smell. Lepidopterists had assumed that the species active in the daytime would have less developed olfactory and taste nerves. But the new sequencing project found that Heliconius also has a rich variety of genes for both smell and taste and are readily able to use them in hunting for nutrition.