Last modified: Thursday, August 22, 2002
IU scientist finds snakes that are keeping pace in toxic arms race
Nerve cell resistance to tetrodotoxin the key
The rough-skinned newt, whose skin is poisonous enough to kill an adult human many times over if it is eaten, poses no threat to a tiny garter snake that has evolved changes in its nerves and muscles, say scientists from Indiana University and Utah State University.
The researchers present new evidence in this week's Science magazine that a co-evolutionary "arms race," in which a western U.S. garter snake has acquired the ability to eat the mega-toxic newts, has also made the snakes physically sluggish. Their finding is among the first to link small changes in physiology to real-world species interactions in the wild.
"The newt toxin, tetrodotoxin or TTX, affects a sodium channel that the snakes' nerve and muscle cells need to function," said IU biologist Edmund Brodie III, who directed the research. "The snakes have evolved resistance to TTX, but the resistance itself seems to have a negative impact on the snakes' ability to crawl."
TTX is a defensive compound found in many different animals, including pufferfish, octopuses and primitive chordates called tunicates. It is used in low concentrations to treat morphine and heroine addicts, and it has been identified as Haitian voodoo's "zombie" drug.
Evolutionary biologists have long been able to see the results of arms races between predators and prey. Prey often acquire wildly exaggerated traits, such as flying fishes' airborne abilities or the porcupine's armor of quills. But the exact way predators, prey and their genes change over time during such races has largely stumped the scientists.
With their new report, the researchers pinpoint a basic mechanism by which the garter snakes keep themselves alive and maintain access to a valuable food source -- even if the cost is that the snakes are a little less spry.
"Normally you might expect a highly advantageous trait -- like TTX-resistance -- to sweep the population, but that hasn't happened here," Brodie said. "Clearly a trade-off between the snakes' speed and their ability to resist the newt's toxin is playing a role in the variation we're seeing."
The research team examined four populations of the garter snake Thamnophis sirtalis that eat toxic newts of the genus Taricha in California, Oregon and Washington. They found not only variation in the amount of TTX produced by individual newts, but also a large variation among individual snakes in their tolerance of the newt toxin. The most resistant snakes can survive a toxin concentration 1,000 times that tolerated by the least resistant snakes.
The researchers also learned that snake TTX-resistance varies geographically. Garter snakes from the Washington population tend to be more sensitive to TTX but live among newts that produce less toxin. The California garter snakes, found to be considerably more toxin-resistant than their Washington and Oregon counterparts, live among newts laden with TTX.
Brodie said the garter snakes and newts provide an excellent model for researchers to learn how vastly different levels of biology interact.
"Linking evolution, ecology and physiology has been a major challenge for scientists," Brodie said. "But doing so allows us to start addressing some really interesting questions, such as how nerves change over evolutionary time in a particular ecological setting."
Utah State University biologists Shana Geffeney, Edmund Brodie Jr. and Peter C. Ruben also contributed to this report. Geffeney was responsible for the neurophysiological work.
This research was supported by grants from the National Institutes of Health and the National Science Foundation.
Citation: "Mechanisms of Adaptation in a Predator-Prey Arms Race: TTX-Resistant Sodium Channels," Science, Aug. 23, 2002, vol. 297, no. 5585
To speak with Brodie, contact David Bricker at 812-856-9035 or brickerd@indiana.edu.
