Pink bollworm resistance to Bt cotton: Genetics and molecular mechanisms

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. 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 describe the genetics of resistance to Cry1Ac and Cry2Ab in the pink bollworm. For

resistance to Cry1Ac, we found that both laboratory-selected resistance in strains from Arizona

and field-evolved resistance to Cry1Ac in Bt cotton in India are associated with mutations in a

gene encoding a cadherin protein that binds Cry1Ac. Of the 25 cadherin mutations we found in

lab strains and field populations, 20 involve alternative splicing or mis-splicing of messenger

RNA. We used two rounds of laboratory selection to achieve extremely high resistance to

Cry2Ab. 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. Our results demonstrate the risk of evolution of resistance to pyramided Bt

plants, particularly when field populations are already resistant to one toxin in the pyramid and

refuges are scarce, as seen with Bt cotton and pink bollworm in India.