Credit: Wikimedia Commons user Pierre Fidenci
Poison is a mystery writer’s best friend. The writer’s criminal mastermind just slips a little tasteless, odorless poison in the unwitting victim’s wine glass, then waits for his or her inevitable demise. The ideal poison, from this perspective, is the one that’s most portable and potent, and least traceable by prying law-enforcement officers, spinsters, and other would-be detectives.
Many of the “best” poisons aren’t manufactured by humans, but created by other organisms—mostly, as you might expect, for attack or defense—but not always. And just as in mystery novels, where some of the most intriguing plots involve antidotes or immunity to poisons, so, too, in nature, some of the thorniest questions about poisons involve the creatures that are immune to them.
Consider the case of the poison frog, Eleutherodactylus iberia. This tiny frog, the size of your fingernail, might seem harmless, but its skin is packed with a toxic alkaloid, PTX 323A. This poison appears to be primarily for defense: Potential predators wouldn’t need more than a taste to learn that E. iberia is not worth messing with. The evolutionary biologist who writes as Grrlscientist wrote two weeks ago about recent efforts to identify the source of the poison in this frog.
Most poison frogs don’t manufacture their own poison; they ingest other poisonous creatures, then sequester the poison in their skin, where it acts to deter predators. A team led by Miguel Vences spent months in Cuba painstakingly tracking and capturing the E. iberia, then examining its stomach contents to suss out the source of its poison. The researchers found that 70 percent of the frog’s diet was a type of mite that produced the same toxin found in the frog’s skin. E. iberia is not only immune to PTX 323A, it eats bugs laced with the stuff. How does this tie in with the frog’s evolution? Much work remains to be done, but so far the hypothesis with the most support suggests that the frogs first specialized their predatory habits, then developed the ability to isolate and concentrate the poison in their skin, which in turn allowed them to seek prey in the daylight.
For another poisonous creature, however, the evolutionary path was likely very different. Lucas Brouwers, who studies molecular mechanisms of disease in Nijmegen, the Netherlands, last week recounted the story of a group of fishermen who were nearly killed by saxitoxin in a batch of mussels they caught near Nantucket. As in E. iberia, the mussels don’t produce the toxin themselves, but get it from cyanobacteria they filter from the surrounding waters. But why do the cyanobacteria produce the poison? The process of synthesizing saxitoxin is not simple, involving dozens of steps. Do the bacteria make the poison to ward off potential predators? A team of researchers led by Shauna Murray analyzed the genes responsible for producing saxitoxin in several cyanobacteria strains. While most of the genes are found in other (non-poisonous) cyanobacteria as well, a critical portion of the genes actually come from a completely different bacterial family, the gammaproteobacteria. The common ancestor of saxitoxin producing cyanobacteria, the researchers estimate, lived about 2.1 billion years ago—meaning the ability to produce saxitoxin is also at least that old!
Saxitoxin acts on humans (and other organisms) by blocking the sodium channels in cell membranes in the nervous system and brain—preventing neurons from firing and leading to numbness and paralysis. But 2.1 billion years ago, animals and even eukaryotes hadn’t evolved yet. In fact, no living organism had a sodium channel. What’s the point of that? It’s as if humans had evolved non-lethal laser guns that will only be effective in fighting the aliens who’ll be attacking the Earth in the year 2764.
It’s possible that saxitoxin could work as a chemical defense against other bacteria, perhaps via the potassium channel in bacteria that functions similarly to the sodium channel in higher organisms. But Brouwers believes a more mundane explanation is more plausible: The toxin serves some entirely benign purpose for the cyanobacteria; it’s only coincidence that it’s toxic to us.
For other creatures, immunity to poison becomes a means of survival. Texas biologist Zen Faulkes writes about a particularly dramatic case, the garter snake’s immunity to otherwise-deadly tetrodotoxin. A rough-skinned newt, one of the garter’s favorite bits of prey, can contain enough tetrodotoxin to kill seven humans. How does the tiny garter snake avoid death? Like saxitoxin, tetrodotoxin targets sodium channels. The poison-resistant garters seem to have modified sodium channels: They don’t collapse in the presence of toxin, but they also aren’t as good at their primary job of activating neurons. When tetrodotoxin-resistant snakes are tested in various physiological tasks, they don’t perform as well as their non-resistant cousins. It’s a trade-off that pays dividends for garters, however, as they are able to prey on a species that practically no other predator will touch.
Dave Munger is editor of ResearchBlogging.org, where you can find thousands of blog posts on this and myriad other topics. Each week, he writes about recent posts on peer-reviewed research from across the blogosphere. See previous Research Blogging columns »
Originally published November 24, 2010