Genes. Spots and Butterflies

How different species can evolve the same colors and pattern has always puzzled biologists. Now, scientists at Cambridge University have found "hot spots" in the butterflies’ genes that might one of the most extraordinary examples of mimicry in the natural world. A gene is a hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism. Genes undergo mutation when their DNA sequence changes.

Heliconius, or passion vine butterflies, live in the American continents from southern United States to southern South America. Although they cannot interbreed two types from two different physical locations (H. melpomene and H. erato) have evolved to mimic one another perfectly.

These delicate butterflies have splashes of red and yellow on their black wings, signaling to birds that they contain toxins and are extremely bad to eat. The butterflies are identical in colors and patterns as a visible warning to predators.

Scientists have studied these butterflies for over a century as a classic case of parallel evolution in action, but only now is modern sequencing technology unlocking the underlying genetics. Parallel evolution have other examples such as old and new world porcupines, leaf patterns, and gliding mammals such as the flying lemurs and squirrels.

The Cambridge team of researchers from United Kingdom and US universities has been searching for the genes responsible for the butterflies' wing patterns and the answer to the question of whether the same genes in two different species are responsible for the parallel evolution of colors and patterns.

According to Dr Chris Jiggins of the Department of Zoology at the University of Cambridge, one of the authors of the study: "The mimicry is remarkable. The two species that we study - erato and melpomene - are quite distantly related, yet you can't tell them apart until you get them in your hand. The similarity is incredible even down to the spots on the body and the minute details of the wing pattern."

That the two species have evolved to look exactly the same is due to predation by birds. "The birds will try anything that looks different in the hope that it's good, so they learn that certain wing patterns are unpalatable and avoid them, but anything that deviates slightly from what they've experienced before is more likely to be attacked," he explains.

For years, scientists have pondered whether when different species evolve to look the same, they share a common genetic mechanism.

Because there are thousands of genes in the butterflies' genome, most scientists felt it was unlikely that the same genes should be involved. But the results of this study suggest that this is, in fact, the case.

The new results (published today in two parallel papers in the journal PLoS Genetics) show that the regions of the genome associated with the wing patterns are very small or genetic "hot spots".

"This tells us something about the limitations on evolution, and how predictable it is. Our results imply that despite the many thousands of genes in the genome there are only one or two that are useful for changing this color pattern. It seems like evolution might be concentrated in quite small regions of the genome, or hot spots, while the rest of it does not change very much," says Jiggins.

The next stage of the research is to look at other traits, such as behavior, because the butterflies have preferences for particular colors and use wing patterns to select mates. "It seems the same regions of the genome control this behavior as well as the wing pattern. We'd like to understand this," he says.

For further information see: http://www.admin.cam.ac.uk/news/dp/2010020405 or http://en.wikipedia.org/wiki/Parallel_evolution