Of insects & their wings

Beetle wings are often hidden. Nestled behind armoured shields on the beetle's back, they unfurl in whirring sheets, whisking their clumsy owners from danger. Beetles don't have more than two sets of wings - unless they're in Yoshinori Tomoyasu's lab. In research recently published in the Proceedings of the National Academy of Sciences, Yoshinori and his co-author, David Linz, genetically engineered beetle larvae with wings on their abdomens, part of an attempt to unpack one of evolution's greatest mysteries: how insects gained the ability to fly.

Insects took to the empty skies sometime between 300 million and 360 million years ago, long before birds, bats or pterosaurs. Wings allowed them to conquer new habitats and ecological niches, and Insecta quickly established themselves as one of the most diverse and successful animal classes, a position they still hold today. The vast majority of living insects either have wings or evolved from flying ancestors, said David, an evolutionary biologist now at Indiana University, USA. "When the average person thinks about an insect wing, they think about a dragonfly - these two pairs of really pretty, long wings," he said. "But it's different in different lineages."

There's a frustrating lack of fossil evidence from the period when insect flight evolved, said Yoshinori, an evolutionary biologist at Miami University. "There's as much variety in origin ideas for insect wings as in insect wings themselves," he said. "With the flight wing in vertebrates, there's a clear origin." But insect wings evolved so long ago, he added, "it's hard to tell what happened." That hasn't stopped researchers from trying to figure it out. According to Floyd Shockley, an entomologist at the Smithsonian's National Museum of Natural History, USA, there have long been two competing hypotheses. The 'tergal hypothesis' suggests that wings originated on the tergum. The 'pleural hypothesis' argues that wings were created from ancient leg segments that merged with the body before ending up on the back.

The rise of evolutionary developmental biology, along with advances in genetics, has lent weight to a third possibility, David said. Originally proposed in 1974, the 'dual origin' hypothesis suggests that insect wings actually began with a fusion of the two separate tissues: the dorsal body wall provided the membrane, while its articulation arose from leg segments. This sort of evolutionary fusion sounds bizarre, David said, but there is some precedent. The ancient ancestors of insects probably had relatively symmetrical body segments, each with a pair of legs. These segments have modified over the millenniums.

Surprising find

Yoshinori and David worked with Tribolium, a common subject because of its fully sequenced genome. The beetles don't fly well, David said, and are easy to keep in a laboratory. In an initial study, the team used master switches in the beetles' genome to manipulate which segments of the body had wings. To their surprise, doing so disrupted portions of anatomy that had seemed unconnected to flight. This offered some support for the idea that wings were composite tissues. But how might the ancestral wing structures have formed? The researchers turned their attention to the pupae, which have defensive sets of miniature pincers along their abdomens. These so-called gin-traps sit near the top of the insect, which make them likely models for early wing structures.

To add support for the dual origin hypothesis, David said, evolution would have had to fuse a structure on the dorsal region of the segment and one from the pleural tissue. The team introduced a fluorescent green protein into the beetles that marked the expression of certain wing-related genes, making it easy to tell which tissues were being affected by genetic tampering. After manipulating genes of the abdomen, they were delighted to see two green tissues: one at the dorsal gin-trap, and one down in the pleural tissue. And by doing so, they were able to produce pupae in which both tissues fused to form pairs of tiny wings.

While he found the study interesting, Floyd said the idea that embryonic or larvae development is like a fast-forwarding tape of prior evolutionary modifications has largely been discredited. Still, he acknowledged that manipulating gene expression is useful for trying to piece together details of structures that are hard to visualise.

The New York Times