The fungal pathogen Batrachochrytrium dendrobatridis (Bd), which has been known to cause the disease chytridiomycosis and decimate frog populations for close to half a century, is causing frogs to evolve around it, according to a new study which took a step toward identifying the genetic mechanisms that makes some frogs resistant to Bd infections in their study of lowland leopard frogs in Arizona. 

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 chytridiomycosis, an infectious disease that typically causes skin to deteriorate. In the current 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.

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 super type.

"We found a very specific and significant relationship between having particular immune system genetic variants and being susceptible to this disease," said Anna Savage, the study's first author and an assistant professor of biology at the University of Central Florida. 

"These findings confirm that, at least under some environmental conditions, frogs can evolve tolerance to pathogens - even deadly ones - in their surroundings," said  Kelly Zamudio, the Goldwin Smith Professor of Ecology and Evolutionary Biology
at Cornell.

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 said. At the same time, alleles that were associated with susceptibility to Bd did not show signs of positive selection.

 Published in Proceedings of Royal Society B: Biological Sciences.