Some Frogs May Be Developing a Resistance to the Disastrous Chytrid Fungus
The dreaded chytrid fungus Batrachochytrium dendrobatidis horrifies even the most sober-minded scientists. It grows on a frog’s ultra-sensitive skin, disrupting the organ's ability to absorb water and air. Considering the number of species the fungus impacts and its ability to rapidly drive them to extinction—as it’s done to hundreds of species so far—it’s considered “the worst infectious disease ever recorded among vertebrates.”
Which makes a new study out today in the journal Science both surprising and exhilarating: Populations of certain frog species in Panama are rebounding after the fungus invaded their territory over a decade ago. And that probably isn’t because the fungus got any less deadly—it’s that the frogs may be developing a resistance the pathogen, perhaps on an evolutionary level. That could change the way conservationists think about saving the rainforest’s vulnerable species.
“The reason why this is so deadly for amphibians is that their skin is really a physiological organ,” says University of Nevada, Reno disease ecologist Jamie Voyles, lead author of the new paper. “Amphibians use their skin to drink water, to take in respiratory gases, and to balance things like electrolytes.” Afflicted frogs eventually die of cardiac arrest.
To protect themselves against nasties like the chytrid fungus, frogs secrete antimicrobial peptides, essentially an immune system for the outside of the body. But this fungus is so nasty, so virulent, that it quickly overwhelms its victims, peptides be damned. Some species, like the clown frog Atelopus varius, have seen 100 percent mortality rates.
You might assume, then, that certain frog species in Panama are bouncing back because the fungus has in some way weakened. To test this, the scientists took samples of the fungus collected by chytrid researcher Joyce Longcore when it first arrived in 2004, and compared that to samples collected 10 years later. They looked at things like growth rate and how many infectious spores the samples produced—even their genetics.
The verdict? The villain hadn’t changed much at all. “At the outset I expected we'd find the opposite, that the pathogen had weakened,” says Voyles. “I was completely wrong.”
So something must be going on with the frogs. To test this, the researchers took frogs from a region ravaged by the fungus, as well as the captive-bred descendants of frogs collected before the fungus hit, and exposed both groups to the pathogen. Sure enough, the group that had been living in the wild with the fungus produced skin secretions that better fought off the pathogen.
Which is odd, for one, because something like a fungus goes through life cycles quicker than one of these frogs (they reach sexual maturity in about half a year, on the quick end). Theoretically, the fungus can evolve more rapidly to overwhelm whatever defenses the frogs come up with. “We normally think of pathogens, because they have short life cycles, as responding more quickly to natural selection and evolution,” says University of Pittsburgh biologist Cori Zawacki, coauthor on the paper. “But instead we're seeing the potential for the amphibians themselves evolving better defenses against the pathogen.”
Now, this kind of bounce-back is only happening in a handful of frog species in sampled regions. We’re still dealing with a very, very nasty fungus. “This is good news in some ways, but it doesn't get us necessarily closer to being able to do anything about this pathogen,” says Zawacki. “The good news is that perhaps nature is solving its own problem here, rather than us being able to do anything about it.”
Not that conservationists will just sit back and hope evolution goes their way. Right now, researchers are fighting the fungus by capturing vulnerable frogs and bringing them into the lab for captive breeding that focuses on creating a genetically diverse population, ensuring the species doesn’t go extinct if the fungus decimates the wild population. That strategy has pitfalls, though; if you wanted to release the frogs back in the wild, it’s hard to know they’d be able to resist the fungus.
Another strategy is to coat frogs with a solution containing a fungus-fighting bacterium, though that of course wouldn't eradicate the fungus itself. Plus, scientists are just beginning to understand the amphibian skin microbiome; perhaps inoculating frogs like this would mean they’d be less likely to evolve resistance on their own.
Given these new findings of fungal resistance in certain species, it’s possible that scientists could breed frogs that put up a better fight. “If we can selectively breed more resistant animals, generating captive offspring that are more resistant, perhaps the loss in overall genetic diversity is worth the benefit in terms of survival,” says Zawacki. "It is unlikely that the fungus is going anywhere anytime soon."
With more confidence that the frogs can manage on their own, conservationists could give the critters a boost of sorts. “Maybe we should be spending more time and effort making sure that other factors are not hindering the frogs,” says Jodi Rowley, a curator at the Australian Museum who works in frog conservation. “So the streams are protected from deforestation and pollution and things like that, so they're able to perhaps take on the disease themselves.”
Is the situation ideal? Far from it. But the better scientists understand about how frogs can fight this thing on their own, the better the odds of stopping a vicious pathogen.
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