New Scientist January 15, 2003

Stick insect forces evolutionary rethink

by Nicola Jones

The lowly stick insect has forced a rethink of one of the key rules of evolution - that complex anatomical features do not disappear and reappear over the course of time.

Researchers have discovered that on a number of occasions in the past 300 million years, stick insects have lost their wings, then re-evolved them. Entomologists have described the revelation as "revolutionary".

Michael Whiting, an evolutionary biologist from Brigham Young University in Provo, Utah, and his team stumbled upon the finding while examining the DNA of 37 different phasmids, the stick and leaf insects famous for camouflaging themselves against plants, in a bid to work out their family tree.

Entomologists have assumed that wings only evolved once in insects. The received wisdom is that a winged ancestor produced the winged phasmids we see today. The 60 per cent of stick insects that do not sport wings will, this thinking goes, have jettisoned them along their evolutionary journey so they could expend more energy on reproduction and less on flying.

But Whiting's analysis shows that the very first stick insect, which appeared 300 million years ago, had already lost its wings and that stick insects re-evolved the structures at least four times (see graphic). The study covers only 14 of the 19 known sub-families of phasmids, so it is possible that wings reappeared even more often.

Beyond repair

Researchers assumed wings could not come back once lost as the genes needed to create them would mutate beyond repair once the wings disappeared. But Whiting says there is evidence from the fruit fly Drosophila that the same genes contain instructions for forming wings and legs.

If the same were true for stick insects, there would be an evolutionary pressure to stop wing genes from mutating, even in the insects that did not have wings. Those genes could then be turned back on in the future.

Whiting says, however, that while wing re-evolution may seem unlikely to insect researchers, the basic idea of switching regulatory genes off and on is well accepted. Even a single gene can sometimes switch on the growth of a complex structure - studies indicate that a master gene called Pax-6, for example, might control the development of eyes in all creatures that have them.

So Whiting suggests that eyes too could have disappeared and reappeared in animals over time. "I remember sitting down with entomologists and hearing them say 'impossible, impossible, impossible'," he says. But "re-evolution is probably more common than we thought".

Journal reference: Nature (vol 421, p 264)

File Date: 01.20.03