Treehoppers redux

Or how I learned (again) that there are no truly simple stories in biology.

In the name of fairness and plain old intellectual integrity, I should mention some interesting new developments in the treehopper-helmet-novelty issue. Back in the first treehopper post I acknowledged that I’m a far cry from an entomologist, and a new study argues that Benjamin Prud’homme and the entire crew on Prud’homme et al. (2011) may share that attribute with me.

A paper published recently in the open-access online journal PLoS ONE (Mikó et al., 2012) questions basically every interpretation the previous study made about those funky thoracic appendages. After dissecting, CT-ing and microscoping several treehoppers and related insects, they conclude that:

  • The helmet is not an appendage that articulates with the first thoracic segment – it’s actually most of the first thoracic segment itself.
  • The joint at the base of the helmet is the articulation between the first two thoracic segments.
  • The paired “helmet buds” Prud’homme et al. reported are more likely to be artefacts of the way they sectioned their specimens, since Mikó et al. couldn’t find any in treehoppers of a similar developmental stage.

If all of this is correct, that would suggest that the helmet has nothing to do with wings, it’s just like other less extreme outgrowths of the thorax that you find in a large variety of insects.

What about the genes?

If you take a gander at the first treehopper post or Prud’homme et al. (2011) itself, you’ll see that they supported their microscopic observations with gene expression data including two appendage-specific genes and one that they considered specific to wings. However, even I had a note of caution about using Dll/Dlx genes – which seem to be there whenever anything starts sticking out of an animal’s body – as evidence of homology to anything. Mikó et al. (2012) point out that nubbin, the supposed “wing gene” actually has quite variable roles in wing and other appendage development when you look at more insect species besides fruit flies. The Hth-Dll combo, it appears, is also involved in the development of more obviously non-wing thoracic outgrowths, like beetle horns.

Where does that leave us?

Seeing as I’m still no entomologist, I can’t really take sides in the anatomical arguments. The genetics? What immediately springs to my mind is Keys et al. (1999), and how some butterflies grow their eyespots by the wholesale co-option of a genetic regulatory circuit from wing development. Did the same sort of thing happen to beetles and treehoppers, then?

This, in fact, only reinforces my general opinion about novelties and the nature of genetic evidence. Evolution rarely, if ever, works from scratch, and the boundary between “novelty” and “tinkering” is as blurry as it gets. Thus, “homology” is rarely a clear-cut and straightforward issue. All of that still stands [1], even if treehoppers might have shifted on some sliding scales. (Which direction is an interesting question. Is a re-activated wing homologue more or less “novel” than a generic thoracic outgrowth patterned by some wing circuitry? Does the distinction even make sense?)

All in all, this is getting quite interesting. It feels decidedly like the beginning of a heated debate [2]. I’ll certainly keep an eye out for future episodes of the treehopper saga.

***

[1] Though I have to say, I have a couple of papers on my reading list that may mess with my opinions… Don’t want to jinx it, so I won’t say more, but I’m hoping to make a post out of them one day.

[2] Or a beautiful friendship. *ducks*

***

References:

Keys DN et al. (1999) Recruitment of a hedgehog regulatory circuit in butterfly eyespot evolution. Science 283:532-534

Mikó I et al. (2012) On dorsal prothoracic appendages in treehoppers (Hemiptera: Membracidae) and the nature of morphological evidence. PLoS ONE 7:e30137

Prud’homme B et al. (2011) Body plan innovation in treehoppers through the evolution of an extra wing-like appendage. Nature 473:83-86

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