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Home » Archive » 2019 » Biology Session

Biology session

The hormonal background of inducible toxin production of common toad (Bufo bufo) tadpoles
Kalina Csenge III. évfolyam
University of Veterinary Medicine Budapest, Department of Ecology
Supervisors: Veronika Bókony, Bálint Üveges

Abstract:

Many species of wildlife use toxic compounds as defence against their natural enemies. Such toxins are the bufadienolides synthesized by toads in their granular skin glands. Previous studies have shown that common toad (Bufo bufo) tadpoles not only respond to the presence of predators by increased toxic production, but also to several other stressors (e.g. starvation, high conspecific density, pollutants). However, we know very little about the physiological regulation of bufadienolide synthesis. In our experiment, we studied how corticosterone, which is the major stress hormone of amphibians, influences the predator-induced toxin production of tadpoles. We tested two hypotheses: that the synthesis of bufadienolides is stimulated by corticosterone (H1) or by the same hormones that stimulate corticosterone (H2). Tadpoles were divided into three treatment groups. Individuals in the control group were treated with 1 µl/l ethanol (solvent control), the other group with corticosterone (125 nM/l), while the third group was treated with metyrapone (110 µM/l), which inhibits the synthesis of corticosterone. We predicted that, in case of H1, corticosterone treatment would increase toxin production, while metyrapone would decrease it. In case of H2 the opposite is expected, as corticosterone reduces the production of glucocorticoid-stimulating hormones via negative feedback, whereas metyrapone stimulates the production of such hormones. The treatments lasted 48 and 144 hours. Half of the individuals also received predator treatment: we added olfactory cues indicating the presence of common perch (Perca fluviatilis) in their water during the hormone treatments. At the end of the experiment, total bufadienolide content of tadpoles was measured by liquid chromatography (HPLC-DAD-MS) and analyzed by linear mixed models. We found that 144 hours of corticosterone treatment resulted in a significant decrease in toxin production compared to the control group, however, the metyrapone treatment had no effect. The presence of predators did not affect these results. Our results support the H2 hypothesis rather than the H1 hypothesis, i.e. that the synthesis of bufadienolides may be stimulated by the same hormones as the corticosterone synthesis. Henceforth, we would like to examine whether the treatments had an expected effect on endogenous stress hormone levels by measuring the amount of corticosterone excreted by the tadpoles. This might help to clarify the lack of effect of metyrapone.



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