By looking at blue butterflies, UC Berkeley researchers found that certain structures affected by a specific gene in butterflies’ wings determine their color.

The more exotic colors of butterflies’ wings, including blues and purples, are generated by small nanostructures rather than pigments. The bottom of each scale of a butterfly’s wing has a thin film on it that both reflects and refracts light. This leads to an iridescent effect, like that of oil on water or a soap bubble.

The study found that brown butterflies’ wings and blue butterflies’ wings had a major difference.

“The bottom part of the scale … is much thicker in blue butterflies,” said Rachel Thayer, UC Berkeley graduate student and first author of the study. “I looked at scale structures in a bunch of related species and found that all the species had structural colors. They had different thicknesses, so there were magenta-colored thin films, blue, green, purple. … Evolving thickness has been important for butterflies to achieve different color effects on their wings.”

Thayer realized this by looking at selectively bred buckeye butterflies and comparing them to wild buckeyes.

While these butterflies are normally brown, butterfly breeder Edith Smith saw a hint of blue in some of their wings and decided to selectively breed the ones with hints of blue in their wings.

“I was more shocked than anyone, I think, when their entire wings turned a metallic blue (some were metallic green, some were metallic purple, and about five percent metallic black),” Smith said in an email. “I saw small changes within two months. It took nine months or so for them to be all blue.”

Thayer compared the lamina — a thin layer of tissue — from Smith’s blue butterflies to the wild brown ones, coming to the conclusion that the butterflies evolved into a blue color through the increase in lamina thickness.

Additionally, Thayer studied butterflies from which Cornell University researchers had removed a gene called optix. Thayer found that deleting this gene led to blue wings in the same way that Smith’s artificial selection did.

“We showed in this paper if you experimentally knock off the gene, it can change the structures,” Thayer said. “When (Smith) bred the bluest butterflies, she’s selecting particular genes. … We have some experiments involving crosses and genetic mapping between the blue and wild brown butterflies.”

Peter Oboyski, executive director of UC Berkeley’s Essig Museum of Entomology, said he believes this study is interesting both in a theoretical way and in an applied way. Theoretically, this study addresses how diversity is generated, which is important as a butterfly’s color is used for camouflage and mate recognition, according to Oboyski. He added that it was historically thought that blue was the only structural color; the other colors were believed to be pigments. This study, however, demonstrated that there is a vast array of structural colors.

“One of the biggest takeaways for me is there is a single simple underlying process that can lead to an amazing amount of diversity in butterflies,” Oboyski said.