Abstract
Environmental conditions can alter the outcomes of symbiotic interactions. Many amphibian species have declined due to chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), but many others persist despite high Bd infection prevalence. This indicates that Bd's virulence is lower, or it may even be a commensal, in some hosts. In the Australian Wet Tropics, chytridiomycosis extirpated Litoria nannotis from high-elevation rain forests in the early 1990 s. Although the species is recolonizing many sites, no population has fully recovered. Litoria lorica disappeared from all known sites in the early 1990 s and was thought globally extinct, but a new population was discovered in 2008, in an upland dry forest habitat it shares with L. nannotis. All frogs of both species observed during three population censuses were apparently healthy, but most carried Bd. Frogs perch on sun-warmed rocks in dry forest streams, possibly keeping Bd infections below the lethal threshold attained in cooler rain forests. We tested whether short-term elevated temperatures can hamper Bd growth in vitro over one generation (four days). Simulating the temperatures available to frogs on strongly and moderately warmed rocks in dry forests, by incubating cultures at 33°C for one hour daily, reduced Bd growth below that of Bd held at 15°C constantly (representing rain forest habitats). Even small decreases in the exponential growth rate of Bd on hosts may contribute to the survival of frogs in dry forests.
Highlights
The distributions, abundances, life history strategies, and virulences of microbial symbionts can all be influenced by the environment, and can, in turn, affect the development and outcome of disease [1,2,3]
Thereafter, one plate was kept at 15uC 24 hours a day to simulate constant, cool conditions at rain forest sites, a second was kept at 15uC 23 hours a day with one hour at 28uC to simulate daily exposure to moderately-warmed rocks as frogs emerge from diurnal retreat sites in dry forests, and a third was kept at 15uC 23 hours a day with one hour at 33uC to simulate emergence onto warmer rocks in dry forests
Carey et al [19] demonstrated that in highly susceptible species, the development of chytridiomycosis can be explained as a consequence of unregulated exponential growth of the population of Batrachochytrium dendrobatidis (Bd) on the host
Summary
The distributions, abundances, life history strategies, and virulences of microbial symbionts can all be influenced by the environment, and can, in turn, affect the development and outcome of disease [1,2,3]. Higher elevation rainforest sites, which correspond to areas of high amphibian diversity and endemism, have been especially hard hit by this disease [6,7,8]. Frogs infected with Bd can consistently lose their infections after relatively short (16 h) exposures to high temperatures (37uC), as the fungus perishes rapidly at this temperature [11]. Constant temperatures as low as 28–30uC resulted in death of the fungus after several days in culture [9], suggesting that even small elevations in environmental temperature are likely to tip the hostpathogen balance in favor of the host. The response of Bd to realistic thermal regimes experienced by persisting and declining populations in the wild has not been tested
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