| As an important pest in wheat crops,studying the resistance of wheat varieties to the English grain aphid,Sitobion avenae(Fabricius),is particularly important.In addition to searching for potential insect resistant genes in plants and studying the mechanisms by which S.avenae adapt to different resistant varieties.Based on the identification and research of different S.avenae biotypes on different regions and barley/wheat in the early stage of the laboratory,in order to further explore the role of host factors in biotype maintenance and evolution,this experiment collected biotypes of S.avenae clones from graminaceous weeds as sources,identified biotypes using the aphid ratio method,induced biotype transformation by switching hosts,and determined the fitness of different biotypes on various cultivars.After studying the effects of secondary metabolite changes on biotype fitness and gene expression,a preliminary exploration of the transformation mechanism of S.avenae biotypes was conducted,and the main results are as follows:Individuals of S.avenae on different grass hosts were collected and genetically identified using microsatellite markers.The results showed that the genetic backgrounds of these individuals were different and could be divided into eight different genotypes.At the same time,these S.avenae individuals were identified for biotypes using the ratio of aphid counts.Five different biotypes were identified,including biotypes10 and 11.biotypes10 and 11 were identified for the first time.The difference between biotypes10 and 11 was reflected in their different resistance to barley cultivar "Zaoshu No.3",while the difference between biotypes2 and 5 was reflected in their different resistance to barley cultivar "Xiyin No.2".By analyzing the life history parameters of four different biotypes(2,5,10,11),it was found that their life history parameters on different cultivars were consistent with the biotypes results.For example,compared with biotype2,biotype5 had a shorter nymphal developmental duration,greater adult weight,and higher 10-d fecundity on the Xiyin No.2 barley cultivar,indicating that biotype5 had a higher host suitability for Xiyin No.2 barley cultivar.Compared with biotype11,biotype10 had no significant difference in the nymphal developmental duration on Zaoshu No.3 barley cultivar,but had a larger adult weight and significantly increased 10-d fecundity,indicating that biotype10 had a higher host suitability for Zaoshu No.3 barley cultivar.Through continuous multi-generation rearing on a particular host,biotype2 and biotype11 were induced to undergo biotype conversion.The results showed that biotype11 successfully transformed into biotype3 after 7 generations of induction,but biotype2 did not undergo biotype conversion after induction.Life history parameters were statistically analyzed for biotype11 and biotype2 in different generations during the conversion process.It was found that biotype11 showed significantly increased in 10-d fecundity,adult weight on the particular host,Zaoshu No.3,before and after conversion.On the other hand,biotype2 showed a decrease in 10-d fecundity and an extension of the nymphal developmental duration when reared on the particular host,barley Xiyin No.2,for multiple generations.Therefore,biotype11 did not reduce host suitability when reared for multiple generations on the Zaoshu No.3(resistant),whereas biotype2 reduced host suitability when reared for multiple generations on the Xiyin No.2(resistant).This indicates that biotype11 overcame host resistance during continuous multi-generation rearing,with a increase in host suitability and biotype conversion occurred.However,biotype2 did not overcome host resistance,with a decrease in host suitability and no biotype conversion occurred.By reintroducing the converted biotype11 back to its original host for experiments,it was found that biotype11 can revert back to its original biotype after9 continuous generations of reintroduction.This suggests that the biotype conversion of the S.avenae may be influenced by plasticity regulation.After comparing the contents of secondary metabolites,such as gramine and gallic acid,in the host plants,it was found that the overall concentration of these two secondary metabolites was highest in Zaoshu No.3 and lowest in Dongmu No.70.Subsequently,two different concentrations of secondary metabolites were added to the artificial diet to determine the fecundity and survival time of different biotypes.The transformed biotype11 had a longer survival time,higher fecundity,and higher fitness compared to the non-transformed biotype11 at the 9.35 m M gallic acid concentration and 6.34 m M gramine concentration.Compared to the untransformed biotype11,the transformed biotype11 is more similar to the untransformed biotype3 in terms of adaptability to secondary metabolites.Therefore,it can be inferred that adaptation to secondary metabolites may be an important change in the process of biotype11 transformation.Subsequently,using fluorescence quantitative PCR to determine the expression of Sa GST genes under different conditions,it was found that 1)the gene expression of Sa GST1 and Sa GST2 in biotype11 fed with artificial diets supplemented was significantly increased at the concentration of 9.35 m M gallic acid and 6.34 m M gramine artificial diet compared to the concentration of 2.57 m M gallic acid and 4.82 m M gramine artificial diet;2)During the conversion process,Sa GST1 increased slightly but not significantly,while Sa GST2 increased significantly with increasing generations.In summary,a distinct biotype was discovered in S.avenae originating from grass hosts,which could gradually overcome host resistance during the process of switching hosts and cause a change in biotype.Secondary metabolites of the hosts played an important role in this process by altering the fitness of the biotype,and S.avenae responded to the increase in secondary metabolite concentration by upregulating the detoxifying enzyme gene Sa GST2 during biotype transition.Furthermore,this study found that after inducing transition for seven generations,biotype11 of the S.avenae could undergo a change,but the change was reversible,which suggests that the evolution of wheat aphid biotypes may be influenced by certain phenotypic plasticity changes. |
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