In a feat of versatility, nature long ago co-opted the genes for wing development in butterflies to paint a giant eyespot that helps confuse predators. But that's not the end of the genetic ingenuity, scientists report: Some of the same genes involved in determining the eyespot locations also set up the exact eyespot pattern.
About a year ago, Sean Carroll, an evolutionary developmental biologist at the University of Wisconsin, Madison, and his team showed that eyespots aren't put in position by single genes. Instead, an entire developmental pathway involving a suite of genes had been recruited to specify where eyespots would appear. This suggested that the evolution of new features doesn't require the evolution of new genes or pathways, just a change in how those pathways are used.
Now Carroll and his colleagues have identified some of the genes that apply finishing touches such as size and, very likely, the colors of the central spot and any surrounding rings. Carroll and Wisconsin's Craig Brunetti decided to track the activity of three genes, called engrailed, spalt, and Distalless, during the stage of development when the outlines of the spot and its rings are actually defined. They just happened to pick those genes from among the many that help the wing form. "We got really lucky," Carroll says, as all three proved active at this time.
There is a great deal of flexibility in how the genes are used. Whereas all four butterfly species studied appear to use the same patterns of gene expression to set up a spot, they each use the genes differently to determine the spot's details. "Everything looks very fixed and conserved to a certain step, and then there's a little riot going on," Carroll says. He suggests that such genetic flexibility yields a rapid and continual experimentation with new eyespots and eyespot patterns, some of which persist because, in the context of entire populations, one distracts predators better than another. Carroll reported his team's findings earlier this month at the annual meeting of the Society for Integrative and Comparative Biology.
The work fills in a "missing link" in understanding how the butterfly sets up the details of spot formation, says Scott Gilbert, a developmental biologist at Swarthmore College in Pennsylvania.