Multiple resistance or cross resistance? A case study of dual resistance to Bacillus thuringiensis toxins Cry1Ac and Cry2Ab in Trichoplusia ni

Development of resistance to Bacillus thuringiensis (Bt) toxins in insect populations is a major threat to the sustainable application of Bt-based insect control technologies. Expression of multiple Bt toxins in transgenic plants by gene pyramiding is one resistance management strategy to delay the development of resistance to Bt crops in insect populations. To study insect resistance to Bt crops with multi-toxins, a strain of Trichoplusia ni resistant to the pyramided Bt cotton Bollgard II® containing Bt toxins Cry1Ac and Cry2Ab was recently selected from a greenhouse-originated Bt resistant T. ni population. To understand whether the dual resistance of T. ni to Bt toxins Cry1Ac and Cry2Ab is conferred by one mechanism (cross resistance) or by two independent mechanisms (multiple resistance), in this study, the dual resistance trait was introgressed into a susceptible laboratory strain of T. ni, and the resulting strain was selected with Cry1Ac and Cry2Ab, respectively, to generate two independent strains. Dose-response assays of these two strains to Cry1Ac and Cry2Ab were conducted to analyze the segregation of the Cry1Ac and Cry2Ab resistance traits and to determine the cross resistance of these two strains to the two Cry toxins. In addition, analysis of backcross progeny from reciprocal crosses between F1 hybrid progeny and the dual-resistant T. ni strain was conducted to determine the genetic linkage between the Cry1Ac and Cry2Ab resistance in T. ni.