On wings, genes, exaptations and the evolution of phenotypic novelty in treehoppers

Last week's lab-meeting on kin selection was cancelled, and since this discussion largely seems to be a semantic one and of little direct relevance to our own work on insects, I thought we should instead discuss an insect paper, now when the field season is rapidly approaching. After all, we are not behavioural ecologists and not so focussed on kin selection, but rather field evolutionary biologists, so this paper might be more central to our ongoing work.

The paper is a wonderful example of how evolution acts like a "tinkerer", rather than as an engineer, when novel structures evolve, and that new traits arise from already existing structures/genes as "exaptations", rather than evolve from scratch. In this case, the remarkable morphological structures we will discuss (see above!) are governed by genes which originally coded for wings! The paper I have in mind is a wonderful example of "evo-devo", i. e. the genetic and developmental basis of animal form and it deals with the morphological structures of treehoppers (Homoptera), a diverse insect group characterized by remarkable appendices on their thorax, of unknown adaptive function (see picture above!). The title is: "Body plan innovation in treehoppers through the evolution of an extra wing-like appendage". 

You can downloadthe paper here, and the abstract is found below. Time and place as usual: "Argumentet" at 14.30 on Wednesday May 25. Any fika volunteer?

Body plan innovation in treehoppers through the evolution of an extra wing-like appendage

 Prud'homme B. et al.

Body plans, which characterize the anatomical organization of animal groups of high taxonomic rank1, often evolve by the reduction or loss of appendages (limbs in vertebrates and legs and wings in insects, for example). In contrast, the addition of new features is extremely rare and is thought to be heavily constrained, although the nature of the constraints remains elusive2, 3, 4. Here we show that the treehopper (Membracidae) ‘helmet’ is actually an appendage, a wing serial homologue on the first thoracic segment. This innovation in the insect body plan is an unprecedented situation in 250 Myr of insect evolution. We provide evidence suggesting that the helmet arose by escaping the ancestral repression of wing formation imparted by a member of the Hox gene family, which sculpts the number and pattern of appendages along the body axis5, 6, 7, 8. Moreover, we propose that the exceptional morphological diversification of the helmet was possible because, in contrast to the wings, it escaped the stringent functional requirements imposed by flight. This example illustrates how complex morphological structures can arise by the expression of ancestral developmental potentials and fuel the morphological diversification of an evolutionary lineage.