Longtime readers know that I get really excited by clear (or not so clear) cases of convergent evolution. Pound for pound, convergence is the most persuasive evidence for the truth of evolution out there; different lineages finding novel paths to the same solution. While I mostly talk about convergences in morphology, genetic convergence is often even more fascinating. Enter the bizarre case of the serpent and the platypus.
Snake venom protein has a host of origins, from acetylcholinesterase to whey acidic protein. The protein I’m most interested in has its origins in the beta-defensin genes.
Defensins are anti-microbial proteins that function as part of the immune systems of most vertebrates. They are effective against both prokaryote and fungal invaders and function by rendering the target cell membrane permeable. Once the defensins enter the cell, their function is less understood, but they somehow inhibit RNA synthesis, a common attack mechnism for immune proteins and toxins. Beta-defensins are a sub-group of defensin proteins that occur in a diverse array of organisms, including (but not limited to) sea anenomes, snakes, platypuses, and humans.
In snakes and platypuses, these proteins function not as part of the immune system, but as a self-defense mechanism in both snake and platypus venom.
It goes without saying that platypuses are extremely weird. As monotremes, they are one of the oldest mammal lineages, and, of course, their morphology is so bizarre that when the first samples were brought back from Australia, scientists thought they were taxodermal farces. Yes, platypuses have venom. It’s produced by male platypuses in spurs on their hind feet. The venom sting can be extremely painful although it is not fatal to humans.
But platypuses can go a long way to informing evolution. By assembling the platypus genome, we can glimpse ancient, shared traits that were present when our lineages diverged. Last year, the platypus genome project was completed.
Among many of the wonderful and odd elements of the platypus genome, the most striking was one of the genes regulating platypus venom. It was a derivative of beta-defensin. Even more surprising, the platypus and snake genes appeared to be almost identical. Almost.
The evolution of both proteins are the result of replication events on the original beta-defensin antimicrobial genes. These events resulted in both an increase in genetic information (for all those creationists arguing that there are no cases of increasing information in the genetics) and a transformation in function. But these replication events are not identical. If they were, that would mean that snakes and platypuses share a common ancestor that occurred long after placental and marsupial mammals diverged. Parsimony dictates that this is wrong.
Instead, it appears as though a gene, common to many animals, suffered separate replication events that resulted in the same protein with the same function, convergent evolution at its most basic. The idea that two random mutations, with slightly different replication mechanisms, have created the same venom protein is mind-blowing.
To be fair, these are not the only proteins involved in either snake or platypus venom. There are a whole host of other proteins that make up the venom, but the beta-defensin complex is an important piece of the puzzle, and one shared across a host of organisms.
This is part of the beauty of the theory of evolution. It allows us to look at features like these and draw conclusions about how those organisms developed. Convergence is the most parsimonious explanation and it highlights how easy it is for advanced systems to develop, given enough time and a system that is both robust and mutable.
~Southern Fried Scientist
- Fry, B. (2005). From genome to “venome”: Molecular origin and evolution of the snake venom proteome inferred from phylogenetic analysis of toxin sequences and related body proteins Genome Research, 15 (3), 403-420 DOI: 10.1101/gr.3228405
- TORRES, A., & KUCHEL, P. (2004). The Beta-defensin-fold family of polypeptides Toxicon, 44 (6), 581-588 DOI: 10.1016/j.toxicon.2004.07.011
- Ganz, T. (2003). Defensins: antimicrobial peptides of innate immunity Nature Reviews Immunology, 3 (9), 710-720 DOI: 10.1038/nri1180
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