Mismatch between lab-generated and field-evolved resistance to transgenic Bt crops in Helicoverpa zea

APN3 Acts as a Functional Receptor of Bacillus thuringiensis Cry1Ac in Grapholita molesta

Grapholita molesta (Busck) (Lepidoptera: Tortricidae) is a worldwide Rosaceae fruit tree pest. Although the Bacillus thuringiensis (Bt) Cry1Ac proteins exhibit excellent insecticidal activity against G. molesta, there are no reported Bt receptors for G. molesta. In the preliminary screening, we identified aminopeptidase N3 (GmolAPN3) as a potential receptor for Cry1Ac in G. molesta. Ligand blot and homologous competition experiments confirmed the simultaneous binding of interaction domains 2 (GluZincin superfamily domain) and 4 (ERAP1-C domain) of GmolAPN3 to activated Cry1Ac. In the cell toxicity bioassay, the overexpression of GmolAPN3 rendered Sf9 cells more susceptible to activated Cry1Ac. The results from dsRNA interference demonstrated that knockdown of GmolAPN3 conferred resistance in G. molesta larvae against Cry1Ac. Our findings provide pioneering evidence for the role of GmolAPN3 as the first identified Bt receptor mediating both binding and toxicity of Cry1Ac in G. molesta and also offer theoretical guidance for understanding the mode of action of Cry1Ac in G. molesta and its field applications.

Insect Pest Control from Chemical to Biotechnological Approach: Constrains and Challenges

The large growth in the global population requires new solutions for the control of harmful insects that compete for our food. Changing regulatory requirements and public perception, together with the continuous evolution of resistance to conventional insecticides, also require, in addition to innovative molecules with different modes of action, new non-chemical control strategies that can help maintain efficient integrated pest management programs. The last 30 years have inaugurated a new era characterised by the discovery of new mechanisms of action and new chemical families. Although European programs also promote a green deal in the crop protection sector, the existing thorough regulations slow down its spread and the adoption of new products. In light of these changes, this review will describe in more detail the dynamics of discovery and registration of new conventional insecticides and the difficulties that the agrochemical industries encounter. Subsequently, the different innovative control strategies alternative to conventional insecticides based on natural substances of different origin, entomopathogenic microorganisms, semiochemical and semiophysical compounds, and classical and augmentative biological control will be described. The advantages of these green strategies will be illustrated and also the constrains to their diffusion and commercialisation. Finally, the main biotechnological discoveries will be described, from transgenic plants to symbiotic control, classical genetic control, and, more recently, control based on insect genomic transformation or on RNAi. These new biotechnologies can revolutionise the sector despite some constrains related to the regulatory restrictions present in different countries.

Crop diversity induces trade-offs in microbial biopesticide susceptibility that could delay pest resistance evolution

Pathogens often exert strong selection on host populations, yet considerable genetic variation for infection defence persists. Environmental heterogeneity may cause fitness trade-offs that prevent fixation of host alleles affecting survival when exposed to pathogens in wild populations. Pathogens are extensively used in biocontrol for crop protection. However, the risks of pest resistance evolution to biocontrol are frequently underappreciated: the key drivers of fitness trade-offs for pathogen resistance remain unclear, both in natural and managed populations. We investigate whether pathogen identity or host diet has a stronger effect on allelic fitness by quantifying genetic variation and covariation for survival in an insect pest across distinct combinations of fungal pathogen infection and plant diet. We demonstrate substantial heritability, indicating considerable risks of biopesticide resistance evolution. Contrary to conventional thinking in host-pathogen biology, we found no strong genetic trade-offs for surviving exposure to two different fungal pathogen species. However, changes in plant diet dramatically altered selection, revealing diet-mediated genetic trade-offs affecting pest survival. Our data suggest that trade-offs in traits not strictly related to infection responses could nevertheless maintain genetic variation in natural and agricultural landscapes.

Functional redundancy in the toxic pathway of Bt protein Cry1Ab, but not Cry1Fa, against the Asian corn borer

Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have been used extensively to control some major crop pests, but their benefits decrease when pests evolve resistance. Better understanding of the genetic basis of resistance is needed to effectively monitor, manage, and counter pest resistance to Bt crops. Resistance to Bt proteins in at least 11 species of Lepidoptera, including many important crop pests, is associated with naturally occurring mutations that disrupt one or more of three larval midgut proteins: cadherin and ATP-binding cassette proteins ABCC2 and ABCC3. Here, we determined how CRISPR/Cas9-mediated mutations disrupting cadherin, ABCC2, and ABCC3 singly and in pairs affect resistance to Bt proteins Cry1Ab and Cry1Fa in the Asian corn borer (Ostrinia furnacalis), which is the most damaging pest of corn in Asia and is closely related to the European corn borer (Ostrinia nubilalis), a major pest in Europe and North America. The results from bioassays of six knockout strains and their parent susceptible strain support a model in which Cry1Ab can kill larvae via one path requiring ABCC2 or another path requiring cadherin and ABCC3, whereas Cry1Fa uses only the first path. The model's predictions are generally supported by results from genetic linkage analyses and responses to Cry1Ab and Cry1Fa of Sf9 cells and Xenopus oocytes modified to produce cadherin, ABCC2, and ABCC3 singly or in pairs. The functional redundancy identified here for Cry1Ab could sustain its efficacy against O. furnacalis and may exemplify a widespread natural strategy for delaying resistance.

Polygenic response to selection by transgenic Bt-expressing crops in wild Helicoverpa zea and characterization of a major effect locus

Strong and shifting selective pressures of the Anthropocene are rapidly shaping phenomes and genomes of organisms worldwide. Crops expressing pesticidal proteins from Bacillus thuringiensis (Bt) represent one major selective force on insect genomes. Here we characterize a rapid response to selection by Bt crops in a major crop pest, Helicoverpa zea. We reveal the polygenic architecture of Bt resistance evolution in H. zea and identify multiple genomic regions underlying this trait. In the genomic region of largest effect, we identified a gene amplification event, where resistant individuals showed variation in copy number for multiple genes. Signals of this amplification increased over time, consistent with the history of field-evolved Bt resistance evolution. Modern wild populations from disparate geographical regions are positive for this variant at high, but not fixed, allele frequencies. We also detected selection against single copy variants at this locus in wild H. zea collected from Bt expressing plants, further supporting its role in resistance. Multiple genes were annotated in this genomic region, and all appeared to be significantly upregulated in Bt resistant H. zea. We functionally characterized genes within the copy number variant (CNV), providing insight into their potential roles in resistance evolution. Our findings reveal the nature of rapid genome evolution in a major crop pest following anthropogenic selection and highlight the role that CNVs can have in rapid evolutionary responses.

Mismatch between lab-generated and field-evolved resistance to transgenic Bt crops in Helicoverpa zea

Transgenic crops producing crystalline (Cry) proteins from the bacterium Bacillus thuringiensis (Bt) have been used extensively to control some major crop pests. However, many populations of the noctuid moth Helicoverpa zea, one of the most important crop pests in the United States, have evolved practical resistance to several Cry proteins including Cry1Ac. Although mutations in single genes that confer resistance to Cry proteins have been identified in lab-selected and gene-edited strains of H. zea and other lepidopteran pests, the genetic basis of field-evolved resistance to Cry proteins in H. zea has remained elusive. We used a genomic approach to analyze the genetic basis of field-evolved resistance to Cry1Ac in 937 H. zea derived from 17 sites in seven states of the southern United States. We found evidence for extensive gene flow among all populations studied. Field-evolved resistance was not associated with mutations in 20 single candidate genes previously implicated in resistance or susceptibility to Cry proteins in H. zea or other lepidopterans. Instead, resistance in field samples was associated with increased copy number of a cluster of nine trypsin genes. However, trypsin gene amplification occurred in a susceptible sample and not in all resistant samples, implying that this amplification does not always confer resistance and mutations in other genes also contribute to field-evolved resistance to Cry1Ac in H. zea. The mismatch between lab-generated and field-evolved resistance in H. zea is unlike other cases of Bt resistance and reflects challenges for managing this pest.