LTREB : Collaborative Research: Long-term dynamics of amphibian populations following disease-driven declines

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Outbreaks of emerging infectious diseases are increasingly recognized as major threats to wildlife populations. The initial invasion of a novel pathogen into a susceptible host population can cause a disease outbreak resulting in high levels of mortality and declines in population size. When this happens, natural selection can occur on both the host and pathogen populations during the disease outbreak. This can result in evolutionary changes in the host's susceptibility and tolerance to infection by the pathogen. It can also change the pathogen's ability to damage the host (virulence). These changes can in turn determine whether the host population can persist and recover from the disease. Understanding these evolutionary processes is crucial in development of conservation strategies for threatened species. This project will examine these processes for the case of a fungal pathogen that causes the disease Chytridiomycosis in frogs and salamanders. This disease has had catastrophic effects on amphibians worldwide, with numerous species extinctions documented in recent decades and many more species at risk. The researchers will investigate the patterns of evolutionary change in both the pathogen and the host (mountain yellow-legged frogs), following the invasion of the disease into hundreds of high elevation lakes in the California Sierra Nevada. This project will contribute to the understanding of the role of infectious diseases, such as Chytridiomycosis, as agents of evolutionary change in natural populations. It will provide critical information to state and federal agencies. It will also train and educate numerous, undergraduates and graduate students as well as the general public. This research builds on data from a long-term study of the population dynamics of mountain yellow-legged frogs (Rana sierrae and Rana muscosa) in the complex landscape of the California Sierra Nevada, and the impacts of Batrachochytrium dendrobatidis (Bd), as it has invaded and spread through hundreds of frog populations. In most cases, invasion of Bd results in outbreaks of the disease Chytridiomycosis, rapid frog population declines, and local extinctions. In some cases however long-term persistence of frog populations occurs with Bd in an enzootic state in which the impact of the pathogen is greatly reduced. The research will make use of recent advances in molecular approaches, and the extensive dataset and archive of samples from the R. sierrae/R. muscosa - Bd system, to investigate how populations of both host and pathogen change during the transition from pre-pathogen arrival, to disease outbreak, to enzootic disease, to potential recovery of the pre-disease host population abundances. This dataset will be used to investigate the genetic basis for differences in host resistance/tolerance and pathogen virulence. Cutting-edge genomic analysis of existing swab samples will be combined with continued surveys of field populations to identify new disease outbreaks and describe the transition from outbreak to enzootic state, collection of Bd cultures and frog mucosal samples from field populations, and laboratory experiments on Bd virulence and frog