Bt maize and frogs : an investigation into possible adverse effects of Bt toxin exposure to amphibian larvae

Abstract

Genetically modified maize expressing the Bt-protein Cry1Ab (Bt maize) is planted widely in South Africa. Crop residues of Bt maize often end up in aquatic ecosystems where aquatic organisms are exposed to Cry1Ab protein. The effect of this protein on non-target aquatic organisms has not yet been studied in South Africa. The aim of this study was to evaluate the possible effect of exposure to Bt maize on morphological development of Xenopus laevis and Amietophrynus gutturalis tadpoles. Three experiments were conducted with each of X. laevis and A. gutturalis. Five of these were conducted in the bio-secure Amphibian Biology laboratory and one with A. gutturalis in a shade-house facility where microcosms were exposed to natural conditions. In the first experiment of X. laevis and A. gutturalis, which was replicated three times, large portions of maize leaves were placed in the bottoms of microcosms. X. laevis received supplementary pulverised leaves in suspension while A. gutturalis tadpoles fed on provided leaves. For both control and experimental groups microcosms were divided in three groups receiving respectively 15, 30 and 45 g of maize leaves. In the second and third experiment tadpoles only received pulverised Bt maize leaves in suspension. Each replicate (microcosm) contained 50 one-day old tadpoles. Experiment two was conducted to determine whether the Bt-protein has adverse effects on A. gutturalis tadpoles when tadpoles are exposed to the protein in the water but not feeding on the plant material. A total of 100 tadpoles were used during the experiment and tadpoles were placed individually in 250 ml plastic cups that were filled with 100 ml water witch contained an extract of either Bt and non-Bt maize leaves. Tadpoles were fed twice a week with TetraTabimin bottom-feeding fish pellets in suspension. Experiment three was conducted to determine whether the Bt-protein will have adverse effects on A. gutturalis tadpoles when tadpoles feed on Bt maize leaves. Tadpoles were divided into a treatment in which 50 tadpoles were fed Bt maize leaves and a control treatment in which 50 tadpoles were fed non-Bt maize leaves. Tadpoles were placed individually in 250 ml plastic cups that were each filled with 100 ml borehole water. On a weekly basis 10 randomly selected tadpoles were collected, measured and staged for morphological development, using the Nieuwkoop and Faber Normal Table for X. laevis and Gosner stages for A. gutturalis tadpoles. The significant effects observed in some life history parameters of tadpoles exposed to Cry1Ab protein cannot be ascribed to the effect of the protein. Poor husbandry turned out to be the single most important confounding factor. Before follow-up studies are conducted husbandry practices should be optimized.