The incredible diversity of eyespots and other patterns on the wings of butterflies all comes down to the timing of gene expression, a new study finds.
More than 10 years ago, Frederik Nijhout, a developmental physiologist at Duke University in North Carolina, proposed that symmetrical patterns on the wings of butterflies and moths (Lepidoptera) might be triggered by a developmental process common to all species. Once this process evolved, he argued, minor modifications might have been enough to generate the great range of colored patterns observed in nature. Many scientists found the idea attractive, but so far they'd found little direct support for it. Now, research published in the 13 July issue of Current Biology identifies a sequence of gene expression that might turn out to be the common developmental process Nijhout proposed.
Robert Reed, a postdoc in Nijhout's lab, and Michael Serfas, a molecular biologist at the University of Wisconsin, Madison, used antibodies tagged with a dye to monitor the expression of two key developmental genes in eight species of Lepidoptera: six butterfly species with symmetrical patterns and two moths without such patterns. In all of the butterflies, the genes Notch and Distal-less seem to be switched on in sequence where wing patterns subsequently emerge. Differences from species to species in the timing of expression of these and other genes may be what determine whether the pattern will turn out to be an eyespot, an ellipse, or an inky streak across the wing, Reed and Serfas suggest.
Notch and Distal-less may well be common to all patterned butterflies, agrees Jim Mallet, an evolutionary ecologist at University College London. If so, he says, the research is an important step toward understanding the evolution of color patterns.
The authors believe that the Notch and Distal-less signaling pathway arose after the evolutionary split between moths and butterflies, because the two genes were not expressed in two unpatterned species of moth they studied. However, Antónia Monteiro, a developmental biologist at the State University of New York, Buffalo, thinks that the patterning system could have arisen in moths before the first butterfly ever fluttered its wings. Some moths near the base of the lepidopteran order do have eyespots, she notes, and it remains to be seen whether these moths express Notch and Distal-less or whether they gain their symmetry by some other means.