Two developmental biologists Nicholas Gompel and Benjamin Prudhomme, when they were together at the Institute of Developmental Biology of Marseilles-Luminy in France, have unraveled the origin and evolution of helmets in treehoppers. This means these hemipterans have achieved what no other insect was able to in more than 300 million years of evolution : a third pair of wing in first segment of thorax which is modified in helmet by tinkering the developmental cues required for dorsal appendage formation.
Treehoppers are a diverse group of plant-feeding insects consisting of roughly 3,200 species worldwide. These species are divided into three orders namely: Melizoderidae, Aetalionidae, and Membracidae, of which Membracidae is by far the largest and most widespread. Most membracids may be easily distinguished from related Hemipterans by their enlarged and often highly decorative pronotum (the dorsal part of the first thoracic segment) referred to AS “HELMETS”. These helmets can resemble anything from a seed to a thorn, sometimes animal droppings, and even dreadful ants, which makes treehoppers as masters of mimicry.
The thorax of all insects can be divided into three segments T1, T2 and T3 segments (T = Thorax ), each bearing a pair of legs. In most orders, there are also two pairs of wings, one on T2 and another on T3 segment. Other orders like dipetrans have only one set of wings, while most ants have no wings at all. No present day insects have wings in the T1 segment , while fossil evidence clearly show that primitive insects living some 350 million years ago had wings on all of their body segments including T1 segment. This loss of wings from T1 and rest of the abdominal segments can be traced to function of Hox genes.
These Hox genes encode Homeo domain containing proteins which play a vital role in patterning along the anterior posterior axis in almost all the animals present on this planet. Previous study in the field showed us that Hox protein called sex comb reduced (Scr) suppress formation of wing in T1 and Abdominal Hox proteins like Ultrabithorax (Ubx) and abdomainal-A (abd-A)
Helmets extend from just behind the head on the first segment of thorax. Entomologists always had this idea that helmet is a folded extension of the thorax’s exoskeleton. Just like wings, helmets also have veins, which prompted one researcher in the 1950s to suggest that the helmet was a modified wing. But entomologists did not buy his version.
However, Nicolas Gompel and Benjamin Prud’homme began having second thoughts after an amateur entomologist friend complained that in trying to catch treehoppers he often wound up holding onto just the helmet, “One wouldn’t expect an insect could afford to lose part of its thorax, even to escape capture and stay alive,” says Prud’homme.
In the article which appeared on cover of journal Nature, authors carefully observed the treehopper’s development from embryo to adult, showed that this headgear began as a pair of buds attaching to the sides, just like wings — that fused together as they grew. In other words the helmet is not simply an extension of the thorax ( as previously thought ), but is connected to thorax, as wings and other appendages ( elytra or halters), by means of a flexible joint. This is the only known example of a modern insect that has grown a third pair of wings. Not only the architecture but the development of helmets also shares significant similarities with wing, by expressing important genes required for formation of wing like Nubbin, Distal-less, and homothorax.
Based on these observations its quite clear that treehoppers evolved a manner to form wing like structures using a developmental pathway somewhat on similar lines used for formation of traditional wings , but at a place where wing development is inhibited by Hox gene Scr in all modern insects.
These findings that treehoppers were able to form a unique T1 appendage in helmet prompted a very important question how the insect body plan has been modified in treehoppers ? According to fossil records insects existed some 350 million years ago bore wings in all thoracic and abdominal segments unlike extant insects where in wings or dorsal appendages are restricted to only T2 and T3 segments.
Next the authors looked into more details related to role of Scr and formation of helmets. As Scr inhibits formation of wing in T1 segment of all modern insects , they wanted to see if Scr is expressed in T1 segment of treehoppers. Antibody staining against Scr showed nice expression of Scr protein in T1 segment and further experiments done in Drosophila ( over expression of treehoppers Scr in Drosophila melanogaster) proved that treehoppers Scr retain ability to suppress wing formation in T1 segment.
Previous work done in the field shows that this transition (from wings to no wings) is brought by Hox genes by acting at various wing patterning genes and different levels of developmental pathway required for formation of dorsal appendages. Scr does repress wing formation by repressing various wing genes as shown by knock down of Scr in coleopter Tribolium leads to expression of Nubbin protein in T1 segment. This result demonstrates that Hox protein Scr repress T1 wing formation by at least repressing transcription of nubbin gene. The expression of nubbin in T1 helmets of treehoppers clearly shows that this structure evolved because Scr no longer exerted its ancestral repressive effect on wing formation.
This works demonstrates a very important aspect related to evo devo – How developmental cues silenced over million years is reused in slightly different manner leads to the evolution of a complex and novel appendage (helmet).
Reference:
Body plan innovation in treehoppers through the evolution of an extra wing-like appendage
Prud’homme B, Minervino C, Hocine M, Cande JD, Aouane A, Dufour HD, Kassner VA, Gompel N.
Nature. 2011 May 5;473(7345):83-6.
Image credit : Pixabay