In the war between predators and their prey, a poisonous newt protects itself with a toxin powerful enough to stop most animals dead in their tracks. But one of the newt's main enemies, the garter snake, has regained the advantage by evolving small genetic modifications that shield it from this chemical defense, according to a new study.
The newt Taricha granulose often carries the deadly toxin tetrodotoxin in its skin. When a snake eats it, this poison blocks sodium channels, protein-lined gateways that let ions pass through the cell membrane. As a result, nerves and muscles shut down, leading to paralysis and death. Pufferfish and octopi also carry tetrodotoxin in their chemical arsenals.
Physiologist Shana Geffeney of Utah State University in Logan and colleagues found Thamnophis sirtalis garter snakes with varying degrees of resistance to the toxin and wondered how some of them were able to eat the newts with impunity.
Scientists had known for decades that the tetrodotoxin's impact depends on the exact sequence of amino acids that make up the sodium channel protein. A change in the channel's outer opening keeps the toxin from binding to the protein and short-circuits its deadliness. Geffeney's team found that some snakes with specific changes in a different section of the outer opening were thousands of times more resistant to the newt toxin than normal snakes were. Moderately resistant snakes had a slightly different amino acid makeup from the most resistant animals. The team, which reports its findings today in Nature, says the work demonstrates how eating a toxic newt has facilitated the evolution of molecular defenses.
"This model of evolution of resistance by a predator to a prey's defenses is one of the better examples of how [natural] selection can shift the balance back to the side of the predator," comments Roy Caldwell for the University of California, Berkeley.