The sad song of the Yosemite toad: The role of the amphibian chytrid fungus in an enigmatic decline

Background/Question/Methods

The Yosemite toad, Bufo (Anaxyrus) canorus, has undergone recent enigmatic declines in the Sierra Nevada of California. The role of the amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd) in these declines has been suggested, but remains largely unexplored. Closely related species (Bufo boreas) and sympatric amphibian species (Rana sierrae) are known to be susceptible to Bd driven population crashes. Another sympatric species (Pseudacris regilla) is thought be a competent disease reservoir. The aim of this study was to understand i) whether Bd invasion drove B. canorus declines, ii) the current prevalence and Bd infection intensity in B. canorus populations and iii) the Bd susceptibility of B. canorus. Skin swabs were collected from live and preserved specimens, and DNA was extracted from swabs using Prepman sample preparation reagent. Host infection intensity was measured as the number of Bd zoospores per swab (Z_swab) and was quantified by a real-time PCR assay.

Results/Conclusions

A retrospective survey of 196 archived B. canorus specimens indicated that Bd began to infect toads as early as 1961, became highly prevalent during the period of observed decline and decreased in more recent decades. A peak in prevalence was observed in specimens collected during the 1990’s, in which 11 of 13 (84.6%) specimens tested positive. Between the years 2006-2011 1,266 swabs were collected from wild B. canorus populations throughout the species range. The highest infection intensities were detected on juvenile toads, with a maximum of 23,468 Z_swab as compared to 158 Z_swab on adults and zero detection in tadpoles. In the most intensively sampled populations the prevalence of Bd on adults was 17.2% and 25.6% on juveniles. The mechanism of infection and the level of susceptibility of juvenile B. canorus was tested in a cross-species transmission lab trial. Subjects were collected as tadpoles from a population where archived specimens collected as early as 1975 tested Bd positive. Six B. canorus toadlets were exposed to small volumes of water in which individually assigned, field-infected and negative control Psuedacris regilla were soaked. Soak durations of P. regilla were increased from 15 minutes to one hour after two weeks. One of the four toadlets exposed to Bd in the described manner became infected after one week, and died of chytridiomycosis with an infection intensity 2 orders of magnitude greater than those known to be lethal to sympatric species (e.g. the Vredenburg 10,000 zoospore rule).