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Abstract
Cry2Ab2 is a Bacillus thuringiensis (Bt) protein that has been pyramided with Cry1A.105 in transgenic maize and Cry1Ac in cotton to control some major lepidopteran pests including the corn earworm/bollworm, Helicoverpa zea (Boddie). However, the widespread occurrence of resistance of this pest to the pyramided Cry1A/Cry2A crops in the southern region of the United State has become a threat to the sustainability of the technology. In this study, multiple genetic crosses and backcrosses were developed to characterize the inheritance of Cry2Ab2 resistance in two H. zea populations resistant to the single protein, Cry2Ab2 (RR2Ab) and the dual proteins in Bt maize, Cry1A.105/Cry2Ab2 (RRVT2P). Diet-overlay bioassays with F1 hybrids from reciprocal crosses between a susceptible and the resistant populations showed that the Cry2Ab2 resistance in both RR2Ab and RRVT2P was inherited autosomally and non-recessively. Segregation tests in F2 and backcrossed generations indicated that the resistance was likely controlled by more than one locus. The effective dominance levels of the resistance estimated at each of three discriminating concentrations were similar between RR2Ab and RRVT2P, ranging from incompletely dominant to incompletely recessive. The similar inheritance observed in RR2Ab and RRVT2P suggests that the Cry2Ab2 resistance is independent from the Cry1A.105 resistance. The non-recessive inheritance of the resistance could be an important factor causing the widespread resistance of this insect to the Cry1A/Cry2A crops in the southern region. Implications of the observed inheritance properties in Bt crop resistance management are discussed.
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