You can get a jump on the class—I’ve posted a list of the textbooks you’ll need on the class website.
Pharyngula
Evolution, development, and random biological ejaculations from a godless liberal
You can get a jump on the class—I’ve posted a list of the textbooks you’ll need on the class website.
Surely, you haven’t had enough information about pycnogonids yet, have you? Here’s another species, Tanystylum bealensis, collected off the British Columbian coast. That’s a ventral view of the male, and those bunches of grapes everywhere are eggs and babies—males do the childcare in this group. These animals also live in relatively shallow water, in the lower intertidal zone, so it was possible to collect thousands of them and develop a complete staging series. Below the fold I’ve put some illustrations of the larvae, which are even cuter.
I’ve been getting swamped with links to this hot article, “Evolution reversed in mice,” including one from my brother (hi, Mike!). It really is excellent and provocative and interesting work from Tvrdik and Capecchi, but the news slant is simply weird—they didn’t take “a mouse back in time,” nor did they “reverse evolution.” They restored the regulatory state of one of the Hox genes to a condition like that found half a billion years ago, and got a viable mouse; it gives us information about the specializations that occurred in these genes after their duplication early in chordate history. I am rather amused at the photos the news stories are all running of a mutant mouse, as if it has become a primeval creature. It’s two similar genes out of a few tens of thousands, operating in a modern mammal! The ancestral state the authors are studying would have been present in a fish in the Cambrian.
I can see where what they’ve actually accomplished is difficult to explain to a readership that doesn’t even know what the Hox genes are. I’ve written an overview of Hox genes previously, so if you want to bone up real quick, go ahead; otherwise, though, I’ll summarize the basics and tell you what the experiment really did.
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.
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.
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.
The other day, I was asked a simple question that I knew the answer to, right off the top of my head, and since I’m nothing but lazy and lovin’ the easy stuff, I thought I’d expand on it a bit here. The question was, “How do flounder get to be that way, with their eyes all on one side of the head?” And the answer is…pedantic and longwinded, but not too difficult.
The Pleuronectiformes, or flatfish, are a successful teleost order with about 500 known species, some of which are important commercially and are very tasty. The key to their success is their asymmetry: adults are camouflaged ambush predators who lurk on the sea bottom, taking advantage of their flat shape to rest cryptically and snap up small organisms that wander nearby. They lie on their sides, and have peculiarly lop-sided heads in which one eye has drifted to the other side, so both eyes are peering out from either the left or right side (which side is consistent and characteristic for a particular species, although there is at least one species with random assignment of handedness to individuals, and mutant strains are known in others that reverse the handedness.)
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.
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.
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.)
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.
