Gene patterns decorate butterflies' wings.

Nature photographer Kjell B. Sandved made his reputation exploring the diversity of the world's approximately 200,000 moths and butterflies. Over the years he has found the alphabet, numbers, and many other human symbols inscribed in their wings (SN: 6/16/90, p.376).

But it took the plain fruit fly (Drosophila melanogaster) to help scientists understand how nature can be so creative.

In fruit flies, and presumably all insects, the activation, or expression, of certain genes in particular cells for short periods helps define body parts. Just before an insect larva changes into a flying adult, it develops white globs of cells called imaginal disks that later become its wings, explains Sean B. Carroll, a geneticist at the Howard Hughes Medical Institute at the University of Wisconsin-Madison.

Carroll's group used fruit fly genes to study these disks in the buckeye butterfly, Precis coenia, which is common throughout the United States. They found six genes very similar to the fruit fly's. In fruit flies, these genes specify the top and bottom, back and front, and inner and outer margins of the developing wings.

The same holds for butterflies, the Wisconsin group now reports in the July 1 SCIENCE. For example, in butterflies, as in fruit flies, the cells that make up the top wing surface contain an activated apterous gene, while those destined to frame the wing perimeter express one called Distal-less. "The same genes are expressed in the same relative nates that we know operate in the fruit fly wing," says Carroll.

Fruit flies began evolving separately from butterflies about 200 million years ago. "So it looks like that program was very much conserved," he adds.

But then the butterfly goes one step further, reactivating those genes to generate colorful wing designs. "Distal-less is expressed in a new and unexpected pattern," comments H. Frederik Nijhout of Duke University in Durham, N.C.

Carroll and his colleagues observed that within each section of developing wing, a wedge of cells activates Distalless genes. Over time, that wedge narrows and finally shrinks to a small spot, just as Nijhout had predicted years earlier when he was theorizing about how patterns should arise.

"It confirms that all butterfly color patterns are derived from a common ground plan," says Nijhout. On each wing, several small groups of cells act as organizing centers that guide the patterning of the entire wing. "Different parts of the color pattern are uncoupled. You can think of it as a mosaic system," he adds. This uncoupling leads to great variety.

Thus, while all wing sections can generate eyespots, only certain ones do. Carroll and Nijhout think the expression of other genes helps define other types of butterfly wing patterns and that natural selection guides the final outcome. For example, showy colors on the upper surface help attract mates, while duller bottom surfaces help the insect blend in with its background to avoid predators, they note.