COS 213-3 - Performance of the amphibian Bufotes viridis under experimental infection with the chytrid fungus Batrachochytrium dendrobatidis in different temperature regimes

Abstract: Amphibians around the world are facing population declines and extinctions. A driving force, along with habitat degradation and destruction, are emerging infectious diseases. Among them chytridiomycosis, caused by the parasitic fungus Batrachochytrium dendrobatidis, is decimating amphibian populations and is implicated in almost one hundred species extinctions in the last decades. While infection outcome is multifactorial and often uncertain, the thermal mismatch hypothesis, stating that a host is outcompeted by a parasite when outside of its thermal optimum, can be central in understanding infection dynamics. Understanding the temperature-dependence of host responses and consequently infection outcome is also very significant in the face of climate change. Our aim was to test the thermal mismatch hypothesis in a temperate amphibian species, and investigate plasticity in the amphibian immune system gene expression and how it affects survival and growth under infection in different temperatures. The relatively warm adapted green toad Bufotes viridis, which is a red-listed species in Sweden with an ongoing re-introduction program, was chosen for the experiment. Individuals were raised from eggs in the lab, infected upon metamorphosis, placed in three different temperature regimes and monitored for 40 days. Subsequently, fungal load on each toad was calculated and skin and liver RNA was isolated, sequenced and analysed for differential gene expression.
Toadlets showed increased mortality and decreased growth when infected in the two colder temperatures, in line with the predictions of the thermal mismatch hypothesis. Conversely, infected toadlets in the warmest treatment exhibited the same survival and growth as their non-infected counterparts, and cleared the infection by the end of the experiment. 6375 (18.5% of) skin RNA transcripts and 2582 (8.9% of) liver transcripts were differently expressed depending on the temperature and treatment. Gene ontology terms associated with the differentially expressed transcripts included immune system processes, that could explain differential survival in the different temperature – treatment combinations, as well as broader metabolic processes, potentially connected to differential growth. Our results suggest that host temperature adaptations may play a significant role in regulating gene expression and thus influencing host growth and mortality under disease and should thus be a key consideration when planning conservation actions, for Bufotes viridis and other amphibians. Though at first glance a warming climate does not herald bad news for this amphibian, high probability of extreme weather events, like cold winters or droughts, points to cautious planning for the species’ longevity.