Detection Of Secondary Symbiotic Bacteria And Effects Of Three Secondary Endosymbionts On Adaptation Of Sitobion Avenae Under Different Temperatures

Sitobion avenae(Fabricius)is an important agricultural pest with a world-wide distribution.Through feeding directly on the plant phloem sap and spreading plant viruses,it can cause serious damage to wheat production.Symbiotic bacteria are widely present in various insects,and in the long-term evolution process,they form a close symbiotic relationship with the host.At present,the research on the diversity of symbiotic bacteria and the influence of secondary symbiotic bacteria on the ecological adaptability of S.avenae under different temperature conditions are scarce.In this paper,the species and infection rates of secondary symbiotic bacteria of S.avenae from 8 different geographical populations in China were determined by PCR,and three secondary symbiotic bacteria with high infection frequencies were selected for further studies.The effects of these three secondary symbiotic bacteria on the adaptability of S.avenae were evaluated under different temperatures using bioassays and molecular means.The main results are as follows:1.Six secondary symbiotic bacteria(256 samples)of 8 different geographical populations of S.avenae in China were detected by diagnostic PCR.The results showed that the secondary symbiotic bacteria H.defensa,R.insecticola and Rickettsia were detected in these 8 geographical populations,but the infection rates of S.avenae were different among different geographical populations.The infection rates of H.defensa in Hami,Zhangye,Yangling or Jiashan were significantly higher than those in Xiangyang,Zhumadian,Huaibei or Yancheng.The infection rate of R.insecticola in Zhangye was significantly higher than that in Hami and Zhumadian.The infection rate of Rickettsia in Yangling and Yancheng was significantly lower than that the other six regions.The secondary symbiotic bacteria Spiroplasma was detected only in Yangling,Xiangyang and Jiashan,and the infection rate was low,the infection rate in Yangling area(17%)was higher than that in the other two areas.However,the secondary commensal bacteria R.insecticola and PAXS were not detected in all of the individuals.Secondary symbiotic bacteria are co-infected in all regions,the combination of H.defensa and Rickettsia is most common in Jiashan,whereas the combination of H.defensa,R.insecticola and Rickettsia is most common in Zhangye.However,three species of co-infection were not detected in Zhumadian and Yancheng.2.The effects of three secondary symbiotic bacteria(H.defensa,R.insecticola and Rickettsia)on the life history parameters of S.avenae were determined at different temperatures.Under the condition of feeding at 14 °C,H.defensa significantly prolonged the nymph development period of each instar and significantly reduced the adult weight and fecundity of S.avenae;while at 26 °C,H.defensa significantly enhanced the fecundity of S.avenae,which is consistent with the performance at 30 °C.The results indicated that under low temperatures,H.defensa has a negative effect on S.avenae;but it could help S.avenae to cope with temperature stress in a certain range of high temperatures.Under all temperature conditions,the secondary symbiotic bacteria R.insecticola not only prolonged the development period of nymph,but also reduced the weight and fecundity of the adult aphid,suggesting that carrying R.insecticola is a burden on the aphid.Under the condition of feeding at 14 °C,the secondary symbiotic bacteria Rickettsia significantly shortened the nymphal developmental duration of the larvae;at 26 °C,Rickettsia increased the adult weight and fecundity of S.avenae;under 30 °C,the weight and fertility of S.avenae carrying Rickettsia decreased,which indicated that 30 °C might exceed the physiological tolerance temperature range for the mutually beneficial symbiosis between Rickettsia and S.avenae,such that growth and reproduction of S.avenae was negatively affected under this temperature.3.The copy numbers of primary and secondary symbiotic bacteria in S.avenae were determined by quantitative methods,and then the changes of the content of symbiotic bacteria at different temperatures were compared.The results showed that there were some differences in the content of symbiotic bacteria in S.avenae at different temperatures.Compared with 22 °C,the content of secondary symbiotic bacteria H.defensa was significantly increased at 14 °C;the content of primary symbiotic bacteria B.aphidicola was not significantly different between 14 °C and 22 °C,However,at 26 °C,the content of B.aphidicola was significantly higher than 22 °C.Compared with 22 °C,high temperature(26 °C and 30 °C)or low temperature(14 °C),the content of secondary symbiotic bacteria R.insecticola increased,at 30 °C,the copy number of R.insecticola is higher than 26 °C;the content of primary symbiotic bacteria B.aphidicola at 26 °C or 30 °C is lower than 22 °C,but the difference did not reach a significant level.The content of secondary symbiotic bacteria Rickettsia was the highest at 22 °C,and it decreased with the increase of temperatures;at 14 °C,the content of primary symbiotic bacteria was significantly higher than at other temperatures,and it gradually decreased with increasing temperatures.Therefore,changes in symbiotic bacteria contents may affect the temperature adaptability of S.avenae.