Abstract
Transgenic crops producing Bacillus thuringiensis (Bt) proteins kill key insect pests, providing economic and environmental benefits. However, the evolution of pest resistance threatens the continued success of such Bt crops. To delay or counter resistance, transgenic plant “pyramids” producing two or more Bt proteins that kill the same pest have been adopted extensively. Field populations of the pink bollworm (Pectinophora gossypiella) in the United States have remained susceptible to Bt toxins Cry1Ac and Cry2Ab, but field-evolved practical resistance to Bt cotton producing Cry1Ac has occurred widely in India. Here we used two rounds of laboratory selection to achieve 18,000- to 150,000-fold resistance to Cry2Ab in pink bollworm. Inheritance of resistance to Cry2Ab was recessive, autosomal, conferred primarily by one locus, and independent of Cry1Ac resistance. We created a strain with high resistance to both toxins by crossing the Cry2Ab-resistant strain with a Cry1Ac-resistant strain, followed by one selection with Cry2Ab. This multi-toxin resistant strain survived on field-collected Bt cotton bolls producing both toxins. The results here demonstrate the risk of evolution of resistance to pyramided Bt plants, particularly when toxins are deployed sequentially and refuges are scarce, as seen with Bt cotton and pink bollworm in India.
Highlights
Reference(s)Cry1Ac Cry1Ac Cry1Ac Cry1Ac, Cry2Ab Cry1Ac, Cry2Ab Cry1Ac, Cry2AbUSDA lab strain Derived from Bt4R Arizona pooled strain Derived from Bt4R AZP-R X Bt4R-2 Derived from AZP-R2, reared on control diet 33 this study 31, 34, 53 this study this study this study None NoneUSDA lab strain APHIS-S × SOM-07a 49 this studyThe lepidopteran pests targeted by Bacillus thuringiensis (Bt) cotton include the pink bollworm, Pectinophora gossypiella (Saunders), which has been a major pest in many countries including the world’s three leading cotton producers (China, India, and the United States)[20]
Table S1): susceptible strain APHIS-SOM, Cry1Ac-resistant strain AZP-R, Bt4-R2, which had moderate resistance to Cry1Ac and high resistance to Cry2Ab (Tables 2 and 3), and AZP-R2U, a subset of AZP-R2 that was not selected with Cry2Ab
We started with pink bollworm strains that were susceptible to Cry2Ab and achieved 18,000- to 150,000-fold resistance to Cry2Ab in strain Bt4-R2 after only two generations of laboratory selection (Fig. 1, Tables 2 and 3)
Summary
Reference(s)Cry1Ac Cry1Ac Cry1Ac Cry1Ac, Cry2Ab Cry1Ac, Cry2Ab Cry1Ac, Cry2AbUSDA lab strain Derived from Bt4R Arizona pooled strain Derived from Bt4R AZP-R X Bt4R-2 Derived from AZP-R2, reared on control diet 33 this study 31, 34, 53 this study this study this study None NoneUSDA lab strain APHIS-S × SOM-07a 49 this studyThe lepidopteran pests targeted by Bt cotton include the pink bollworm, Pectinophora gossypiella (Saunders), which has been a major pest in many countries including the world’s three leading cotton producers (China, India, and the United States)[20]. Laboratory selection with either Cry1Ac only or both Cry1Ac and Cry2Ab yielded pink bollworm strains that survived on Bt cotton producing Cry1Ac, but not on Bt cotton producing both toxins[30,31]. Whereas some of these strains had > 1000-fold resistance to Cry1Ac, the highest previously reported pink bollworm resistance to Cry2Ab was 240-fold[11,31]. After we crossed Bt4-R2 with a Cry1Ac-resistant strain (AZP-R)[30,32] and selected once on diet with Cry2Ab, the resulting multi-toxin resistant strain (AZP-R2) survived on bolls of Bt cotton producing both toxins. We used Bt4R-2, AZP-R2 and other strains to assess inheritance and fitness costs of resistance to Cry2Ab, as well as cross-resistance between Cry1Ac and Cry2Ab
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