Evolution of median fins

Often, as I’ve looked at my embryonic zebrafish, I’ve noticed their prominent median fins. You can see them in this image, although it really doesn’t do them justice—they’re thin, membranous folds that make the tail paddle-shaped.

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These midline fins are everywhere in fish—lampreys have them, sharks have them, teleosts have them, and we’ve got traces of them in the fossil record. Midline fins are more common and more primitive, yet usually its the paired fins, the pelvic and pectoral fins, that get all the attention, because they are cousins to our paired limbs…and of course, we completely lack any midline fins. A story is beginning to emerge, though, that shows that midline fin development and evolution is a wonderful example of a general principle: modularity and the reuse of hierarchies of genes.

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Voices of science

If you’re at work, I hope you have headphones; if you don’t, check in once you get home. Here are a couple of audio recordings of good science.

Ancient rules for Bilaterian development

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Assuming that none of my readers are perfectly spherical, you all possess notable asymmetries—your top half is different from your bottom half, and your front or ventral half is different from you back or dorsal half. You left and right halves are probably superficially somewhat similar, but internally your organs are arranged in lopsided ways. Even so, the asymmetries are relatively specific: you aren’t quite like that Volvox to the right, a ball of cells with specializations scattered randomly within. People predictably have heads on top, eyes in front, arms and legs in useful locations. This is a key feature of development, one so familiar that we take it for granted.

I’d go so far as to suggest that one of the most important events in our evolutionary history was the basic one of taking a symmetrical ball of cells and imposing on it a coordinate system, creating positional information that allowed cells to have specific identities in particular places in the embryo. When the first multicellular colony of identical cells set aside a particular patch of cells to carry out a particular function, say putting one small subset in charge of reproduction, that asymmetry became an anchor point for establishing polarity. If cells could then determine how far away they were from that primitive gonad, evolution could start shaping function by position—maybe cells far away from the gonad could be dedicated to feeding, cells in between to transport, etc., and a specialized multicellular organism could emerge. Those patterns are determined by interactions between genes, and we can try to unravel the evolutionary history of asymmetry with comparative studies of regulatory molecules in early development.

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Flap those gills and fly!

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I am going mildly nuts right now—somehow, I managed to arrange things so multiple deadlines hit me on one day: tomorrow. I’ve got a new lecture to polish up for our introductory biology course, a small grant proposal due, and of course, tomorrow evening is our second Café Scientifique. Let’s not forget that I also have a neurobiology lecture to give this afternoon, and I owe them a stack of grading which is not finished yet. I’m really looking forward to Wednesday.

Anyway, so my new lecture for our introductory biology course is on…creationism, yuck. What I’m planning to do is to describe some of the most common creationist arguments and then give a biologist’s rebuttal. Creationism is really a waste of our class time, but using it to explain some general concepts that any informed biologist should understand (and that the creationists, including Mike Behe, are astonishingly clueless about) will make it a little more productive, I hope. We’ll find out tomorrow.

One of the common creationist claims I plan to shoot down is the whole idea of “irreducible complexity” as an obstacle to evolution. I was going to bring up two ideas that invalidate it: the principle of scaffolding (which I discussed here), and exaptation, in which features evolved for some other purpose than the one that they play in an organism we observe today. I was looking for a good example, and then John Wilkins fortuitously sent me a paper that filled the bill (we evilutionists, you know, are sneakily sending each other data behind the scenes to help in our assault on ignorance. We’re devious that way.)

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Coming to Life

Books from Nobel laureates in molecular biology have a tradition of being surprising. James Watson(amzn/b&n/abe/pwll) was catty, gossipy, and amusingly egotistical; Francis Crick(amzn/b&n/abe/pwll) went haring off in all kinds of interesting directions, like a true polymath; and Kary Mullis(amzn/b&n/abe/pwll) was just plain nuts. When I heard that Christiane Nüsslein-Volhard was coming out with a book, my interest and curiousity were definitely piqued. The work by Nüsslein-Volhard and Wieschaus has shaped my entire discipline, so I was eagerly anticipating what her new book, Coming to Life: How Genes Drive Development(amzn/b&n/abe/pwll) would have to say.

It wasn’t what I expected at all, but I think readers here will be appreciative: it’s a primer in developmental biology, written for the layperson! Especially given a few of the responses to my last article, where the jargon seems to have lost some people, this is going to be an invaluable resource.

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Generic bumps and recycled genetic cascades

How do you make a limb? Vertebrate limbs are classic models in organogenesis, and we know a fair bit about the molecular events involved. Limbs are induced at particular boundaries of axial Hox gene expression, and the first recognizable sign of their formation is the appearance of a thickened epithelial bump, the apical ectodermal ridge (AER). The AER is a signaling center that produces, in particular, a set of growth factors such as Fgf4 and Fgf8 that trigger the growth of the underlying tissue, causing the growing limb to protrude. In addition, there’s another signaling center that forms on the posterior side of the growing limb, and which secretes Sonic Hedgehog and defines the polarity of the limb—this center is called the Zone of Polarizing Activity, or ZPA. The activity of these two centers together define two axes of the limb, the proximo-distal and the anterior-posterior. There are other genes involved, of course—this is no simple process—but that’s a very short overview of what’s involved in the early stages of making arms and legs.

Now, gentlemen, examine your torso below the neck. You can probably count five protuberances emerging from it; my description above accounts for four of them. What about that fifth one? (Not to leave the ladies out, of course—you’ve also got the same fifth bump, it’s just not quite as obvious, and it’s usually much more tidily tucked away.)

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Diploblasts and triploblasts

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Carl Zimmer wrote on evolution in jellyfish, with the fascinating conclusion that they bear greater molecular complexity than was previously thought. He cited a recent challenging review by Seipel and Schmid that discusses the evolution of triploblasty in the metazoa—it made me rethink some of my assumptions about germ layer phylogeny, anyway, so I thought I’d try to summarize it here. The story is clear, but I realized as I started to put it together that jeez, but we developmental biologists use a lot of jargon. If this is going to make any sense to anyone else, I’m going to have to step way back and explain a collection of concepts that we’ve been using since Lankester in the 19th century.

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