Sharon and Danny Brower
Sharon and Danny Brower Different species of snakes, like this black-tailed rattlesnake, kept the genes for some types of toxins but shed others, new research shows.
As you hike Wisconsin this summer, be sure to appreciate evolution if you’re bit by a rattlesnake.
Well, maybe get help, first. And the kind of help you’ll receive is improving thanks to UW-Madison research deep into the rattlesnake family tree.
By examining snakes’ DNA and reconstructing their evolutionary histories, the researchers solved a mystery about the animals’ venom.
“We found something surprising,” says Sean Carroll, professor of molecular biology and genetics. “Some of that surprise has something to do with designing a 21st century antidote. But part of that is also just understanding how life works.”
There are 36 species of rattlesnake, and they’re found only in the Americas. Two species, both endangered, are native to Wisconsin: Eastern Massasauga and Timber Rattler. But don’t worry too much about them.
“Death from rattlesnake bite is very rare,” says Carroll, who also serves as vice president for science education at the Maryland-based Howard Hughes Medical Institute. “We can go years without a death [by rattlesnake bite] in the U.S. When it occurs, it’s usually somebody who can’t get to help — say, a lone hiker or jogger, a couple hours away from help.”
Eve’s snake in Eden gave up knowledge pretty easily. By contrast, Carroll’s search for snake wisdom has taken considerable time. “I kept snakes as a kid,” he recalls. “I just thought they were really a fascinating animal.”
The research, published last fall, uncovered why the snakes are so varied. Carroll and his team found that the ancestors of all rattlesnakes, which lived millions of years ago, didn’t specialize. Each snake had toxins to poison blood, toxins to damage muscle, and neurotoxins to attack the nervous system.
It was a clever genetic strategy. If you were a prehistoric rattler, after all, “you’d like to have sort of a Swiss Army knife, with a weapon for every occasion,” he says.
Over time, however, different branches of the rattlesnake family tree began to specialize, claiming only certain of the toxins on their shelf of ancestral poisons. Why?
“Snakes are in an arms race with their prey,” explains Carroll. “That’s because various prey evolve resistance to venoms. Some of the most famous are hedgehogs and mongoose. Opossums feed on rattlesnakes, and they’ve developed resistance. Ground squirrels in the California area are heavy prey for rattlesnakes. They’ve developed resistance to venom, too.”
As certain toxins become less effective on prey, “evolution selects for other components in the mixture,” he says. “If that selection shifts far enough, then there’s no pressure to retain all of those toxins. Away it goes. That’s a phenomenon we know of in general from evolution. It’s sort of use it or lose it.”
For example, the Eastern and Western Diamondbacks of North America left their nervous-system poison behind. They kept only toxins that damage muscles and blood vessels. The Mojave Rattlesnake, however, held onto its neurotoxin, and lost genes for producing other poisons.
The team’s findings have many potential benefits. “The toxins in these venoms have potent physiological effects,” notes Carroll. “That’s why they work. Some venoms, for example, have been known to rapidly drop blood pressure. Now you can say, ‘Let’s use that snake venom to control blood pressure.’ ” But there’s more and subtler science involved.
“More likely that snake toxin is telling me something about how blood pressure is controlled,” he says. “You use that information to design something that controls blood pressure.”
One toxin has already “led directly to a multi-million dollar drug,” says Carroll.
All this derives, he notes, from basic research, a corner of science that’s often viewed as “useless.” It was funded by the Howard Hughes Medical Institute and the National Institutes of Health.
“The thing about basic research is that we don’t know where it’s going to lead us,” explains Carroll. “That was part of the discovery path: realizing that the way venoms work is to screw up something in physiology — blood coagulation, platelet formation, nervous system functions, blood pressure.”
Such basic research is fuel that drives innovation, he argues. “It’s the sort of long-term investment we need to keep earning the scientific and technological dividends that we depend upon. Again and again it opens up avenues in all sorts of directions in science, technology, medicine, and other fields.”
Carroll emphasizes, “most snakes that you’re going to encounter in North America are completely harmless. They control a lot of populations of things that we don’t want to have around: rodents and all sorts of things. They’re a valuable part of the ecosystem.”
