Novel genetic basis of resistance to Bt toxin Cry1Ac inHelicoverpa zea

Abstract

Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have advanced pest management, but their benefits are diminished when pests evolve resistance. Elucidating the genetic basis of pest resistance to Bt toxins can improve resistance monitoring, resistance management, and design of new insecticides. Here, we investigated the genetic basis of resistance to Bt toxin Cry1Ac in the lepidopteran Helicoverpa zea, one of the most damaging crop pests in the United States. To facilitate this research, we built the first chromosome-level genome assembly for this species. Using a genome-wide association study, fine-scale mapping, and RNA-seq, we identified a 250kb QTL on chromosome 13 that was strongly associated with resistance in a strain of H. zea that had been selected for resistance in the field and lab. This QTL contains no genes with a previously reported role in resistance or susceptibility to Bt toxins. However, within this QTL, we discovered a premature stop codon in a kinesin gene. We hypothesize that this mutation contributes to resistance. The results indicate the mutation on chromosome 13 was necessary but not sufficient for resistance, and therefore conclude that mutations in more than one gene contributed to resistance. Moreover, we found no changes in gene sequence or expression consistently associated with resistance for 11 genes previously implicated in lepidopteran resistance to Cry1Ac. Thus, the results reveal a novel and polygenic basis of resistance and extend the list of genes contributing to pest resistance to Bt toxins.

Utility and challenges of using whole‐genome resequencing to detect emerging insect and mite resistance in agroecosystems

“…When resistance is detected in a pest population, quantitative genetic and genomic experiments can link changes in a pest's resistance status to one or more causal mutations across the genome (i.e., genome architecture). Commonly used approaches include quantitative trait locus (QTL) analysis (e.g., Gahan et al, 2001; Heckel et al, 1999; Taylor et al, 2021; Wondji et al, 2007), bulked segregant analysis (BSA; e.g., Benowitz et al, 2022; Fotoukkiaii et al, 2021; Van Leeuwen et al, 2012), and evolve and resequence studies (Snoeck et al, 2019; Wybouw et al, 2019). QTL and BSA approaches use offspring from planned crosses between pesticide‐resistant and susceptible populations and SNPs from across the genome to identify genomic regions where genotypic variation is strongly associated with variation in resistance phenotypes.…”

A chromosome-scale genome assembly of aBacillus thuringiensisCry1Ac insecticidal protein resistant strain ofHelicoverpa zea

“…Resistance to Bt insecticidal proteins is associated with changes in midgut receptors or signal transduction pathways (Soberón et al 2009). Evidence suggests H. zea Bt Cry1Ac resistance is influenced by point mutations in tetraspanin (Jin et al 2019) or kinesin (Benowitz et al 2021) genes, or at several loci (Taylor et al 2021). These reductions in H. zea insecticide susceptibility (Yang et al 2020;Santiago González et al 2021) contribute to increased levels of crop damage and lower yields (Reisig and Kurtz 2018;Reay-Jones 2019), presenting a threat to global food security (Coates et al 2015).…”