Genetic and biochemical approach for characterization of resistance to Bacillus thuringiensis toxin Cry1Ac in a field population of the diamondback moth, Plutella xylostella

Four subpopulations of a Plutella xylostella (L.) strain from Malaysia (F-4 to F-8) were selected with Bacillus thuringiensis subsp. kurstaki HD-1, Ba cillus thuringiensis subsp. aizawai, Cry1Ab, and Cry1Ac, respectively, whil e a fifth subpopulation was left as unselected (UNSEL-MEL). Bioassays at F- 9 found that selection with Cry1Ac, Cry1Ab, B. thuringiensis subsp. kurstak i, and B. thuringiensis subsp. aizawai gave resistance ratios of >95, ill, 7, and 3, respectively, compared with UNSEL-MEL (>10,500, 500, >100, and 26 , respectively, compared with a susceptible population, ROTH). Resistance t o Cry1Ac, Crg1Ab, B. thuringiensis subsp. kurstaki, and B. thuringiensis su bsp. aizawai in UNSEL-MEL declined significantly by F-9. The Cry1Ac-selecte d population showed very little cross-resistance to Cry1Ab, B. thuringiensi s subsp. kurstaki, and B, thuringiensis subsp. aizawai (5-, 1-, and 4-fold compared with UNSEL-MEL), whereas the Cry1Ab-, B. thuringiensis subsp. kurs taki, and B. thuringiensis subsp. aizawai-selected populations showed high cross-resistance to Cry1Ac (60-, 100-, and 70-fold). The Cry1Ac-selected po pulation,vas reselected (F-9 to F-13) to give a resistance ratio of >2,400 compared with UNSEL-MEL. Binding studies,vith I-125-labeled Cry1Ab and Cry1 Ac revealed complete lack of binding to brush border membrane vesicles prep ared from Cry1Ac-selected larvae (F-15). Binding was also reduced, although less drastically, in the revertant population, which indicates that a modi fication in the common binding site of these two toxins was involved in the resistance mechanism in the original, population. Reciprocal genetic cross es between Cry1Ac-reselected and ROTH insects indicated that resistance was autosomal and showed incomplete dominance. At the highest dose of Cry1Ac t ested, resistance was recessive while at the lowest dose it was almost comp letely dominant. The F-2 progeny from a backcross of F-1 progeny with ROTH was tested with a concentration of Cry1Ac which would kill 100% of ROTH mot hs. Eight of the 12 families tested had 60 to 90% mortality, which indicate d that more than one allele on separate loci was responsible for resistance to Cry1Ac.