How does temperature affect the severity of Bd infection in the common toad (Bufo bufo)? : A test of thermal mismatch hypothesis

Abstract: Batrachochytrium dendrobatidis (Bd) is a fungus that causes chytridiomycosis in amphibians. This fungus is an invasive species that has spread all over the world and is causing mass deaths in many areas. Bd has an efficient life cycle which targets suitable hosts and causes disturbances in amphibian osmoregulation and eventually death. It has caused more than 90 presumed extinctions worldwide. However, even though this fungus is commonly found in Sweden, no cases of chytridiomycosis has been found in the wild. This raises the questions i) what factors makes the infection in amphibians more severe and ii) does temperature affect the severity of Bd infection? According to the thermal mismatch hypothesis pathogens should be more efficient at temperatures where the performance gap between the host and pathogen is the widest. We chose the common toad as a study organism since it can be found all over Sweden and is generally considered to be adapted to colder climates. Our aim was to test the thermal mismatch hypothesis and see if the effect of infection differed between different temperatures. Common toad eggs were collected from two ponds in southern Sweden, raised in the laboratory and metamorphosed juvenile toads were reared in three different temperature rooms, either 14℃, 18℃ or 22℃. There they were either infected with Bd or exposed to a sham infection and monitored for 40 days. We show that Bd infection negatively affects growth and survival of common toads in accordance with previous studies. Furthermore, we found that temperature affects the effect of infection and shows patterns of the thermal mismatch hypothesis.