Biologists at Indiana University Bloomington, Utah State University and the University of Utah present evidence in this week's Nature that a toxin produced by the rough skinned newt, Taricha granulosa, has forced several evolutionary changes in the garter snake Thamnophis sirtalis or, more specifically, in the snake nerve cell protein that endures the toxin's attacks.
Embedded in the surface of garter snake nerve cells is tsNa(V)1.4, a tube-shaped protein that allows sodium ions to flow into the cell. When nerve cells' ability to move sodium in and out is hampered, paralysis and death can result. Tetrodotoxin (TTX), a powerful paralytic poison concentrated in the newts' skin, can bind to garter snake nerve cell channels and prevent sodium ions from flowing freely.
"These channels are absolutely fundamental to every aspect of nerve and muscle function and are highly specific gateways for sodium ions," said IUB biologist Edmund Brodie III, one of the paper's coauthors. "If the channels change too much or in the wrong way, they can't perform their basic, everyday functions. Sodium channel genes in different vertebrates are virtually identical to each other, but not in these snakes. We're finding a molecular arms race is driving rapid and repeated changes in the gene within this group of beasts."
For TTX to bind successfully to the sodium channel, the toxin needs something to bind to. At this moment in the garter snake's evolutionary history, TTX infiltrates a hole on tsNa(V)1.4's surface, binding to an aromatic amino acid and causing enough of a change in the sodium channel's shape to impair its function. Three of the four Pacific Northwest snake populations the scientists examined have evolved som