Field-Evolved Mode 1 Resistance of the Fall Armyworm to Transgenic Cry1Fa-Expressing Corn Associated with Reduced Cry1Fa Toxin Binding and Midgut Alkaline Phosphatase Expression

Abstract

dInsecticidal protein genes from the bacterium Bacillus thuringiensis (Bt) are expressed by transgenic Bt crops (Bt crops) for effective and environmentally safe pest control. The development of resistance to these insecticidal proteins is considered the most serious threat to the sustainability of Bt crops. Resistance in fall armyworm (Spodoptera frugiperda) populations from Puerto Rico to transgenic corn producing the Cry1Fa insecticidal protein resulted, for the first time in the United States, in practical resistance, and Bt corn was withdrawn from the local market. In this study, we used a field-collected Cry1Fa corn-resistant strain (456) of S. frugiperda to identify the mechanism responsible for field-evolved resistance. Binding assays detected reduced Cry1Fa, Cry1Ab, and Cry1Ac but not Cry1Ca toxin binding to midgut brush border membrane vesicles (BBMV) from the larvae of strain 456 compared to that from the larvae of a susceptible (Ben) strain. This binding phenotype is descriptive of the mode 1 type of resistance to Bt toxins. A comparison of the transcript levels for putative Cry1 toxin receptor genes identified a significant downregulation (>90%) of a membrane-bound alkaline phosphatase (ALP), which translated to reduced ALP protein levels and a 75% reduction in ALP activity in BBMV from 456 compared to that of Ben larvae. We cloned and heterologously expressed this ALP from susceptible S. frugiperda larvae and demonstrated that it specifically binds with Cry1Fa toxin. This study provides a thorough mechanistic description of field-evolved resistance to a transgenic Bt crop and supports an association between resistance and reduced Cry1Fa toxin binding and levels of a putative Cry1Fa toxin receptor, ALP, in the midguts of S. frugiperda larvae.

Broad-spectrum resistance to Bacillus thuringiensis toxins by western corn rootworm (Diabrotica virgifera virgifera)

“…Studies characterizing Bt resistance by pest insects often emphasize the role of modification in the binding sites of Bt toxins404142, and the alteration of gut membrane receptors is often reported as a mechanism of Bt resistance4344. The development of cross-resistance among Bt toxins in target insects can depend on shared binding sites in the insect’s midgut and amino acid similarities between Bt toxins2045.…”

Nutrition affects insect susceptibility to Bt toxins

“…1a), as evidenced by the fact that all definitions for the term resistance describe a change in susceptibility that is genetically-determined and heritable4. This is true even though the underlying genetics responsible for resistance are difficult to identify and rarely known in instances of field-evolved resistance, except for a few species/populations5678910111213. We should note, that our use of the term susceptibility throughout the paper refers only to the extent or likelihood of an organism being negatively affected by a Bt trait, and this usage makes no assumptions about the mode of susceptibility or its underlying heritability.…”

Resistance to Bt Crops; Influence, mechanisms and management strategies

“…Furthermore, in armyworm, S. frugiperda resistance to Cry1Fa in Bt corn was related to reduced toxin binding and expression of midgut alkaline phosphatase (Jakka et al, 2015). In H. armigera and H. punctigera, resistance to Cry2Ab was conferred by mutations in ABC transporter (Coates and Siegfried, 2015;Tay et al, 2015).…”

“…Diabrotica virgifera virgifera showed resistance to Cry3Bb1 in Bt maize (Gassmann et al, 2011). S. frugiperda evolved resistance to Cry1Fa toxin in Bt corn (Farias et al, 2015;Jakka et al, 2015;Monnerat et al, 2015).…”