Some frogs are evolving to survive a killer fungus

"These findings confirm that, at least under some environmental conditions, frogs can evolve tolerance to pathogens—even deadly ones—in their surroundings," says Kelly Zamudio. (Credit: Johnida Dockens/Flickr)

Some populations of frogs are rapidly adapting to a fungal pathogen that has decimated them for close to half a century.

As reported in a new study in the the journal Proceedings of Royal Society B: Biological Sciences, scientists have taken a step toward identifying the genetic mechanisms that makes some lowland leopard frogs in Arizona resistant to Batrachochrytrium dendrobatridis (Bd) infections that cause the disease chytridiomycosis.

A previous lab study by the same researchers isolated an immune system allele (a variant of a gene) called allele Q that gave frogs immunity to the infectious disease that typically causes skin to deteriorate.

In the new study, the researchers attempted to validate those lab results in the field by taking skin swabs and tissue samples from eight natural populations. These samples yielded more than 80 alleles. The researchers observed and recorded whether a frog was infected with chytridiomycosis and then analyzed what immune system alleles each frog inherited.

[Does deadly frog fungus lurk in insects?]

They verified that frogs in the field with allele Q did indeed survive Bd infections. Additionally, all of the members of one population survived Bd, but upon genetic analysis, these frogs each had alleles that were part of a group of functionally similar alleles (called a supertype) that also gave them immunity.

The alleles of this supertype were not found within the populations previously studied in the lab or in any other populations in the field. Allele Q was not part of this supertype.

“We found a very specific and significant relationship between having particular immune system genetic variants and being susceptible to this disease,” says first author Anna Savage, assistant professor of biology at the University of Central Florida. The study culminates work Savage began as a graduate student in the lab of Kelly Zamudio, professor of ecology and evolutionary biology at Cornell University, a coauthor of the study.

“These findings confirm that, at least under some environmental conditions, frogs can evolve tolerance to pathogens—even deadly ones—in their surroundings,” Zamudio says.

[This frog took a bizarre path to see blue]

The variations in immune system genes that give frogs tolerance to Bd infections are associated with a frog’s ability to identify pathogens and launch an immune response.

Genetic analyses revealed that these immune system alleles and supertypes associated with survival in the field showed signs of positive selection, providing evidence that these alleles were increasingly inherited and rapidly evolving, Savage says. At the same time, alleles that were associated with susceptibility to Bd did not show signs of positive selection.

Along with genetics, environment plays a big role in disease dynamics. For example, the researchers previously used a predictive computer modeling approach incorporating genetic and environmental data and found “a significant relationship” between genetic diversity and whether a frog was infected or whether a frog died.

But when it came to the intensity of those infections, environmental factors such as temperature and precipitation, rather than genetics, were the driving force. Since populations in Arizona only get chytridiomycosis in winter when temperatures drop and Bd becomes more virulent, “much of the year, there is no selective pressure for these populations to adapt at all,” Savage says.

Such inconsistent, less pervasive pressure “is likely going to reduce how quickly populations might adapt,” she adds.

The National Science Foundation and the National Geographic Society funded the work.

Source: Cornell University