In recent years, hundreds of species of amphibians have experienced precipitous declines largely due to amphibian chytridiomycosis. Chytridiomycosis is a disease of amphibian skin caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd). The effects of Bd has on its hosts are highly variable, depending on host species and life stage, environment, and strain of the pathogen. Variation in the virulence of different Bd strains may be an important driver of varying disease outcomes, but we have limited understanding of the phenotypes associated with this variation. Identifying traits linked to virulence would help elucidate mechanisms responsible for virulence, as well as inform conservation and management decisions for declining amphibians.
In this study, we examined the association between zoosporangium size (a putative virulence trait) and virulence at multiple scales (within and between sites) using a laboratory exposure experiment. Cascades frogs (Rana cascadae) collected from montane lakes in the Klamath mountains of northern California were exposed to one of nine Bd isolates collected from two sites (n = 8-9 frogs per isolate). We used Bd load two weeks post-exposure to estimate virulence, and measured maximum zoosporangium size from photographs of Bd cultures.
Zoosporangium size was associated with virulence—there was a positive correlation between zoosporangium size and Bd load two weeks post-exposure. Neither site nor size-site interaction were good predictors of virulence, despite the positive correlation between size and virulence and that Section Line lake isolates had a larger mean zoosporangium size than Found lake. This may be due to the large range of zoosporangium size and virulence in Section Line isolates. Though the general trend was for larger isolates to be more virulent, Section Line had one isolate with the lowest measured virulence of all.
These results suggest that zoosporangium size may be a good predictor of virulence, supporting our hypothesis that larger sporangia result in a deadlier disease. Larger zoosporangia may produce more zoospores, cause damage to a larger surface area of skin, or produce more immunotoxic compounds. These results also demonstrate the wide diversity of Bd size both within and between sites, prompting questions about the evolution of virulence on multiple scales. Using easy-to-measure phenotypic characteristics, like zoosporangium size, to predict pathogen virulence may prove to be a highly valuable tool for conservation and management of infected host populations.