Characterisation of gonadal responses in Xenopus laevis to exposures of atrazine in semi-natural microcosms

Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2009.

The African Clawed frog (Xenopus laevis) is most likely the most studied amphibian to date. This animal is widely used as a laboratory screening model for the testing of various chemicals. In recent years, there has been considerable controversy over the possible effects of the widely-used herbicide atrazine on amphibians. There were claims that this broadleaf herbicide causes gonadal abnormalities in amphibians, including feminisation and the promotion of a form of abnormality in the testes characterised by the presence of ovarian follicles (oocytes). Clawed frogs are native to Africa and were used in this study to test the reproductive success and development of F2 offspring after the F1 parent animals were exposed to known atrazine concentrations from 96 hrs to 24 month-old mature frogs. Animals were exposed to four nominal concentrations of atrazine (0, 1, 10, 25 µg/£). Male and female frogs were paired off according to the atrazine concentration in which they were reared and spawning was induced. Clutch size and survival of offspring were used to evaluate developmental success. Gonads of metamorphs as well as breeding F1 frogs were examined for gross anomalies. Testes were serially sectioned and screened for anomalies at the microscopic level. We were unable to find any concentration response to hatching success, time to metamorphosis or sex ratios. No indication of a transgenerational effect of atrazine on spawning success or reproductive development of X. laevis was observed. Adult X. laevis collected along a north-south transect from the south-west Western Cape region to the north-east were analysed and screened for gonadal anomalies. We found that, irrespective of exposure to atrazine, male X. laevis from north-east sites contained testicular ovarian follicles whereas none of the animals from the Western Cape sites had any. Differences between these populations of X. laevis have been reported and it cannot be excluded that they belong to separate species, in which case the phenomenon of testicular ovarian follicles could be associated with the northern form and with no relevance to atrazine usage. Atrazine has been widely used in South Africa for more than 40 years, and still robust populations of X laevis with balanced sex ratios occur throughout its distribution range -which include the major maize production area in South Africa. Our data does not support the hypothesis that atrazine impacts negatively on amphibians in natural situations and at environmentally relevant concentrations.

Masters