Submitted to: Society for Invertebrate Pathology Annual Meeting
Publication Type: Proceedings
Publication Acceptance Date: 8/3/2008
Publication Date: 8/3/2008
Citation: Zhu, Y., Wu, X., Yang, Y., Ottea, J., Leonard, R.B., Abel, C.A., Huang, F. 2008. Study on Bt Susceptibility and Resistance Mechanisms in the Sugarcane Borer, Diatraea saccharalis. Society for Invertebrate Pathology Annual Meeting. 2008:52. Interpretive Summary: In recent years, the sugarcane borer has expanded its geographic range and has become a dominant corn borer species in some areas in the mid-south of the United States, especially in Louisiana and Texas. Transgenic Bt corn has been rapidly adopted in Louisiana since 1999 because of emerging lepidopteran pest problems, especially with the sugarcane borer. A Bt-resistant strain of sugarcane borer capable of completing larval development on commercial Cry1Ab corn has been selected using novel F2 screening techniques. This Bt-resistant strain provides opportunities to explore the physiological mechanisms of Bt resistance in this species and may serve as a model for other corn stalk boring caterpillar pests. This study was conducted to determine the susceptibility of the Cry1Ab-resistant sugarcane borer to other Bt Cry proteins including Cry1Aa, Cry1Ac, to examine enzymatic activities of midgut proteinases (trypsins and chymotrypsins), aminopeptidase, and alkaline phosphatase, to clone and compare cDNA sequences and gene expression levels of Bt resistance related genes in Bt-susceptible and -resistant strains of sugarcane borer.
Technical Abstract: Dose response and growth inhibition of Cry1Ab-susceptible and -resistant strains of the sugarcane borer, Diatraea saccharalis, were evaluated with Cry1Aa and Cry1Ac toxins. The median lethal concentration (LC50) of the Cry1Ab-resistant strain was estimated to be >80- and 45-fold greater than that of the susceptible larvae to the Cry1Aa and Cry1Ac, respectively. Cry1Aa was more effective protein against both of the susceptible and resistant strains than Cry1Ac. Inhibition to larval growth for both Cry proteins was observed in both resistant and susceptible strains, but the inhibition to the resistant strain was less intensive than that to the susceptible larvae. To determine if Bt resistance in D. saccharalis is associated with proteolytic and/or binding changes in midgut, enzymatic activities of trypsins, chymotrypsins, aminopeptidase, and alkaline phosphatase from both susceptible and resistant larvae were assayed in vitro with BApNA, SAApNA, LpNA, and pNPP as the substrates, respectively. By using cDNA library sequencing and RT-PCR, cDNAs of several Bt resistance related genes were cloned and sequenced. Results indicated that the susceptible and resistant strains showed different enzyme activities. In addition, three aminopeptidase cDNAs, three alkaline phosphatase cDNAs, and one cadherin cDNA were cloned and sequenced. Potential sequence modifications were examined between S and R strains. Gene expression levels of these resistance candidate genes were further examined and compared using realtime-PCR.