Paraquat induced embryotoxicity on Xenopus laevis development

Paraquat (PQ, 1-1'-dimethyl-4,4'-bipyridylium dichloride) is an effective herbicide widely used in agriculture with a rate of application for aquatic weed control ranging from 0.1 to 2 parts per million. Considering its wide-spread presence in Italian wetlands, we studied its embryotoxic effects with the Frog Embryo Teratogenesis Assay-Xenopus (FETAX). The percentage of mortality as well as the percentage of malformed larvae was investigated by probit analysis. The results showed that PQ was highly embryolethal. From a LC(50) of 0.138 mg/l and TC(50) of 0.267 mg/l, a TI(50) of 0.52 was derived; indicating that PQ is to be considered a non-teratogenic compound. Remarkable was the presence of a specific malformation, classified as ventral tail flexure, in the 29% of living larvae exposed to 0.125 mg/l PQ concentration. Their histological examination showed several zones of abnormal somites containing severely affected myocytes. This confirmed the molecular mechanism of PQ toxicity in cell microfilaments. Even at the lowest concentration of 0.0625 mg/l, the difference between the mean head-tail length of control and exposed larvae was statistically significant, a sign of growth retardation. All our data emphasize that PQ must be consider highly embryotoxic on amphibian development, and suggest that this herbicide should be strictly regulated in weed control programs.