Exposure to Batrachochytrium dendrobatidis affects chemical defences in two anuran amphibians, Rana dalmatina and Bufo bufo

1. Batrachochytrium dendrobatidis (Bd) is the causative agent of chytridiomycosis, one of the major
causes of worldwide amphibian biodiversity loss. Many amphibians exhibit
skin-based chemical defences which may play an important role against invading
pathogens, but whether the synthesis of these chemical compounds is enhanced or
suppressed in the presence of pathogens is largely unknown.

2. Here we investigated direct and indirect effects of
larval exposure to the globally distributed and highly virulent Bd-GPL strain on skin secreted chemical
defences and life history traits during early ontogeny of agile frogs (Rana dalmatina) and common toads (Bufo bufo).

3. Exposure to Bd
during the larval stage did not result in enhanced synthesis of the
antimicrobial peptide Brevinin-1Da in R.
dalmatina tadpoles or in increased production of bufadienolides in B. bufo tadpoles. However, exposure to Bd during the larval stage had a
carry-over effect reaching beyond metamorphosis: both R. dalmatina and B. bufo
froglets contained smaller quantities of defensive chemicals than their Bd-naïve conspecifics in the control
treatment.

4. Prevalence of Bd
and infection intensities were very low in both larvae and metamorphs of R. dalmatina, while in B. bufo we observed high Bd prevalence and infection intensities,
especially in metamorphs. At the same time, we did not find a significant effect
of Bd-exposure on survival, body mass
or development rate in larvae or metamorphs in either species.

5. The lack of detrimental effect of Bd-exposure on life history traits, even parallel with high
infection intensities in case of B. bufo
individuals, is surprising and suggests high tolerance of local populations of
this two species against Bd. However,
the lowered quantity of defensive chemicals may compromise antimicrobial and
antipredatory defences of froglets, which may ultimately contribute to population
declines also in the absence of conspicuous mass-mortality events.