Detection Of Resistance Allele Frequencies To Cry2Ab In Field Populations Of Helicoverpa Armigera And Separation Of Different Bt Resistance Genes In An2Ab Strain

Cotton bollworm,Helicoverpa armigera(Hubner),is a global agricultural pest with biological characteristics of polyphagy,high fecundity,facultative diapause and migration,and one of the important cotton pest in China.The economic and social benefits have been presented since the cultivation of transgenic Bt cotton,because it is effective to suppress field populations of cotton bollworm and some other main lepidopteran pests.With the evolution of resistance of target pests to Bt cotton,the gene pyramiding strategy,which could express Cry2Ab together with Cry 1 Ac,has become an important tactic to delay the evolution of resistance in the target pests to single Bt toxin Cry1Ac.To detect the initial frequencies of resistance alleles to Cry2Ab in the field populations of H.armigera and to monitor the field resistance changes will be helpful to improve Bt resistance management measures and provide useful knowledge for the sustainable development of Bt transgenic cotton.Furthermore,the different Bt resistance genes in a Cry2Ab-selected strain(An2Ab)of H.armigera were separated and three strains with the isolated single resistance gene were set up respectively.Contribution of each of the three resistance genes on resistance to Cry2Ab and Cry 1 Ac was compared.This work is helpful to further investigate resistance characters and mechanisms of Cry2Ab resistance and cross-resistance between Cry2Ab and Cry 1 Ac in H.armigera.1.Detection of Cry2Ab resistance allele frequencies in the field populations of H.armigera with F2 screenInitial frequencies of resistance alleles to Cry2Ab were detected with F2 screen in field populations collected from Anyang of Henan province,Langfang of Hebei province and Xiajin of Shandong province of China in 2015.A total of 213 iso-female families of H.armigera were established for Anyang population.73 single-pair families were detected and 6 of them had resistance alleles.The frequency of resistance allele for Anyang population was 0.023(0.010-0.050).A total of 187 iso-female families of H.armigera were established from Langfang population.62 single-pair families were detected and 3 of them had resistant alleles.The frequency of resistance alleles was 0.012(0.005-0.042)for Langfang population.A total of 215 iso-female families of H.armigera were established from Xiajin population.62 single-pair families were detected and 6 of them had resistance alleles.The frequency of resistance allele was 0.027(0.012-0.058)for Xiajin population.Further screen to some iso-female families with resistance alleles using Cry2Ab showed that resistance to Cry2Ab in these field-derived populations was controlled by multiple genes with minor effect.Although dual Bt cotton producing Cry 1 Ac and Cry2Ab has not been planted in China,resistance allele frequencies to Cry2Ab are at higher level than expected.So,it should be very caucious to consider using the dual Bt cotton as a resistance management tactic for Bt cotton in China.2.Separation and isolation of different Bt resistance genes in the An2Ab strainPrevious studies in our group indicated that Cry2Ab resistance in a Cry2Ab-selected strain(An2Ab)of H.armigera was inherited as dominant and polygenic loci.Dominant resistance to Cry2Ab and potential cross-resistance between Cry2Ab and Cry1Ac will pose significant challenge to the current resistance management strategy for Bt cotton in China.In this study,An2Ab strain was backcrossed with the susceptible GR strain and resistance genes in Chr09,Chr10 and Chr13 were separated with SNP marker-assisted selection method.Three single-gene strains were accordingly established and named as A9,B10 and C13,respectively.The A9 strain had 15-and 5.09-fold resistance to Cry2Ab and Cry1Ac respectively.The B10 strain showed 38.4-and 6.91-fold resistance to Cry2Ab and Cry1Ac respectively.The C13 strain possessed 12.3-and 19-fold resistance to Cry2Ab and Cry 1 Ac respectively.The resistance gene in Chr10 gave the greatest contribution to Cry2Ab resistance,and the resistance gene in Chr13 made the greatest contribution to CrylAc resistance.Inheritance analyses indicated that resistance in the three resistance strains was autosomal and dominant to both Cry2Ab and CrylAc.The three single-gene strains of H.armigera established in the present study will provide an important basis for further investigation of mechanisms of dominant resistance to Cry2Ab and cross-resistance between Cry2Ab and Cry1Ac in H.armigera.