Thursday, October 6, 2011

Rapid Venom Evolution in Pit Vipers May Be Defensive

Recent research published by the Am
erican Museum of Natural History has been able to deliver some important insight into the evolution of toxins in snake venom. Although originally thought to be a feeding, or trophic, adaptation, these studies have revealed this evolution to be a critical defensive technique against predators like hedgehogs, mongooses, and opossums.
Robert Voss, curator in the Department of Mammalogy at the museum, explained how for many years herpetologists thought of the evolution of toxins merely based on interactions with prey. "But if that were true," he continues, "[one] would see equally rapid
evolution in toxin-targeted molecules of prey species, which has not yet been seen." Instead, what's been found is a curious mutation in predators of pit vipers.Snake venom is so incredibly dangerous because it contains highly toxic compounds that will attack blood proteins, or serum proteins. These are responsible for, among other things, regulation
and functioning within the immune system. When they are targeted, massive internal hemorrhaging will occur in non-resistant, warm-blooded species.
It was during research on snake-eating opossums, not the snakes themselves, that this new insight came into being. Phylogenetic study suggested incredibly rapid evolution in the gene that codes for the von Willebrand factor, an important blood-clotting protein. While sequencing several genes, this one stood alone as an unusual outlier. It was evolving much more rapidly than expected in a group of opossums discovered to be resistant to pit viper venom.
What the researchers discovered was that the rate of replacement substitutions was much higher than the rate of silent substitutions in the von Willebrand factor gene among these opossums. "The specific amino acids in vWF that interact with toxin proteins show unexpectedly high rates of replacement substitutions," explained research associate Sharon Jansa, "[they] undoubtedly affect protein function, suggesting that the vWF protein can no longer be attacked by these snake toxins."
This meant this gene was under strong positive selection, actively coevolving with the increasingly dangerous snake venom toxins. Therefore, researchers concluded it was the predator driving the snakes' rapid evolution, not the prey.

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