BINDING OF INSECTICIDAL CRYSTAL PROTEINS OF BACILLUS-THURINGIENSIS TOTHE MIDGUT BRUSH-BORDER OF THE CABBAGE-LOOPER, TRICHAPLUSIA NI (HUBNER) (LEPIDOPTERA, NOCTUIDAE), AND SELECTION FOR RESISTANCE TO ONE OF THE CRYSTAL PROTEINS

The susceptibility of Trichoplusia ni larvae to several Bacillus thuri ngiensis insecticidal crystal proteins (ICPs) was tested. Neonatal lar vae proved to be susceptible to solubilized trypsin-treated CrgIA(a), CryIA(b), acid CryIA(c) (50% lethal concentrations [LC(50)s], 570, 580 , and 320 ng/cm(2), respectively) but showed little susceptibility to CryIB and CryID (LC(50)s, 5,640 and 2,530 ng/cm(2), respectively). The toxicity of ICPs was correlated to binding to the epithelial brush bo rder of the midgut, as revealed by immunocytochemical staining with mo noclonal antibodies. In vitro binding experiments,vith iodinated ICPs and brush border membrane vesicles indicated that CryIA(b) and CryIA(c ) share the same high-affinity binding site, whereas CryIA(a) binds to a different one. The affinities of CryIA(b) and CryIA(c) for the bind ing site were similar (K-d = 3.6 and 4.7 nM, respectively), and the me an binding site concentration was 0.71 pmol/mg of vesicle protein. Sel ection of a population with increasing concentrations of CryIA(b) prod uced 31-fold resistance in seven generations. The realized heritabilit y (h(2)) was 0.19. The increase of homozygosity (for resistance factor s) as selection proceeded was reflected in the increase in the slopes of the dose-mortality curves. Resistance was specific for CryIA(b) and did not extend to CryIA(a) or even to CryIA(c). This result was not p redicted by the binding-site model, in which CryIA(b) and CryIA(c) bin d to the same high-affinity binding site. This result may suggest a mo re complicated relationship between in vitro binding of ICPs to specif ic sites in the epithelial membrane of the midgut and the in vivo toxi c effect.