Abstract
In Africa, the target pests of genetically modified Bt maize are lepidopteran stem borers, notably Busseola fusca (Lepidoptera: Noctuidae). Gene flow between Bt maize hybrids and open pollinated varieties (OPVs) that do not contain the Bt trait is highly likely in areas where both types of maize are cultivated. Consequently, introgression of the cry1Ab transgene into local OPVs will result in unknown patterns of Cry1Ab protein expression in plants during follow-up seasons when recycled seed of OPVs is planted. Too low concentrations of Cry protein in such plants may result in selection for resistant alleles and accelerate resistance evolution. The aim of this study was to determine the effects of introgression of the cry1Ab transgene into an OPV, on Cry protein concentration levels and pest survival. Bt transgene introgression was done by crossing a transgenic donor hybrid containing the cry1Ab gene with a non-Bt OPV as well as with a non-Bt near-isogenic hybrid. F1 and F2 crosses as well as back crosses were done yielding 11 genotypes (treatments). Cry1Ab protein concentrations in leaf tissue of these crosses were determined by means of ELISAs. All crosses that contained the transgene had similar or higher Cry1Ab concentrations when compared to the Bt parental hybrid, except for the Bt x OPV F1-cross that had a significantly lower Cry1Ab concentration. Survival B. fusca larvae were evaluated in assays in which larvae were reared for 14 days on whorl leaf tissue of the different treatments. Larval survival did not differ between any of the maize plant treatments which contained the Bt gene. Results suggest that Bt transgene introgression into OPVs may produce plant progenies that express Cry1Ab protein at sufficient concentrations, at last up to the F2 seed, to control B. fusca larvae. Resistance evolution is however not only influenced by the frequency of pest individuals that survive exposure to the Cry proteins but also by factors such as genetics of the pest and recipient OPV, pest biology and migration behaviour.
Highlights
Since the first deployment of genetically modified Bt crops concerns were raised about resistance evolution in target pest species [1,2]
Transgene introgression into open pollinated maize seed is unlikely to have a major effect on resistance evolution [21]. Even if scenarios such as those discussed above lead to the increased rate of transgene introgression into open pollinated varieties (OPVs), the results of this study indicate a stable expression of the transgene up to F2 seed which should have no major effect on resistance evolution
The evolution of resistance to Cry proteins in target pests is a major threat to the continued successful use of Bt crops [60,61], emphasizing the importance of knowing the effects of varying Cry protein concentrations in OPVs and Bt-introgressed non-Bt hybrids in uncontrolled environments
Summary
Since the first deployment of genetically modified Bt crops concerns were raised about resistance evolution in target pest species [1,2]. The rates at which pests develop resistance to Bt toxins can be exacerbated by increased larval migration, low-dose expression and non-compliance to refuge requirements [9,10,11,12]. Insect resistance management (IRM) in Africa is faced by several challenges such as recycling of seed, gene flow between varieties and small units of land. Low dose expression of Cry toxins is a challenge to IRM, especially in subsistence farming where crop fields are small, Bt maize OPV’s may be cultivated in adjacent fields, and seed is recycled. Gene flow in environments where Bt maize and OPVs co-exist will result in introgression of transgenes into OPV’s [13,14,19,20], providing challenges to IRM in the African context [17,21,22]
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