Written By Annie Peterson

BIO + MED

written by ANNIE PETERSON

Snakes Could you dedicate your life to a newt? Stanford’s postdoctoral biologist, Dr. Charles Hanifin, probably will. In a study published on March 11, 2008 in the renowned journal, PLoS Biology, Hanifin and his colleagues contributed their extensive data on the rough-skinned newt (Taricha granulosa) and its co-evolutionary relationship with the common garter snake (Thamnophis sirtalis) to the biological community, significantly enhancing current thinking about evolutionary theory. The study’s far-reaching geographical underpinnings, along with Hanifin’s 3month education in an advanced Japanese lab technique, were among many factors that secured its credibility. Hanifin added to decades of his colleagues’ garter snake data, collecting 383 newts from 28 sites covering over 15,000 miles of Pacific coast, from British Columbia, Canada, to southern California.

Taricha granulosa

While it may look cute, the rough-skinned newt secretes an incredibly potent neurotoxin from glands on the surface of its skin. This poison, called tetrodotoxin (TTX), is powerful enough to kill thousands of mice, or a dozen humans. You might ask: why expend the energy to produce enough poison to kill a potential predator several times over? The answer lies in the “arms race” dynamic between newts and the common garter snake. Snakes’ resistance to the newts’ secreted TTX places selective pressure on the toxicity of the newt. As the newt population becomes more toxic, the snake population must respond by becoming more resistant in order to continue to survive ingesting the toxin. Outside the bounds of current evolutionary thinking, however, is the “escape” phenomenon shown in Hanifin’s study: even TTX, the most potent toxin on Earth, proves



stanford scientific

and Newts at War

Advantages on the Snake’s side

Fortunately for Hanifin and his coresearchers, newts exert their defense with a well-studied neurotoxin. TTX had been used to study the mechanism of voltage-gated sodium channels for over 30 years. Thus, Hanifin knew that tetrodotoxin poisons victims by binding to sodium channels in nerve and muscle membranes, blocking the propagation of electrical signals that are necessary for proper communication between cells, which in turn causes respiratory failure, and paralysis. How, then, were garter snakes able to resist these effects? And how had they managed to develop this defense in such a small number of generations? A member of Hanifin’s team discovered that the snakes’ sodium channels were uniquely structured to inhibit the toxin’s binding capacity and its deadly effects with only a slight alteration: one mutation, a change in a single nucleotide, had been responsible for the many subsequent physiological changes that allowed for the rapid surge in snakes’ resistance. The potential of escape for the newts, he says, depends upon whether the genes involved in toxin production are few, with large phenotypic effect (like the garter snakes), or whether many genes are involved, each having a small effect on

the biological outcome. TTX production in newts is thought to be the result of multiple genes, each with a small effect, but extensive study is required to be sure.

toxin levels to again be effective. In other words, the snakes’ apparent “escape” would only be temporary.

In an interview with Stanford Scientific Magazine, Hanifin humbly added, “For my end, that’s just dumb luck to have a system that was so well-known. It was a real advantage to be able to go in there, knowing it was one molecule, one pathway, one set of proteins, and understanding… the physiology that others haven’t been able to do in the same way.”

Years of studies are needed to answer any number of questions raised by the data.

Methods & Implications The University of Oregon, Oregon State University, researchers in Washington and California, and the California Department of Fish and Wildlife were all involved in the arduous task of gathering and testing the newts and snakes.

In the lab, Hanifin measured newt toxin levels by removing a half-centimeter circle of skin from anesthetized newts (similar to biopsy punches), grinded up the skin and injected varying amounts of the toxin into garter snakes. They then estimated snake resistance based on the animal’s crawling performance after the injections. The most resistant snakes showed faster crawl speeds than snakes with little or no resistance. Hanifin and his team could imagine a shift in the species’ co-evolutionary trajectory. If TTX resistance came at a price, and if there were no advantage for snakes that were more resistant than necessary, a costinduced de-escalation could allow newts’

Future Questions

In several of the locations, researchers discovered that the levels of both newts’ toxicity and snakes’ resistance were very low—the two species had not yet entered into a “race.” Hanifin related that his team is still trying to figure out what actually alters the selection dynamic status quo and triggers the animals’ escalation into an arms race. Moreover, scientists will need to acquire additional examples of an escape from the populations’ co-evolutionary dynamic, similar to that of garter snakes and newts, in order to draw conclusions about the universality of this type of interaction. As Hanifin put it, “We’re still trying to see how population-specific this data really is. The temptation is to worry about a similar escape by antibiotic resistant bacteria. But it’s very possible that this data is just specific to these species.” Hanifin does not seem to be fazed by what he says are a remaining lifetime’s worth of questions. “I do what I do because I want to know answers. Even though the next set of questions seem to be a lot more difficult to answer, that’s what makes my work interesting.”

ANNIE PETERSON is senior double majoring in Human Biology and English. In addition to bioethics, she is interested in childern’s development, art and art history. Credit: sxc.com

no defense against some garter snakes. These snakes have developed an extreme resistance to the newt’s toxin, and seem to have escaped the arms race. In fact, in about one third of the geographical areas in Hanifin’s study, even the least TTX-resistant snake can eat the most toxic newt. So how have the snakes managed to break the toxicity and resistance escalation? Hanifin and his lab decided to look at the molecular mechanisms behind the newts poisonous effects and the garter snake’s co-evolved resistance.

Dueling organisms seek to escape the arms race

To Learn More

You can find Dr. Hanifin’s article at http://biology.plosjournals.org/perlserv/

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