Studies On Evolution And Inheritance Of Resistance For Bt Cry1Type Toxins In Asian Corn Borer

Asian corn borer, Ostrinia furnacalis (Guenee), is one of the most important insect pests of maize in China. Transgenic Bacillus thuringiensis (Bt) maize provide an effective mean to control this insect pest. However, evolution of resistance by target pests will be the great threat to the continued success of Bt toxins used in insecticide formulations or expressed by transgenic maize. It is theoretically and practically important for well-characterized resistant strains, which will provide the only way to empirically validate proposed management strategies. In this study, selection for CrylAb, Cry1Fa, CrylIe and CrylAh resistance in Asian corn borer were conducted through diet incoperation with thoses toxins in the laboratory; inherentance of CrylAb and CrylAc resistance stains (ACB-AbR and ACB-AcR) which had been selected for more than100generations with CrylAb and/or CrylAc and a susceptible strain without exposure to any Bt toxins were determined with backcrosses experiments.The susceptibilities of Asian corn borer to CrylAb, CrylFa, CrylIe, and CrylAh were gradually decreased when the selection pressure and generation increased. After14generations selection, resistant indexes were increased by28-,52-,23-, and9-fold in CrylAb, CrylFa, Crylle, and CrylAh selected populations, respectively. Compared with unselected population, larval development times of four selected populations were increased by7.4,4.4,5.7, and5.4d; pupal weights were decreased by24.4,18.6,13.7, and15.9%; numbers of eggs laid per femail were reduced by51.0,28.3,40.0, and49.0%. Meanwhile, the selected populations had evolved significant cross resistance to the Bt toxins, for instance, CrylAb selected population evolved11-and15-fold resistance to CrylF and CrylAc, but noncross resistance was detected in CrylIe; CrylFa selected population evolved3.8-and3.7-fold resistance to CrylAb and CrylAc, but noncross resistance was detected in Crylle; CrylIe selected population did not evolve resistance to CrylAb, CrylAc, and CrylF tonxins. These results showed that there was high cross resistance between CrylAb and CrylAc toxins, and there were lower cross resistance between CrylF and CrylAb and/or CrylAc; there were noncross resistance between Crylle and CrylAb or CrylAc or CrylF. These suggested that CrylAb and CrylAc have more common binding sites, CrylF has one common bingding site, but CrylIe do not have any noncommon binding sites with CrylAb, CrylAc, and CrylF.ACB-AbR and ACB-AcR stains evolved more than66and142-fold resistance. Resistance was pramarilly autosomal. The degree of dominance decreased as the toxins concentration increased. The resistance of ACB-AbR to CrylAb and CrylAc was controlled by more than one locus or multiple alleles at one locus. However, the resistance of ACB-AcR was polygenic to CrylAc, but monogenic to CrylAb. The degree of dominance of cross resistance in two resistanct strains to CrylF decreased as the toxin concentration increased. The resistance levels of F1offspring of reciprocal crosses between ACB-AbR and ACB-AcR were not significant different than their parents. However, they showed high cross resistance to CrylF. This indicated that ACB-AbR and ACB-AcR had similar common binding sites, but those binding sites had additive effects to CrylF. The resistance levels of F1offspring of reciprocal crosses between ACB-AbR and ACB-AcR were as susceptible as there parents to CrylIe, indicating there was not any common binding site for CrylIe in ACB-AbR and ACB-AcR.