Knockout of chitin synthase gene confers resistance to Bt toxin Vip3Aa in Helicoverpa zea

BACKGROUND: Genetically engineered crops that produce insecticidal proteins from Bacillus thuringiensis (Bt) have many benefits and are used globally to manage key insect pests, including Helicoverpa zea (Lepidoptera: Noctuidae), a major pest of crops in the Americas. However, pests of at least 11 species, including H. zea, have evolved resistance to Bt crops, diminishing their effectiveness and benefits. For H. zea in the United States, practical resistance to Bt corn and cotton producing crystalline (Cry) Bt proteins is widespread and early warning of resistance to the vegetative insecticidal protein Vip3Aa has been reported. Thus, a better understanding of the genetic basis of resistance to Vip3Aa is needed to monitor, manage and counter resistance. In some strains of lepidopteran pests, resistance to Vip3Aa is associated with disruptive mutations in the chitin synthase 2 (CHS2) gene but this association had not been investigated previously in H. zea. RESULTS: Here, we show that mutations introduced by CRISPR/Cas9 editing of the CHS2 gene can cause resistance to Vip3A in H. zea. Disruptive mutations in CHS2 facilitated the creation of strain CHS2-KO that had 29 000-fold resistance to Vip3Aa relative to its unedited parental susceptible strain. Resistance to Vip3Aa in CHS2-KO was autosomal, recessive, and did not cause cross-resistance to Cry1Ac or Cry2Ab. CONCLUSION: Results of this study indicate that CHS2 plays an important role in Vip3Aa intoxication in H. zea. It will be important to determine if mutations in CHS2 contribute to field-evolved resistance to Vip3Aa in H. zea and other pests.