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Benowitz KM, Allan CW, Degain BA, Li X, Fabrick JA, Tabashnik BE, Carrière Y, Matzkin LM. Novel genetic basis of resistance to Bt toxin Cry1Ac in Helicoverpa zea. Genetics 2022; 221:iyac037. [PMID: 35234875 PMCID: PMC9071530 DOI: 10.1093/genetics/iyac037] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2021] [Accepted: 02/25/2022] [Indexed: 11/14/2022] Open
Abstract
Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis have advanced pest management, but their benefits are diminished when pests evolve resistance. Elucidating the genetic basis of pest resistance to Bacillus thuringiensis toxins can improve resistance monitoring, resistance management, and the design of new insecticides. Here, we investigated the genetic basis of resistance to Bacillus thuringiensis 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, which has 31 chromosomes containing 375 Mb and 15,482 predicted proteins. Using a genome-wide association study, fine-scale mapping, and RNA-seq, we identified a 250-kb quantitative trait locus on chromosome 13 that was strongly associated with resistance in a strain of Helicoverpa zea that had been selected for resistance in the field and lab. The mutation in this quantitative trait locus contributed to but was not sufficient for resistance, which implies alleles in more than one gene contributed to resistance. This quantitative trait locus contains no genes with a previously reported role in resistance or susceptibility to Bacillus thuringiensis toxins. However, in resistant insects, this quantitative trait locus has a premature stop codon in a kinesin gene, which is a primary candidate as a mutation contributing to resistance. 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.
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Affiliation(s)
- Kyle M Benowitz
- Department of Entomology, University of Arizona, Tucson, AZ 85721, USA
- Department of Biology, Austin Peay State University, Clarksville, TN 37040, USA
| | - Carson W Allan
- Department of Entomology, University of Arizona, Tucson, AZ 85721, USA
| | - Benjamin A Degain
- Department of Entomology, University of Arizona, Tucson, AZ 85721, USA
| | - Xianchun Li
- Department of Entomology, University of Arizona, Tucson, AZ 85721, USA
| | - Jeffrey A Fabrick
- U.S. Department of Agriculture, Agricultural Research Service, U.S. Arid Land Agricultural Research Center, Maricopa, AZ 85138, USA
| | - Bruce E Tabashnik
- Department of Entomology, University of Arizona, Tucson, AZ 85721, USA
| | - Yves Carrière
- Department of Entomology, University of Arizona, Tucson, AZ 85721, USA
| | - Luciano M Matzkin
- Department of Entomology, University of Arizona, Tucson, AZ 85721, USA
- Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA
- Bio5 Institute, University of Arizona, Tucson, AZ 85721, USA
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Kerns DD, Yang F, Kerns DL, Stewart SD. Evaluation of Bt resistance in Helicoverpa zea (Lepidoptera: Noctuidae) strains using various Bt cotton plant tissues. PEST MANAGEMENT SCIENCE 2022; 78:95-103. [PMID: 34448529 DOI: 10.1002/ps.6611] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/05/2021] [Accepted: 08/27/2021] [Indexed: 06/13/2023]
Abstract
BACKGROUND Diet-overlay bioassays suggest that Helicoverpa zea (Lepidoptera: Noctuidae) field populations have developed resistance to some of the Bt insecticidal proteins that are constituents of the pyramids expressed in the second and third generation Bt cotton technologies. Unfortunately, these bioassays are not always a reliable indicator for how a seemingly resistant population will perform in an actual cotton field, and thus, leaf tissue bioassays have been suggested as a method to better assess field performance. However, bollworm larvae typically prefer to feed on floral tissue rather than leaf tissue, and an alternative cotton structure type may be more ideal for use in plant tissue-based bioassays. A series of diet-overlay bioassays using Bt proteins and Bt cotton plant tissue were conducted with laboratory susceptible (Bz-SS) and resistant (Cry-RR, resistant to Cry1Ac and Cry2Ab) H. zea strains to determine if plant tissue overlays could detect resistance and which cotton plant structure type would be most ideal for use in bioassays. RESULTS Results suggest that diet overlays using lyophilized plant tissue were able to detect resistance. Lyophilized tissue from white flowers was most ideal for use in bioassays, whereas tissue from non-Bt bolls and leaves affected larval health and behavior, confounding assay results. CONCLUSION Overlays using white flower tissue could potentially be used to supplement Bt protein overlays and provide an improved assessment of larval performance on Bt cotton technologies. © 2021 Society of Chemical Industry.
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Affiliation(s)
- Dawson D Kerns
- Department of Entomology and Plant Pathology, University of Tennessee, Knoxville, Tennessee, USA
| | - Fei Yang
- Department of Entomology, Texas A&M University, College Station, Texas, USA
| | - David L Kerns
- Department of Entomology, Texas A&M University, College Station, Texas, USA
| | - Scott D Stewart
- Department of Entomology and Plant Pathology, University of Tennessee, Knoxville, Tennessee, USA
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López-Molina S, do Nascimento NA, Silva-Filha MHNL, Guerrero A, Sánchez J, Pacheco S, Gill SS, Soberón M, Bravo A. In vivo nanoscale analysis of the dynamic synergistic interaction of Bacillus thuringiensis Cry11Aa and Cyt1Aa toxins in Aedes aegypti. PLoS Pathog 2021; 17:e1009199. [PMID: 33465145 PMCID: PMC7846010 DOI: 10.1371/journal.ppat.1009199] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2020] [Revised: 01/29/2021] [Accepted: 11/30/2020] [Indexed: 12/22/2022] Open
Abstract
The insecticidal Cry11Aa and Cyt1Aa proteins are produced by Bacillus thuringiensis as crystal inclusions. They work synergistically inducing high toxicity against mosquito larvae. It was proposed that these crystal inclusions are rapidly solubilized and activated in the gut lumen, followed by pore formation in midgut cells killing the larvae. In addition, Cyt1Aa functions as a Cry11Aa binding receptor, inducing Cry11Aa oligomerization and membrane insertion. Here, we used fluorescent labeled crystals, protoxins or activated toxins for in vivo localization at nano-scale resolution. We show that after larvae were fed solubilized proteins, these proteins were not accumulated inside the gut and larvae were not killed. In contrast, if larvae were fed soluble non-toxic mutant proteins, these proteins were found inside the gut bound to gut-microvilli. Only feeding with crystal inclusions resulted in high larval mortality, suggesting that they have a role for an optimal intoxication process. At the macroscopic level, Cry11Aa completely degraded the gastric caeca structure and, in the presence of Cyt1Aa, this effect was observed at lower toxin-concentrations and at shorter periods. The labeled Cry11Aa crystal protein, after midgut processing, binds to the gastric caeca and posterior midgut regions, and also to anterior and medium regions where it is internalized in ordered "net like" structures, leading finally to cell break down. During synergism both Cry11Aa and Cyt1Aa toxins showed a dynamic layered array at the surface of apical microvilli, where Cry11Aa is localized in the lower layer closer to the cell cytoplasm, and Cyt1Aa is layered over Cry11Aa. This array depends on the pore formation activity of Cry11Aa, since the non-toxic mutant Cry11Aa-E97A, which is unable to oligomerize, inverted this array. Internalization of Cry11Aa was also observed during synergism. These data indicate that the mechanism of action of Cry11Aa is more complex than previously anticipated, and may involve additional steps besides pore-formation activity.
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Affiliation(s)
- Samira López-Molina
- Departamento de Microbiología Molecular, Instituto de Biotecnología, Universidad Nacional Autónoma de México (UNAM), Cuernavaca, Morelos, Mexico
| | | | | | - Adán Guerrero
- Laboratorio Nacional de Microscopía Avanzada, Instituto de Biotecnología, UNAM, Cuernavaca, Morelos, Mexico
| | - Jorge Sánchez
- Departamento de Microbiología Molecular, Instituto de Biotecnología, Universidad Nacional Autónoma de México (UNAM), Cuernavaca, Morelos, Mexico
| | - Sabino Pacheco
- Departamento de Microbiología Molecular, Instituto de Biotecnología, Universidad Nacional Autónoma de México (UNAM), Cuernavaca, Morelos, Mexico
| | - Sarjeet S. Gill
- Department of Molecular, Cell and Systems Biology, University of California, Riverside, Riverside, California, United States of America
| | - Mario Soberón
- Departamento de Microbiología Molecular, Instituto de Biotecnología, Universidad Nacional Autónoma de México (UNAM), Cuernavaca, Morelos, Mexico
| | - Alejandra Bravo
- Departamento de Microbiología Molecular, Instituto de Biotecnología, Universidad Nacional Autónoma de México (UNAM), Cuernavaca, Morelos, Mexico
- * E-mail:
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Mutations in a Novel Cadherin Gene Associated with Bt Resistance in Helicoverpa zea. G3-GENES GENOMES GENETICS 2020; 10:1563-1574. [PMID: 32179620 PMCID: PMC7202007 DOI: 10.1534/g3.120.401053] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
Transgenic corn and cotton produce crystalline (Cry) proteins derived from the soil bacterium Bacillus thuringiensis (Bt) that are toxic to lepidopteran larvae. Helicoverpa zea, a key pest of corn and cotton in the U.S., has evolved widespread resistance to these proteins produced in Bt corn and cotton. While the genomic targets of Cry selection and the mutations that produce resistant phenotypes are known in other lepidopteran species, little is known about how selection by Cry proteins shape the genome of H. zea. We scanned the genomes of Cry1Ac-selected and unselected H. zea lines, and identified twelve genes on five scaffolds that differed between lines, including cadherin-86C (cad-86C), a gene from a family that is involved in Cry1A resistance in other lepidopterans. Although this gene was expressed in the H. zea larval midgut, the protein it encodes has only 17 to 22% identity with cadherin proteins from other species previously reported to be involved in Bt resistance. An analysis of midgut-expressed cDNAs showed significant between-line differences in the frequencies of putative nonsynonymous substitutions (both SNPs and indels). Our results indicate that cad-86C is a likely target of Cry1Ac selection in H. zea. It remains unclear, however, whether genomic changes at this locus directly disrupt midgut binding of Cry1Ac and cause Bt resistance, or indirectly enhance fitness of H. zea in the presence of Cry1Ac by some other mechanism. Future work should investigate phenotypic effects of these nonsynonymous substitutions and their impact on fitness of H. zea larvae that ingest Cry1Ac.
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Tessnow AE, Behmer ST, Walsh TK, Sword GA. Protein-carbohydrate regulation in Helicoverpa amigera and H. punctigera and how diet protein-carbohydrate content affects insect susceptibility to Bt toxins. JOURNAL OF INSECT PHYSIOLOGY 2018; 106:88-95. [PMID: 28733239 DOI: 10.1016/j.jinsphys.2017.07.004] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/11/2017] [Revised: 06/27/2017] [Accepted: 07/17/2017] [Indexed: 06/07/2023]
Abstract
Many animals, including insects, demonstrate a remarkable ability to regulate their intake of key macronutrients (e.g., soluble protein and digestible carbohydrates), which allows them to optimize fitness and performance. Additionally, regulating the intake of these two macronutrients enhances an animal's ability to defend itself against pathogens, mitigate the effects of secondary plant metabolites, and decrease susceptibility to toxins. In this study, we first compared how Bt-resistant and -susceptible lines of Helicoverpa armigera and Helicoverpa punctigera regulate their intake of protein (p) and digestible carbohydrates (c). We found that there was no difference in the self-selected protein-carbohydrate intake target between resistant and susceptible genotypes of either species. We then explored the extent to which food protein-carbohydrate content altered the susceptibility of these species to three Bt toxins: Cry1Ac, Cry2Ab, and Vip3Aa. We found that H. armigera on diets that had protein-carbohydrate profiles that matched their self-selected protein-carbohydrate intake target were significantly less susceptible to Cry1Ac. In contrast, diet protein-carbohydrate content did not affect H. punctigera susceptibility to Cry1Ac. For both H. armigera and H. punctigera, susceptibility to Cry2Ab and Vip3Aa toxins did not change as a function of diet protein-carbohydrate profile. These results, when combined with earlier work on H. zea, suggest food protein-carbohydrate content can modify susceptibility to some Bt toxins, but not others. An increased understanding of how the nutritional environment can modify susceptibility to different Bt toxins could help improve pest management and resistance management practices.
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Affiliation(s)
- Ashley E Tessnow
- Department of Entomology, Texas A&M University, College Station, TX 77843, USA.
| | - Spencer T Behmer
- Department of Entomology, Texas A&M University, College Station, TX 77843, USA; Ecology and Evolutionary Biology Interdisciplinary Program, Texas A&M University, College Station, TX 77843, USA
| | - Tom K Walsh
- CSIRO, Black Mountain Laboratories, Canberra, ACT 2601, Australia
| | - Gregory A Sword
- Department of Entomology, Texas A&M University, College Station, TX 77843, USA; Ecology and Evolutionary Biology Interdisciplinary Program, Texas A&M University, College Station, TX 77843, USA
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Carrière Y, Degain BA, Unnithan GC, Harpold VS, Heuberger S, Li X, Tabashnik BE. Effects of seasonal changes in cotton plants on the evolution of resistance to pyramided cotton producing the Bt toxins Cry1Ac and Cry1F in Helicoverpa zea. PEST MANAGEMENT SCIENCE 2018; 74:627-637. [PMID: 28967711 DOI: 10.1002/ps.4746] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/07/2017] [Revised: 07/13/2017] [Accepted: 09/25/2017] [Indexed: 06/07/2023]
Abstract
BACKGROUND In pests with inherently low susceptibility to Bacillus thuringiensis (Bt) toxins, seasonal declines in the concentration of Bt toxins in transgenic crops could accelerate evolution of resistance by increasing the dominance of resistance. Here, we evaluated Helicoverpa zea survival on young and old cotton plants that produced the Bt toxins Cry1Ac and Cry1F or did not produce Bt toxins. RESULTS Using a strain selected for resistance to Cry1Ac in the laboratory, its parent strain that was not selected in the laboratory, and their F1 progeny, we showed that resistance to Cry1Ac + Cry1F cotton was partially dominant on young and old plants. On Cry1Ac + Cry1F cotton, redundant killing was incomplete on young plants but nearly complete on old plants. No significant fitness costs on non-Bt cotton occurred on young plants, but large recessive costs affected survival on old plants. Simulation models incorporating the empirical data showed that the seasonal changes in fitness could delay resistance to Cry1Ac + Cry1F cotton by inducing low equilibrium frequencies of resistance alleles when refuges are sufficiently large. CONCLUSION Our results suggest that including effects of seasonal changes in fitness of pests on Bt crops and refuge plants can enhance resistance risk assessment and resistance management. © 2017 Society of Chemical Industry.
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Affiliation(s)
- Yves Carrière
- Department of Entomology, University of Arizona, Tucson, AZ, USA
| | - Ben A Degain
- Department of Entomology, University of Arizona, Tucson, AZ, USA
| | | | | | | | - Xianchun Li
- Department of Entomology, University of Arizona, Tucson, AZ, USA
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Deans CA, Sword GA, Behmer ST. Nutrition as a neglected factor in insect herbivore susceptibility to Bt toxins. CURRENT OPINION IN INSECT SCIENCE 2016; 15:97-103. [PMID: 27436738 DOI: 10.1016/j.cois.2016.04.005] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/14/2016] [Revised: 03/23/2016] [Accepted: 04/13/2016] [Indexed: 06/06/2023]
Abstract
The widespread global adoption of Bt crops elevates concerns about the evolution of Bt resistance in insect pest species. Current insecticide resistance management (IRM) strategies focus solely on genetic variation as a causal factor in the evolution of resistance, but ignore the role that environmental factors, such as nutrition, may play. In this opinion paper, we discuss the benefits that insect herbivores gain from consuming foods with protein-carbohydrate content that matches their self-selected protein-carbohydrate intake, and show that even within monocultures there is amply opportunity for insect herbivores to regulate their macronutrient intake. Next we review new data that show that dietary protein and carbohydrates can: firstly, have predictably strong effects on the survival and performance of caterpillars challenged with Bt toxins, and secondly, mediate plasticity in susceptibility to Cry1Ac, which can account for large differences in LC50 values. Nutrition-Bt interactions such as these have important implications for IRM, particularly given that diet-incorporated Bt bioassays commonly use artificial diets that vary substantially from their self-selected optimal diets, which likely results in underestimates of resistance in the field. Failing to bioassay larvae on ecologically-relevant diets can seriously confound the results of Bt resistance monitoring bioassays and undermine our ability to detect resistance in the field.
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Affiliation(s)
- C A Deans
- Department of Entomology, Texas A&M University, College Station, TX 77843, USA; Department of Entomology, University of Minnesota, St. Paul, MN 55108, USA.
| | - G A Sword
- Department of Entomology, Texas A&M University, College Station, TX 77843, USA; Ecology & Evolutionary Biology Graduate Program, Texas A&M University, College Station, TX 77843, USA
| | - S T Behmer
- Department of Entomology, Texas A&M University, College Station, TX 77843, USA; Ecology & Evolutionary Biology Graduate Program, Texas A&M University, College Station, TX 77843, USA
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