No abstract

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 U.S. Environmental Protection Agency required a rigorous investigation conducted under standardized procedures. X. laevis tadpoles were exposed to atrazine at concentrations of 0.01, 0.1, 1, 25, or 100 μg/l from day 8 postfertilization (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. 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 X. 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 μg/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 X. laevis to 0.2 μg 17β-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 X. laevis to atrazine at concentrations ranging from 0.01 to 100 μg/l does not affect growth, larval development, or sexual differentiation.

No abstract

Abstract-Effects of fenoxycarb on growth and reproduction of Daphnia magna were examined under realistic exposure conditions as defined by predictions from laboratory fate studies and observations in the field. Initial concentrations were administered in a single pulse dose and altered to mimic the reduction of fenoxycarb that occurs following field application to natural waters (halflife Ӎ 10 hours). Potential effects on the developmental stage of test organisms were determined by exposure of four age groups: less than 24 h, 4 to 6 d, 8 d, and 11 d. There were no significant effects on survival or time to first brood of first-and secondgeneration daphnids in any age group. The number of young per daphnid was significantly lower than controls only among daphnids that were less than 24 h old at test initiation and exposed to the highest initial measured concentration of fenoxycarb (45 g/L). The estimated maximum acceptable toxicant concentration (MATC) was 26 g/L, calculated from exposure of the most sensitive age group (less than 24 h old) to environmentally realistic levels of fenoxycarb. This represents a substantial reduction in toxicity when compared to the MATC of 0.0016 g/L previously reported from a standard, constant-exposure study.

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 17β-estradiol (E2), at concentrations from 0.2 to 6.0 μg E2 L −1 in Experiment 1A, and 0.015 to 2.0 μg 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 μg E2 L −1 . The final verified protocol is suitable for determining effects of EAC on development and sexual differentiation in X. laevis.