The new research came out of a previous phylogenetic study of marsupials, published as a Bulletin of the American Museum of Natural History, that suggested unusually rapid evolution in one gene among a group of snake-eating opossums. The rapidly evolving gene codes for von Willebrand's factor, an important blood-clotting protein that is known to be the target of several snake-venom toxins. The association of rapid evolution in a venom-targeted gene among just those opossums known to eat pitvipers was the essential clue that prompted further study.
"This finding took us by surprise," says Sharon Jansa, associate professor in the Department of Ecology, Evolution and Behavior at the University of Minnesota and a Museum research associate. "We sequenced several genesincluding the one that codes for von Willebrand Factor (vWF)to use in a study of opossum phylogeny. Once we started to analyze the data, vWF was a real outlier. It was evolving much more rapidly than expected in a group of opossums that also, as it turns out, are resistant to pitviper venom."
The recently published research demonstrates that the rate of replacement substitutions (nucleotide changes that result in amino-acid changes) is much higher than the rate of silent substitutions (nucleotide changes that have no effect on the protein) in the von Willebrand Factor gene among pitviper-eating opossums. Typically, high rates of replacement substitutions means that the gene is under strong, sustained natural selection. That only happens in a few evolutionary circumstances.
"Most nucleotide substitutions have little or no effect on protein function, but that doesn't seem to be the case with vWF in these venom-resistant opossums," says Jansa. "The specific amino acids in vWF that interact with toxin proteins show unexpectedly high rates of replacement substitutions. These
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American Museum of Natural History