Bt Cry Toxins Research Articles

Inheritance of Resistance to Bacillus thuringiensis Cry1Ac Toxin in a Greenhouse-Derived Strain of Cabbage Looper (Lepidoptera: Noctuidae)

A population of cabbage looper, Trichoplusia ni (Hübner), collected from commercial greenhouses in the lower mainland of British Columbia, Canada, in 2001 showed a resistance level of 24-fold to Dipel, a product of Bacillus thuringiensis (Bt) subspecies kurstaki. This population was selected with Cry1Ac, the major Bt Cry toxin in Dipel, to obtain a homogenous population resistant to Cry1Ac. The resulting strain of T. ni, named GLEN-Cry1Ac, was highly resistant to Cry1Ac with a resistance ratio of approximately 1000-fold. The larvae from the GLEN-Cry1Ac strain could survive on Cry1Ac-expressing transgenic broccoli plants that were highly insecticidal to T. ni and diamondback moth, Plutella xylostella (L.). The inheritance of Cry1Ac resistance in this T. ni strain was autosomal and incompletely recessive. The degree of dominance of the resistance was -0.402 and -0.395, respectively, for the neonates in reciprocal crosses between the GLEN-Cry1Ac and a laboratory strain of T. ni. Using chi2 goodness-of-fit test, we demonstrated that the inhibition of larval growth resulting from testing 12 toxin doses in the progeny of the backcross fit the predicted larval responses based on a monogenic inheritance model. Therefore, we conclude that the inheritance of the resistance to Cry1Ac in the T. ni larvae is monogenic.

IAPPS Newsletter Number V May 2004

Adaptation of insects to plant protease inhibitors and development of resistance to Bacillus thuringiensis (Bt) insecticidal toxins has been laid the concept of combination of different insecticidal proteins. While using the combination of insecticidal proteins, it is important to have complete understanding of the target insect physiology and midgut biochemistry. In the present investigation, we report the combined effects of Bt Cry1Ac toxin and protease inhibitor (PMSF) on the growth and activities of midgut digestive proteases of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). We also checked the interactions between H. armigera midgut soluble aminopeptidases and Bt Cry1Ac toxin. As expected, the larval growth and survival was found to be reduced on the diets supplemented with Cry1Ac and PMSF as compared to control larvae. Significant reduction in the growth and survival rate of the larvae was observed on the diet incorporated with the combination of Cry1Ac and PMSF. H. armigera midgut soluble aminopeptidase activity was strongly decreased in the larvae reared on Cry1Ac as compared to larvae fed on control diet. Serine proteinase activities were significantly declined in the larvae reared on PMSF containing diet as compared to control larvae. However, the serine proteinase activities were notably increased in the larvae fed on Cry1Ac. Reduction in the aminopeptidase activity with LpNA in the H. armigera larvae was compensated with an induction of aminopeptidase activity with ApNA. Our findings could be helpful to further dissect the roles of midgut soluble aminopeptidases in the perception of Bt toxins. Studies also suggest that combination of different types of insecticidal proteins such as Bt Cry toxin and protease inhibitors might be a promising pest management strategy.