Common, but Complex, Mode of Resistance of Plutella xylostella to Bacillus thuringiensis Toxins Cry1Ab and Cry1Ac

Abstract

A field collected population of Plutella xylostella (SERD4) was selected in the laboratory with Bacillus thuringiensis endotoxins Cry1Ac (Cry1Ac-SEL) and Cry1Ab (Cry1Ab-SEL). Both subpopulations showed similar phenotypes: high resistance to the Cry1A toxins and little cross-resistance to Cry1Ca or Cry1D. A previous analysis of the Cry1Ac-SEL showed incompletely dominant resistance to Cry1Ac with more than one factor, at least one of which was sex influenced. In the present study reciprocal mass crosses between Cry1Ab-SEL and a laboratory susceptible population (ROTH) provided evidence that Cry1Ab resistance was also inherited as incompletely dominant trait with more than one factor, and at least one of the factors was sex influenced. Analysis of single pair mating indicated that Cry1Ab-SEL was still heterogeneous for Cry1Ab resistance genes, showing genes with different degrees of dominance. Binding studies showed a large reduction of specific binding of Cry1Ab and Cry1Ac to midgut membrane vesicles of the Cry1Ab-SEL subpopulation. Cry1Ab-SEL was found to be more susceptible to trypsin-activated Cry1Ab toxin than protoxin, although no defect in toxin activation was found. Present and previous results indicate a common basis of resistance to both Cry1Ab and Cry1Ac in selected subpopulations and suggest that a similar set of resistance genes are responsible for resistance to Cry1Ab and Cry1Ac and are selected whichever toxin was used. The possibility of an incompletely dominant trait of resistant to these toxins should be taken into account when considering refuge resistance management strategies.

Gut Bacteria Are Not Required for the Insecticidal Activity of Bacillus thuringiensis toward the Tobacco Hornworm, Manduca sexta

“…Cry1Ac protoxins were produced as inclusion bodies in E. coli cultures and purified as previously described (30). Cry1Ac crystals were diluted in sterile distilled water and incorporated into sterile diet at five concentrations as described previously by Gilliland et al (12).…”

“…Fifty percent lethal concentration (LC 50 ) or 50% lethal dose (LD 50 ) estimates and their 95% fiducial limits (FLs) were calculated by logit regression using GLIM 3.77 (1985; Numerial Algorithms Group) as previously described (30)…”

Bacillus thuringiensis resistance in Plutella — too many trees?

“…Although this finding was consistent with a defect in toxin activation the authors noted that alternative explanations existed -such as preferential sequestration of the protoxin form. In a related paper a resistant population was once again found to be more susceptible to activated toxin, although no defect in toxin activation could actually be established [10]. Another example where an indirect observation could have had several explanations was seen in a paper by Sayyed et al [11] in which it was observed that an esterase inhibitor could synergise the activity of a Cry toxin against a resistant population.…”

“…One such paper [10] suggests that resistance was inherited in an incompletely dominant fashion and showed some maternal influence. These differences may represent multiple mutations/mechanisms of resistance but may also reflect significant differences in genetic backgrounds that can confound the analysis of the major resistance-causing mutation(s).…”

Bacillus thuringiensis Vip3Aa Toxin Resistance in Heliothis virescens (Lepidoptera: Noctuidae)

“…Other populations resistant to Cry toxins have also been shown to exhibit polygenic resistance: for example, populations of H. virescens (46) and H. armigera (47) resistant to Cry2A and populations of P. xylostella (32) and P. gossypiella (48) resistant to Cry1Ac. Nevertheless, monogenic resistance has been found in other populations of H. virescens (37), H. armigera (41), P. xylostella (49), and Ostrinia nubilalis (50).…”