Is resistance to Bacillus thuringiensis toxin Cry2Ab associated with mutations of Cry1Ac-receptor genes in Trichoplusia ni?

Simultaneous expression of multiple Bacillus thuringiensis (Bt) toxins with different binding sites in plants is one of the strategies for delaying development of insect resistance to Bt crops. Bt toxin Cry2Ab is a major toxin being stacked with Cry1Ac in the new generation of Bt crops. We have established a strain of Trichoplusia ni resistant to the second-generation Bt cotton Bollgard II^® which produces both toxins Cry1Ac and Cry2Ab from the T. ni strain Glen-DiPel that developed resistance to DiPel^® in commercial greenhouses. However, the genetic mechanism of resistance to Cry2Ab in insects is unknown. In this study, the genetic linkages of Cry1Ac-receptor genes with the Cry2Ab resistance in T. ni were examined. The Cry1Ac-receptor genes analyzed included the genes coding for the midgut cadherin, the membrane-bound alkaline phosphatase, and six aminopeptidases N. Single-nucleotide polymorphism sites (SNPs) in these genes were identified and used as molecular markers of different gene alleles for the linkage analyses. Our genetic linkage analyses determined that the Cry2Ab resistance in T. ni is not associated with the eight genes analyzed. Therefore, we conclude that Cry2Ab resistance in T. ni is not conferred by mutations of the candidate Cry1Ac-receptor genes.