An incredibly exciting paper came out in Nature last week about my favorite group of insects–the treehoppers, Membracidae. This group is instantly recognizable by their enlarged pronotum, or thoracic shield. It’s usually big and strangely shaped, letting these tiny insects mimic thorns, leaves, and even ants.
This paper is one that even non-pointy-headed academics should be excited about, because it provides a neat explanation to one of the central questions of evolution: how do really complex structures evolve?
Jerry Coyne and Ed Yong already wrote about this paper, but because I have huge brass ones, I’m going to take a stab at explaining this paper too, hopefully in a lay-reader-friendly way. I’m doing this in part because news coverage of the paper makes it clear that a lot of people don’t understand what homology is. It’s a confusing concept, because homology can be described at many levels, from basic anatomy to molecular biology.
In its simplest definition, homology means that organisms share a common ancestor. Most of us are introduced to this concept with a diagram showing how the bones in human arms, horse legs, and bird wings all share the same pattern. These are homologous structures.
Let’s begin at the beginning of insects (and most animals) with segmentation. Segmentation is a wonderful way to make an animal–it allows the same pattern to be used over and over. Segmentation allows parts of an animal to have separate and specialized functions–business in front, party in the rear, if you will.
If you look at an ancestral arthropod like a trilobite, what you see is that they have a segmented body with appendages on each segment. Over the history of arthropod evolution, those appendages have specialized in different ways–or been lost all together.
So, the first three segments’ appendages became mouthparts or antennae and were lumped together into the head; and different numbers of appendages were lost or joined into the basic body plans for centipedes, spiders, and insects.
Here’s a nice detailed chart showing homologous segments between trilobites (ancestral arthropod) and modern arthropods like spiders and insects. We know that these animals all share a common ancestry and homologous structures.
Somehow, in the evolution of these arthropods, different sets of genes were modified and turned on and off, changing structures and where they appear. Organisms have the same DNA in each cell, but only some of it is “turned on” to make organs or different tissue types. This is the domain of “evo-devo” or evolutionary developmental biology.
How does your body “know” that genitals only go in one specific spot during development? Why doesn’t it build a scrotum on your head like a jaunty beret? Or, in the case of an insect, why do legs and wings develop only on the thorax, in specific spots? Regulatory genes give the instructions. By looking at the way in which regulatory genes control the change from an egg to an adult, biologists can also infer how changes from ancestral organisms have evolved.
The key that unlocked a whole lot of evo-devo knowledge was the discovery of Hox genes. Hox genes are genes that specify identity—whether a segment of the embryo will form part of the head, thorax, or abdomen of an insect, for instance. (If you want a detailed explanation of Hox genes, check out this post by PZ; I’m trying to keep this post light on molecular bio.)
A classic mutation in a hox gene is called Antennapedia. It shows clearly what goes wrong when the command for “put a leg here” is garbled. This fly has a nice looking thoracic leg in place of its antenna.
It would be correct to say that this leg is homologous to an antenna; it wouldn’t be correct to say that it IS an antenna. And that is where the news coverage of this research paper on treehoppers (we’ll get there eventually, I promise!) doesn’t get it quite right.
The way this paper has been reported has been, for the most part:
That’s not technically correct. However, it’s a lot more marketable to a general audience than saying:
“Treehoppers have a fused prothoracic pair of appendages serially homologous to the wings on the meso and metathorax!”
Both statements are pretty exciting (if you are a bug dork, anyway.) The treehopper paper used a clever combination of molecular biology, anatomical studies, and developmental biology to illustrate the evolutionary history of a really complex structure. Here’s the actual paper citation:
Prud’homme, B., Minervino, C., Hocine, M., Cande, J., Aouane, A., Dufour, H., Kassner, V., & Gompel, N. (2011). Body plan innovation in treehoppers through the evolution of an extra wing-like appendage Nature, 473 (7345), 83-86 DOI: 10.1038/nature09977
Treehoppers and their pointy helmets start showing up around 40 million years ago. And the variety is amazing. (Apparently they use the same hatters as the one that created the fallopian monstrosity on Princess Beatrice for the Royal Wedding.)
Just HOW membracids evolved all their strange pointy hats, and what the structure was derived from, has been a source of argument amongst entomologists for many years. Prud’homme and his co-authors have shown fairly conclusively that hopper hats are related to activation of a long dormant genetic control sequence.
In modern insects, wings occur only on the second and third thoracic segments. This is controlled by regulatory hox genes like the antennapedia one I mentioned above. Somehow, the sequence that suppresses wing formation on the first thoracic segment was lost in hoppers. And that provided the raw material for evolution to create endless forms most wonderful.
Prud’homme et al.’s evidence is based upon:
- Anatomical studies showing that the helmet structure has a hinge–like wings do.
- Morphological studies on wing-venation-like structures on helmets, and demonstrating that helmets inflate during a moult like wings do.
- Developmental studies found that the helmet structure is formed from tissue that is similar to wing precursors.
- Molecular studies identified a wing gene (nubbin) that is activated in the tissue of the helmet. This is unusual, because in all other insects, a hox gene called Scr blocks wings from developing on the first thoracic segment.
In fact, the researchers actually manipulated their suspect regulatory genes in another insect to see if they could get wing tissue to develop on the first thoracic segment (T1) of an insect.
The photo at the right shows a mealworm, which is a beetle larva. The top photo shows a normal mealworm, with tissue that will eventually become wings on the 2nd and 3rd thoracic segments (T2 & T3) highlighted using a molecular marker.
The second photo shows what happens when they disable the hox gene that they suspect suppresses wing formation–wing tissue occurs in T1.
It’s a compelling way to wrap up a great evolutionary story. Regulatory genes that control how organisms develop over their lifetime also provide the raw material for many different structures. The crazy hats of hoppers didn’t evolve from new genetic material, but from modification of what already existed.
So that’s my attempt to explain this wonderful discovery to folks who may not have a molecular biology or entomology background. This paper will certainly become a classic in evolutionary biology, as it nicely provides an explanation for the evolution of an elaborate trait using multiple lines of evidence.
And if you’ve read all this way, here is your reward! A video from the author’s supplemental material that shows a hopper in its final molt, first inflating its wings and then expanding it’s pronotum. The similarity to wings is pretty remarkable! (Alas, I was unable to embed it.)