Baseline Susceptibility To Cry1A Toxins, Polymorphism And MRNA Expression Of The Cadherin Gene In Field Populations Of Chilo Suppressalis

The rice stem borer Chilo suppressalis (Walker) is one of the most serious pests of rice in China. Chemical insecticides have been intensively used to control C. suppressalis. Overuse of chemical insecticides resulted in serious insecticide resistance in C. suppressalis, and negative impacts on ecological environment. Bt transgenic rice gives a effective and safe alternative to control the rice stem borers. Bt transgenic rice lines passed through biosafety risk assessment in China in 2009, and Bt transgenic rice could be commercialized in a few years. Resistance risk of C. suppressalis (one of main target pests of Bt rice) is one of the key factors that determine the success of transgenic Bt rice in the field. Before its extensive commercialization in China, to determine baseline susceptibility to CrylA toxins of field populations is important for resistance risk assessment and resistance monitoring of C. suppressalis in future.In this study, we investigated the toxicity of 3 CrylA toxins in 5 field populations and 1 laboratory susceptible strain of C. suppressalis. We also analyzed amino acid polymorphism and mRNA expression of a cadherin gene in field populations of C. suppressalis. The results are summarized as follows:1. Susceptibility to 3 CrylA toxins in field populations of C. suppressalisSusceptibility to3 Cry1A toxins (Cry1Aa, Cry1Ab, Cry 1 Ac) in 5 field populations of C. suppressalis collected from Wenzhou (WZ), Ruichang (RC), Dafeng (DF) and Jingzhou (JZ) in 2011, and a lab strain (CN) were investigated. For the susceptible CN strain, LC50S of CrylAb, CrylAa and Cry1Ac were 0.229,0.240 and 0.277?g/cm2, respectively. LC5Os of CrylAb, CrylAa and Cryl Ac in 5 field populations were between 0.147-0.304, 0.152?0.335, and 0.170?0.351?g/cm2, respectively. Five field populations had limited difference in susceptibility to 3 CrylA toxins (less than 2-fold), and the toxicity of three CrylA toxins were generally ranked as:CrylAb>CrylAa>CrylAc. Considering limited variations of susceptibility to Cry1A toxins in field populations, Bt rice is expected to have good efficiency on C. Suppressalis in the field for a long period. However, early resistance monitoring and preventative resistance management tactics must be developed in advance.2. Amino acid polymorphism and mRNA expression level of the cadherin gene in field populations of C. suppressalisBased on the deposited cDNA sequence of the cadherin gene of C. suppressalis (CsCAD) in GenBank (DQ821523.2), the full-length sequences of cDNA of CsCAD in susceptible population CN and 4 field populations (WZ, RC, HX, JZ) were cloned and compared. Fifteen full-length cDNAs (5148 bp) were sequenced from 5 populations, and there were 3.48% polymorphic bases (179 bases) and 5.83% polymorphic amino acids (100 aa). There was about 99% similarity in amino acid sequence between CsCAD (DQ821523.2) and the alleles of field populations. Polymorphic amino acids were evenly distributed in the coding region of the cadherin gene.Using real-time quantitative PCR, mRNA expression level of cadherin gene were determined in the susceptible CN strain and the 4 field populations. The mRNA expression level among the 5 populations was not significantly different. Compared with the CN strain, the relative mRNA expressions in JZ, WZ, HX and RC were 1.292,0.656,0.809 and 0.639, respectively.The limited variations were observed in both sequence polymorphism and mRNA expression of CsCAD of C. suppressalis. CsCAD was confirmed previously as a CrylA-binding protein in C. suppressalis, but whether or not CsCAD is a CrylA receptor needs further investigation.