In a paper in the July 13, 2004, issue of Current Biology, biologists Robert Reed and Michael Serfas add a new piece to the evolutionary puzzle of the butterfly wing. By comparing among species the molecular machinery that controls wing development, the researchers are revealing how the regulation of two key genes has evolved in association with specific color patterns. The color patterns they studied vary among species, existing in a continuum including simple lines, teardrops and rounded spots.
Reed is currently at Duke University, and Serfas is at the University of Wisconsin at Madison. Their work was supported by the National Science Foundation and the Human Frontier Science Program.
Said Reed, "The wing pattern is really the way the butterfly communicates with the world around it. And if we can understand the developmental basis of wing patterns in terms of butterfly ecology and evolution, we can start asking some profound questions about fundamental mechanisms of biodiversification."
In their study, Reed and Serfas sought to compare how genes control the evolution of line and eyespot patterns among a number of butterfly and moth species -- including species that have such patterns and those that either lost them or that evolved before the patterns appeared in nature.
Among the species they studied were those with such evocative names as buckeye butterfly, painted lady, passion vine butterfly, Gulf fritillary, cabbage white, hornworm moth and pink bollworm.
The biologists concentrated on following the activity of two genes, called Notch and Distal-less. Other scientists had shown Distal-less to be a good candidate as a controller gene for eyespot patterns. However, the researchers' discovery that the Notch gene, which encodes a protein that enables signaling among cells, controls butterfly wing pattern development was a new discovery.
The researchers not only compared the activity of the two genes across species, but also took molecular "snapshots" of how the genes' activity changed over time as individual species developed their wing patterns. These analyses implicated Notch and Distal-less regulation in the very early stages of color pattern development. Also importantly, the moth species showed no color patterns related to Notch or Distal-less activity, said Reed.
"In this paper we showed that line and spot color patterns share a similar underlying developmental network," said Reed. "And we can trace a switch between line and spot patterns to the timing of the deployment of the genes in this network. We can also trace the origin of the line and spot network to some time after the evolutionary split of moths and butterflies.
"These gene expression patterns give us a good model of how a character can evolve, switching between two states, in this case a line and a spot," Reed continued. "This study provides an unusually large sampling of a temporal pattern formation process from a group of closely related organisms," said Reed. "And to have all of these temporal time points for multiple genes from so many species allows a unique perspective onto the issue of how changes in temporal gene regulation can underlie the evolution of morphology."
The new findings represent only the beginning of the scientific story of Notch, Distal-less and the evolution of color patterns, said Reed. The mystery still remains of how the two genes actually function to determine wing patterns. And as the scientists pursue the mystery, they hope to gain new insights into the intricacies of evolution at the molecular level and how natural selection drives evolution.
Reed's work, in fact, carries on a long tradition of Duke studies of butterflies as examples of evolution. Professor of Biology Fred Nijhout is considered among the pioneers in using the elegant patterns of butterfly wings as laboratories for studying evolution.