Synergistic epistasis confers strong and dominant resistance to Bacillus thuringiensis toxin Cry1Ac in Helicoverpa armigera

Evolution of resistance to insecticidal proteins from Bacillus thuringiensis (Bt) threatens the sustained use of transgenic Bt crops, underscoring the need for elucidating genetic mechanisms of resistance to improve development of resistance management strategies. Bt cotton and Bt corn have been extensively used to control some major lepidopterans including Helicoverpa armigera , a global pest of many major crops. Previous work with the strains LF256 and SCD-r1 of H. armigera indicated that resistance to Cry1Ac involves epistasis resulting in dominant resistance in the F₁ progeny from crosses between these strains. Cry1Ac resistance in SCD-r1 is recessive and conferred by a disruptive mutation in the cadherin gene HaCad whereas resistance in LF256 is recessive and attributed to one or more unknown loci independent of HaCad . To confirm the presence of epistasis, we generated an LF256-dCad strain by CRISPR-mediated knockout of HaCad in LF256. We hypothesized that integration of the different resistance alleles in LF256-dCad would result in synergistic interactions between resistance loci and dominant inheritance of resistance. Compared to the susceptible SCD strain, SCD-r1, LF256 and LF256-dCad exhibited 480-, 143- and 3443-fold resistance to Cry1Ac, respectively, with no cross-resistance to Cry2Ab and Vip3Aa. The F₁ progeny from crosses between LF256-dCad and SCD were resistant to Cry1Ac, indicating dominant inheritance of resistance. Our results demonstrate that synergistic epistasis between HaCad and one or more resistance loci in LF256 confers strong and dominant resistance to Cry1Ac, potentially enhancing resistance evolution in field populations of H. armigera .