Debate and controversy exists concerning the potential for the herbicide atrazine to cause gonadal malformations in developing Xenopus laevis. Following review of the existing literature the US Environmental Protection Agency required a rigorous investigation conducted under standardized procedures. Xenopus laevis tadpoles were exposed to atrazine at concentrations of 0.01, 0.1, 1, 25 or 100 mug/L from day 8 post-fertilization (dpf) until completion of metamorphosis or dpf 83, whichever came first. Nearly identical experiments were performed in two independent laboratories: Experiment 1 at Wildlife International Ltd.(WLI) and Experiment 2 at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB). Both experiments employed optimized animal husbandry procedures and environmental conditions in validated flow-through exposure systems. The two experiments demonstrated consistent survival, growth and development of Xenopus laevis tadpoles, and all measured parameters were within the expected ranges and were comparable in negative control and atrazine-treated groups. Atrazine, at concentrations up to 100 mug/L, had no effect in either experiment on the percentage of males or the incidence of mixed sex as determined by histological evaluation. In contrast, exposure of larval Xenopus laevis to 0.2 mug 17ss-estradiol/L as the positive control resulted in gonadal feminization. Instead of an even distribution of male and female phenotypes, percentages of males:females:mixed sex were 19:75:6 and 22:60:18 in Experiments 1 and 2, respectively. These studies demonstrate that long-term exposure of larval Xenopus laevis to atrazine at concentrations ranging from 0.01 to 100 mug/L does not affect growth, larval development or sexual differentiation.

Xenopus laevis has been introduced as a model to study effects of endocrine-active compounds (EAC) on development and sexual differentiation. However, variable and inconsistent data have raised questions about the reliability of the test methods applied. The current study was conducted in two laboratories to develop, refine, and standardize procedures and protocols. Larvae were exposed in flow-through systems to 17beta-estradiol (E2), at concentrations from 0.2 to 6.0 mug E2 L(-1) in Experiment 1A, and 0.015 to 2.0 mug E2 L(-1) in Experiment 1B. In both studies survival (92%, 99%) and percentage of animals that completed metamorphosis (97%, 99%) indicated reproducible biological performance. Furthermore, minor variations in husbandry led to significant differences in snout-to-vent length, weight, and gonad size. In Experiment 1A, almost complete feminization occurred in all E2 treatment groups whereas a concentration response was observed in Experiment 1B resulting in an EC(50) of 0.12 mug E2 L(-1). The final verified protocol is suitable for determining effects of EAC on development and sexual differentiation in X. laevis.

Xenopus laevis has been introduced as a model to study effects of endocrine-active compounds (EAC) on development and sexual differentiation. However, variable and inconsistent data have raised questions about the reliability of the test methods applied. The current study was conducted in two laboratories to develop, refine, and standardize procedures and protocols. Larvae were exposed in flow-through systems to 17beta-estradiol (E2), at concentrations from 0.2 to 6.0 mug E2 L(-1) in Experiment 1A, and 0.015 to 2.0 mug E2 L(-1) in Experiment 1B. In both studies survival (92%, 99%) and percentage of animals that completed metamorphosis (97%, 99%) indicated reproducible biological performance. Furthermore, minor variations in husbandry led to significant differences in snout-to-vent length, weight, and gonad size. In Experiment 1A, almost complete feminization occurred in all E2 treatment groups whereas a concentration response was observed in Experiment 1B resulting in an EC(50) of 0.12 mug E2 L(-1). The final verified protocol is suitable for determining effects of EAC on development and sexual differentiation in X. laevis.

Debate and controversy exists concerning the potential for the herbicide atrazine to cause gonadal malformations in developing Xenopus laevis. Following review of the existing literature the US Environmental Protection Agency required a rigorous investigation conducted under standardized procedures. Xenopus laevis tadpoles were exposed to atrazine at concentrations of 0.01, 0.1, 1, 25 or 100 mug/L from day 8 post-fertilization (dpf) until completion of metamorphosis or dpf 83, whichever came first. Nearly identical experiments were performed in two independent laboratories: Experiment 1 at Wildlife International Ltd.(WLI) and Experiment 2 at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB). Both experiments employed optimized animal husbandry procedures and environmental conditions in validated flow-through exposure systems. The two experiments demonstrated consistent survival, growth and development of Xenopus laevis tadpoles, and all measured parameters were within the expected ranges and were comparable in negative control and atrazine-treated groups. Atrazine, at concentrations up to 100 mug/L, had no effect in either experiment on the percentage of males or the incidence of mixed sex as determined by histological evaluation. In contrast, exposure of larval Xenopus laevis to 0.2 mug 17ss-estradiol/L as the positive control resulted in gonadal feminization. Instead of an even distribution of male and female phenotypes, percentages of males:females:mixed sex were 19:75:6 and 22:60:18 in Experiments 1 and 2, respectively. These studies demonstrate that long-term exposure of larval Xenopus laevis to atrazine at concentrations ranging from 0.01 to 100 mug/L does not affect growth, larval development or sexual differentiation.