Xenopus laevis a versatile animal model to study chemical embryotoxicity: the fungicide Triadimefon.

Triazole-derivatives are potent antifungal agents widely used as systemic agricultural fungicides and against fungal diseases in humans and domestic animals. They act by inhibiting the cytochrome P-450 conversion of lanosterol to ergosterol resulting in faulty fungal cell wall synthesis. Some data have been published about the teratogenic activity of triazoles on rodent embryos: hypoplasias, abnormal shape, agenesis of the pharyngeal arches were reported as typical induced malformations. Unfortunately, no data are available on the embryo-toxicity of these compounds in amphibians, despite the increasing concern among the scientific community about the phenomenon of global amphibian population declines. The aim of the present work is to evaluate the embryo-lethal and teratogenic potentials of Triadimefon (FON), a triazole-derivative used as antimicotic in agriculture, by the test FETAX (Frog Embryos Teratogenic Assay, Xenopus) with particular attention to the analysis of pharyngeal arch malformations. Moreover, to verify which period of development is the most sensitive to FON, 2h exposures were performed at gastrula or neurula stage. The assay has estimated LC50 and TC50 values of 63.46 μM and 2.73 μM, respectively and neurula as the most sensitive developmental stage. The resulting TI (Teratogenic Index= LC50/TC50) value of 23.4 has underlined the high teratogenic risk associated to this compound. Interestingly, 100% of malformed embryos showed alterations at anterior cartilages that were reduced, missing, fused or incorrectly positioned. These malformations were correlated to defective pharyngeal arch development possibly caused by abnormal Neural Crest Cell (NCCs) migration into the pharyngeal mesenchyme. As the migration of NCCs is controlled by the HOX code and by an anteroposterior retinoic acid (RA) gradient, the expression of CYP26, a key enzyme in RA metabolism, was analyzed following FON exposure. The increased mRNA levels of this gene and the ability of citral (a RA inhibitor) to reduce the teratogenic effects of the fungicide confirm the hypothesis that endogenous RA is involved in the mechanism of action of FON.