6 May 2011

Full-circle evolution: when a wing is not a wing

By
Agence France-Presse
The extravagant headgear of small bugs called treehoppers are in fact wing-like appendages that grew back 200 million years after evolution had supposedly cast them aside, according to a new study.
Sphongophorus ballista

Treehoppers display an endless diversity of forms, most of which are conveyed by a bizzare structure called the helmet, an novel homologue to wings no longer involved in flight. Credit: Nicolas Gompel, Nature

treehopper

Credit: Nicolas Gompel, Nature

treehopper

Credit: Nicolas Gompel, Nature

treehopper

Credit: Nicolas Gompel, Nature

PARIS: The extravagant headgear of small bugs called treehoppers are in fact wing-like appendages that grew back 200 million years after evolution had supposedly cast them aside, according to a new study.

That’s probably shocking news if you are an entomologist, and challenges some very basic ideas about what makes an insect an insect, the researchers said, publishing in this week’s Nature.

“Primitive insects 350 million years ago had wings on all of their body segments,” said lead author Benjamin Prud’homme, a researcher at the Development Biology Institute of Marseille-Luminy in France.

“We don’t know if they were all for flight, but we do know – from fossil records – that these wing-like structures were present on each and every body segment.”

Non functional flappers in the head

The thorax of all insects is by definition divided into three segments, each with a pair of legs. In most orders, there are also two pairs of wings, one on the middle segment of the thorax and another at the rear.

Other orders such as flies and mosquitoes have only one set of wings, at the rear, and a few – most ants, for example – have no wings at all.

But no insects today have functional flappers in the first segment next to the head. Their forebear, however, did.

Losing wings and gaining them

For 100 million years following the first primitive insects, Prud’homme explained, wings on the first segment of the thorax and the abdomen dropped away entirely.

But then, some 50 million years ago, something strange happened to the cicada-like treehoppers: they once again sprouted wing-like structures from the top of the first segment of the thorax.

Some of these wildly divergent extrusions resemble thorns, others look like antlers, and still others like aggressive ants or animal droppings, creating one of Nature’s most exotic menageries.

Experts had long assumed that these so-called ‘helmets’ were armour-like expansions of the insects’ exoskeletons.

Full-circle evolution

But by carefully observing the treehopper’s development into adulthood, Prud’homme and colleagues showed that this headgear began as a pair of buds – attached at the sides, and articulated like wings – that fused together as they grew.

Evolution is usually described as linear, but these modified wings suggested the process had come full circle. “This is the only known example of a modern insect that has grown a third pair of wings,” Prud’homme said by phone. “It is a modification of the basic body plan of insects.”

Putting renewed appendages to use

Just how this happened remains a mystery. For 200 million years, certain genes prevented wing-like structures from emerging on this part of any insect’s anatomy.

The researchers speculated that these genes had lost their inhibiting capacity, but experiments on other insect species demonstrated that their repressive powers remain intact.

However it happened, the evolutionary process found a way to put the renewed appendages to use, the researchers speculate.

Raw evolution material

“This extra pair of wings was not needed for flight, but nor did it prevent it,” Prud’homme said. “So it became raw material for evolution to play with.”

Many of the helmets appear to serve as camouflage, helping the insects to avoid predators.

The study shows “how development abilities can be lost or silenced over millions of years, only to be redeployed to contribute to the evolution of a complex and beautiful appendage,” commented Armin Moczek, a professor at Indiana University, also writing in Nature.

NEWSLETTER

Sign up to our free newsletter and have "This Week in Cosmos" delivered to your inbox every Monday.

>> More information
Latest
issue
CONNECT
Like us on Facebook
Follow @COSMOSmagazine
Add COSMOS to your Google+ circles