The Serpent and the Platypus

ResearchBlogging.org

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.

rattlerSnake 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.

platypus_spurIt 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


Interesting Resources

  1. 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
  2. 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
  3. Ganz, T. (2003). Defensins: antimicrobial peptides of innate immunity Nature Reviews Immunology, 3 (9), 710-720 DOI: 10.1038/nri1180
  4. Warren, W., Hillier, L., Marshall Graves, J., Birney, E., Ponting, C., Grützner, F., Belov, K., Miller, W., Clarke, L., Chinwalla, A., Yang, S., Heger, A., Locke, D., Miethke, P., Waters, P., Veyrunes, F., Fulton, L., Fulton, B., Graves, T., Wallis, J., Puente, X., López-Otín, C., Ordóñez, G., Eichler, E., Chen, L., Cheng, Z., Deakin, J., Alsop, A., Thompson, K., Kirby, P., Papenfuss, A., Wakefield, M., Olender, T., Lancet, D., Huttley, G., Smit, A., Pask, A., Temple-Smith, P., Batzer, M., Walker, J., Konkel, M., Harris, R., Whittington, C., Wong, E., Gemmell, N., Buschiazzo, E., Vargas Jentzsch, I., Merkel, A., Schmitz, J., Zemann, A., Churakov, G., Ole Kriegs, J., Brosius, J., Murchison, E., Sachidanandam, R., Smith, C., Hannon, G., Tsend-Ayush, E., McMillan, D., Attenborough, R., Rens, W., Ferguson-Smith, M., Lefèvre, C., Sharp, J., Nicholas, K., Ray, D., Kube, M., Reinhardt, R., Pringle, T., Taylor, J., Jones, R., Nixon, B., Dacheux, J., Niwa, H., Sekita, Y., Huang, X., Stark, A., Kheradpour, P., Kellis, M., Flicek, P., Chen, Y., Webber, C., Hardison, R., Nelson, J., Hallsworth-Pepin, K., Delehaunty, K., Markovic, C., Minx, P., Feng, Y., Kremitzki, C., Mitreva, M., Glasscock, J., Wylie, T., Wohldmann, P., Thiru, P., Nhan, M., Pohl, C., Smith, S., Hou, S., Renfree, M., Mardis, E., & Wilson, R. (2008). Genome analysis of the platypus reveals unique signatures of evolution Nature, 453 (7192), 175-183 DOI: 10.1038/nature06936
  1. This is so cool! Never would I have thought a platypus to have nearly the same venom as a snake. Maybe somewhere down the line humans will evolve this awesome trait..just kidding.

  2. That’s gross David, but I would have never thought of that to be a venomous trait. This was really interesting to read about. I never even thought that a platypus had any kind of venom so this is all news to me. I can’t believe a playpus could be dangerous. Snakes terrify me so the thought of another animal being as dangerous is scary.

  3. This is really interesting, and I like how random these animals are because it makes it even better. I wonder how they discovered this, or why they decided to study and compare the proteins in these 2 animals, considering how different they are. How comparable are these proteins, or are they common in many animals and not actually as closely linked as this makes them sound?

  4. This post is very interesting. It always amazes me how two different species can be traced back to a common ancestor using DNA information. Two species that you would never think might be realted because of how they look and interact today could have easily decended from the same ancestor. This makes me wonder what other animals contain the beta-defensin complex and other charactistics that may link their “heritage” that we would not normally think or know about.

  5. This is fascinating! I had no idea that male platypuses had venom in their hind feet! It just goes to show how drastically our world has evolved. If snakes and platypuses are actually are related that would be an amazing scientific discovery!

  6. So Platypus and snake share the same venom or some of their venom have similar traits? Either way, that is so interesting. Was this discovery an accident? or someone thought of this specifically?

  7. Folks, there seems to be a lot of confusion going on here.

    First off, beta-defensins are common in most vertebrates and many invertebrates and function as part of the immune system. In snakes and platypuses, two DIFFERENT evolutionary events created nearly identical toxic proteins. Snakes and platypuses are not more closely related than platypuses and placental mammals.

    Second, all living organisms share a common ancestor, everything is related. It’s the pathways from that common ancestor to current forms which are interesting.

    Finally, the discovery of the mutation event in the platypus came with the sequencing of the entire platypus genome. Snake venom complexes have been studied in detail for decades.

    And for a point of clarification: Convergence is an evolutionary process whereby separate events and pathways develop the similar or identical traits because they’re fulfilling the same function.

  8. Ahh yes, the beauty of evolution! Being a staunch evolutionary-atheist, the theory- or should I say fact- of convergence evolution is just another proof that, creationism is bs and that evolution is the truth.
    Anyways, I have recently found a way to get onto the roof of my house in Charleston. So, the other night while sharing a few Coronas with friends, we all witnessed convergent evolution at its finest. What I am speaking of here is the “wing.” Around four- five pm (we started early) we first watched cardinals, blue jays, and even a curious hawk, “socialize” and feed, flying above our heads. As the day turned to night, we started to see several bats dart back and forth in the air. It was simply amazing to see convergent evolution at its best. Although bats are mammals and birds are well birds, both have “wings,” however, they but evolutionary converged through natural selection completely separate of one another. Simply amazing.

    • Nope, you should say Theory. Facts are discrete units of information, Theories are grand concepts that unify facts into a cohesive system which describes a piece of how the universe works. Don’t get caught up in the “just a theory” game, a Theory is the strongest statement science can make.

      Theory of Gravity…
      Theory of Evolution…
      Theory of Plate Tectonics…
      Theory of Relativity…

  9. “or should I say fact- of convergence evolution is just another proof that, creationism is bs and that evolution is the truth.”

    Sounds like circular logic, with a little strawman thrown in for the side that is “bs”. Perhaps you should read Carl Sagans baloney detector test (he was a religious skeptic in case you are afraid to look).

    Modern car “evolved” from model T, Windows XP evolved from windows 2000, the more complex humans make stuff, the more likely it is closely related to a previous design. Richard Dawkings in “The Blind Watchmaker” claims that software can evolve, but so far most programmers understand the limits of that, Microsoft is only able to “tune” an SQL server rather than engineer new features using chance and selection. It is easy in some cases such as SQL server to test for fitness and allow the “fitter” mutations to survive and billions of generations could happen in a day, but programmers know what the results would be, brute force has its limits.

    Human intelligent design sometimes uses gene splicing to transfer a feature from one species to another. Why is X a fact and Y sure bs, when we ourselves do Y?

    • Except that the development of venom in snakes and platypuses aren’t based on the same feature just “spliced from one species to another”. They evolved separately, caused by very different processes randomly acting on the same gene, which is present in most vertebrates. No need for a “designer” at all, and no reason to arbitrarily insert one just because you feel like it.

      No circular logic there, and you’re the only one throwing up strawmen.