When the butterfly emerged from its pupa, Robert Reed was stunned. It was a Gulf fritillary—a bright-orange species with a few tigerlike stripes. But this butterfly had no trace of orange anywhere. It was entirely black and silver. “It was the most heavy-metal butterfly I’ve ever seen,” Reed says. “It was amazing to see that thing crawl out of the pupa.”
Reed’s team at Cornell University had created the metal butterfly by deleting just one of its genes, using the revolutionary gene-editing technique known as CRISPR. And by performing the same feat across several butterfly species, the team showed that this one gene, known as optix, controls all kinds of butterfly patterns. Red becomes black. Matte becomes shiny. Another gene, known as WntA, produces even wilder variations when it’s deleted. Eyespots disappear. Boundaries shift. Stripes blur.
These experiments prove what earlier studies had suggested—that optix and WntA are “paintbrush genes,” says Anyi Mazo-Vargas, one of Reed’s students. “Wherever you put them, you’ll have a pattern.”
Biologists have long been smitten by butterflies, and not just for their pretty colors. These insects are perfect subjects for addressing two of the most fundamental questions in the study of evolution. First, where do new things come from? Butterflies all evolved from a moth ancestor, so how did a presumably dull-winged insect give rise to a kaleidoscopic dynasty of some 18,000 species, each with a distinctive pattern of colors and shapes plastered on its wings? Also, what are the genes behind these patterns? How did a limited set of DNA come to produce patterns of such astonishing diversity and often-baffling complexity?