Preliminary Studies On Biotype Differentiation And The Adaptive Mechanism For Sitobion Avenae From Gramineous Weeds

The English grain aphid,Sitobion avenae is a major pest of wheat crops worldwide,which seriously affects the yield and quality of wheat.Many insects can develop different biotypes on different host plants.S.avenae have been found to be differentiated to a certain extent on different barley(Hordeum vulgare L.)and wheat(Triticum aestivum L.)varieties,and can be divided into different biotypes according to their performance on different resistant hosts.In addition to wheat crops,it can also survive on a variety of gramineous weeds,but there are no reports about biotype differentiation and composition of S.avenae on gramineous weeds.In this respect,this study collected S.avenae clones from gramineous weeds in Qinling Mountains(Shaanxi Province)far away from contiguous growing areas of wheat,compared their performance differences on wheat and gramineous weed hosts(i.e.,ryegrass and brome),analyzed host-aphid interactions,and explored the adaptation and differentiation mechanisms of S.avenae biotypes from gramineous weeds.The main results are as follows:(1)From six genera(i.e.,Elymus,Bromus,Avena,Lolium,Achnatherum,and Poa)of gramineous weeds,we collected S.avenae samples,and identified seven different genotypes by using methods based on molecular markers of mitochondrial cytochrome oxidase subunit 1(COI gene)and microsatellites.Five biotypes were identified from the seven genotypes of S.avenae.Among them,three newly discovered biotypes were named as biotype7,biotype 8 and biotype 9,while biotype 1 and biotype 5 exist both on wheat crops and gramineous weed hosts.Based on the comparison of survival rate and 13-d fecundity,it was found that the five biotypes had significantly different adaptations to the above-mentioned different hosts,and the performance on wheat was better than that on gramineous weeds(ie,ryegrass and brome).Compared with other biotypes,biotype 8 has relatively higher adaptability on the two gramineous weeds.(2)After each biotype was reared on wheat and gramineous weed hosts(Lolium perenne L.and Bromusinermis Leyss.)for 13 days,the total phenolic content of the host plants and the expression of glutathione S-transferase genes(i.e.,Sa GST1,Sa GST2 and Sa GST3)of each biotype were determined.It was found that the total phenolic content of wheat Ai Kang 58 and ryegrass were significantly changed when eaten by different biotypes,while the total phenolic content of brome had no significant change.The effects on Sa GST gene expression in S.avenae biotypes on the three host plants induced by feeding were different,and there were significant differences in different regularities among different biotypes.Biotype 1 and biotype 9 had high expression of Sa GST(Sa GST1,Sa GST2 and Sa GST3)genes induced by wheat Ai Kang 58 feeding.The expression level of Sa GST2 genes of Biotype 7 and biotype 8induced by gramineous weeds was once more than that induced by wheat Aikang 58 feeding.In addition,the expression of Sa GST2 of these two biotypes induced by gramineous weeds was 100% more than that of biotype 1.Correlation analysis found that Sa GST2 gene expression levels of biotype 8 was significantly correlated with its fecundity,the high expression of Sa GST2 gene induced by feeding on gramineous weeds may be the key factor that makes biotype 8 more adaptable to ryegrass and brome than other biotypes.Thus,modulation of Sa GST gene expression can help S.avenae to counter secondary defenses of its weed hosts,and can be closely associated with its bioptye differentiation on gramineous weeds.To sum up,compared barley and wheat,the biotype numbers and compositions of S.avenae from gramineous weeds are significantly different.In this study,biotype 8 was found to show certain levels of adaptability on gramineous weeds.We have preliminarily identified the close relationship between the adaptability of S.avenae biotypes on gramineous weeds and Sa GST gene expression.Modulation of Sa GST gene expression may be also closely related to the biotype differentiation of this aphid.The research on the biotypes of S.avenae from gramineous weeds can help to understand the evolutionary potential of its host adaptability,and clarify its outbreak mechanisms,thereby providing a theoretical basis and reference for ecological control of of S.avenae.