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Inheritance of resistance to Bacillus thuringiensis Cry1Ac toxin in Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) from India.

BACKGROUND
The cotton bollworm, Helicoverpa armigera (Hübner), is one of the most serious insect pests of cotton. It has developed resistance to almost all groups of chemical insecticides because of their intensive use. The failure of insecticides to control H. armigera has been a strong incentive for the adoption of transgenic cotton (Bt cotton). However, the value of Bt could be diminished by widespread resistance development to Bt toxins in insect populations. Therefore, understanding the genetic basis of resistance is essential for developing and implementing strategies to delay and monitor resistance.


RESULTS
A resistant strain designated as BM-R was obtained from the cross of adults from Bathinda (male) and Muktsar (female), Punjab, India, which showed the highest survival (60.68%) and LC(50) value (1.396 µg mL(-1) diet). Similarly, a laboratory-maintained strain from Hoshiarpur, Punjab, showed maximum susceptibility to Cry1Ac toxin with the lowest LC(50) value (0.087 µg mL(-1)), and was designated as HP-S. The genetic purity of both strains was confirmed by RAPD profile analysis at each generation, and genetic similarity reached more than 90% after the third generation. Continuous maintenance of the resistant BM-R strain on Cry1Ac resulted in an increase in LC(50) from 0.531 µg mL(-1) in F(0) to 4.28 µg mL(-1) in F(14) and 7.493 µg mL(-1) in F(19) , while the LC(50) values for HP-S larvae on diet without Cry1Ac increased to 0.106 and 0.104 µg mL(-1) , which lay within the fiducial limits of the baseline LC(50) value. The mode of inheritance of resistance was elucidated through bioassay response of resistant, susceptible heterozygotes and backcross progeny to Cry1Ac incorporated in semi-synthetic diet.


CONCLUSION
Based on dominance, degree of dominance and backcross values, resistance was inferred to be polygenic, autosomal and inherited as a recessive trait.