Mary’s Monday Metazoan: Coming in for a landing
Pharyngula
Evolution, development, and random biological ejaculations from a godless liberal
This sad chimpanzee at the Mysore Zoo has lost all of his hair. He looks rather Caucasian, I think…
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.
An aquarium was having a problem with their coral reef and fish disappearing overnight. It wasn’t the cephalopod! They dismantled the reef rock and discovered a 4-foot long polychaete worm.
They’ve given the beast its own tank now. Good thing — I’d pay to go see that.
The history of venoms is a wonderful example of an evolutionary process. We’re all familiar with the idea of venomous snakes, but the cool thing is that when we examine exactly what it is they’re injecting into their prey, it’s a collection of proteins that show a nested hierarchy of descent. Ancient reptiles had a small and nasty set of poisons they would use, and to improve their efficacy, more and more have been added to the cocktail; so some lizards produce venomous proteins, while the really dangerous members of the Serpentes produce those same proteins, plus a large array of others.
So something like CRISP (Cystein RIch Secretory Protein) is common to all, but only the most refined predators add PLA2 (Phosopholipase A2) to the mix.
Now lethally poisonous snakes are nice and cute and all, but we all know where the interesting action really is: cephalopods. Let’s leave the vertebrates altogether and look at a venomous protostome clade to see what they do.
Brian Fry, who did all that excellent work characterizing and cataloging the
pharmacy of venoms secreted by poisonous snakes, has also turned his hand to the cephalopods. He examined the products of the venom glands of octopus, squid, and cuttlefish, and found a range of proteins, some unique, and others familiar: CAP (a CRISP protein), chitinase, peptidase S1, PLA2 and others. There are a couple of interesting lessons in that list.
First, evolution doesn’t just invent something brand new on the spot to fill a function — what we find instead is that existing proteins are repurposed to do a job. This is how evolution generally operates, taking what already exists and tinkering and reshaping it to better fulfill a useful function. Phospholipase A2, for instance, is a perfectly harmless and extremely useful non-venomous protein in many organisms — we non-toxic humans also make it. We use it as a regulatory signal to control the inflammation response to infection and injury — in moderation, it’s a good thing. What venomous animals can do, though, is inject us with an overdose of this regulator to send our local repair and recovery systems berserk, producing swelling that can incapacitate a tissue. Similarly, a peptidase is a useful enzyme for breaking down proteins in the digestive system…but a poisonous snake or cephalopod biting your hand can squirt it into the tissues, and now it’s being used to digest your muscles and connective tissue. Some effective venoms are simply common proteins used inappropriately (from the perspective of the target).
Another interesting observation is that cephalopods and vertebrates have independently converged in using some of the same venoms. In part, this is a consequence of historical availability — all animals have phospholipases,, since they are important general signalling molecules, so it’s part of the collection of widgets in the metazoan toolbox from which evolution can draw. It’s also part of an inflammation pathway that can be exploited by predators, in the same way that we have shared proteins used in the operation of the nervous system that can be targeted by neurotoxins. So there is independent convergence on a specific use of these proteins as toxins, but one of the things that facilitates the convergence is a shared ancestry.
In fact, some very diverse groups seem to consistently settle on the same likely suspects in their venoms.
But finally, there must also be physical and chemical proteins of these particular proteins that must also predispose them to use as toxins. After all, animals aren’t coopting just any protein for venoms — they aren’t injecting large quantities of tubulin or heat shock proteins into their prey. There must be something about each of the standard suspects in venoms that make them particularly dangerous. What the comparative evolutionary approach allows us to do is identify the common molecular properties that make for a good venom. As Fry explains it,
Typically the proteins chosen are from
widely dispersed multigene secretory protein families with
extensive cysteine cross-linking. These proteins are collectively much more numerous than globular enzymes,
transmembrane proteins, or intracellular protein. Although
the relative abundance of these protein types in animal
venoms may reflect stochastic recruitment processes, there
has not been a single reported case of a signal peptide
added onto a transmembrane or intracellular protein or a
hybrid protein expressed in a venom gland. A strong bias is
also evident for all of the protein-scaffold types, whether
from peptides or enzymes. Although the protein scaffolds
present in venoms represent functionally and structurally
versatile kinds, they share an underlying biochemistry that
would produce toxic effects when delivered as an “overdose”. Toxic effects include taking
advantage of a universally present substrate to cause
physical damage or causing changes in physiological
chemistry though agonistic or antagonistic targeting. This allows the new venom gland protein to have an
immediate effect based on overexpression of the original
bioactivity. Furthermore, the features of widely dispersed
body proteins, particularly the presence of a molecular
scaffold amenable to functional diversification, are features
that make a protein suitable for accelerated gene duplication and diversification in the venom gland.
To simplify, killing something with a secreted poison typically involves reusing an extant protein, but not just any protein — only a subset of the proteins in an animal’s proteome has just the right properties to make for a good venom. Therefore, we see the same small set of proteins get independently coopted into the venom glands of various creatures.
Fry BG, Roelants K, Norman JA (2009) Tentacles of venom: toxic protein convergence in the Kingdom Animalia. J Mol Evol Mar 18. [Epub ahead of print].
