Infection with the chytrid fungus (Batrachochytrium dendrobatidis) may distort sex ratio via sex reversal but not sex-dependent mortality in a common Eurasian amphibian

One of the major factors driving the currently ongoing biodiversity crisis is the anthropogenic spread of infectious diseases. Diseases can have conspicuous consequences, such as mass mortality events, but may also exert covert but similarly severe effects, such as sex ratio distortion via sex-biased mortality or sex reversal. Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd) is among the most important threats to amphibian biodiversity. Yet, whether Bd infection can skew sex ratios in amphibians is currently unknown, although such a hidden effect may cause the already dwindling amphibian populations to collapse. To investigate this possibility, we collected common toad (Bufo bufo) tadpoles from a natural habitat in Hungary, and continuously treated them until metamorphosis with sterile Bd culture medium (control), or a liquid culture of a Hungarian or a Spanish Bd isolate. Three months after metamorphosis we dissected the individuals and diagnosed their phenotypic and genetic sex. Bd prevalence was high in animals that died during the experiment but was almost zero at sexing. Survival was generally low in the control group, but it was further lowered by the Bd treatments. We did not observe sex-dependent mortality in either treatment. However, treatment with the Spanish Bd isolate significantly increased the frequency of sex reversal: 3 out of 9 genetic females developed into phenotypic males. Based on our results, Bd infection may have the potential to affect sex ratio in common toads through female-to-male sex reversal, but future research is needed to ascertain the generality of these findings.