| Endosymbionts are widely distributed in insects.They can improve the multiply and spread of insects by manipulating the reproduction of hosts,participating in nutrient metabolism,and improving resistance of hosts to poor environment.Endosymbionts can expand their host range through vertical and horizontal transmission.Studying the relationship between insects and endosymbionts can help us understand the mechanisms of insect development,as well as the spread of endosymbionts.Bemisia tabaci is a worldwide agricultural pest.Due to its strong invasiveness,the development of B.tabaci in China is rapidly,which brings significant losses to agricultural production,especially greenhouse cultivation.B.tabaci harbor many endosymbionts,which are play important roles in improving the spread of B.tabaci.However,the infection status and role of endosymbionts are different in B.tabaci biotypes.Whiteflies can be parasitized by a variety of wasps in the fields.And wasps also harbor endosymbionts.There may have the horizontal transmission of endosymbionts between wasps and whitefly.Although phylogenetic analysis shows that horizontal transmission of endosymbionts was very common between parasitoids and hosts,this phenomenon was not easily observed in natural.And given the potential of endosymbionts in the biological control of B.tabaci,it is necessary to understand the mechanism of horizontal transmission in wild environment and the influence of endosymbionts obtained by horizontal transmission on the fitness of B.tabaci.This paper uses field collection and laboratory tests to explore the horizontal transmission of endosymbionts and their impacts on the host.Weexamined endosymbionts infection frequency of B.tabaci and parasitoids by sampling from three region of Jiangsu Province.Factors affecting the infection frequency of symbionts,and the phylogenetic of the endosymbionts were also analysed.Radiation treatment was used to achieve the horizontal transmission of endosymbionts between B.tabaci and parasitoids.Through the establishment of infected and uninfected lines,the effects of endosymbionts on the fitness of B.tabaci and their role in population spreading were also studied.Results are summarized as follows:1.Infection status and horizontal transmission of endosymbionts in B.tabaci and parasitoid from fieldsWhitefly nymphs were collected from three different sample times and three common host plants in three regions of Jiangsu Province.Total 1,350 whiteflies and 36 wasps emerged from the nymph were investigated.Results showed that all B.tabaci were belong to MED biotype,and four secondary endosymbionts were detected,they are Hamiltonella,Cardinium,Wolbachia and Rickettsia respectively.Among the four endosymbionts,the infection frequency of Hamiltonella was 100%in all B.tabaci populations,and the infection frequency of other endosymbionts was varied among different populations.Wasps were identified as the genus Encarsia and Eretmocerus.Only Wolbachia and Rickettsia were detected in the parasitoids.The infection status of B.tabaci was mainly affected by geographical location and sampling time.Affecting factors are different between endosymbionts.There are also found obvious interactions between different factors.Phylogenetic analysis of the endosymbionts detected in B.tabaci and parasitoids revealed that,bacteria detected in B.tabaci were grouped into one clade on the phylogenetic tree except for Wolbachia,shows a consistent phylogenetic relationship.And Wolbachia was grouped into different clade,there are multiple strains of Wolbachia in different populations of B.tabaci,even within the same population.It was also found that some of the Wolbachia and Rickettsia strains detected from B.tabaci and parasitoids were grouped together.The haplotype analysis of Wolbachia and Rickettsia found that both B.tabaci and parasitoids have unique and shared haplotypes,indicating that horizontal transmission of Wolbachia and Rickettsia between B.tabaci and their parasitoids have occured in the fields.2.Horizontal transmission of Wolbachia between B.tabaci and E.formosa in semi-natural environmentFields experiment found that Wolbachia was transmitted from E.formosa to B.tabaci,and the proportion of females in whitefly population decreased after infected with Wolbachia.But the occurrence of horizontal transmission in natural is very low.The alignment and phylogenetic analysis of the Wolbachia wsp gene sequence revealed that the Wolbachia strain infected by B.tabaci and E.formosa was very similar.Which indicates that horizontal transmission of Wolbachia was occurred between B.tabaci and E.formosa.3.Horizontal transmission of Wolbachia was promoted by radiation treatmentIn order to explore the horizontal transmission mechanism,E.formosa was treated by radiation and the effects of radiation on the Wolbachia titre and parasitic were also observed.Results showed there had serious toxic effects on the physiological and biochemical characteristics of E.formosa after radiation,especially for the damage to reproductive system.The shape of ovary became smaller and eggs melted.The ability of parasitic was significantly reduced in E.formosa after radiation,and the Wolbachia titer was also decreased.However,since the viability of the egg was decreased,the successful parasitic rate reduced.Nymph of whitefly can not be killed after being parasitized,and the acquisition of Wolbachia by B.tabaci significantly increased.The same Wolbachia strains were found in B.tabaci and E.formosa by MLST analysis.Wolbachia was found in eggs produced by E.formosa which located in nymph of whitefly by using FISH methods.Wolbachia was also found in whitefly adults.Our results show that the horizontal transmission of Wolbachia from E.formosa to B.tabaci has successfully achieved by radiation treatment.4.Vertical transmission of Wolbachia and its effects on fitness of B.tabaciTo establish a successful horizontal transmission,endosymbionts must be transmitted vertically in a new host.Continuous detection of Wolbachia in newly infected B.tabaci found that the vertical transmission efficiency of Wolbachia keeping low(20%-30%)in the generation of B.tabaci.But the vertical transmission efficiency showed a slight increase with the development of generations.Compared with Wolbachia uninfected B.tabaci,the number of offspring of infected B.tabaci was less,with a reduced hatching rate of eggs and increased mortality before emergence,and the proportion of females in the offspring also lower,but the development time between two populations have no difference.The data of different cross type showed that the affects of Wolbachia on B.tabaci was mainly reflected in females.Our study showed that the vertical transmission rate of Wolbachia was low in the new host,and also has an unfavorable influence on B.tabaci.5.Spreading of Rickettsia in B.tabaci and its effect on host fitnessAfter bringing back to the laboratory for cultivation from fields,it was found that the infection frequency of Rickettsia increased with time in whitefly.To explore the impact of Rickettsia on the fitness of B.tabaci,the Rickettsia infected and uninfected B.tabaci lines were established in the lab,and the influence of Rickettsia on the fitness of B.tabaci was also observed by setting different cross types of whitefly.Results showed that the infection of Rickettsia in whitefly brings reduced fecundity and increased developmental time.But there were no differences in the hatching rate of eggs and survival rate before adulthood.The number of male offspring produced by Rickettsia-infected whitefly was significantly lower,but there was no significant difference in the number of female offsprings.The vertical transmission rate of Rickettsia from mother to their offspring reached 100%.Quantitative PCR also showed that the titer of Rickettsia increased first and then falls.Our results showed that Rickettsia can be established and stabilized in the whitefly population depending on the perfect vertical transmission rate.However,due to the reproductive disadvantage in infected B.tabaci,Rickettsia could not achieve complete infection in the B.tabaci population. |
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