Multi-Toxin Resistance Enables Pink Bollworm Survival on Pyramided Bt Cotton

CRISPR/Cas9-Mediated Knockout of the PxJHBP Gene Resulted in Increased Susceptibility to Bt Cry1Ac Protoxin and Reduced Lifespan and Spawning Rates in Plutella xylostella

Juvenile hormone binding protein (JHBP) is a key regulator of JH signaling, and crosstalk between JH and 20-hydroxyecdysone (20E) can activate and fine-tune the mitogen-activated protein kinase cascade, leading to resistance to insecticidal proteins from Bacillis thuringiensis (Bt). However, the involvement of JHBP in the Bt Cry1Ac resistance of Plutella xylostella remains unclear. Here, we cloned a full-length cDNA encoding JHBP, and quantitative real-time PCR (qPCR) analysis showed that the expression of the PxJHBP gene in the midgut of the Cry1Ac-susceptible strain was significantly higher than that of the Cry1Ac-resistant strain. Furthermore, CRISPR/Cas9-mediated knockout of the PxJHBP gene significantly increased Cry1Ac susceptibility, resulting in a significantly shorter lifespan and reduced fertility. These results demonstrate that PxJHBP plays a critical role in the resistance to Cry1Ac protoxin and in the regulation of physiological metabolic processes associated with reproduction in adult females, providing valuable insights to improve management strategies of P. xylostella.

Genome-wide analysis reveals distinct global populations of pink bollworm (Pectinophora gossypiella)

The pink bollworm (Pectinophora gossypiella) is one of the world's most destructive pests of cotton. This invasive lepidopteran occurs in nearly all cotton-growing countries. Its presence in the Ord Valley of North West Australia poses a potential threat to the expanding cotton industry there. To assess this threat and better understand population structure of pink bollworm, we analysed genomic data from individuals collected in the field from North West Australia, India, and Pakistan, as well as from four laboratory colonies that originated in the United States. We identified single nucleotide polymorphisms (SNPs) using a reduced-representation, genotyping-by-sequencing technique (DArTseq). The final filtered dataset included 6355 SNPs and 88 individual genomes that clustered into five groups: Australia, India-Pakistan, and three groups from the United States. We also analysed sequences from Genbank for mitochondrial DNA (mtDNA) locus cytochrome c oxidase I (COI) for pink bollworm from six countries. We found low genetic diversity within populations and high differentiation between populations from different continents. The high genetic differentiation between Australia and the other populations and colonies sampled in this study reduces concerns about gene flow to North West Australia, particularly from populations in India and Pakistan that have evolved resistance to transgenic insecticidal cotton. We attribute the observed population structure to pink bollworm's narrow host plant range and limited dispersal between continents.

Molecular Genetic Basis of Lab- and Field-Selected Bt Resistance in Pink Bollworm

Transgenic crops producing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) control some important insect pests. However, evolution of resistance by pests reduces the efficacy of Bt crops. Here we review resistance to Bt cotton in the pink bollworm, Pectinophora gossypiella, one of the world's most damaging pests of cotton. Field outcomes with Bt cotton and pink bollworm during the past quarter century differ markedly among the world's top three cotton-producing countries: practical resistance in India, sustained susceptibility in China, and eradication of this invasive lepidopteran pest from the United States achieved with Bt cotton and other tactics. We compared the molecular genetic basis of pink bollworm resistance between lab-selected strains from the U.S. and China and field-selected populations from India for two Bt proteins (Cry1Ac and Cry2Ab) produced in widely adopted Bt cotton. Both lab- and field-selected resistance are associated with mutations affecting the cadherin protein PgCad1 for Cry1Ac and the ATP-binding cassette transporter protein PgABCA2 for Cry2Ab. The results imply lab selection is useful for identifying genes important in field-evolved resistance to Bt crops, but not necessarily the specific mutations in those genes. The results also suggest that differences in management practices, rather than genetic constraints, caused the strikingly different outcomes among countries.

Slowing virulence adaptation in Asian rice planthoppers through migration-based deployment of resistance genes

Advances in molecular biology have accelerated rice breeding for resistance to Asian planthoppers. However, experience shows that planthoppers quickly adapt to resistance in tropical overwintering areas. With only limited sources available, the large-scale deployment of resistance genes can rapidly reduce the utility of these public goods. Planthoppers that migrate from tropical to temperate Asia carry virulence against many resistance genes, but adapt more slowly to resistant rice in temperate regions. Therefore, by restricting deployment of selected genes to temperate regions, virulence-adaptation rates and the volume of migrants returning to overwintering sites could be reduced. The current open exchange of breeding materials throughout Asia urgently requires an international, multidisciplinary, stakeholder coalition to promote a more sustainable deployment of planthopper-resistant rice.

Segregation of Cry Genes in the Seeds Produced by F1 Bollgard® II Cotton Differs between Hybrids: Could This Be Linked to the Observed Field Resistance in the Pink Bollworm?

Indian populations of the Pink Bollworm (PBW) are resistant to Bt (Bacillus thuringiensis) cotton hybrids containing Cry1Ac and Cry2Ab genes. Segregation of these Cry genes in F₁ hybrids could subject PBW to sublethal concentrations. Moreover, planting hybrids with varying zygosities of Cry genes could produce diverse segregation patterns and expose PBW populations to highly variable toxin concentrations. This could potentially promote the rate of resistance development. Therefore, we studied the segregation patterns of Cry genes in different commercial Bt hybrids cultivated in India. Results showed that two hybrids segregated according to the Mendelian mono-hybrid ratio, three segregated according to the Mendelian di-hybrid ratio, and one showed a mixed segregation pattern. The assortment of seeds containing Cry genes varied between bolls of the same hybrid. In India, different Bt cotton hybrids are cultivated in small patches next to each other, exposing PBW populations to sublethal doses and wide variations in the occurrence of Cry genes. It is necessary to avoid segregation of Cry genes in the seeds produced by F₁ hybrids. This study recommends using Bt parents homozygous for Cry genes in commercial Bt cotton hybrid development. This breeding strategy could be effective for similar transgenic crop hybrids as well.

Responses to Bt toxin Vip3Aa by pink bollworm larvae resistant or susceptible to Cry toxins

BACKGROUND

Transgenic crops that make insecticidal proteins from Bacillus thuringiensis (Bt) have revolutionized management of some pests. However, evolution of resistance to Bt toxins by pests diminishes the efficacy of Bt crops. Resistance to crystalline (Cry) Bt toxins has spurred adoption of crops genetically engineered to produce the Bt vegetative insecticidal protein Vip3Aa. Here we used laboratory diet bioassays to evaluate responses to Vip3Aa by pink bollworm (Pectinophora gossypiella), one of the world's most damaging pests of cotton.

RESULTS

Against pink bollworm larvae susceptible to Cry toxins, Vip3Aa was less potent than Cry1Ac or Cry2Ab. Conversely, Vip3Aa was more potent than Cry1Ac or Cry2Ab against laboratory strains highly resistant to those Cry toxins. Five Cry-susceptible field populations were less susceptible to Vip3Aa than a Cry-susceptible laboratory strain (APHIS-S). Relative to APHIS-S, significant resistance to Vip3Aa did not occur in strains selected in the laboratory for > 700-fold resistance to Cry1Ac or both Cry1Ac and Cry2Ab.

CONCLUSIONS

Resistance to Cry1Ac and Cry2Ab did not cause strong cross-resistance to Vip3Aa in pink bollworm, which is consistent with predictions based on the lack of shared midgut receptors between these toxins and previous results from other lepidopterans. Comparison of the Bt toxin concentration in plants relative to the median lethal concentration (LC₅₀ ) from bioassays may be useful for estimating efficacy. The moderate potency of Vip3Aa against Cry1Ac- and Cry2Ab-resistant and susceptible pink bollworm larvae suggests that Bt cotton producing this toxin together with novel Cry toxins might be useful as one component of integrated pest management. © 2022 Society of Chemical Industry.

Functional Diversity of the Lepidopteran ATP-Binding Cassette Transporters

The ATP-binding cassette (ABC) transporter gene family is ubiquitous in the living world. ABC proteins bind and hydrolyze ATP to transport a myriad of molecules across various lipid-containing membrane systems. They have been studied well in plants for transport of a variety of compounds and particularly, in vertebrates due to their direct involvement in resistance mechanisms against several toxic molecules/metabolites. ABC transporters in insects are found within large multigene families involved in the efflux of chemical insecticides and toxic/undesired metabolites originating from food and endogenous metabolism. This review deals with ABC transporter subfamilies of few agronomically important Lepidopteran pests. The transcriptional dynamics and regulation of ABC transporters during insect development emphasizes their functional diversity against insecticides, Cry toxins, and plant specialized metabolites. To generate insights about molecular function and physiological roles of ABCs, functional and structural characterization is necessary. Also, expansion and divergence of ABC transporter gene subfamilies in Lepidopteran insects needs more systematic investigation. We anticipate that newer methods of insect control in agriculture can benefit from an understanding of ABC transporter interactions with a vast range of natural specialized molecules and synthetic compounds.

Mutation in the Cadherin Gene Is a Key Factor for Pink Bollworm Resistance to Bt Cotton in China

Transgenic crops producing Bacillus thuringiensis (Bt) toxins are widely planted for insect control, but their efficacy may decrease as insects evolve resistance. Understanding the genetic basis of insect resistance is essential for developing an integrated strategy of resistance management. To understand the genetic basis of resistance in pink bollworm (Pectinophora gossypiella) to Bt cotton in the Yangtze River Valley of China, we conducted an F2 screening for alleles associated with resistance to the Bt (Cry1Ac) protein for the first time. A total of 145 valid single-paired lines were screened, among which seven lines were found to carry resistance alleles. All field parents in those seven lines carried recessive resistance alleles at the cadherin locus, including three known alleles, r1, r13 and r15, and two novel alleles, r19 and r20. The overall frequency of resistance alleles in 145 lines was 0.0241 (95% CI: 0.0106-0.0512). These results demonstrated that resistance was rare and that recessive mutation in the cadherin gene was the primary mechanism of pink bollworm resistance to Bt cotton in the Yangtze River Valley of China, which will provide a scientific basis for implementing targeted resistance management statics of pink bollworm in this region.

CRISPR-mediated mutations in the ABC transporter gene ABCA2 confer pink bollworm resistance to Bt toxin Cry2Ab

Crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt) have many benefits and are important globally for managing insect pests. However, the evolution of pest resistance to Bt crops reduces their benefits. Understanding the genetic basis of such resistance is needed to better monitor, manage, and counter pest resistance to Bt crops. Previous work shows that resistance to Bt toxin Cry2Ab is associated with mutations in the gene encoding the ATP-binding cassette protein ABCA2 in lab- and field-selected populations of the pink bollworm (Pectinophora gossypiella), one of the world’s most destructive pests of cotton. Here we used CRISPR/Cas9 gene editing to test the hypothesis that mutations in the pink bollworm gene encoding ABCA2 (PgABCA2) can cause resistance to Cry2Ab. Consistent with this hypothesis, introduction of disruptive mutations in PgABCA2 in a susceptible strain of pink bollworm increased the frequency of resistance to Cry2Ab and facilitated creation of a Cry2Ab-resistant strain. All Cry2Ab-resistant individuals tested in this study had disruptive mutations in PgABCA2. Overall, we found 17 different disruptive mutations in PgABCA2 gDNA and 26 in PgABCA2 cDNA, including novel mutations corresponding precisely to single-guide (sgRNA) sites used for CRISPR/Cas9. Together with previous results, these findings provide the first case of practical resistance to Cry2Ab where evidence identifies a specific gene in which disruptive mutations can cause resistance and are associated with resistance in field-selected populations.

Transgenic cotton and sterile insect releases synergize eradication of pink bollworm a century after it invaded the United States

Invasive organisms pose a global threat and are exceptionally difficult to eradicate after they become abundant in their new habitats. We report a successful multitactic strategy for combating the pink bollworm (Pectinophora gossypiella), one of the world's most invasive pests. A coordinated program in the southwestern United States and northern Mexico included releases of billions of sterile pink bollworm moths from airplanes and planting of cotton engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt). An analysis of computer simulations and 21 y of field data from Arizona demonstrate that the transgenic Bt cotton and sterile insect releases interacted synergistically to reduce the pest's population size. In Arizona, the program started in 2006 and decreased the pest's estimated statewide population size from over 2 billion in 2005 to zero in 2013. Complementary regional efforts eradicated this pest throughout the cotton-growing areas of the continental United States and northern Mexico a century after it had invaded both countries. The removal of this pest saved farmers in the United States $192 million from 2014 to 2019. It also eliminated the environmental and safety hazards associated with insecticide sprays that had previously targeted the pink bollworm and facilitated an 82% reduction in insecticides used against all cotton pests in Arizona. The economic and social benefits achieved demonstrate the advantages of using agricultural biotechnology in concert with classical pest control tactics.

Cadherin repeat 5 mutation associated with Bt resistance in a field-derived strain of pink bollworm

Evolution of resistance by pests reduces the benefits of transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt). Here we analyzed resistance to Bt toxin Cry1Ac in a field-derived strain of pink bollworm (Pectinophora gossypiella), a global pest of cotton. We discovered that the r14 allele of the pink bollworm cadherin gene (PgCad1) has a 234-bp insertion in exon 12 encoding a mutant PgCad1 protein that lacks 36 amino acids in cadherin repeat 5 (CR5). A strain homozygous for this allele had 237-fold resistance to Cry1Ac, 1.8-fold cross-resistance to Cry2Ab, and developed from neonate to adult on Bt cotton producing Cry1Ac. Inheritance of resistance to Cry1Ac was recessive and tightly linked with r14. PgCad1 transcript abundance in midgut tissues did not differ between resistant and susceptible larvae. Toxicity of Cry1Ac to transformed insect cells was lower for cells expressing r14 than for cells expressing wild-type PgCad1. Wild-type PgCad1 was transported to the cell membrane, whereas PgCad1 produced by r14 was not. In larval midgut tissue, PgCad1 protein occurred primarily on the brush border membrane only in susceptible larvae. The results imply r14 mediates pink bollworm resistance to Cry1Ac by reduced translation, increased degradation, and/or mislocalization of cadherin.

Shared and Independent Genetic Basis of Resistance to Bt Toxin Cry2Ab in Two Strains of Pink Bollworm

Evolution of pest resistance threatens the benefits of crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt). Field populations of the pink bollworm (Pectinophora gossypiella), a global pest of cotton, have evolved practical resistance to transgenic cotton producing Bt toxin Cry2Ab in India, but not in the United States. Previous results show that recessive mutations disrupting an autosomal ATP-binding cassette gene (PgABCA2) are associated with pink bollworm resistance to Cry2Ab in field-selected populations from India and in one lab-selected strain from the United States (Bt4-R2). Here we discovered that an independently derived, lab-selected Cry2Ab-resistant pink bollworm strain from the United States (BX-R) also harbors mutations that disrupt PgABCA2. Premature stop codons introduced by mis-splicing of PgABCA2 pre-mRNA were prevalent in field-selected larvae from India and in both lab-selected strains. The most common mutation in field-selected larvae from India was also detected in both lab-selected strains. Results from interstrain crosses indicate BX-R has at least one additional mechanism of resistance to Cry2Ab that does not involve PgABCA2 and is not completely recessive or autosomal. We conclude that recessive mutations disrupting PgABCA2 are the primary, but not the only, mechanism of resistance to Cry2Ab in pink bollworm.

Evidence for population expansion of Cotton pink bollworm Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae) in India

Pink bollworm, Pectinophora gossypiella (Saunders) infestation on Bt cotton is a major concern to cotton production in India. The genetic diversity and phylogeographic structure of the insect in light of PBW resistance needs to be revisited. The objective of this study was to identify different haplotypes of pink bollworm and their distribution in India. To achieve this we studied the population structure in 44 cotton growing districts of India. The partial mitochondrial COI sequence analyses of 214 pink bollworm populations collected from 44 geographical locations representing 9 cotton growing states of India were analysed. Genetic diversity analysis exhibited presence of 27 haplotypes, among them Pg_H1 and Pg_H2 were the most common and were present in 143 and 32 populations, respectively. Distributions of pairwise differences obtained with partial COI gene data from the overall Indian populations are unimodal, suggesting population expansion in India. Significant neutrality test on the basis of Tajima’ D and Fu’s Fs presented a star-shaped haplotype network together with multiple haplotypes. The unimodal mismatch distribution, rejection of neutrality test with significant negative values supported the theory of demographic expansion in cotton pink bollworm populations in India. Genetic data not only provides us with a perspective of population genetics, but also that the two populations of pink bollworm, those occurring early in the season are genetically close to the late season populations with respect to their partial CO1 region. Resistance to Cry toxins does not seem to have had an impact on this region of the mt DNA in populations of pink bollworm.

Bt resistance alleles in field populations of pink bollworm from China: Similarities with the United States and decreased frequency from 2012 to 2015

BACKGROUND

Although most monitoring of pest resistance to widely cultivated transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) relies on bioassays, DNA screening for alleles associated with resistance has some advantages, particularly for rare, recessively inherited resistance. In China's Yangtze River Valley, where farmers first planted transgenic cotton producing Bt toxin Cry1Ac in 2000, bioassays have been used to monitor the recessive resistance of pink bollworm (Pectinophora gossypiella). Previous bioassay results show a small but significant increase in resistance to Cry1Ac during 2008-2010, followed by a significant decrease in resistance during 2011-2015 associated with extensive planting of second-generation hybrid cotton seeds that boosted the percentage of non-Bt cotton. Here we screened DNA from 19 748 pink bollworm collected during 2012-2015 from the Yangtze River Valley for seven alleles associated with resistance to Cry1Ac. These alleles were previously identified from lab-selected strains; three from the U.S. and four from China.

RESULTS

The most common resistance allele was first identified from the U.S. and accounted for over 71% of all resistance alleles detected. Resistance was rare, with the total frequency of the seven resistance alleles showing a significant, 2.3-fold decrease from 0.0105 (95% CI: 0.0084-0.0132) in 2012 to 0.0046 (0.0031-0.0067) in 2015.

CONCLUSIONS

The DNA screening data confirm results from bioassays showing pink bollworm resistance to Cry1Ac remained rare in the Yangtze River Valley from 2012-2015. The prevalence in China of the resistance allele identified from the U.S. implies a shared genetic basis of resistance that could facilitate molecular monitoring of resistance. © 2019 Society of Chemical Industry.

Reduced cadherin expression associated with resistance to Bt toxin Cry1Ac in pink bollworm

BACKGROUND

Better understanding of the molecular basis of resistance is needed to improve management of pest resistance to transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt). Here we analyzed resistance of the pink bollworm (Pectinophora gossypiella) to Bt toxin Cry1Ac, which is used widely in transgenic Bt cotton. Field-evolved practical resistance of pink bollworm to Cry1Ac is widespread in India, but not in China or the United States. Previous work with laboratory- and field-selected pink bollworm indicated that resistance to Cry1Ac is caused by changes in the amino acid sequence of a midgut cadherin protein (PgCad1) that binds Cry1Ac in susceptible larvae.

RESULTS

Relative to a susceptible strain, the laboratory-selected APHIS-R strain had 530-fold resistance to Cry1Ac with autosomal recessive inheritance. Unlike previous results, resistance in this strain was not consistently associated with insertions or deletions in the expected amino acid sequence of PgCad1. However, this resistance was associated with 79- to 190-fold reduced transcription of the PgCad1 gene and markedly lower abundance of PgCad1 protein.

CONCLUSION

The ability of pink bollworm and other major pests to evolve resistance to Bt toxins via both qualitative and quantitative changes in receptor proteins demonstrates their remarkable adaptability and presents challenges for monitoring and managing resistance to Bt crops. © 2019 Society of Chemical Industry.

Phenotypic performance of nine genotypes of Cry1A.105/Cry2Ab2 dual-gene resistant fall armyworm on non-Bt and MON 89034 maize

BACKGROUND

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is the first target pest that has developed resistance to Bt crops across several countries. Leaf tissue and whole plant assays were employed to determine the survival, development, progeny production, and net reproductive rate of all nine possible genotypes of Cry1A.105/Cry2Ab2-dual gene resistant S. frugiperda on non-Bt and MON 89034 maize expressing the Cry1A.105/Cry2Ab2 proteins.

RESULTS

The homozygous resistant genotype was highly resistant to Bt plants. Genotypes possessing only Cry2Ab2 resistance alleles (RAs) or two Cry1A.105 RAs only were susceptible to MON 89034 with a functional dominance level (D_ML ) of 0.0-0.07 on MON 89034 plants. In contrast, genotypes containing two Cry1A.105 plus one Cry2Ab2 RA performed well on Bt plants, with a D_ML of 0.11-0.78. Significant numbers of survivors on Bt plants were also observed for genotypes containing a single Cry1A.105 RA, or a Cry1A.105 plus one or two Cry2Ab2 RAs, with a D_ML of 0.0-0.47.

CONCLUSIONS

The fitness of individual resistant genes on pyramided Bt plants varied in the dual-gene resistance system. Genotypes containing more Cry1A.105 RAs performed better than those possessing more Cry2Ab2 RAs. The functional dominance level of an individual resistant gene in this system is related to the D_ML level in its corresponding single-gene system. Data generated from this study should fill gaps in understanding dual-/multiple-gene Bt resistance, as well as providing useful information for refining resistance modeling, improving resistance risk assessment, and developing management strategies for the sustainable use of pyramided Bt maize technology. © 2019 Society of Chemical Industry.

A long non-coding RNA regulates cadherin transcription and susceptibility to Bt toxin Cry1Ac in pink bollworm, Pectinophora gossypiella

Extensive planting of transgenic crops producing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) has spurred increasingly rapid evolution of resistance in pests. In the pink bollworm, Pectinophora gossypiella, a devastating global pest, resistance to Bt toxin Cry1Ac produced by transgenic cotton is linked with mutations in a gene (PgCad1) encoding a cadherin protein that binds Cry1Ac in the larval midgut. We previously reported a long non-coding RNA (lncRNA) in intron 20 of cadherin alleles associated with both resistance and susceptibility to Cry1Ac. Here we tested the hypothesis that reducing expression of this lncRNA decreases transcription of PgCad1 and susceptibility to Cry1Ac. Quantitative RT-PCR showed that feeding susceptible neonates small interfering RNAs (siRNAs) targeting this lncRNA but not PgCad1 decreased the abundance of transcripts of both the lncRNA and PgCad1. Moreover, neonates fed the siRNAs had lower susceptibility to Cry1Ac. The results imply that the lncRNA increases transcription of PgCad1 and susceptibility of pink bollworm to Cry1Ac. The results suggest that disruption of lncRNA expression could be a novel mechanism of pest resistance to Bt toxins.

Transposon insertion causes cadherin mis-splicing and confers resistance to Bt cotton in pink bollworm from China

Transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) are cultivated extensively, but rapid evolution of resistance by pests reduces their efficacy. We report a 3,370-bp insertion in a cadherin gene associated with resistance to Bt toxin Cry1Ac in the pink bollworm (Pectinophora gossypiella), a devastating global cotton pest. We found the allele (r15) harboring this insertion in a field population from China. The insertion is a miniature inverted repeat transposable element (MITE) that contains two additional transposons and produces two mis-spliced transcript variants (r15A and r15B). A strain homozygous for r15 had 290-fold resistance to Cry1Ac, little or no cross-resistance to Cry2Ab, and completed its life cycle on Bt cotton producing Cry1Ac. Inheritance of resistance was recessive and tightly linked with r15. For transformed insect cells, susceptibility to Cry1Ac was greater for cells producing the wild-type cadherin than for cells producing the r15 mutant proteins. Recombinant cadherin protein occurred on the cell surface in cells transformed with the wild-type or r15A sequences, but not in cells transformed with the r15B sequence. The similar resistance of pink bollworm to Cry1Ac in laboratory- and field-selected insects from China, India and the U.S. provides a basis for developing international resistance management practices.

Field-evolved resistance of pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), to transgenic Bacillus thuringiensis (Bt) cotton expressing crystal 1Ac (Cry1Ac) and Cry2Ab in India

BACKGROUND

Pink bollworm (PBW) adaptation to transgenic Bacillus thuringiensis (Bt) cotton expressing crystal 1Ac (Cry1Ac) (Bt-I) and Cry1Ac + Cry2Ab (Bt-II) was assessed in India during 2010-2017 in 38 districts of the 10 major cotton-growing states.

RESULTS

PBW larval incidence on Bt cotton was nil in northern India, wherein the resistance ratios (RRs) to Cry1Ac were 26-262 and those to Cry2Ab were 1-108. In central and southern India, the annual average PBW larval recovery from Bt II cotton was high at 28.85-72.49% during 2014-2017. In central and southern India, the 50% lethal concentration (LC₅₀ ) of Cry1Ac increased from a mean of 0.330 µg mL^-1 (range 0.126-0.849 µg mL^-1 ) in 2013 to a mean of 6.938 µg mL^-1 (range 3.52 to 10.30 µg mL^-1 ) in 2017 and the RR increased from a mean of 47.12 (range 18-121) in 2013 to a mean of 1387 (704-2060) in 2017, whereas the LC₅₀ value for Cry2Ab increased from a mean of 0.014 µg mL^-1 (range 0.004-0.094 µg mL^-1 ) in 2013 to a mean of 12.51 µg mL^-1 (range 3.92 to 28.10 µg mL^-1 ) in 2017 and the RR increased from a mean of 5.4 (range 1-31) in 2013 to a mean of 4196 (1306-9366) in 2017.

CONCLUSION

High PBW larval recovery on Bt-II in conjunction with high LC₅₀ values for Cry1Ac and Cry2Ab in major cotton-growing districts of central and southern India provides evidence of field-evolved resistance in PBW to Bt-I and Bt-II cotton. © 2018 Society of Chemical Industry.

ABC transporter mis-splicing associated with resistance to Bt toxin Cry2Ab in laboratory- and field-selected pink bollworm

Evolution of pest resistance threatens the benefits of genetically engineered crops that produce Bacillus thuringiensis (Bt) insecticidal proteins. Strategies intended to delay pest resistance are most effective when implemented proactively. Accordingly, researchers have selected for and analyzed resistance to Bt toxins in many laboratory strains of pests before resistance evolves in the field, but the utility of this approach depends on the largely untested assumption that laboratory- and field-selected resistance to Bt toxins are similar. Here we compared the genetic basis of resistance to Bt toxin Cry2Ab, which is widely deployed in transgenic crops, between laboratory- and field-selected populations of the pink bollworm (Pectinophora gossypiella), a global pest of cotton. We discovered that resistance to Cry2Ab is associated with mutations disrupting the same ATP-binding cassette transporter gene (PgABCA2) in a laboratory-selected strain from Arizona, USA, and in field-selected populations from India. The most common mutation, loss of exon 6 caused by alternative splicing, occurred in resistant larvae from both locations. Together with previous data, the results imply that mutations in the same gene confer Bt resistance in laboratory- and field-selected strains and suggest that focusing on ABCA2 genes may help to accelerate progress in monitoring and managing resistance to Cry2Ab.

Interaction between Insects, Toxins, and Bacteria: Have We Been Wrong So Far?

Toxins are a major virulence factor produced by many pathogenic bacteria. In vertebrates, the response of hosts to the bacteria is inseparable from the response to the toxins, allowing a comprehensive understanding of this tripartite host-pathogen-toxin interaction. However, in invertebrates, this interaction has been investigated by two complementary but historically distinct fields of research: toxinology and immunology. In this article, I highlight how such dichotomy between these two fields led to a biased, or even erroneous view of the ecology and evolution of the interaction between insects, toxins, and bacteria. I focus on the reason behind such a dichotomy, on how to bridge the fields together, and on confounding effects that could bias the outcome of the experiments. Finally, I raise four questions at the border of the two fields on the cross-effects between toxins, bacteria, and spores that have been largely underexplored to promote a more comprehensive view of this interaction.

A new observation on feeding behaviour of pink bollworm and its application in screening Bt-resistant population

The pink bollworm (PBW), Pectinophora gossypiella (Saund.) does not feed on leaves in natural circumstances. We made an attempt with the first instar larva (5 days old) and found it eats the leaf in the absence of other foods. Utilizing this new feeding behaviour, a simple methodology was developed for screening of resistance in PBW against Bt cotton plants. The PBW collected from BGII Bt cotton fields (BGII-resistant population) and NBAIR culture (susceptible population) were reared under laboratory conditions for two generations. Laboratory reared 5-day old larvae of PBW were released on cotton leaf discs individually for screening. The BGII-resistant and susceptible larvae fed an average leaf area of 75.52 ± 16.68 and 5.95 ± 0.93 mm² with survival rate of 90 and 4% respectively, in BGII Bt cotton leaves. Whereas in case of non-Bt cotton (MCU13), the BGII-resistant and susceptible larvae consumed average leaf area of 114.84 ± 23.70 and 116.80 ± 24.14 mm² with survival rate of 93.34 and 95.33%, respectively. In addition to the screening process, the larval survivors were transferred to an artificial diet after 7 days of experiment and observed up to their emergence as adults. To confirm the development of resistance in PBW, sequencing of larval DNA amplicons was carried out and it revealed mutation in the cadherin gene of the BGII-resistant PBW population. Hence, the detached leaf bit feeding assay described here could be used in a simple manner for screening of resistance developed by PBW against Bt cotton and also for evaluating Bt cotton plants for their inherent resistance to PBW. This method could also be used for studying toxicity of Bt isolates by coating spore-crystal mixture on non-transgenic cotton leaf discs.

Resistance to Bacillus thuringiensis linked with a cadherin transmembrane mutation affecting cellular trafficking in pink bollworm from China

Evolution of pest resistance reduces the efficacy of insecticidal proteins from the gram-positive bacterium Bacillus thuringiensis (Bt) used widely in sprays and transgenic crops. In some previously studied strains of three major lepidopteran pests, resistance to Bt toxin Cry1Ac is associated with mutations disrupting the extracellular or cytoplasmic domains of cadherin proteins that bind Cry1Ac in the midgut of susceptible larvae. Here we report the first case of a cadherin transmembrane mutation associated with insect resistance to Bt. We discovered this mutation in a strain of the devastating global cotton pest, the pink bollworm (Pectinophora gossypiella), derived from a field population in the Yangtze River Valley of China. The mutant allele analyzed here has a 207 base pair deletion and encodes a cadherin protein lacking its transmembrane domain. Relative to a susceptible strain, a strain homozygous for this allele had 220-fold resistance to Cry1Ac and 2.1-fold cross-resistance to Cry2Ab. On transgenic cotton plants producing Cry1Ac, no susceptible larvae survived, but the resistant strain completed its life cycle. Inheritance of resistance to Cry1Ac was autosomal, recessive and tightly linked with the cadherin gene. Transportation of cadherin protein to the cell membrane and susceptibility to Cry1Ac occurred in transfected insect cells expressing the wild type cadherin allele, but not in transfected insect cells expressing the mutant cadherin allele. The results imply that the mutant allele analyzed here confers resistance to Cry1Ac by disrupting cellular trafficking of cadherin.

Comparisons of Transcriptional Profiles of Gut Genes between Cry1Ab-Resistant and Susceptible Strains of Ostrinia nubilalis Revealed Genes Possibly Related to the Adaptation of Resistant Larvae to Transgenic Cry1Ab Corn

A microarray developed on the basis of 2895 unique transcripts from larval gut was used to compare gut gene expression profiles between a laboratory-selected Cry1Ab-resistant (R) strain and its isoline susceptible (S) strain of the European corn borer (Ostrinia nubilalis) after the larvae were fed the leaves of transgenic corn (MON810) expressing Cry1Ab or its non-transgenic isoline for 6 h. We revealed 398 gut genes differentially expressed (i.e., either up- or down-regulated genes with expression ratio ≥2.0) in S-strain, but only 264 gut genes differentially expressed in R-strain after being fed transgenic corn leaves. Although the percentages of down-regulated genes among the total number of differentially expressed genes (50% in S-strain and 45% in R-strain) were similar between the R- and S-strains, the expression ratios of down-regulated genes were much higher in S-strain than in R-strain. We revealed that 17 and 9 significantly up- or down-regulated gut genes from S and R-strain, respectively, including serine proteases and aminopeptidases. These genes may be associated with Cry1Ab toxicity by degradation, binding, and cellular defense. Overall, our study suggests enhanced adaptation of Cry1Ab-resistant larvae on transgenic Cry1Ab corn as revealed by lower number and lower ratios of differentially expressed genes in R-strain than in S-strain of O. nubilalis.

Resistance to Bacillus thuringiensis toxin Cry2Ab and survival on single-toxin and pyramided cotton in cotton bollworm from China

Evolution of Helicoverpa armigera resistance to Bacillus thuringiensis (Bt) cotton producing Cry1Ac is progressing in northern China, and replacement of Cry1Ac cotton by pyramided Bt cotton has been considered to counter such resistance. Here, we investigated four of the eight conditions underlying success of the refuge strategy for delaying resistance to Cry1Ac+Cry2Ab cotton, a pyramid that has been used extensively against H. armigera outside China. Laboratory bioassays of a Cry2Ab‐selected strain (An2Ab) and a related unselected strain (An) reveal that resistance to Cry2Ab (130‐fold) was nearly dominant, autosomally inherited, and controlled by more than one locus. Strong cross‐resistance occurred between Cry2Ab and Cry2Aa (81‐fold). Weaker cross‐resistance (18‐ to 22‐fold) between Cry2Ab and Cry1A toxins was also present and significantly increased survival of An2Ab relative to An on cotton cultivars producing the fusion protein Cry1Ac/Cry1Ab or Cry1Ac. Survival on Cry1Ac+Cry2Ab cotton was also significantly higher in An2Ab than in An, showing that redundant killing on this pyramid was incomplete. Survival on non‐Bt cotton did not differ significantly between An2Ab and An, indicating an absence of fitness costs affecting this trait. These results indicate that a switch to three‐toxin pyramided cotton could be valuable for increasing durability of Bt cotton in China.

Bt resistance in Australian insect pest species

Bt cotton was initially deployed in Australia in the mid-1990s to control the polyphagous pest Helicoverpa armigera (Hübner) which was intractably resistant to synthetic chemistries. A conservative strategy was enforced and resistance to first generation single toxin technology was managed. A decade later, shortly after the release of dual toxin cotton, high baseline frequencies of alleles conferring resistance to one of its components prompted a reassessment of the thinking behind the potential risks to this technology. Several reviews detail the characteristics of this resistance and the nuances of deploying first and second generation Bt cotton in Australia. Here we explore recent advances and future possibilities to estimate Bt resistance in Australian pest species and define what we see as the critical data for enabling effective pre-emptive strategies. We also foreshadow the imminent deployment of three toxin (Cry1Ac, Cry2Ab, Vip3A) Bollgard 3 cotton, and examine aspects of resistance to its novel component, Vip3A, that we believe may impact on its stewardship.