These must be some of the craziest-looking animals I’ve ever seen.
(Yes, they are actually bugs, as in they belong to order Hemiptera)
Apparently, those extravagant shapes are all due to one special body part called the helmet – an outgrowth of the first thoracic segment of these insects. (Here‘s a little reminder of insect anatomy.) It only occurs in treehoppers, according to Prud’homme et al. (2011). I confess, I know very little about insects in general, and nothing about treehoppers in particular, but talk of evolutionary novelties always gives me a little kick.
[NOTE: I won’t define “novelty” exactly. You can probably figure out what it means, and it’s one of those funny concepts that defies an easy definition. Which is kind of the point of this post, though I didn’t originally intend it to come out that way.]
Evolutionary novelty, at least in complex, multicellular organisms like animals, is usually thought to come from tinkering more than “true” innovation. This is thought to hold on all levels; new genes are often modified versions of old genes, new cell types originate from old cell types, and new body parts are built on old body parts. If you think about it, this makes perfect sense: the old parts are already there, doing jobs that can be used as a starting point, whereas sticking a mutation in a piece of DNA that doesn’t encode anything and stumbling on a useful new gene is not exactly the likeliest event in evolution.
[ASIDE: Whole new body parts practically have to come from old parts on some level – the probability of evolution assembling a complex organ entirely from scratch has many times more zeroes after the decimal point than the probability of accidentally making a new gene. The question is how much of the new part is new. Is it built almost completely from an old structure, such as a whole arm – individual bones, muscles and everything – being modified into a wing, or does it only borrow basic building blocks and put them together in a completely new way?]
The outlandish helmets of treehoppers (sort of) uphold the prevailing view. Prud’homme et al. (2011) tell us that this has been a matter of some controversy – most held that they were “true” novelties that were not homologous to any other body part, but there were clues that there’s more to the story than that. And, indeed.
The first hints were anatomical. Helmets don’t simply grow out of the animal’s back – they are attached by a joint. Above that, they share a few other details, including their tissue structure and their veins, with the appendages almost all insects bear on their other thoracic segments: wings. What’s more, although the mature helmet is a single structure, it develops from two precursors that eventually fuse together. Two wings, two helmet primordia, you get the picture.
Prud’homme et al.‘s investigation involved more than dismantling the thoraxes of baby treehoppers. Homologous structures often share a common genetic underpinning, so they checked the expression of some “wingy” genes (or, to be precise, their protein products) to see just how deep the similarity between helmets and wings extended. The first of these, Nubbin, is wing-specific in better-studied insects. As expected if helmets are homologous to wings, the developing helmet was chock full of Nubbin. The two other genes they analysed, Distal-less (Dll) and homothorax (hth), are more generally expressed in insect appendages (wings, legs and antennae), defining their different regions from base (hth) to tip (Dll). They showed the same expression pattern in the helmet – which doesn’t necessarily mean that helmets are modified wings, but it does suggest they are based on some kind of appendage. And, given what appendages the other thoracic segments bear in the same position…
[NOTE: Well, I don’t know much about hth, but Dll is a bit problematic in this respect. It’s not just an “appendage gene” in insects, but also in a wide variety of other animals. Were it not for Dll expression, no one would suggest homology between, say, the tube feet of a starfish and the legs of a fly (Panganiban et al., 1997) – it’s pretty likely that Dll was originally more of an “anything that sticks out of the body” gene than an “appendage”, never mind a “wing”, gene proper. Dll/Dlx genes also do other stuff, like making neurons migrate in vertebrate brains (Anderson et al., 1997). So Dll expression alone doesn’t mean something is an appendage, let alone a specific type of appendage. Luckily, it’s not alone here. Incidentally, this is lesson number one of comparative/evolutionary developmental genetics. When the question is homology of a structure or process, always look at combinations of genes.]
This is not too surprising given the evolutionary history of wings, or what the fossil record was kind enough to preserve for posterity. The first known winged insects (link leads to drawing of Stenodictya lobata in Grimaldi and Engel, 2005) actually had winglets on the first thoracic segment as well, but those were lost before the last common ancestor of living insects. (How that happened in genetic terms, and how it may have been reversed in treehoppers, is also discussed in the paper, but it isn’t directly relevant to the novelty issue) In a way, treehoppers’ “invention” is a giant laugh in the face of Dollo’s Law, which proposes that complex features don’t re-evolve once they are lost (I kind of touched on this “law” here).
Nevertheless, helmets look nothing like wings and function nothing like wings. (To be fair, they look nothing like one another, either.) They are so dissimilar to their proposed evolutionary sisters that apparently their relationship eluded most researchers. How “novel” are they, then? It’s something of a philosophical question. Since, at this level of complexity, literally nothing comes from scratch, at what point do we stop calling something “tinkering” and start calling it “true novelty”?
As with most philosophical questions, I don’t think this one has a correct answer. That doesn’t mean these questions are not worth pondering. The way we word things influences the way we think about them. Exactly where (or even if) we draw a line between two fuzzy concepts isn’t important in my opinion. But to be aware that there is a dilemma about that line, and that other people may draw it in different places, is. Effective communication is one of my Big Issues, and being critical of your own thinking is an issue that ought to be Big for anyone doing science. (Or for anyone, full stop.) Thinking about unanswerable questions like this is a great way of exercising those (self-)critical muscles.
(Originally, I just wanted to gush about the excitement of figuring out the origin of novelties, but I managed to turn it into a philosophical treatise. Whoda thunk that? <.< )
Anderson SA et al.(1997) Interneuron migration from basal forebrain to neocortex: dependence on Dlx genes. Science 278:474-476
Grimaldi D and Engel MS (2005) Evolution of the Insects. Cambridge University Press.
Panganiban G et al. (1997) The origin and evolution of animal appendages. PNAS 94:5162-5166
Prud’homme B et al. (2011) Body plan innovation in treehoppers through the evolution of an extra wing-like appendage. Nature 473:83-86