Butterfly Color Patterns Reveal Clues About Genes That Build Insect Wings
By Viviane Callier
Butterfly wings are natural canvases decorated with elaborate color patterns, but how these patterns develop and evolve is still incompletely understood. Now, a new study in Scientific Reports has identified the genetic code by which butterflies can assign color patterns to different parts of their wings during development. The code is based on a set of genes called transcription factors that establish compartments in most—perhaps all—insect wings. Each compartment, whose “address” is determined by the combination of genes that are active in that sector of the wing, can evolve different color patterns independently from the other compartments.
“Now we have a model for the addressing system that butterflies and moths use to make different parts of their wings individuated,” explained biologist Jeffrey Marcus, Ph.D., of the University of Manitoba, who led the study. “It is in part because of this capacity to produce color pattern diversity that so many beautifully different species of butterflies have evolved.”
Earlier genetic studies in Drosophila melanogaster showed that the forewing is divided into two developmental compartments: The posterior compartment is defined by the activity of a gene called engrailed, while the anterior compartment is defined by the absence of this gene. At the border between anterior and posterior compartments, cells express a gene called decapentaplegic (or dpp). These cells secrete dpp proteinthat diffuses out, creating a gradient and acting as a signal. Depending on the concentration of dpp, other genes become activated and determine where the wing veins are laid down. These other genes also create patterns like eyespots at a specific distance from the boundary.
Marcus and his former student Roohollah Abbasi, Ph.D., noticed that in butterflies in the genus Vanessa, eyespots 3 and 4 are always correlated with each other: they might be both quite large or both absent. The two eyespots that bracket those, eyespots 2 and 5, are also highly correlated with one another. A similar pattern had already been observed in other butterfly species (Bicyclus anynana and species in the genus Junonia). This arrangement resembles the dimensions and organization of gene expression patterns in Drosophila wings around the anterior-posterior (A-P) boundary, but “the only problem was that this didn’t correspond to the A-P boundary; it’s far posterior to where the A-P boundary is,” Marcus said. That’s what led them to suspect there might be another wing compartment.
To demonstrate that they had in fact identified another compartment, Marcus and Abbasi looked at butterflies that naturally had lost the w chromosome in some of their cells, thus producing female/male mosaics. By looking at the regions of male patches (where the w chromosome was lost) in female butterflies, the researchers found a predictable clonal boundary between the center eyespots 3 and 4.
Next the researchers looked for this boundary in Drosophila wings. Using genetic methods, the researchers generated wing cells with yellow hairs and watched how those clones divided through development. “We made a bunch of clones, and lo and behold we found the clonal boundary in Drosophila,” Marcus said. That boundary is in the very posterior part of the Drosophila wing.
The researchers believe this compartment is present in not just Drosophila and butterflies but probably all holometabolous insects. They don’t know yet what genes are responsible for making this compartment boundary, but each compartment is likely defined by a different combination of transcription factors that uniquely identify it, Marcus said.
“Our work also shows that research using nontraditional model organisms has the potential to teach the scientific community many fundamental aspects of developmental biology that may not be apparent from research in more traditional systems like fruit flies,” Marcus added. “Fruit flies also possess the far-posterior developmental compartment in the wing that we discovered in butterflies, but because their wings are small and have fewer visible landmarks, four decades of research on wing development in fruit flies consistently missed it. In butterflies, we can use the color patterns as landmarks, making the underlying developmental architecture of all insect wings more obvious.”