Studies On Susceptibility Of Cry1AB-tolerant Strain And Field Population Of Chilo Suppressalis To Bt Toxin

Chilo suppressalis?Walker?is one of the most important rice stem borers,and its damage seriously impacts on rice production every year.The development of insect-resistant transgenic rice provides an effective strategy for the control of the rice pests,with the development of genetic engineering technology.However,the evolution of the resistance of target pests to Bt crops is one of the most important factors for influencing the sustainable utility of Bt rice.It is also one of the focused contents of ecological safety evaluation for insect-resistant genetically modified crops before its commercial application.Moreover,the resistant risk of C.suppressalis to Bt rice has not yet been reported.Further,the definition of resistance risk of C.suppressalis is based on the clarification of susceptibility of target pests to Bt insecticidal protein/Bt rice.Based on this,the survival of Cry1Ab-susceptible and resistant strains of C.suppressalis on Bt rice and susceptibility monitoring of field population of C.suppressalis to CryAb insecticide protein were conducted and initial frequency of resistant alleles was determined.This study will provide importantly scientific data and theoretical basis for ecological biosafety assessment of Bt rice and formulation of precautionary resistance management strategy.The specific results are as follows:The survival of Cry1Ab-susceptible and resistant strains of C.suppressalis on different developmental stages of Bt rice were observed.The results showed that:?1?The expression levels of Bt insecticidal protein in the three transgenic rice lines were measured by ELISA.It was found the levels of Cry1Ab protein expressed in transgenic cry1Ab and cry1Ab+cry1C rice lines decreased significantly with the increase of developmental stages.However,Cry1C protein level expressed in transgenic cry1C and cry1Ab+cry1C lines first decreased and then increased,with the increase of growth stages;?2?The mortalities of Cry1Ab-susceptible strains on different growth stages of insect-resistant transgenic cry1Ab,cry1C and cry1C+cry1Ab rice lines were all 100%and the lethal times for C.suppressalis at the tillering of three rice lines were 2 d,3 d and 4 d;the elongation of three rice lines were 2 d,3 d and 5d;the booting of three rice lines were 2 d,3 d and 4 d,respectively;?3?The mortalities of Cry1Ab-resistant strain on three developmental stages of cry1C and cry1C+cry1Ab rice lines were100%,but the survival rate of Cry1Ab-resistant strain on cry1Ab rice line significantly increased,compared with other transgenic rice lines.Moreover,survival rate of resistant C.suppressalis on booting stage of cry1Ab rice line was significantly higher than that on tilling and elongation stages of cry1Ab line.These indicated that the Cry1Ab-resistant strain of C.suppressalis was tolerant to cry1Ab rice line.But it was also high susceptible to cry1C and cry1Ab+cry1C rice lines.Simultaneously,it was also showed that Cry1Ab-resistant strain was not cross-resistant to Cry1C protein.?4?There was a positive correlation between the expression level of Cry1Ab and mortality of C.suppressalis larvae,which indicated that the expression level of Cry1Ab protein expressed in transgenic cry1Ab rice determined the control efficacy against C.suppressalis.Susceptibility monitoring of field populations of C.suppressalis from Xingan,Guangxi Province and Fuzhou,Fujian Province,to Cry1Ab insecticidal protein was conducted,using F₂ screen mothod.A total of 172 isofemale families from Xingan population provided sufficient eggs for the F₂ screen and only three isofemale C.suppressalis?#108,#127 and#144?were confirmed to be resistant to Cry1Ab protein.The estimated initial frequency of resistant alleles was 0.0057.A total of 185 isofemale families from Fuzhou population were successfully bioassayed by F₂ screen.Three isofemale families?#4,#161and#179?were found to be resistant to Cry1Ab protein and the estimated initial frequency of resistant alleles was 0.0053.The initial frequencies of Cry1Ab resistant alleles in Xingan and Fuzhou were lower than that previously reported in other areas of China,which showed that the field population of C.suppressalis was still susceptible to Cry1Ab insecticidal protein.