Bt transgenic crops do not have favorable effects on resistant insects

Sayyed et al. (Ecology Letters (2003) 6: 167–169) hypothesized that insecticidal Bacillus thuringiensis (Bt) toxins produced by transgenic crops could have nutritionally favorable effects that increase the fitness of resistant insects eating such crops. This idea was based on increased pupal weight of resistant larvae of diamondback moth, Plutella xylostella (L.), fed leaf discs treated externally with a Bt toxin. We summarize evidence from diamondback moth and other pests showing that the Bt toxins in transgenic crops do not enhance performance of resistant insects. Aside from a few notable exceptions in which performance of resistant insects did not differ between Bt and non-Bt crops, Bt crops had adverse affects on resistant insects. Abbreviation: / Bt: Bacillus thuringiensi

Similar Genetic Basis of Resistance to Bt Toxin Cry1Ac in Boll-Selected and Diet-Selected Strains of Pink Bollworm

Genetically engineered cotton and corn plants producing insecticidal Bacillus thuringiensis (Bt) toxins kill some key insect pests. Yet, evolution of resistance by pests threatens long-term insect control by these transgenic Bt crops. We compared the genetic basis of resistance to Bt toxin Cry1Ac in two independently derived, laboratory-selected strains of a major cotton pest, the pink bollworm (Pectinophora gossypiella [Saunders]). The Arizona pooled resistant strain (AZP-R) was started with pink bollworm from 10 field populations and selected with Cry1Ac in diet. The Bt4R resistant strain was started with a long-term susceptible laboratory strain and selected first with Bt cotton bolls and later with Cry1Ac in diet. Previous work showed that AZP-R had three recessive mutations (r1, r2, and r3) in the pink bollworm cadherin gene (PgCad1) linked with resistance to Cry1Ac and Bt cotton producing Cry1Ac. Here we report that inheritance of resistance to a diagnostic concentration of Cry1Ac was recessive in Bt4R. In interstrain complementation tests for allelism, F1 progeny from crosses between AZP-R and Bt4R were resistant to Cry1Ac, indicating a shared resistance locus in the two strains. Molecular analysis of the Bt4R cadherin gene identified a novel 15-bp deletion (r4) predicted to cause the loss of five amino acids upstream of the Cry1Ac-binding region of the cadherin protein. Four recessive mutations in PgCad1 are now implicated in resistance in five different strains, showing that mutations in cadherin are the primary mechanism of resistance to Cry1Ac in laboratory-selected strains of pink bollworm from Arizona

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

Synergism between entomopathogenic nematodes and Bacillus thuringiensis crops: integrating biological control and resistance management

Summary 1. The past decade has witnessed a continual increase in the use of crops genetically modified to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt). This presents the challenge of designing agricultural systems to manage pests and the evolution of resistance to Bt. 2. We tested whether entomopathogenic nematodes might act synergistically with Bt crops by killing pests in non-Bt refuges and by increasing the fitness costs of resistance to Bt. We also tested whether insect mortality and fitness costs were affected by the cotton phytochemical gossypol. 3. The entomopathogenic nematode Steinernema riobrave increased the fitness cost of Bt resistance, indicating that its presence in refuges may slow pest adaptation to Bt crops. No effect on fitness costs was detected for the nematode Heterorhabditis bacteriophora . Gossypol did not alter nematode-imposed fitness costs. 4. Simulation modelling supported the hypothesis that nematodes in refuges may slow resistance evolution. 5. The effects of gossypol on insect mortality from nematodes and nematode reproduction differed between nematode species. Gossypol increased insect mortality caused by H. bacteriophora but did not affect mortality caused by S. riobrave . Gossypol enhanced reproduction of H. bacteriophora and decreased reproduction of S. riobrave . 6. Synthesis and applications. Our results point to the value of developing integrated pest management strategies for Bt crops that include non-Bt refuges in which entomopathogenic nematodes are used as a pest-management agent. Because entomopathogenic nematodes can magnify fitness costs of Bt resistance, their presence in refuges may delay resistance by pests to Bt crops. Moreover, entomopathogenic nematodes can serve as biological control agents thereby decreasing dependence on conventional insecticides to manage pest populations in refuges

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 wildtype 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.China's Key Project for Breeding Genetically Modified Organisms [2016ZX08012-004]; Agriculture and Food Research Initiative Competitive Grant from the USDA National Institute of Food and Agriculture [2018-67013-27821]Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]