Gene expression in Xenopus laevis embryos after Triadimefon exposure.

The triazole derivative Triadimefon (FON) is a systemic fungicide used to control powdery mildews, rusts, and other fungal pests. Some data have already been published about the teratogenic activity of this compound: craniofacial malformations were found in mouse, rat, and Xenopus laevis embryos exposed to FON. These alterations were correlated to defective branchial arch development possibly caused by abnormal neural crest cell (NCC) migration into the branchial mesenchyme. As the migration of NCCs is controlled by the HOX code and by an anteroposterior retinoic acid (RA) gradient, we analyzed the expression of CYP26, a key enzyme in RA metabolism, following FON exposure. The increased expression of this gene and the ability of citral (a RA inhibitor) to reduce the teratogenic effects of the fungicide support the notion that endogenous RA is involved in the mechanism of action of FON. Moreover, by in situ hybridization, we studied the effects of FON exposure at gastrula stage on the expression of some genes involved in craniofacial development, hindbrain patterning, and NCC migration. We observed abnormal localization of xCRABP, Hoxa2 and Xbap signal expression at the level of migrating NCC domains, whereas in the hindbrain, we did not find any alteration in Krox20 and Hoxa2 expression.