Transgenic cotton producing Bacillus thuringiensis (Bt) toxins have been planted widely to control a number of lepidopteran pests including Heliothis virescens, Helicoverpa armigera, and Pectinophora gossypiella. Evolution of resistance by these pests can reduce the benefits of Bt cotton. Intensive planting of Bt cotton producing Cry1Ac has resulted in field-evolved resistance in P. gossypiella from western India, and increases in resistance gene frequencies in some populations of H. armigera from northern China. A number of strains of the three cotton pests with resistance to Cry1Ac were selected in the laboratory or isolated from the field. Understanding resistance mechanisms of these Cry1Ac-resistant strains is essential for designing early detection methods and predicting resistance evolution. The most common type of resistance mechanisms to Cry1Ac is caused by mutations in the Bt receptors that reduce binding of Bt toxins to larval midgut proteins. Mutations at a toxin-binding cadherin genetically linked with Cry1Ac resistance were first identified in a laboratory-selected strain of H. virescens, and then in both laboratory-selected and field-selected strains of P. gossypiella and H. armigera. Mutations in an ABCC2 transporter gene are linked with resistance to Cry1Ac in a laboratory-selected strain of H. virescens. Reduced levels of membrane-bound alkaline phosphatases (ALPs) associated with Cry1Ac resistance were detected in some laboratory-selected strains of H. virescens and H. armigera. Apart from mutations in the Bt receptors, improper activation of Cry1Ac protoxin caused by down-regulation of a proteinase was responsible for 72-fold resistance to Cry1Ac protoxin in a H. armigerastrain from India. It is demonstrated that a diverse of genetic options can be employed by the three cotton pests to evolve resistance to Bt toxin Cry1Ac. However, predicting what mechanisms of Bt resistance will be evolved in the field is a challenging task.

Resistance mechanisms of three cotton pests to Bt toxin Cry1Ac

248th National Meeting (2014)

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