Study finds RNA molecule controls butterfly wing coloration

A team of international researchers has discovered a surprising genetic mechanism that influences the vibrant and complex patterns on butterfly wings. In a study published in the journal Proceedings of the National Academy of SciencesThe team, led by Luca Livraghi of George Washington University and the University of Cambridge, discovered that an RNA molecule, rather than a protein as previously thought, plays a key role in determining the distribution of black pigment on butterfly wings.

How butterflies create the vibrant patterns and colors on their wings has fascinated biologists for centuries. The genetic code contained in the cells of a developing butterfly’s wings dictates the specific arrangement of color on the wing scales (the microscopic tiles that form wing patterns), similar to the way colored pixels are arranged to form a digital image. Deciphering this code is fundamental to understanding how our own genes construct our anatomy. In the lab, researchers can manipulate this code in butterflies using gene-editing tools and observe the effect on visible traits, such as the coloration of a wing.

Scientists have long known that protein-coding genes are essential to these processes. These types of genes create proteins that can determine when and where a specific scale should generate a particular pigment. When it comes to black pigments, researchers thought the process would be no different and initially implicated a protein-coding gene. The new study, however, paints a different picture.

The team discovered a gene that produces an RNA molecule – not a protein – that controls where the black pigments are produced during the butterflies’ metamorphosis. Using the genome-editing technique CRISPR, the researchers showed that when the gene that produces the RNA molecule is deleted, the butterflies completely lose their black pigmented scales, showing a clear link between RNA activity and the development of black pigments.

“What we found is amazing,” said Livraghi, a postdoctoral researcher at GW. “This RNA molecule directly influences where the black pigment appears on the wings, shaping the butterfly’s color patterns in ways we hadn’t anticipated.”

The researchers studied in more detail how the RNA molecule works during wing development. By examining its activity, they observed a perfect correlation between where the RNA is expressed and where the black scales form.

“We were amazed to find that this gene is turned on right where the black scales develop on the wing, with exquisite precision,” said Arnaud Martin, associate professor of biology at GW. “It’s really an evolutionary paintbrush in that sense, and a creative one, judging by its effects on multiple species.”

The researchers examined the newly discovered RNA in several other butterflies whose evolutionary histories diverged about 80 million years ago. They found that in each of these species, the RNA had evolved to control new locations in the dark pigment patterns.

“The consistent results obtained from CRISPR mutants in multiple species really demonstrate that this RNA gene is not a recent invention, but a key ancestral mechanism for controlling wing pattern diversity,” said Riccardo Papa, professor of biology at the University of Puerto Rico-Río Piedras.

“We and others have studied this genetic trait in many butterfly species and found that this same RNA is used repeatedly, from long-winged butterflies to monarch butterflies to painted lady butterflies,” said Joe Hanly, a postdoctoral researcher and visiting scholar at GW. “This is clearly a crucial gene for the evolution of wing patterns. I wonder what other similar phenomena biologists might have missed because they didn’t pay attention to the dark matter of the genome.”

These findings not only challenge long-held assumptions about genetic regulation, but also open new avenues for studying how visible traits evolve in animals.

The study, “A long noncoding RNA at the cortex locus controls adaptive coloration in butterflies,” was published Aug. 30, 2024, in the journal Proceedings of the National Academy of Sciences.