Wings of colour

The brilliant, intricate patterns on butterfly wings — from haunting eye spots to iridescent splashes of blue — look as if they were painted on by teams of artists. Researchers thought that a complex collection of genes might be responsible, interacting to build up the final pattern. But two studies now suggest that two genes play an outsize role in determining the wing’s lines and colours. Turning off these ‘master’ genes disrupts the canvas, dulling the colours or turning the insects monochromatic. The studies, published in Proceedings of the National Academy of Sciences, challenge the old paradigm of wing pattern development, says Bob Reed, an evolutionary developmental biologist at Cornell University, USA, and lead author of one of the papers and a co-author on the other. Understanding how wing patterns are controlled gives scientists greater insight into the evolution of traits that help the insects to avoid predation and attract mates. “The two different genes are complementary. They are painting genes specialised, in a way, for making patterns,” says Arnaud Martin, a developmental biologist at George Washington University, USA, and lead author of one of the studies.

Previous studies that used conventional genetic mapping and knockout genes showed that WntA and optix are involved in wing pattern development. The work showed that the two genes are ‘adaptive hot spots’ because they are linked to physical changes in the organism that appear to be adaptations to their environment. The researchers used the CRISPR–Cas9 technique to tinker with WntA and optix in several butterfly species. Scientists switched each of the genes on and off to demonstrate how much influence they had over what appeared on the butterflies’ wings.

The new WntA study involved seven species of butterfly, including the charismatic monarch butterfly. In most species, when WntA was switched off, colours bled, patterns faded or markings disappeared. In the monarchs, for example, deep-black contouring at the wings’ edges lightened to grey. WntA sets borders and boundaries, says Arnaud. The optix study showed that this “paintbrush gene,” as Bob calls it, has a large role in pigmentation. Previous work had suggested that it was involved in red and orange colour patterns, but it took CRISPR to finally confirm that, says Bob. He and his colleagues knocked out optix in four butterfly species. Parts of the wings — as well as other areas of the butterflies’ bodies — turned black or grey. “One went entirely jet black,” says Bob, citing an example of an individual Gulf fritillary.

Most surprising, Bob says, in the common buckeye, its wings also developed spots of bright, iridescent blue, an indication of structural change beyond pigmentation. The results indicate that optix affects colouring in a way that goes beyond simple pigmentation. Iridescence is created by certain microscopic structural features of the wing scales, so optix seems to be influencing both the pigment and the architecture of butterfly wings.