A second locus associated with high-level resistance to Bt Cry1Ac in Trichoplusia ni

Previous studies on the toxicity of the insecticidal protein Cry1Ac from Bacillus thuringiensis (Bt) have indicated the presence of multiple pathways of Cry1Ac toxicity in the cabbage looper, Trichoplusia ni. Consistently, the high-level resistance to Cry1Ac evolved in greenhouse populations of T. ni has been identified to be associated with an ABCC2 gene mutation and an additional yet-to-be-identified gene mutation. In this study, the second resistant trait to Cry1Ac in T. ni, Cry1Ac-R2, was genetically mapped by biphasic linkage analysis, using bulked segregant analysis by whole-genome sequencing (BSA-seq) and amplicon sequencing (Amp-seq). The Cry1Ac-R2 was localized in a 675-kb region on chromosome 9. Of the 37 genes in the Cry1Ac-R2 locus, 25 were expressed in the midgut of T. ni larvae. Among those midgut genes are a cluster of aminopeptidase N (APN) genes, including APN1 and APN6. Analysis of the dominance of Cry1Ac-R2 revealed that the dominance at a high dose of Cry1Ac could be significantly influenced by the genetic background of T. ni strains. The results from this study demonstrated that resistance to Cry1Ac in an insect may involve multiple gene mutations with various types of dominance, and the dominance can be influenced by the genetic background of an insect population.

The soil bacterium Bacillus thuringiensis (Bt) and its insecticidal Cry proteins are widely used for insect control in agriculture and public health. Continuing success of Bt biotechnology requires a better understanding of the insect genes involved in the toxicity of Bt proteins and their mutations leading to Bt resistance. Cry1Ac is a major Bt protein used for Lepidoptera pest control, and resistance to Cry1Ac has occurred in field populations of insects, including the cabbage looper, Trichoplusia ni. The results from this study confirmed that Cry1Ac resistance in an insect may involve multiple gene mutations and both dominant and recessive resistant traits. This study also revealed that the dominance of a resistance trait can be influenced by the genetic background of an insect population, which could complicate research data interpretation and may affect the efficacy of Bt toxins for control of different insect populations in the field.