Butterfly on a leaf (photo by Melissa Hanes)

How the butterfly got its spots

By Bill Steele
periodiCALS, Vol. 6, Issue 2, 2016

The wings of many butterflies, like the common buckeye, sport large round “eyespots” that help them attract mates and deflect would-be predators. Associate professor of ecology and evolutionary biology Robert Reed and postdoctoral researcher Linlin Zhang discovered that tweaking just one or two genes can erase, enlarge and multiply the round markings. It’s a major genetic clue to understanding how butterfly wing patterns have evolved and perhaps to how color patterns and shapes have evolved in other species. 

The experiment was made possible by recently developed genome editing technology that can target and snip out a single section of DNA in a living cell. The researchers used it to cut out the gene known as spalt. The butterfly that emerged lacked eyespots altogether. In another experiment, they removed a gene known as distal-less. That butterfly’s wings had more eyespots, which were larger, and showed other changes in wing design as well.

How did the butterfly get its spots? By co-opting genes that had other functions in butterfly development. For example, the distal-less gene revealed itself to be a jack-of-all-trades that plays roles in shaping several parts of the body. Deleting it not only caused the butterfly to have extra eyespots but to have shorter legs and antennae as well. According to Reed, this role in appendage development is known as the ancestral function of the gene.

“People suspected these genes had something to do with wing patterns, but nobody proved it,” Reed said. “It probably takes dozens or hundreds of genes to make an eyespot, so it was remarkable to find that only one or two genes are required to add or subtract these complex patterns. It is a beautiful demonstration of how animals are assembled as modules, much like a model kit.”

Butterfly wing patterns are of special interest to evolutionary biologists because they provide an easily accessible model of how natural selection chooses from many possible variations. The patterns on wings are key to their interactions with other organisms, from attracting mates to warning birds of poison within—or tricking birds into thinking they are poisonous by mimicking the wing markings of toxic kin. Reed and Zhang’s discovery is an insight into the genetics behind the tremendous variation in butterfly wing patterning. 

“Variation is the raw material of evolution,” Reed said. “And we are just beginning to tap gene editing’s great potential for understanding how this variation originates.”