The effect of glyphosate and Cry1Ab proteins on the growth and survival of tadpoles of two amphibian species

Abstract

Studies have shown that pesticides have negative effects on non-target organisms. Lately more studies are focusing on amphibians due to their rapid world-wide decline. This study addressed the potential effects of glyphosate (herbicide) and Cry proteins produced by genetically modified Bt maize (insecticide), and focuses on the development of tadpoles. Crops can be genetically modified by inserting a specific gene, in this case that of the soil bacterium Bacillus thuringiensis (Bt), into their genomes. This gene encodes for proteins which have insecticidal characteristics. This protein can then protect the crops from certain Lepidopteran pests. Glyphosate, the herbicide used in this study, is a non-selective systemic herbicide chosen due to its popularity as an active ingredient in many herbicides. Genetically modified Bt maize was selected because of its wide-scale cultivation and possible adverse effects that the insecticidal protein (Cry1Ab) found in aquatic systems may have on amphibians. Xenopus laevis and Amietophrynus gutturalis tadpoles were chosen based on their wide distribution in South Africa and because they are found in areas where herbicide tolerant and Bt maize are planted. It is believed that each amphibian species thrive in different conditions. For this reason a husbandry study was developed for X. laevis, where it was found that X. laevis thrives while in groups and A. gutturalis thrives individually. The aims of our study were to expose these tadpoles to different concentrations of glyphosate (laboratory based study) and genetically modified Bt maize leaves (laboratory and mesocosm based study) and monitor their development. We hypothesised that glyphosate will affect the development of the tadpoles exposed to the higher dosages and that the tadpoles exposed to Bt maize leaf infusions will develop slower than in the control treatment. After exposing tadpoles to Bt and non-Bt infusions, it was found that both treatments have adverse effects on the development of X. laevis tadpoles. However, the severity of the toxicity could depend on the hybrid or its genetic background. Exposure to the three highest glyphosate concentrations resulted in 100% mortality after three weeks. However, tadpoles exposed to the lowest concentration of glyphosate had a similar growth and developmental pattern as those in the control group, although only 50% reached metamorphosis when compared to that of the control treatment. The results for the degradation of the Cry1Ab proteins were inconclusive. However, an increase in degradation was followed by a sudden decrease before reverting to a steady increase at the end of the test. The Cry1Ab protein concentration flattened off between 10 and 21°C temperatures, and no decrease in concentration was seen during the exposure periods between one hour and sixteen days. During a mesocosm experiment it was also found that the development and growth of tadpoles was less affected in the Bt and nonBt infusions than in the control. Considering these results, we conclude that the presence of the Bt toxin at environmentally relevant concentrations, does not effect on the development or survival of Amietophrynus gutturalis tadpoles.