Comparative susceptibility and binding of Cry1 Bacillus thuringiensis toxins to the Asian (Ostrinia furnacalis Guenee) and European (Ostrinia nubilalis Hubner) corn borers (Lepidoptera: Crambidae)

Maize genetically engineered to express Bacillus thuringiensis (Bt) was introduced into the Philippines for the control of Asian corn borer (Ostrinia furnacalis Guenee) in 2002. Ostrinia furnacalis is closely related to the European corn borer, Ostrinia nubilalis (Hubner). An enormous amount of information has been generated on Bt corn as it relates to O. nubilalis and the current research was undertaken to determine whether such knowledge could be leveraged for O. furnacalis. Both corn borer species exhibited similar pattern and high sensitivity to Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ba and Cry1F, suggesting that they share toxin receptors and similar mechanisms of toxicity. Both corn borer species were less susceptible to Cry1Ba compared to an outgroup species, the sugarcane borer, Diatraea saccharalis Fabricius, although significantly more susceptible to Cry1F. Ligand and immuno --blots allowed comparisons of Cry1Ab and Cry1F binding patterns, as well as expression of putative toxin receptors in brush border membrane vesicles prepared from these three insect species. Both Ostrinia spp. shared a common receptor that binds to Cry1Ab toxin and it is most likely a putative cadherin-like receptor. Both species exhibited comparable expression of putative cadherin and aminopeptidase-N-like receptors, with high homology of cDNA sequences. Interestingly, both Ostrinia spp. exhibited slightly different binding patterns to Cry1F toxin. Diatraea saccharalis showed different binding patterns compared to both Ostrinia spp. Although both Ostrinia spp. are genetically closely related, generalizations regarding binding characteristics across Bt toxins as shown in the differences in binding patterns between the Cry1Ab and Cry1F should be avoided. Ostrinia furnacalis clustered into two sister clades with a low bootstrap support (19%), and both clades show high similarity with respect to the mitochondrial cytochrome oxidase II gene. These results show that the sensitivity and binding of Bt toxins obtained by using specimens from Malaysia and Philippines, respectively are representative of O. furnacalis from surrounding Southeast Asia, as well as China and Japan. As such, we have a high chance of leveraging existing knowledge generated on O. nubilalis, particularly on the use of Cry1Ab-expressing Bt corn against O. furnacalis in Southeast Asia. The results of this research have important practical implications on the commercialization, regulatory decisions, stewardship and resistance management of Bt corn in Asia, and more importantly, provide the first comparative study between both Ostrinia spp. with regard to the sensitivity and mechanism of toxicity to Bt toxins.