In 250 million years of insect evolution, the appearance of new wings is unprecedented. Transformations and losses of wings, yes, but additions, never. A team from the Institut de Biologie du Développement de Marseille-Luminy (CNRS/Université Aix-Marseille 2) has shattered this belief by providing proof that the exuberant helmet of Membracidae, a group of insects related to cicadas, is in fact a third pair of profoundly modified wings.
The discovery is published in the journal Nature on May 5, 2011 and is also featured on its cover.
Membracidae, or treehoppers, are a family of insects, related to cicadas, whose various species vie with each other as regards the originality of their shapes, textures and colors. This diversity is to a large extent conferred by a rather surprising structure covering a large part of their bodies: a 'helmet'. This sometimes resembles an ant in attack position, sometimes a bird dropping, a dead leaf, a thorn, etc. Before the team headed by Nicolas Gompel and Benjamin Prud'homme, both CNRS researchers, studied them with an electronic microscope, the evolutionary origin of this structure was still much debated.
Unlike the horn of the rhinoceros beetle, the helmet of Membracidae is not a simple outgrowth of the cuticle, but a dorsal appendage attached to each side of the thorax by an articulation, with muscles and a flexible membrane that allow it to move. These anatomical observations have been confirmed by the team of researchers at the genetic level: the same genes are involved in the development of the helmet and the wings. Membracidae could thus be insects with three pairs of wings, one of which is heavily modified and unrecognizable.
This discovery is the first example of a change in the body plan of insects by the addition of an evolutionary innovation. This plan is defined by a body divided into three parts (head, thorax and abdomen), a pair of antennas, three pairs of legs and, most frequently, two pairs of wings, always present on the second and third segment of the thorax. However, variations in this general plan exist. In Diptera, such as houseflies or mosquitoes," for example, the hind wings are reduced to small round appendages known as balancers. In Coleoptera (ladybugs, beetles, maybugs, etc.), the first pair is transformed into elytra, hard and often colored "wings" that protect the hind wings. In certain insects, the wings have even totally disappeared. This is the case with fleas and lice, which have a parasitic lifestyle, or red bugs, commonly known as Fire Bugs.
How could a new pair of wings appear in Membracidae? "In insects, wing formation is normally repressed on all segments by Hox genes, except on the second and third thoracic segment," explains Gompel. Could it be that the Hox gene, which intervenes in the first segment of the thorax, is not expressed in Membracidae? No, the Hox protein, the product of the gene, is indeed detected in helmets in formation. Could the Hox gene be inactive? Once again, the answer is no: when injected into Drosphila, it clearly inhibits the formation of wings. "We are faced with a paradox: a Hox gene that is capable of repressing the wing formation, but does not do so. We think that the evolutionary changes instead affect the genetic program of wing formation: these genes may have become insensitive to repression by the Hox gene," adds Gompel. These results also run counter to the idea that the body plan is solely governed by Hox genes. In fact, the Hox gene has not changed, whereas the body plan has changed.
Since its appearance around 40 million years ago, the helmet of Membracidae has totally freed itself of the structural constraints linked to flight. "In short, it's a wing that's no longer a wing. Freed of its flight function in this group of insects, this wing has been able to diversify its shape and texture without moderation," concludes Prud'homme.
- Benjamin Prud’homme, Caroline Minervino, Mélanie Hocine, Jessica D. Cande, Aïcha Aouane, Héloïse D. Dufour, Victoria A. Kassner, Nicolas Gompel. Body plan innovation in treehoppers through the evolution of an extra wing-like appendage. Nature, 2011; 473 (7345): 83 DOI: 10.1038/nature09977
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