Toxicity of Bacillus thuringiensis Cry2Ab and the inheritance of Cry2Ab resistance in the Pink bollworm, Pectinophora gossypiella (Saunders).

Though the insecticidal crystal (Cry) proteins produced by Bacillus thuringiensis (Bt) are effective against insect pests, evolution of resistance remains an issue of great concern. Here, we explored the ability of pink bollworm, Pectinophora gossypiella (Lepidoptera: Gelechiidae), a key pest of cotton, to evolve resistance to Cry2Ab and characterized it in terms of inheritance. Sixteen day bioassay of Bt Cry2Ab toxin against 5-day old pink bollworm larvae showed median lethal concentration (LC50) ranging 0.16-1.44 µg/g diet for five different populations collected from Srivilliputtur (Tamil Nadu), Jalgaon (Maharashtra), Bharuch (Gujarat), New Delhi (Delhi) and Sri Ganganagar (Rajasthan). Selection of pink bollworm for evolution of Cry2Ab resistance led to the maximal of 37.75-fold resistance vis-à-vis the most susceptible strain. Further studies on inheritance using above parental populations showed autosomal and semidominant nature of Cry2Ab resistance, with dominance h values of 0.69 and 0.79 for two reciprocal crosses. The inheritance of Cry2Ab resistance appeared to be governed by multiple alleles/genes. Cry2Ab resistance was associated with fitness costs in terms of prolonged larval and pupal period when resistant parent population was reared on the diet without toxin. Fitness cost in terms of larval period appeared to be inherited in F1, F2 and backcross progenies. F2 progeny also inherited these differences in larval and pupal periods. These studies clearly advocate rigorous monitoring of Cry2Ab resistance and compliance of resistance management in the pink bollworm for ensuring Bt cotton sustainability.

Assessment of Bt trait purity in different generations of transgenic cottons.

Adequate expression of Bt (Bacillus thuringiensis) toxins and purity of seeds of Bt-transgenic cottons are important for controlling bollworms, and thereby increasing the cotton productivity. Therefore, we examined the variability in expression of Bt toxin proteins in the seeds and in leaves of different cotton (Gossypium hirsutum (L.) hybrids (JKCH 226, JKCH 1947, JKCH Durga, JKCH Ishwar, JKCH Varun KDCHH 441 and KDCHH 621) expressing Bt toxins in F1 and F2 generations, using bioassays against the cotton bollworm, Helicoverpa armigera (Hübner), and the lateral flow strip (LFS) test. Toxicity of Bt toxin proteins in the seeds of Bt-transgenic cottons to H. armigera correlated with their toxicity in the leaves in one-toxin Bt cotton hybrids. The Bt-F1 and Bt-F2 seeds of JKCH 1947 were more toxic to H. armigera than those of JKCH Varun seeds. The seeds and leaves of F1s showed greater toxicity than the F2 seeds or leaves of one-toxin (cry1Ac) Bt cotton hybrids. However, no significant differences were observed for the two-toxin (cry1Ac and cry2Ab) hybrid, KDCHH 621. Toxicity of leaves to H. armigera increased with crop age, until 112 days after seedling emergence. The Bt trait purity in F1 seeds of four two-toxin Bt cotton hybrids ranged from 86.7 to 100%. The present study emphasizes the necessity of 95% Bt trait purity in seeds of transgenic cotton for sustainable crop production.