Pharyngula
Evolution, development, and random biological ejaculations from a godless liberal
My first column in the Guardian science blog will be coming out soon, and it’s about a recent discovery that I found very exciting…but that some people may find strange and uninteresting. It’s all about the identification of nodal in snails.
Why should we care? Well, nodal is a rather important — it’s a gene involved in the specification of left/right asymmetry in us chordates. You’re internally asymmetric in some important ways, with, for instance, a heart that is larger on the left than on the right. This is essential for robust physiological function — you’d be dead if you were internally symmetrical. It’s also consistent, with a few rare exceptions, that everyone has a stronger left ventricle than right. The way this is set up is by the activation of the cell signaling gene nodal on one side, the left. Nodal then activates other genes (like Pitx2) farther downstream, that leads to a bias in how development proceeds on the left vs. the right.
In us mammals, the way this asymmetry in gene expression seems to hinge on the way cilia rotate to set up a net leftward flow of extraembryonic fluids. This flow activates sensors on the left rather than the right, that upregulate nodal expression. So nodal is central to differential gene expression on left vs. right sides.
What about snails? Snails are cool because their asymmetries are just hanging out there visibly, easy to see without taking a scalpel to their torsos (there are also internal asymmetries that we’d need to do a dissection to see, but the external markers are easier). The assymetries also appear very early in the embryo, in a process called spiral cleavage, and in the adult, they are obvious in the handedness of shell coiling. We can see shells with either a left-handed or right-handed spiral.
Until now, the only organisms thought to use nodal in setting up left/right asymmetries were us deuterostomes — chordates and echinoderms. In the other big (all right, bigger) branch of the animals, the protostomes, nodal seemed to be lacking. Little jellies, the cnidaria, didn’t have it, and one could argue that with radial symmetry it isn’t useful. The ecdysozoans, animals like insects and crustaceans and nematodes, which do show asymmetries, don’t use nodal for that function. This suggests that maybe nodal was a deuterostome innovation, something that was not used in setting up left and right in the last common ancestor of us animals.
That’s why this is interesting news. If a major protostome group, the lophotrochozoa (which includes the snails) use nodal to set up left and right, that implies that the ecdysozoans are the odd group — they secondarily lost nodal function. That would suggest then that our last common ancestor, a distant pre-Cambrian worm, used this molecule in the same way.
Look in the very early mollusc embryo, and there’s nodal (in red, below) switched on in one or a few cells on one side of the embryo, the right. It’s asymmetrical gene expression!
Seeing it expressed is tantalizing, but the next question is whether it actually does anything in these embryos. The test is to interfere with the nodal-Pitx2 pathway and see if the asymmetry goes away…and it does, in a dramatic way. There is a chemical inhibitor called SB-431542 that disrupts this pathway, and exposing embryos to it does interesting things to the formation of the shell. In the photos below, the animal on the left is a control, and what you’re seeing is a coiled shell (opening to the right). The other two views are of an animal treated with SB-431542…and look! Its shell doesn’t have either a left- or right-handed twist, and instead extends as a straight tube.
What this all means is that we’ve got a slightly better picture of what genes were present in the ancestral bilaterian animal. It probably had both nodal and Pitx2, and used them to build up handedness specializations. Grande and Patel spell this out:
Although Pitx orthologues have also been identified in non-deuterostomes such as Drosophila melanogaster and
Caenorhabditis elegans, in these species Pitx has not been reported in
asymmetrical expression patterns. Our results suggest that asymmetrical expression of Pitx might be an ancestral feature of the bilaterians.
Furthermore, our data suggest that nodal was present in the common
ancestor of all bilaterians and that it too may have been expressed
asymmetrically. Various lines of evidence indicate that the last common ancestor of all snails had a dextral body. If this is true, then our
data would suggest that this animal expressed both nodal and Pitx on
the right side. Combined with the fact that nodal and Pitx are also
expressed on the right side in sea urchins, this raises the possibility
that the bilaterian ancestor had left-right asymmetry controlled by
nodal and Pitx expressed on the right side of the body. Although
independent co-option is always a possibility, the hypotheses we present can be tested by examining nodal and Pitx expression and function in a variety of additional invertebrates.
It’s also, of course, more evidence for the unity of life. We are related to molluscs, and share key genes between us.
Grande C, Patel NH (2009) Nodal signalling is involved in left-right asymmetry in snails. Nature 457(7232):1007-11.
A fish is a fish, right? They’re just a blur of aquatic beasties that most people distinguish by flavor, rather than morphology or descent. But fish are incredibly diverse, far more diverse than terrestrial vertebrates, and there are significant divisions within the group. Most people know of one big distinction, between the Chondrichthyes (fish with cartilaginous skeletons, like sharks and rays) and the Osteichthyes (fish with bony skeletons), but there’s another particularly interesting split within the Osteichthyes: the distinction between Sarcopterygians (the word means “fleshy fins”, and we call them lobe-finned fishes colloquially) and the Actinopterygians, the ray-finned fishes. The lobe-finned fishes most distinctive feature is the muscular and bony central core of their fins — extant forms are the coelacanth and lungfish. It is this lineage that led to us terrestrial tetrapods, but other than that successful invasion of the land, the sarcops were something of an aquatic failure, with only a few genera surviving. The ray-finned fishes, on the other hand, are a major success story, with more than 28,000 species today. To put that in context, there are only about 5,500 species of mammals.
The Sarcopterygii and the Actinopterygii must have begun diverging a long time ago, and a couple of questions of interest are a) when did the last common ancestor of both groups live, and b) what did it look like? We don’t have a good and specific answer yet, because Osteichthyes origins are lost far, far back in time, over 400 million years ago, but every new discovery edges us a little closer. What we now have is a well-preserved fossil of a fish that has been determined to be an early sarcopterygian, and it tells us that a) the last common ancestor had to have lived over 419 million years ago, the age of this fossil, and the divergence probably occurred deep in the Silurian, and b) this animal has a mosaic of primitive Osteichthyan features, which tells us that that last common ancestor may well have shared some of these elements. It is another transitional fossil that reveals much about the gradual separation of two great vertebrate groups.
And here it is:
That may be a bit disappointing at first — it looks like Silurian road-kill — but really, that’s a remarkable good and useful specimen. The animal was covered with thick bony scales, and the skull was built of thick bony plates, and so while it was squashed flat by pitiless geology, the pieces are all there, and it can be reassembled into a much more fishy state. This drawing may be more satisfying:
Now it looks like a kind of armored, spiky salmon with a thick muscular body (and yes, I too wonder about flavor, and would like to taste a slab of that). It’s definitely not a salmon, though — the bony structure is a curious set of compromises where some features are distinctly sarcopterygian, some look like they belong on a primitive actinopterygian, and others are unique or show affinities to characters of ancient extinct fishes, like rhipidistians. This is very cool. What we see here are relics of an ancient common osteichthyan ancestor, which are being honed into the specific characteristics of the Sarcopterygii. The analysis of the totality of the animal’s features, though, place it more in the lobe-finned than the ray-finned clade. That places it on a branch of the line leading to us…a very, very old branch, making this your many-times-great grand uncle, or cousin only a few million times removed. Now my curiosity about a taste-test is making me feel mildly cannibalistic.
When you look at that diagram, what should jump out at you is all the diversity in the Devonian, the so-called Age of Fishes, and the paucity of representative fossils from the Silurian…which is exactly where all the interesting branch points in the fish family tree are located. Once again, paleontology is a predictive science, and this tells us where to look for the next batch of exciting and informative fossils.
Zhu M, Zhao W, Jia L, Lu J, Qiao T, Qu Q (2009) The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature 458:469-474.
I’m not going to say much about this since Ed Yong has an excellent write-up, but a new feathered dinosaur has been discovered, called Tianyulong. As you can see in this image of the fossil, it was bristling with a fuzz of thin fibers — proto-feathers.
There are a couple of noteworthy features in this creature. One is apparent: feathers just didn’t bloom suddenly in evolution, but appeared in steps. This animal has ‘feathers’ that don’t branch like those of modern birds, but instead form more of a furry coat than a set of flat blades.
The other cool thing is that this is an ornithischian dinosaur; most of the other dinosaurs that have been discovered to have feathers were saurischian. What that means might be made more clear by this diagram:
It implies that just maybe the last common ancestor of the saurischia and ornithischia were also covered with proto-feathers, which means that feathers may be a primitive state in this lineage.
Zheng X-T, You H-L, Xu X, Dong Z-M (2009) An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. Nature 458:333-336.
Several new and spectacular cephalopod fossils from 95 million years ago have been found in Lebanon. “Spectacular” is not hyperbole — these specimens have wonderfully well-preserved soft parts, mineralized in fine-grained calcium phosphate, and you can see…well, take a look.
The Open University is having an open lecture on 17 March, and you’re all invited! The topic sounds historically, philosophically, and scientifically interesting:
Richard Dawkins suggests that there are four “bridges to evolutionary understanding” and illustrates this with four claimants to the discovery of natural selection: Edward Blyth, Patrick Matthew, Alfred Wallace and Charles Darwin.
The fifth bridge of evolutionary understanding is identified as modern genetics – which he terms digital Darwinism.
It’s all going to be streamed live on the web, if you are awake at 7:30 pm Natural History Museum time, which I won’t, or you can grab it from a webcast after the event.
This is a cool talk: Bill Gross talks about his efforts to tap into solar power. It’s a little bit over-optimistic — how much of the desert Southwest would we have to pave over to collect enough energy for the country? — but the really fun part is where he talks about using unguided evolutionary processes to design solar collectors and heat engines. People who claim that chance and selection can’t produce anything new have never tinkered with genetic algorithms.
So Ray Comfort is now complaining on the revered pages of the respected publication, World Net Daily about me. The article is full of dishonest misquotes, but let’s zip right to Ray’s scientific misunderstandings. They are deep and painful. He has this bizarre idée fixe that the necessity of every species having males and females somehow greatly reduces the probability that new species could arise. It’s total nonsense, and I dismissed it briefly when I commented on it before.
“I know Ray is rather stupid, but who knew he could be that stupid. This has been explained to him multiple times: evolution does explain this stuff trivially. Populations evolve, not individuals, and male and female elephants evolved from populations of pre-elephants that contained males and females. Species do not arise from single new mutant males that then have to find a corresponding mutant female – they arise by the diffusion of variation through a whole population, male and female.”
Ray has read that, and failed to grasp the central concept. Take a look at the workings of Ray Comfort’s mind as he attempts to wrestle with a simple idea: the hamster wheel is wobbling, but the poor beast lies dead with legs up in its cage, and nothing is turning over.
He was probably able to get home before midnight last night. You can now read his description of the social events around Dawkins’ visit, and a much more thorough account of the talk itself.
One other point that I should emphasize. This talk presented an overview of how we should look at the appearance of design in the universe, for a general public. While I heard some complaints that there was nothing new in it, that’s the way this had to be: it was a synthesis of a position.
Dawkins is often given a rap as one of those ultradarwinians who see every detail of life as the explicit product of carefully honed selection. For me, what was interesting in this talk was how clearly he repudiated that position. In several places, he contrasted what he called a “naive Darwinist” perspective with reality, and showed that the strawman didn’t hold up. A major point was also that features that may very well have evolved with a core that was selected for can have side-effects, and been subverted to non-adaptive purposes, and that these features may represent a significant element of the species’ character. He talked quite a bit about the flexibility of the human brain, a property that was the product of selection, yet that same flexibility means it can be reprogrammed into deleterious byways, such as religion or fanaticism or unthinking patriotism.
It was all stuff that I agreed with, and didn’t surprise me at all. Similarly, The God Delusion didn’t contain anything radical or new. The virtue of these kinds of talks and books is that they pull many commonly held ideas together into a coherent fusion that can be more readily absorbed by a larger number of people who haven’t yet taken in all of the underlying evidence.
