Chytridiomycosis is an emerging infectious disease of amphibians caused by the fungal pathogen, Batrachochytrium dendrobatidis ("Bd") that has been implicated as a major cause of amphibian population declines and extinctions around the world. At the current rapid rate of global Bd spread, many amphibian populations will become infected within the next decade, and this will likely result in substantial numbers of species extinctions in this already-imperiled group of organisms. In California's Sierra Nevada mountains, Bd is rapidly spreading through previously uninfected amphibian populations. The mountain yellow-legged frog (Rana muscosa) is highly susceptible to chytridiomycosis, and has experienced hundreds of recent population extinctions due to Bd infection. Interestingly, although the majority of host populations are driven extinct following the arrival of Bd, a small fraction of populations persist with the pathogen, and disease dynamics in these persistent populations are fundamentally different from those during population crashes.
The goal of this research by researchers from University of California is to understand the mechanisms leading to these contrasting disease outcomes. The investigators hypothesize that population extinction versus persistence is the result of between-population differences in (1) density-dependent disease dynamics, (2) Bd virulence, (3) frog susceptibility, or (4) environmental conditions. A model of the R. muscosa- Bd interaction that includes within-host Bd dynamics and host stage-structure will be parameterized and tested. In addition, four non-mutually exclusive hypotheses that could account for different disease outcomes will be tested using field and laboratory experiments. A functional genomics approach that utilizes complete Bd and frog genome sequences will be used to describe the genetic basis of any observed differences in Bd virulence and/or frog susceptibility. The proposed research will contribute significantly to the ability to predict outcomes of future diseases on wildlife and human populations.
In terms of broader impacts, results from the research will be directly relevant to the conservation of amphibians worldwide that are threatened by disease, and will likely be broadly used to inform conservation strategies and policy initiatives. To ensure the rapid incorporation of results into such efforts, results will be communicated to policy makers via (i) continued participation by project researchers in an R. muscosa recovery working group, (ii) annual meetings with federal and state agencies charged with R. muscosa conservation (e.g., National Park Service, U. S. Fish and Wildlife Service, California Department of Fish and Game), and (iii) consultation with non-governmental organizations developing amphibian conservation programs worldwide. The project will promote teaching and training of a diverse group of students and postdoctoral researchers in modeling, genomics, molecular genetics, statistics, and laboratory and field methods through their direct involvement in the research. To provide Bd-specific training to a wider audience, three workshops targeted at federal and state agency biologists and private consultants will be hosted during the project in collaboration with the Amphibian Specialist Group (IUCN, World Conservation Union). Sampling protocols (in text and video formats) will also be provided online in both English and Spanish.
Amphibian populations throughout the world have declined in recent decades. One of the many causes of these declines is the disease chytridiomycosis, which is caused by the fungal pathogen, Batrachochytrium dendrobatidis (abbreviated "Bd"). The outcome of infection with this fungus, however, varies among amphibian species, and among populations of the same species. In some cases, infection with Bd leads to rapid death of individuals and decline or extinction of amphibian populations. In other cases Bd-infected individuals can survive, and infected populations can persist indefinitely with the fungus. In the Sierra Nevada mountains of California, both of these outcomes are occurring in populations of mountain yellow-legged frogs (Rana muscosa and Rana sierrae). In this project, we sought to determine the causes of these different outcomes in the Sierran frog system, through a combination of field surveys, laboratory and field experiments, statistical analysis, and mathematical modeling. We investigated whether the different disease outcomes (population extinction versus persistence) could be explained by differences among populations in: (a) frog density (which can affect the rate transmission of the fungus between frogs), (b) Bd virulence (the ability of the fungus to cause disease symptoms in frogs), (c) frog susceptibility to infection (the ease with which the frog becomes infected when exposed to the fungus), or (d) environmental conditions (such as temperature, which affects the growth rate of the fungus). We found the strongest support for (c), that differences in frog susceptibility are leading to the different disease outcomes. We further showed that a number of factors can affect frog susceptibility to infection, including prior exposure to Bd, and the microbial community associated with the frog skin. Frogs that have been exposed to Bd previously are less likely to become infected, and develop less intense infections, than naïve frogs, suggesting a major role for the frogs’ adaptive immune response. This is an important finding because it suggests that priming the frogs’ immune system could be an effective strategy for reducing the impact of the disease. Additionally, some bacteria associated with the frogs’ skin have anti-Bd properties that reduce the frogs’ susceptibility to infection. These bacteria may be used for probiotic treatments, potentially protecting the frogs from chytridiomycosis. These are among the conservation strategies that are currently being implemented in the California Sierra Nevada, and elsewhere, to stop amphibian population declines due to this disease. We performed long-term field surveys of hundreds of frog populations in the Sierra Nevada, and showed that Bd spread from west to east across Sequoia-Kings Canyon National Park and surrounding areas, at a rate of 1-4 km per year. We found that the chance that an uninfected population would become infected with Bd was determined only by the distance to the nearest infected population, and not by environmental conditions or frog population density. The results of our project are directly relevant to conservation of the threatened amphibian species in the California Sierra Nevada, and throughout the world. Our group is working with the agencies responsible for implementing management plans for mountain yellow-legged frogs, and our results are being immediately translated into conservation actions. Over the course of our grant, our project has been featured in several television, radio, and magazine presentations, reaching a much wider audience than standard scientific journal articles. Our project has contributed significantly to training at the undergraduate, graduate, and postdoctoral levels. Over 25 undergraduate students have worked on the projected, gaining skills in field methods, experimental design, molecular approaches, and statistical analysis. Nine graduate students, and one postdoctoral researcher have also received training on this project.
