Different mechanisms of resistance to Bacillus thuringiensis toxins in theindianmeal moth

Susceptibility to protoxin and toxin forms of Cry1Ab and the binding of I-1 25-labeled Cry1Ab and Cry1Ac has been examined in three Plodia interpunctel la colonies, one susceptible (688(s)) and two resistant (198(r) and Dpl(r)) to Bacillus thuringiensis. Toxicological studies showed that the 198(r) co lony was 11-fold more resistant to Cry1Ab protoxin than to Cry1Ab activated toxin, whereas the Dpl(r) colony was ii-fold more resistant to protoxin ve rsus toxin, Binding results with I-125-labeled toxins indicated the occurre nce of two different binding sites for Cry1Ab in the susceptible insects, o ne of them shared with Cry1Ac. Cry1Ab binding was found to be altered in in sects from both resistant colonies, though in different ways. Compared with the susceptible colony, insects from the Dpl(1) colony showed a drastic re duction in binding affinity (60-fold higher K-d), although they had similar concentrations of binding sites. Insects from the 198(r) colony showed a s light reduction in both binding affinity and binding site concentration (fi ve-fold-higher K-d and ca. three-fold-lower R-t compared with the 688(s) co lony). No major difference in Cry1Ac binding was found among the three colo nies. The fact that the 198(r) colony also has a protease-mediated mechanis m of resistance (B. Oppert, R. Hammel, J. E. Throne, and K. J. Kramer, J. B iol. Chem. 272:23473-23476, 1997) is in agreement with our toxicological da ta in which this colony has a different susceptibility to the protoxin and toxin forms of Cry1Ab. It is noteworthy that the three colonies used in thi s work derived originally from ca, 100 insects, which reflects the high var iability and high frequency of B. thuringiensis resistance genes occurring in natural populations.