Characteristics of resistance to Bacillus thuringiensis toxin Cry2Ab in a strain of Helicoverpa punctigera (Lepidoptera: Noctuidae) isolated from a field population

Genome-scale analysis of ABC transporter genes and characterization of the ABCC type transporter genes in the oriental armyworm, Mythimna separata (Walker)

The oriental armyworm Mythimna separata is a polyphagous, migratory corn pest in China and other Asian countries. Transgenic Bacillus thuringiensis (Bt) corn may effectively control this insect pest. Several reports have suggested that ATP-binding cassette (ABC) transporter proteins may act as receptors that bind Bt toxins. However, our knowledge about ABC transporter proteins in M. separata is limited. We identified 43 ABC transporter genes in the M. separata genome by bioinformatics prediction. Evolutionary tree analysis grouped these 43 genes into 8 subfamilies, ABCA to ABCH. Among the 13 ABCC subfamily genes, the transcript levels of MsABCC2 and MsABCC3 were upregulated. In addition, RT-qPCR analyses of these two potentials showed that both were predominantly expressed in the midgut tissue. Knock-down of MsABCC2, but not MsABCC3, decreased Cry1Ac susceptibility as indicated by increased larval weight and reduced larval mortality. This suggested that MsABCC2 might play a more important role in Cry1Ac toxicity and that it is a putative Cry1Ac receptor in M. separata. Together, these findings provide unique and valuable information for future elucidating of the role of ABC transporter genes in M. separata, which is highly valuable and important for the long-term application of Bt insecticidal protein.

HearNPV susceptibility in Helicoverpa armigera and Helicoverpa punctigera strains resistant to Bt toxins Cry1Ac, Cry2Ab, and Vip3Aa

Genetically engineered crops expressing insecticidal toxins from Bacillus thuringiensis (Bt) have improved the management of targeted lepidopteran pests and reduced the use of insecticide sprays. These benefits explain an increasing adoption of Bt crops worldwide, intensifying the selection pressure on target species and the risk of resistance. Nucleopolyhedroviruses (NPVs) are effective bioinsecticides against numerous important lepidopteran pests. If Bt-resistant insects are shown to be susceptible to NPVs then these bioinsecticides could be a valuable component of Insecticide Resistance Management (IRM) strategies for Bt crops. We assessed the effectiveness of a Helicoverpa nucleopolyhedrovirus (HearNPV) against several different Bt-resistant strains. Utilising a droplet feeding bioassay we confirmed susceptibility to HearNPV in Helicoverpa punctigera and Helicoverpa armigera larvae resistant to the Bt toxins Cry1Ac, Cry2Ab, and Vip3A. Dual resistant H. punctigera, (Cry1Ac/Cry2Ab, and Cry2Ab/Vip3A) and dual resistant H. armigera (Cry2Ab/Vip3A) were also susceptible to HearNPV. Regardless of their specific resistance profile, Bt-resistant larvae displayed statistically similar lethal concentration (LC₅₀) and lethal time (LT₅₀) responses to HearNPV when compared to Bt-sensitive control insects. These results indicate that Bt-resistant H. armigera and H. punctigera are not cross-resistant to HearNPV. Consequently, the use of HearNPV against these pests may be a valuable tool to an IRM strategy for controlling Bt-resistant populations.

An Overview of Mechanisms of Cry Toxin Resistance in Lepidopteran Insects

Arthropods have the capacity to evolve resistance to insecticides and insecticidal traits in genetically modified crops. Resistance development among Lepidoptera is a common phenomenon, and a repertoire of resistance mechanisms to various Cry toxins have been identified from laboratory, greenhouse, and field studies in this insect order. Elucidation of such resistance mechanisms is crucial for developing IRM (insect resistance management) strategies to ensure sustainable use of genetically modified crops. This mini review provides a comprehensive overview of mechanisms of resistance that have been reported for lepidopteran pests. This study demonstrated that resistance mechanisms are highly complex, and the most common mechanism of resistance is altered binding sites. It is yet to be established whether all these altered binding sites are regulated by an MAPK signaling pathway, which might suggest a universal mechanism of resistance in lepidopterans.

Insect Resistance to Bacillus thuringiensis Toxin Cry2Ab Is Conferred by Mutations in an ABC Transporter Subfamily A Protein

The use of conventional chemical insecticides and bacterial toxins to control lepidopteran pests of global agriculture has imposed significant selection pressure leading to the rapid evolution of insecticide resistance. Transgenic crops (e.g., cotton) expressing the Bt Cry toxins are now used world wide to control these pests, including the highly polyphagous and invasive cotton bollworm Helicoverpa armigera. Since 2004, the Cry2Ab toxin has become widely used for controlling H. armigera, often used in combination with Cry1Ac to delay resistance evolution. Isolation of H. armigera and H. punctigera individuals heterozygous for Cry2Ab resistance in 2002 and 2004, respectively, allowed aspects of Cry2Ab resistance (level, fitness costs, genetic dominance, complementation tests) to be characterised in both species. However, the gene identity and genetic changes conferring this resistance were unknown, as was the detailed Cry2Ab mode of action. No cross-resistance to Cry1Ac was observed in mutant lines. Biphasic linkage analysis of a Cry2Ab-resistant H. armigera family followed by exon-primed intron-crossing (EPIC) marker mapping and candidate gene sequencing identified three independent resistance-associated INDEL mutations in an ATP-Binding Cassette (ABC) transporter gene we named HaABCA2. A deletion mutation was also identified in the H. punctigera homolog from the resistant line. All mutations truncate the ABCA2 protein. Isolation of further Cry2Ab resistance alleles in the same gene from field H. armigera populations indicates unequal resistance allele frequencies and the potential for Bt resistance evolution. Identification of the gene involved in resistance as an ABC transporter of the A subfamily adds to the body of evidence on the crucial role this gene family plays in the mode of action of the Bt Cry toxins. The structural differences between the ABCA2, and that of the C subfamily required for Cry1Ac toxicity, indicate differences in the detailed mode-of-action of the two Bt Cry toxins.

Transgenic crops expressing the insecticidal protein Cry2Ab from Bacillus thuringiensis (Bt) are used worldwide to suppress damage by lepidopteran pests, often used in combination with Cry1Ac toxin to delay resistance evolution. Until now, the Cry2Ab mode of action and the mechanism of resistance were unknown, with field-isolated Cry2Ab resistant Helicoverpa armigera showing no cross-resistance to Cry1Ac. In this study, biphasic linkage analysis of a Cry2Ab-resistant H. armigera family followed by EPIC marker mapping and candidate gene sequencing identified three independent INDEL mutations in an ATP-Binding Cassette transporter subfamily A gene (ABCA2). A deletion mutation was identified in the same gene of resistant H. punctigera. All four mutations are predicted to truncate the ABCA2 protein. This is the first molecular genetic characterization of insect resistance to the Cry2Ab toxin, and detection of diverse Cry2Ab resistance alleles will contribute to understanding the micro-evolutionary processes that underpinned lepidopteran Bt-resistance.

Resistance to Bacillus thuringiensis Toxin Cry2Ab in Trichoplusia ni Is Conferred by a Novel Genetic Mechanism

The resistance to the Bacillus thuringiensis (Bt) toxin Cry2Ab in a greenhouse-originated Trichoplusia ni strain resistant to both Bt toxins Cry1Ac and Cry2Ab was characterized. Biological assays determined that the Cry2Ab resistance in the T. ni strain was a monogenic recessive trait independent of Cry1Ac resistance, and there existed no significant cross-resistance between Cry1Ac and Cry2Ab in T. ni. From the dual-toxin-resistant T. ni strain, a strain resistant to Cry2Ab only was isolated, and the Cry2Ab resistance trait was introgressed into a susceptible laboratory strain to facilitate comparative analysis of the Cry2Ab resistance with the susceptible T. ni strain. Results from biochemical analysis showed no significant difference between the Cry2Ab-resistant and -susceptible T. ni larvae in midgut proteases, including caseinolytic proteolytic activity and zymogram profile and serine protease activities, in midgut aminopeptidase and alkaline phosphatase activity, and in midgut esterases and hemolymph plasma melanization activity. For analysis of genetic linkage of Cry2Ab resistance with potential Cry toxin receptor genes, molecular markers for the midgut cadherin, alkaline phosphatase (ALP), and aminopeptidase N (APN) genes were identified between the original greenhouse-derived dual-toxin-resistant and the susceptible laboratory T. ni strains. Genetic linkage analysis showed that the Cry2Ab resistance in T. ni was not genetically associated with the midgut genes coding for the cadherin, ALP, and 6 APNs (APN1 to APN6) nor associated with the ABC transporter gene ABCC2. Therefore, the Cry2Ab resistance in T. ni is conferred by a novel but unknown genetic mechanism.

Resistance of Trichoplusia ni populations selected by Bacillus thuringiensis sprays to cotton plants expressing pyramided Bacillus thuringiensis toxins Cry1Ac and Cry2Ab

Two populations of Trichoplusia ni that had developed resistance to Bacillus thuringiensis sprays (Bt sprays) in commercial greenhouse vegetable production were tested for resistance to Bt cotton (BollGard II) plants expressing pyramided Cry1Ac and Cry2Ab. The T. ni colonies resistant to Bacillus thuringiensis serovar kurstaki formulations were not only resistant to the Bt toxin Cry1Ac, as previously reported, but also had a high frequency of Cry2Ab-resistant alleles, exhibiting ca. 20% survival on BollGard II foliage. BollGard II-resistant T. ni strains were established by selection with BollGard II foliage to further remove Cry2Ab-sensitive alleles in the T. ni populations. The BollGard II-resistant strains showed incomplete resistance to BollGard II, with adjusted survival values of 0.50 to 0.78 after 7 days. The resistance to the dual-toxin cotton plants was conferred by two genetically independent resistance mechanisms: one to Cry1Ac and one to Cry2Ab. The 50% lethal concentration of Cry2Ab for the resistant strain was at least 1,467-fold that for the susceptible T. ni strain. The resistance to Cry2Ab in resistant T. ni was an autosomally inherited, incompletely recessive monogenic trait. Results from this study indicate that insect populations under selection by Bt sprays in agriculture can be resistant to multiple Bt toxins and may potentially confer resistance to multitoxin Bt crops.

Construction and characterisation of near-isogenic Plutella xylostella (Lepidoptera: Plutellidae) strains resistant to Cry1Ac toxin

BACKGROUND

Resistance to insecticidal Bacillus thuringiensis (Bt) toxins has arisen in multiple populations of the worldwide Brassica pest Plutella xylostella (L.). To help elucidate the mechanism of resistance to Bt Cry1Ac toxin in a population from Florida, two pairs of near-isogenic lines (NILs) were developed.

RESULTS

NILs were generated using either backcross or recombinant inbred line methodologies and evaluated for near-isogenicity with inter-simple-sequence-repeat (ISSR) markers. Backcross line BC6F4 maintained a similar level of Cry1Ac resistance to parental strain DBM1Ac-R (>5000-fold) yet showed 98.24% genetic similarity to the susceptible parental strain DBM1Ac-S. Single-pair backcrosses between DBM1Ac-S and BC6F4 revealed that Cry1Ac resistance was controlled by one recessive autosomal locus. BC6F4 exhibited high levels of cross-resistance to Cry1Ab and Cry1Ah but not to Cry1Ca or Cry1Ie.

CONCLUSION

Near-isogenic strains were constructed to provide a reliable biological system to investigate the mechanism of Cry1Ac resistance in P. xylostella. These data suggest that resistance to Cry1Ac, Cry1Ab and Cry1Ah is probably caused by the alteration of a common receptor not recognised by Cry1Ca or Cry1Ie. Understanding Bt toxin cross-resistance provides valuable information to consider when developing pest control strategies to delay resistance evolution. © 2014 Society of Chemical Industry.

A cadherin-like protein influences Bacillus thuringiensis Cry1Ab toxicity in the oriental armyworm, Mythimna separata

Cadherins comprise a family of calcium-dependent cell adhesion proteins that act in cell-cell interactions. Cadherin-like proteins (CADs) in midguts of some insects act as receptors that bind some of the toxins produced by the Bacillus thuringiensis (Bt). We cloned a CAD gene associated with larval midguts prepared from Mythimna separata. The full-length cDNA (MsCAD1, GenBank Accession No. JF951432) is 5642 bp, with an open reading frame encoding a 1757 amino acid and characteristics typical of insect CADs. Expression of MsCAD1 is predominantly in midgut tissue, with highest expression in the 3rd- to 6th-instars and lowest in newly hatched larvae. Knocking-down MsCAD1 decreased Cry1Ab susceptibility, indicated by reduced developmental time, increased larval weight and reduced larval mortality. We expressed MsCAD1 in E. coli and recovered the recombinant protein, rMsCAD1, which binds Cry1Ab toxin. Truncation analysis and binding experiments revealed that a contiguous 209-aa, located in CR11 and CR12, is the minimal Cry1Ab binding region. These results demonstrate that MsCAD1 is associated with Cry1Ab toxicity and is one of the Cry1Ab receptors in this insect. The significance of this work lies in identifying MsCAD1 as a Cry1Ab receptor, which helps understand the mechanism of Cry1Ab toxicity and of potential resistance to Bt in M. separata.

Diverse genetic basis of field-evolved resistance to Bt cotton in cotton bollworm from China

Evolution of pest resistance reduces the efficacy of insecticidal proteins from Bacillus thuringiensis (Bt) used in sprays or in transgenic crops. Although several pests have evolved resistance to Bt crops in the field, information about the genetic basis of field-evolved resistance to Bt crops has been limited. In particular, laboratory-selected resistance to Bt toxin Cry1Ac based on recessive mutations in a gene encoding a toxin-binding cadherin protein has been identified in three major cotton pests, but previous work has not determined if such mutations are associated with field-selected resistance to Bt cotton. Here we show that the most common resistance alleles in field populations of cotton bollworm, Helicoverpa armigera, selected with Bt cotton in northern China, had recessive cadherin mutations, including the deletion mutation identified via laboratory selection. However, unlike all previously studied cadherin resistance alleles, one field-selected cadherin resistance allele conferred nonrecessive resistance. We also detected nonrecessive resistance that was not genetically linked with the cadherin locus. In field-selected populations, recessive cadherin alleles accounted for 75-84% of resistance alleles detected. However, most resistance alleles occurred in heterozygotes and 59-94% of resistant individuals carried at least one nonrecessive resistance allele. The results suggest that resistance management strategies must account for diverse resistance alleles in field-selected populations, including nonrecessive alleles.

Incipient resistance of Helicoverpa punctigera to the Cry2Ab Bt toxin in Bollgard II cotton

Combinations of dissimilar insecticidal proteins (“pyramids”) within transgenic plants are predicted to delay the evolution of pest resistance for significantly longer than crops expressing a single transgene. Field-evolved resistance to Bacillus thuringiensis (Bt) transgenic crops has been reported for first generation, single-toxin varieties and the Cry1 class of proteins. Our five year data set shows a significant exponential increase in the frequency of alleles conferring Cry2Ab resistance in Australian field populations of Helicoverpa punctigera since the adoption of a second generation, two-toxin Bt cotton expressing this insecticidal protein. Furthermore, the frequency of cry2Ab resistance alleles in populations from cropping areas is 8-fold higher than that found for populations from non-cropping regions. This report of field evolved resistance to a protein in a dual-toxin Bt-crop has precisely fulfilled the intended function of monitoring for resistance; namely, to provide an early warning of increases in frequencies that may lead to potential failures of the transgenic technology. Furthermore, it demonstrates that pyramids are not ‘bullet proof’ and that rapid evolution to Bt toxins in the Cry2 class is possible.

Adaptive management of pest resistance by Helicoverpa species (Noctuidae) in Australia to the Cry2Ab Bt toxin in Bollgard II® cotton

In Australia, monitoring Helicoverpa species for resistance to the Cry2Ab toxin in second generation Bacillus thuringiensis (Bt) cotton has precisely fulfilled its intended function: to warn of increases in resistance frequencies that may lead to field failures of the technology. Prior to the widespread adoption of two-gene Bt cotton, the frequency of Cry2Ab resistance alleles was at least 0.001 in H. armigera and H. punctigera. In the 5 years hence, there has been a significant and apparently exponential increase in the frequency of alleles conferring Cry2Ab resistance in field populations of H. punctigera. Herein we review the history of deploying and managing resistance to Bt cotton in Australia, outline the characteristics of the isolated resistance that likely impact on resistance evolution, and use a simple model to predict likely imminent resistance frequencies. We then discuss potential strategies to mitigate further increases in resistance frequencies, until the release of a third generation product. These include mandating larger structured refuges, applying insecticide to crops late in the season, and restricting the area of Bollgard II® cotton. The area planted to Bt-crops is anticipated to continue to rise worldwide; therefore the strategies being considered in Australia are likely to relate to other situations.

Binding site alteration is responsible for field-isolated resistance to Bacillus thuringiensis Cry2A insecticidal proteins in two Helicoverpa species

BACKGROUND

Evolution of resistance by target pests is the main threat to the long-term efficacy of crops expressing Bacillus thuringiensis (Bt) insecticidal proteins. Cry2 proteins play a pivotal role in current Bt spray formulations and transgenic crops and they complement Cry1A proteins because of their different mode of action. Their presence is critical in the control of those lepidopteran species, such as Helicoverpa spp., which are not highly susceptible to Cry1A proteins. In Australia, a transgenic variety of cotton expressing Cry1Ac and Cry2Ab (Bollgard II) comprises at least 80% of the total cotton area. Prior to the widespread adoption of Bollgard II, the frequency of alleles conferring resistance to Cry2Ab in field populations of Helicoverpa armigera and Helicoverpa punctigera was significantly higher than anticipated. Colonies established from survivors of F(2) screens against Cry2Ab are highly resistant to this toxin, but susceptible to Cry1Ac.

METHODOLOGY/PRINCIPAL FINDINGS

Bioassays performed with surface-treated artificial diet on neonates of H. armigera and H. punctigera showed that Cry2Ab resistant insects were cross-resistant to Cry2Ae while susceptible to Cry1Ab. Binding analyses with (125)I-labeled Cry2Ab were performed with brush border membrane vesicles from midguts of Cry2Ab susceptible and resistant insects. The results of the binding analyses correlated with bioassay data and demonstrated that resistant insects exhibited greatly reduced binding of Cry2Ab toxin to midgut receptors, whereas no change in (125)I-labeled-Cry1Ac binding was detected. As previously demonstrated for H. armigera, Cry2Ab binding sites in H. punctigera were shown to be shared by Cry2Ae, which explains why an alteration of the shared binding site would lead to cross-resistance between the two Cry2A toxins.

CONCLUSION/SIGNIFICANCE

This is the first time that a mechanism of resistance to the Cry2 class of insecticidal proteins has been reported. Because we found the same mechanism of resistance in multiple strains representing several field populations, we conclude that target site alteration is the most likely means that field populations evolve resistance to Cry2 proteins in Helicoverpa spp. Our work also confirms the presence in the insect midgut of specific binding sites for this class of proteins. Characterizing the Cry2 receptors and their mutations that enable resistance could lead to the development of molecular tools to monitor resistance in the field.