Strike, counterstrike

Natural History, June, 2005 by Stephan Reebs

Poison can be a formidable weapon--particularly if it's 10,000 times more lethal than cyanide. Tetrodotoxin, the substance in pufferfish organs that may be Japanese cuisine's biggest thrill, turns up in other animals, too--among them the rough-skinned newt. This newt, a resident of the American West, has the stuff in its skin, which acts as an excellent deterrent to predators.

But some populations of garter snakes eat the newt willingly. How is that possible? Shana L. Geffeney, a biologist at Utah State University in Logan, and several colleagues say just a few key mutations in one garter-snake gene are enough to do the trick. Tetrodotoxin kills by paralyzing its victims. It worms its way into a hole in a protein expressed in the membrane of muscle cells that control contraction. There the poison blocks the movement of sodium into the cells. If the sodium can't move, the muscles can't contract. In the poison-tolerant snakes, however, the protein differs from the one in vulnerable snakes by only a few amino acids. That's enough to thwart the tetrodotoxin and keep the muscles going--until either prey or predator evolves a new weapon. (Nature 434:759-63, 2005)