Enter the Cobra

A study published in this week’s PNAS has unraveled the genome of king cobra venom and discovered evidence of a genetic arms race that may have led to snake venom’s genetic complexity and remarkable potency. The group found that genes connected to cobra venom production had undergone repeated duplication within the snake’s genome, allowing those genes to mutate rapidly and thus helping the toxic mechanism of venom to change more easily.

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A king cobra’s venom is coded by genes prone to mutation, making it a constantly evolving threat to prey, and making king cobras an even more terrifying creature to face down (Source: animals.nationalgeographic.com)

The study’s results suggest that snake venom’s evolution is closely linked to how successful a poison it is. The researchers compared the king cobra’s venom genes to those of the only other venomous vertebrate whose genome has been fully sequenced, the platypus. Whereas the cobra’s venom was made up of genes that had been duplicated repeatedly to create a more complex chemical cocktail, the platypus’ venom was fairly simple by comparison. The group suggests that the source of this fundamental difference lies in the differing functions of each poison.

According to the authors, platypus venom is used mostly in male-male combat as a competitive tool to win mates. As such, they really only ever have to compete with one another and evolution is driven by sexual selection towards those concoctions that are most potent against other male platypuses. Snakes on the other hand have to be able to take down a wide variety of prey, and their ability to do so relies heavily on how effective their venom is at killing any number of tasty organisms. As these diverse forms of dinner evolved their own defenses against snake venom over time, snakes had to continue to evolve to ensure that their number one biological weapon stayed relevant. The authors argue that this created a sort of biological arms race where those snakes with more unstable, and thus more dynamically changing, venom genes would survive best because they were capable of remaining one step ahead of prey defenses. This then explains both the complexity of the venom genome and how different it is from the platypus.

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A platypus’ venom is fairly stable genetically speaking, meaning its venom isn’t all that dangerous. One more reason no one has ever been scared of a platypus (Source: http://www.zoo.org.au)

The group’s results also vindicate a recently abandoned idea about how venom glands evolved. It was previously thought that snakes’ venom glands were distantly related to the pancreas, but a lack of evidence had led many scientists to discount this possibility and move towards other explanations. In their analysis of the micro RNA (miRNA) strands in venom glands, this group found that the most abundant miRNA was miR-375, which is associated with pancreatic cells that help secrete the pancreas’ various products (like insulin). What this means is that at some point venom glands co-opted the genes that controlled pancreas secretion to secrete venom, thus connecting the two organs definitively.

Their results could also have more widespread implications about how proteins in general have evolved, but there will need to be a lot more work into how environmental factors have shaped genetics before such broad conclusions can be drawn. For now, king cobras are just that much more intimidating, what with their constantly evolving poison specially designed to outsmart a victim’s defenses. Thanks science. I’ll sleep a lot better now, I’m sure.

Original article:

http://www.pnas.org/content/110/51/20651.full

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