Effect of methoxychlor on various life stages of Xenopus laevis

The toxicological effects of the organochlorine pesticide methoxychlor were evaluated at various life stages of the South African clawed frog, Xenopus laevis, in an effort to determine stage-specific sensitivity. A battery of four separate assays, including a short-term (4-day) early embryo-larval assay (FETAX) (NF stages 8-46 [Nieuwkoop and Faber, 1994]), 30-day hind limb development assay (NF stages 8-54), 18-day metamorphic climax assay (NF stages 58-66), and 30-day adult reproduction assay were performed. Test concentrations for the FETAX, hind limb development, metamorphic climax, and reproductive assays ranged from 0.0001-1.0 mg/l, 0.0001-0.1 mg/l, 0.0001-0.1 mg/l, and 0.001-0.1 mg/l, respectively. Results from the short-term embryo-larval assay indicated that increased embryo-lethality, malformation, and growth inhibition were not induced at /=0.01 mg/l delayed hind limb digit differentiation. Follicular hyperplasia of the thyroid glands was noted in specimens exposed to 0.1 mg/l methoxychlor. Results from the 18-day metamorphic climax assay indicated that methoxychlor inhibited the rate of tail resorption in a concentration-dependent manner. Whole body tissue triiodothyronine (T(3)) profiles showed a reduced and delayed surge during climax compared to controls. For the reproductive assessment, adult female X. laevis were super-ovulated and both female and male were then exposed to varying concentrations of methoxychlor. A concentration-dependent reduction in ovary weight and the number of viable oocytes was observed. In exposed male specimens, a concentration-dependent reduction in testis weight and sperm count was found. Methoxychlor was found to accumulate in the ovary, and to a lesser extent in the testis. Based on breeding studies in which exposed females were bred with control males and exposed males bred with control females, the frequency of amplexus, fertilization, and embryo viability was also affected by adult female methoxychlor exposure, and to a lesser extent by male exposure. Overall, these results suggested that sensitivity to methoxychlor is most dramatic during the reproductive and metamorphic phases of the life cycle and least sensitive during early embryo-larval development.