Distribution Of Cry1Ac Protein In Tissues And Organs And Its Effect On The Nutritional Utilization Of Helicoverpa Armigera

The cotton bollworm,Helicoverpa armigera(Hiibner)is a major insect pest which can do great damage to many crops like cotton,maize,wheat and flowers and vegetables.Crystal(Cry)proteins derived from Bacillus thuringiensis(Bt)have been widely used in transgenic crops due to their toxicity against insect pests.However,the distribution and metabolism of these insecticidal proteins in insect tissues and organs have remained obscure because the target insects do not ingest much insecticidal proteins.In this study,several Cry1Ac-resistant strains of Helicoverpa armigera,fed artificial diets containing high doses of Cryl Ac protein,were used to investigate the distribution and metabolism of Cryl Ac in their bodies.Cryl Ac was only detected in larvae,not in pupae or adults.Furthermore,we found that the Cryl Ac protein could be detected in the hemolymph(0.28 ± 0.13 ng/g)of the 6th instar larvae when fed on an artificial diet containing more than 10.00 mg/L of Cry1Ac,and in the fat body(0.66±0.01 ng/g)when the artificial diets contained more than 30.00 mg/L of Cryl Ac,but the protein was not present in the integument of H.armigera.Metabolic tests revealed that CrylAc degraded most rapidly in the fat body,followed by the hemolymph,peritrophic membrane and its contents.The insecticidal protein was metabolized slowly in the midgut,but was degraded in all locations within 48 h.These findings will improve understanding of the functional mechanism of Bt proteins in target insects and the biotransfer and the bioaccumulation of Bt proteins in arthropod food webs in the Bt crop ecosystem.When Helicoverpa armigera feed on Bt protein,nutritional utilization are probably affected.Most researches investigated the influence of low doses of Bt protein on insect nutritional utilization,however,with the application of integrated pest management "high dose/refuge strategy",the insects might be confronted with high doses of Bt protein.To our knowledge,less studies has investigated the influence of high doses of Bt protein on insect nutritional utilization,even least studies on the nutritional utilization of Bt-resistant insects.In the current study,Bt-susceptible strain(LFS)and three Bt-resistant strains(LF5,LF60 and LF240)of Helicoverpa armigera,fed artificial diets containing four doses of Cryl Ac protein,were used to investigate the influence of Cry1Ac protein on insect development and nutritional utilization.We found similar nutritional indices was observed when Bt-susceptible strain LFS fed on artificial-diet containing low dose of Cry1Ac protein and pure diet containing non-CrylAc protein.Significantly lower consumption,frass,relative growth rate(RGR),efficiency of conversion of digested food(ECD),efficiency of ingested food(ECI),and significantly higher approximate digestibility(AD)were observed when LFS fed on medium and high doses of insecticidal protein.The results suggested that the doses of CrylAc protein influenced the development and nutritional utilization of Bt-susceptible strain significantly.The influence of CrylAc protein on LF5 was similar to LFS,but only a little effects were observed on LF60 an LF240 strains.Thus,we concluded that the resistance to Cry1Ac protein could decrease the impact of Cryl Ac on Helicoverpa armigera,and the influence decreased as the resistant ratio increased.In conclusion,the studies indicated that the influence of CrylAc protein on the development and nutritional utilization of Helicoverpa armigera could be quantified by the nutritional indices of herbivores exposed to different doses of Cry1Ac protein,thus provided data for the control of cotton bollworm.