Temporal Expression Profiles and Bioactivity of Single (bollgard) and Dual-toxin (bollgard Ii) Bt Cotton

Laboratory studies were conducted using ELISA and bioassays (Helicoverpa zea) to investigate temporal Cry protein expression in field-grown single (‘Bollgard’) and dual-toxin (‘Bollgard II’) Bt cotton. In single-toxin Bt cotton, Cry1Ac levels changed significantly as the season progressed, exhibiting ca. 3-fold difference in Cry1Ac expression between the highest date (6.2 ppm—12 June) and the lowest date (1.7 ppm-6 August). In subsequent bioassays for single-toxin Bt cotton (7 DAI), there were also significant differences among dates with ca. 2-fold difference in larval weights between the highest date (37.0 mg—6 August) and the lowest date (18.4 mg —26 June). When comparing results from ELISA to those from H. zea bioassays, Cry1Ac levels were inversely related to mean larval weight across all dates, in that the highest levels of toxin expression resulted in the lowest larval weights among treatments, and vise versa. For dual-toxin Bt cotton, Cry1Ac responded similarly to those from single-toxin Bt cotton, exhibiting ca. 3-fold difference between the highest date (6.43 ppm—12 June) and the lowest date (2.19 ppm—10 July). Cry2Ab levels, however, only exhibited ca. 1.5-fold difference in toxin expression throughout the season (14.6 24.6 ppm). In bioassays for dual-toxin Bt cotton, there were no significant differences in weights of H. zea among dates at 7 DAI (15.6 20 mg). Because the insertion of the cry2Ab gene had no deleterious effect on the performance of the cry1Ac gene in dual-toxin Bt cotton, enhanced efficacy can be attained from the additive effects of both toxins. In our studies, the presence of the Cry2Ab protein played a significant role in reducing the variation in susceptibility of H. zea larvae feeding on transgenic Bt cotton tissue. Therefore, cottons containing two toxins of B. thuringiensis can potentially be valuable tools in resistance management, as well as provide enhanced season-long protection against H. zea.