These Frogs Have Developed a Resistance to Killer Fungus

If you pay attention to science news, you’ve likely heard about the fungal disease chytridiomycosis and how it’s devastating amphibian populations around the world. You’ve heard how fast it’s spreading, and how many species it’s driven to extinction. But you likely haven’t heard the good news: at least one frog species has developed a genetic immunity. These findings were published last month in the journal Proceedings of Royal Society B: Biological Sciences. 

A quick biology refresher: amphibians are little H2O-loving creatures, like frogs and salamanders. They reproduce in the water and spend a lot of time there. Some amphibian species have lungs, but all of them can breathe through their porous skin. This is super cool, but it also makes them super vulnerable to pathogens in the air and water.

One such pathogen is the deadly fungus Batrachochytrium dendrobatidis (Bd), which causes chytridiomycosis (aka chytrid). Scientists have identified antifungal treatments that can help amphibians fight off the disease, but as yet they haven’t found a way to deliver these treatments to wild populations. They’re working as fast as they can, but the disease is faster, and amphibian specialists have had to watch helplessly as the beloved animals sicken and vanish.

There are some notable exceptions. Oak toads and Cuban tree frogs have been able to learn to avoid the fungus altogether, and a few years ago, scientists in Arizona realized that certain populations of local lowland leopard frogs (Lithobates yavapaiensis—say that out loud; it’s really fun) were actually able to fight off infection. For that study, researchers Anna Savage and Kelly Zamudio collected eggs from the wild and reared the tadpoles in the lab. Once the frogs had reached maturity, the researchers exposed them to the fungus and observed which frogs got sick. As expected, adult frogs whose eggs had come from relatively robust populations were less likely to fall ill. To find out why, the researchers tested all the frogs’ DNA. They found a gene variant called allele Q that seemed to make the healthy frogs immune. If these results were accurate, they would be pretty darn exciting, especially in a field where there has been so little good news of late.

For the current study, the same researchers built on those findings, this time focusing on totally wild frogs in Arizona. Savage and Zamudio went to eight different sites in the winter, where L. yavapaiensis mortality rates from Bd infection are highest. They swabbed the skin of both living and dead frogs and collected tissue samples from the frogs’ toes. Once again, they sequenced the frogs’ genomes, and once again, frogs with allele Q were more likely to survive. 

One population was especially hardy, yielding not a single dead frog—yet there wasn’t a single allele Q among them. Instead, the genes of these frogs contained other variants from another group of alleles known to boost immunity. In other words, members of a single species had developed immunity in several different ways simultaneously.

This was good news indeed, co-author Kelly Zamudio said in a press statement. "These findings confirm that, at least under some environmental conditions, frogs can evolve tolerance to pathogens—even deadly ones—in their surroundings."