Monday, February 06, 2006

Snakes and Lizards: Venom

I was blown away by an article I found in Nature this week - an article that has generally escaped the attention of bloggers, or so I thought. It turns out that I am a bit behind the mark - despite only being published in the print version of Nature this week, the article, it seems, is so last year, having appeared at Carl Zimmer's blog The Loom and at Afarensis also, with some discussion.

Nevertheless, whenever the article was published, it seriously impressed me - not so much because of its implications for the evolution of venom systems in snakes and lizards (although there is plenty of that), but more so because of the staggering amount of work that went into drawing some very strong, very detailed conclusions; More so because the research, analysis and communication represents good, nay excellent science.

So many of the articles published in journals (particularly the brevia) discuss the results of a single experiment with a bioactive compound, or a field study of an animal; perhaps a play back experiment in bird song, or a test of olfactory identification by petrels, as two examples I have seen. While these experiments are surely elegant, subtle and well designed, they are inherently limited by their restricted scopes. Too often their validity rests upon their compatibility with the results from other disciplines, and the effect of other variables that may not have been tested concurrently. Their results are statistically significant, for sure, and their conclusions undoubtedly sound, but they are in many cases hamstrung in their subsequent applicability to other phenomena, because the original methodology and execution lacked the robustness of a cross disciplinary approach.

The Nature article is produced by 14 authors, chief among them a chap named Dr Bryan Fry from the Australian Venom Research Unit in Melbourne, and is a sterling example of the power of an holistic, cross-disciplinary approach to resolving scientific questions.

Essentially what the article details is an investigation into the evolution of venom production and delivery in the Squamata, the subgroup of diapsid 'reptiles' containing lizards and snakes. More than that, though, the research challenges a number of major assumptions that we hold about these animals; assumptions about evolutionary relationships, assumptions about which lineages actually produce venom and, perhaps most interestingly, assumptions about how at least one lineage was originally thought to have hunted prey. All of these challenges, these conclusions, gain their power from the combination of extensive research across the disciplines of molecular phylogenetics, histology, molecular modelling, molecular evolution and cDNA library construction, toxicology and pharmacology, all against the backdrop of paleaontological reconstructions of the animals.

The most recent consensus of relationships (phylogeny) between the lineages of the Squamata clade is shown below on the left (from the very excellent Tree Of Life Website). The phylogenetic tree printed in Fry's article is on the right.

[Click to Enlarge]

As you can see, there is a major reshuffling of many of the lineages. Most interestingly, the relationship of the snakes appears to have been confidently resolved, with the phylogram placing them in a single clade with the Iguania and the [Helodermatidae + Anguidae + Varandidae] clade, (collectively the three are known as the Anguimorpha). The new phylogeny also changes the place of the Iguania clade (which includes the Iguana lizards), from being the most ancestral or basal lineage of the squamates to one of the most derived. It also means that Iguanas and anguimorph lizards are more closely related to snakes than they are to other lizards. Lets be clear: this is quite a fundamental shift.

Our understanding of venom systems in the squamates (until this research) basically held that only the Serpentes (snakes) and Helodermatidae (Gila Monster) had venom delivery systems, and that the evolution of venom delivery systems in the Serpentes essentially underscored their massive diversification - it was what made them so successfula as a group of reptiles. If this conclusion were true, then according to either of the phylogenies above, the evolution of these systems happened independently - that is, by convergent evolutionary processes. Analysising the systems would make this a pretty reasonable conclusion: snakes deliver their venom via specialised glands in the upper jaw (maxilla), whereas Gila Monsters produce theirs via a gland on their lower jaw (mandible) which then delivers the venom along grooves in the teeth.

Now comes the beauty of the research: The researchers showed that other squamates possessed gland that produced proteins in a similar fashion; the Iguanas (both upper and lower glands) and the Varanids (lower). 'Libraries' were constructed of the DNA coding for all the active compounds coming from all the glands, upper and lower. All members of the 'Venom' group - the Snakes, the Komodos and Monitors and the Iguanas - were analysed, and the nature of the proteins, and the relationships between them, were investigated.

The article shows that the 'ancestral' state appears to have been a delivery system that incorporated both the upper and lower jaw bones. 9 of the original toxin proteins are today found in both the snakes and lizards, 2 of which are only found in the upper jaw glands of Snakes and Iguanas. It appears that, after the development of these 9 proteins, the early snakes split off and underwent a massive evolution - essentially ceasing production of toxins in the lower jaw (except for a few species), but inventing at least 16 other toxins that they delivered through the upper jaw. The next split was the Iguanians, retaining the ancestral condition both upper and lower jaw venom delivery systems. The remaining Anguimorph lizards went the other way - losing the ability to produce toxin in the upper jaw, but inventing at least 3 new toxins to be delivered through the lower jaw.

These three toxins were originally only thought to have been produced by the Gila monster's group, the Helodermatidae, but the article shows that one of the toxins, PLA2 Type III, is also found in the Varanidae - a group that includes the Komodo Dragon, and the Goanna of Australia.

I'm skimming over a great deal here - work that would have gone into deciphering the relationships between toxin molecules is MASSIVE, work that analysed tissue samples, molecule structure, bioactivity and physical properties, not to mention trying to put all of the molecules together in an evolutionary sense. It is a breathtaking amount of research to do, but the picture it produces is worth it.

We can now see that the evolution of venom systems in snakes and lizards was not an entirely independent process - that many of the conditions which preceded modern the modern systems developed before the first snakes even appeared. We can see that, in each of the lineages, subsequent evolution took its course in a way that led us to believe they were independent, that each lineage 'co-opted' only a part of the ancestral state, producing what appear to be unique systems.

The new picture also has major implications for the predatory behaviour one of the lineages which we thought we understood, and about which we could have been horribly wrong: the lineage of the Komodo Dragon, the Goanna and the extinct Megalania prisca

Conventional wisdom has it that the Varanidae hunt by infection, that is, that the lizards deliver a bite to their prey, injecting it with massive amounts of bacteria (over 50 strains!) that fester in its gums. They wait until the animal is overcome by blood loss, infection and shock and then go in for the kill. It turns out that Fry and his team don't think this is a very good explanation at all - a 'red herring' if you will. Previous work on varanid bites, and the speed at which prey items (or humans!) react to the bite has indicated the presence of an active biological compound (such as venom) rather than a slow, passive bacterial infection.

The article shows that the toxins identified in varanid bites are consistent with the effects seen in human victims, including respiratory problems, intense pain, muscle weakness and increased and irregular heart rates. It appears that there is more to this story, and it definitely needs to be investigated further.

Interestingly enough, this little discovery also influences, or at least affects our ability to accept, a possible model of predatory behaviour in a related but very dead animal: Tyrannosaurus rex. Some paleontologists have attempted to show that Tyrannosaurus jaw, teeth and gum structure was very similar to that of the Komodo Dragon, and that it was possible T. rex may also have hunted by causing infectious bites. If it turns out that the Komodo Dragon doesn't even do that, and instead delivers a venomous bite, we may have to throw that particular explanation out the window.

While I respect Dr Fry's expertise on this matter, I would certainly caution against throwing out the "Infectious Bite" explanation altogether. While it may be that the Varanidae do deliver a bioactive venom in their bites, it is also clear that they do have exceptionally filthy mouths. The two explanations are not mutually exclusive, and I would argue that any bite that ensures death, be it immediately or in a week's time, is likely to be a very useful adaptation.

The article seems small and unassuming, but it's conclusions, and the work behind them, are phenomenal. By combining a whole range of seemingly distinct disciplines, the researchers have been able to produce a robust, profound and clear explanation about some of the most interesting animals and the way that they operate in their own worlds. It's this kind of work that makes me itch to do research. It's this kind of thing that makes me love science and the most profound way, and shows me why there is nothing else I want to do with my life.

And just because it's put me in such a good mood, here is a copy of the Wedge Strategy produced by the Discovery Institute, on how to get Intelligent Design in public schools, Reproduced from the Seattle Weekly, via Pharyngula, the best science blog I've read...Just click to enlarge

Have a Happy Waitangi Day