Biochemistry and Molecular Biology of Cry1Ac Resistance in Cotton Bollworm Helicoverpa armigera

Abstract

The cotton bollworm, Helicoverpa armigera (Hübner) is an important pest of cotton newlineand many other crops. It has an ability to evolve resistance to selection pressure of newlineinsecticides both in laboratory and field. Transgenic cotton incorporating Cry1Ac newlinegene derived from Bt is one of the most promising advances made in cotton pest newlinemanagement. However, the main concerns associated with the widespread adoption of newlineBt cotton is the evolution of resistance in H. armigera. The studies on baseline newlinesusceptibility to Cry toxins and monitoring of resistance to Bt crops are essential newlinecomponent of Bt resistance management. In the present study, the baseline newlinesusceptibility of H. armigera collected from different locations to Cry1Ac toxin newlineshowed wide variation in susceptibility to Cry1Ac. The susceptibility of H. armigera newlineto Cry1Ac ranged from 0.0084 and#956;g/ g to 1.5 and#956;g/ g for 96 h. Populations collected from newlineRaichur and Bharuch showed highest tolerance to Cry1Ac for 96 h as well as 168 h newlinebioassays. The analysis of reciprocal genetic crosses between resistant H. armigera newlinestrain (BH-R) (227.9-fold) with susceptible Vadodara (VA-S) strain showed newlinedominance (h) of 0.65 0.89 and degree of dominance (D) of 0.299 0.782, suggesting newlineCry1Ac resistance as a semi-dominant trait. The D and h values of F1 hybrids of newlinefemale resistant parent were higher than female susceptible parent, showing newlinematernally enhanced dominance of Cry1Ac resistance. Evaluation of some biological newlineattributes showed that larval and pupal periods of progenies of reciprocal F1 crosses, newlinebackcrosses and F2 crosses were either at par with resistant parent or lower than newlinesusceptible parent on treated diet (0.01 and#956;g/ g). In the present study, comparison of the newlinemidgut protease pattern in both resistant and susceptible H. armigera was found to be newlinesimilar. Further studies on the kinetics of degradation of Cry1Ac toxin by midgut newlineproteases of resistant and susceptible showed no difference with varying ratios of newlineprotoxin (and#956;g): protease (TU).