Inheritance of resistance to the Cry1Ab Bacillus thuringiensis toxin in Ostrinia nubilalis (Lepidoptera: crambidae)

Laboratory selection with CrylAb, the predominant Bacillus thuringiensis (Bt) toxin in transgenic corn, Zea mays L., produced > 1000-fold resistance in two laboratory strains of European corn borer, Ostrinia nubilalis (Habner). We tested the offspring of various crosses to determine the mode of inheritance of resistance to CrylAb. Patterns of inheritance of resistance were similar in the two resistant strains. The progeny of reciprocal F-1 crosses (resistant male X susceptible female and vice versa) responded alike in bioassays, indicating autosomal inheritance. The median lethal concentrations (LC50 values) of F-1 were intermediate between the resistant and susceptible parents, indicating approximately additive inheritance. However, the dominance of resistance increased as the concentration of CrylAb decreased. Analysis of progeny from backcrosses (F-1 X susceptible strain) suggests that resistance was controlled by more than one locus. In particular, the fit of observed to expected mortality improved as the number of putative loci increased from 1 to 10. The polygenic nature of resistance in these two laboratory strains suggests that major genes for resistance to CrylAb were not common in the founding populations of O. nubilalis. A low initial frequency of major genes for CrylAb resistance might be an important factor in delaying evolution of resistance to Bt corn in this pest.