Fluttering butterflies may seem far removed from the buzz of hairy, pesky bugs, but their wings betray them: The relatively large, colorful scales appear to have evolved from the minute sensory bristles of their forefathers. The finding, reported in the 15 June Current Biology, highlights how the genetic hard-wiring for one trait can be commandeered to create another over evolutionary time.
The similar early developmental patterns of butterfly scales and fruit fly bristles are a strong hint that the two structures are related. To establish the evolutionary relationship, geneticist Sean Carroll and his collaborators at the University of Wisconsin, Madison, and Washington University in St. Louis set out to probe the masterminding genes in action.
First off, the team searched the genome of Precis coenia butterflies for sequences similar to the achaete-scute (AS-C) genes that guide bristle development in Drosophila fruit flies. Once the researchers had found a similar gene, dubbed AS-C homolog-1, they checked where it was expressed in developing butterfly embryos and larvae. It turned out that until the larvae entered the pupal stage, the butterfly gene was turned on in a pattern similar to that of the bristle genes in Drosophila, implying that the genes have similar functions during early development and that they therefore likely came from the same ancestor. During pupation, however, the butterfly gene was turned on in what became the scale-forming cells, while in flies the bristle genes go on to form bristles.
Carroll's team proposes that a common ancestor of flies and butterflies may have had functioning bristles that over time lost their connection to the nervous system, spread to cover the wings, and mutated into scales. As further evidence, they note that caddisflies--a close relative of butterflies--have wings covered by nonsensory bristles.
The finding of similar genes sculpting quite different body parts adds weight to the notion that some genes can be viewed as a switch that can be plugged into different developmental pathways, says Frederik Nijhout, an insect developmental physiologist at Duke University. "We are getting away from this tight relationship between a particular gene and a particular character," he says. The new finding also helps to explain how species with limited genomes "can do so much with so few genes," by coding for traits like variations on a theme.