Just 2 genes give butterfly wings their stripes and color

A pair of master genes control the complex traits in butterfly wings—one for colors and iridescence and the other for stripe patterns, two new papers suggest.

“It seems like a small number of genes disproportionately drive evolution over and over again.”

In the first paper, scientists describe using CRISPR-Cas9 gene editing technology to “break” the gene, after which the butterflies wings became black and white.

Similarly, the researchers’ second paper shows that when the WntA gene is cut out with CRISPR-Cas9, stripe patterns disappear.

Both papers appear in the Proceedings of the National Academy of Sciences.

The findings are striking because they describe how single genes can have such massive effects. The discovery runs counter to the idea that control of something as complex as butterfly color patterns would require dozens to hundreds of genes.

The papers come on the heels of another by the same group published in Nature Communications that proved that a gene called spalt controlled wing eyespot patterns.

“It looks like, between these two papers and the one we published last year, we basically have the fundamental toolkit of genes that controls most color patterns in butterfly wings,” says Robert Reed, an associate professor of ecology and evolutionary biology at Cornell University and a coauthor of all three papers.

The findings have larger implications for genes involved in evolution. It appears that a few master genes—single genes with large effects such as optix and WntA—play an unusually central role in repeatedly driving evolution in different species, Reed says.

Since there are a finite number of genes, they are often recycled throughout evolutionary time, with the same gene having different functions over time and across species. For example, the optix gene also exists in fruit flies, but there it is involved in eye development, not wing color.

“If you mutate optix in a fruit fly, then you’d essentially have no eye,” Reed says.

Other studies show that a single gene controls coat color in different species of mammals and that gene is repeatedly implicated in evolution of that trait even though other genes might have a similar effect.

Similarly, while there are many other genes that separately influence color in butterfly wings, when evolution and natural selection occur, they are typically driven by mutations in optix, Reed says.

“It seems like a small number of genes disproportionately drive evolution over and over again.”

Scientists turned these brown butterflies violet

While optix controls color pigments, the researchers were surprised to find it also controls iridescence, a completely different mechanism where light is reflected in a particular manner off of microscopic features of the butterfly wing scale cells.

Linlin Zhang, a postdoctoral researcher, is the first author of the optix paper, and graduate student Anyi Mazo-Vargas is first author of the WntA paper. Both are members of Reed’s lab.

The National Science Foundation funded the optix paper. The NSF, the National Institutes of Health, the Leverhulme Trust, the Pew Charitable Trust, and the Smithsonian Institution funded the WntA paper.

Source: Cornell University