DEAR EDITOR,Insertion sequences(ISs)are the simplest structural transposable elements(TEs)in prokaryotes,consisting only of a transposase coding sequence and its bilateral short terminal inverted repeats.Due to their gradually streamlined genomic construction,TEs rarely exist in the genomes of obligate endosymbionts.However,TE content,especially ISs,is abundant in the genome of Wolbachia bacteria,obligate endosymbionts widespread in arthropods and nematodes.

Herbivorous insects have evolved diverse mechanisms enabling them to feed on plants with suboptimal nutrient availability.Low nutrient availability negatively impacts insect herbivore development and fitness.To overcome this obstacle numerous insect lineages have evolved intimate associations with nutritional endosymbionts.This is especially true for insects that specialize on nitrogen-poor substrates,as these insects are highly dependent on intracellular symbionts to provide nitrogen lacking in their insect host's diet.Emerging evidence in these systems suggest that the symbiont's and/or the insect's biosynthetic pathways are dynamically regulated throughout the insect's development to potentially cope with the insect's changing nutritional demands.In this review,we evaluate the evolutionary development of symbiotic insect cells (bacteriocytes) by comparing and contrasting genes and mechanisms involved in maintaining and regulating the nutritional symbiosis throughout insect development in a diversity of insect herbivore-endosymbiont associations.With new advances in genome sequencing and functional genomics,we evaluate to what extent nutritional symbioses are shaped by (i) the regulation of symbiont titer,(ii) the regulation of insect symbiosis genes,and (iii) the regulation of symbiont genes.We discuss how important these mechanisms are for the biosynthesis of essential amino acids and vitamins across insect life stages in divergent insect-symbiont systems.We conclude by suggesting future directions of research to further elucidate the evolutionary development ofbacteriocytes and the impact of these nutritional symbioses on insect-plant interactions.

[目的]DNA甲基化是基因修饰的一种重要方式,主要可以形成5-甲基胞嘧啶(5mC)和6-甲基腺嘌呤(6mA)等.目前关于5mC的研究比较多,而关于6mA在真核生物中的研究则较少.沃尔巴克氏体Wolbachia是昆虫中最常见的共生菌之一,可通过多种方式操纵宿主生殖,其中最常见的就是引起精卵细胞质不亲和(cytoplasmic incompatibility,CI),即感染Wolbachia的雄性与未感染的雌性宿主交配后,胚胎致死,但其机制还不清楚.本研究拟从6mA甲基化的角度,探讨Wolbachia影响果蝇生殖的分子机制.[方法]以模式生物黑腹果蝇Drosophila melanogaster为材料,通过实时荧光定量PCR方法,检测Wolbachia感染对果蝇精巢、卵巢以及3个交配组[TT(对照,父母本都未感染),TW(父本未感染,母本感染,胚胎感染,可发育)和WT(CI,致死)]早期胚胎中DNA 6mA去甲基化酶基因DMAD的表达变化.[结果]Wolbachia感染可显著上调1日龄果蝇精巢中DMAD的表达水平,而对卵巢中DMAD的表达没有显著影响.在产卵后0.5h(中囊胚过渡前)的胚胎中,CI胚胎的DMAD表达水平极显著高于可正常发育的胚胎(TT和TW);在3h的胚胎(中囊胚过渡期)中,TW和CI胚胎中DMAD的表达量都极显著高于对照组;在6h的胚胎(中囊胚过渡后)中,CI胚胎中DMAD的表达量相对最低.[结论]Wolbachia感染可能通过干扰宿主果蝇精巢中6mA甲基化水平对精子产生修饰,导致其与正常未感染的卵子受精后胚胎致死.

Bacterial endosymbionts of sap-sucking insects provide their host with a number of beneficial qualities,including the supply of nutrition,defense against parasitoids,and protection from heat stress.Damage to these bacterial associates can therefore have a negative impact on the fitness of their insect host.We evaluated observational and experimental factors regarding the nonmative hemlock woolly adelgid (Adelges tsugae Annand)(Hemiptera:Adelgidae) to help understand the roles of its three recently identified symbionts,including under heat stress conditions.The prevalence of A.tsugae's facultative symbiont (Serratia symbiotica) was examined at different spatial scales to determine how variable infection rates are for this symbiont.There was no significant difference found in infection rates between adelgids on a tree,within a plot,or within a state.However,significantly more adelgids in Georgia (95％) had S.symbiotica compared to those in New York (68％).Microsatellite genotyping of the adelgids found that this difference was most likely not the result of a second introduction ofA.tsugae into eastern North America.Comparison ofS.symbiotica proportions between first and fourth instars showed that symbiont absence did not affect the ability ofA.tsugae to survive aestivation.Evaluations of symbiont densities within each adelgid found that when S.symbiotica was absent,the density of obligate symbionts was significantly higher.Exposure to heat stress (32.5 ℃) was not consistently correlated with changes in symbiont densities over a 4-d period.Overall,we have shown that symbiont prevalence and densities vary within the broad population of A.tsugae in eastern North America,with potentially significant effects upon the ecology of this important pest.

The endosymbionts play vital roles in growth,development and reproduction in insects.Yeast-like endosymbionts (YLSs) have been well studied in Nilaparvata lugens (N.lugens),but little is known about the tissue-specific bacterial microbiomes,especially on the microbial intersection among internal tissues.Here,the correlation of microbial composition,structure,dispersal ability and functional profiling were illuminated in two tissues,the fat body and ovary in N.lugens.A total of 11 phyla and 105 genera were captured from all samples;Firmicutes and Proteobacteria were the most predominant and accounted for more than 99％ in all samples.However,the relative abundance of Firmicutes and Proteobacteria was significantly different in ovary and fat body through Fisher's Least Significant Difference test.Microbial diversity but not the richness index in the two tissues exhibited significant difference.Furthermore,the microbial community structure of the ovary and fat body were primarily determined by tissue quality.Firmicutes showed strong dispersal ability between ovary and fat body based on the quantitative null model assessing,indicating the frequent interaction of these microbiomes in the two tissues.In addition,the Kyoto Encyclopedia of Genes and Genomes pathways of microbial participation were delineated.The ten most abundant pathways counted for over 46％ of the annotation and were shared between the two tissues,mainly containing Energy Metabolism and Amino Acid Metabolism/Biosynthesis.The results will provide insights into the correlation of microbial community structttre between ovary and fat body ofN.lugens.

Wolbachia can profoundly influence the survival,reproduction,and defenses of insect hosts.These interactions could potentially be harnessed for managing pests or insecttransmitted diseases.Diaphorina citri Kuwayama is a phloem-feeding pest capable of transmitting the putative causal agent of citrus greening,Candidatus Liberibacter asiaticus (CLas).Like many insects,D.citri is also infected with Wolbachia (wDi).Recent studies indicate that the relative abundance of wDi could be associated with the abundance of CLas,and that wDi may contribute to regulating expression of phage lytic cycle genes in CLas,suggesting the need for better understanding of wDi biology in general.This study investigated the genetic diversity of wDi among D.citri in populations spanning eleven countries and two U.S.territories.Six Wolbachia genes,wsp,coxA,fbpA,ftsZ,gatB,and hcpA,were sequenced and compared across samples.Two prevalent wDi strains were identified across the samples,and screening of clone libraries revealed possible coinfection of wDi strains in specific populations.D.cirri mitochondrial cytochrome oxidase subunit Ⅰ gene (mtCOI) were more divergent between D.citri populations that were infected with different wDi strains or had different infection statuses (single infection vs.coinfection).While we could not eliminate the possibility that maternal transmission may contribute to such patterns,it is also possible that wDi may induce cytoplasmic incompatibility in their host.These findings should contribute to the understanding of wDi population ecology,which may facilitate manipulation of this endosymbiont for management of citrus greening disease worldwide.

Bacterial endosymbionts have enabled aphids to adapt to a range of stressors,but their effects in many aphid species remain to be established.The bird cherry-oat aphid,Rhopalosiphum padi (Linnaeus),is an important pest of cereals worldwide and has been reported to form symbiotic associations with Serratia symbiotica and Sitobion miscanthi L-type symbiont endobacteria,although the resulting aphid phenotype has not been described.This study presents the first report ofR.padi infection with the facultative bacterial endosymbiont Hamiltonella defensa.Individuals of R.padi were sampled from populations in Eastern Scotland,UK,and shown to represent seven R.padi genotypes based on the size ofpolymorphic microsatellite markers;two of these genotypes harbored H.defensa.In parasitism assays,survival ofH.defensa-infected nymphs following attack by the parasitoid wasp Aphidius colemani (Viereck) was 5 fold higher than for uninfected nymphs.Aphid genotype was a major determinant of aphid performance on two Hordeum species,a modern cultivar of barley H.vulgare and a wild relative H.spontaneum,although aphids infected with H.defensa showed 16％ lower nymph mass gain on the partially resistant wild relative compared with uninfected individuals.These findings suggest that deploying resistance traits in barley will favor the fittest R.padi genotypes,but symbiontinfected individuals will be favored when parasitoids are abundant,although these aphids will not achieve optimal performance on a poor quality host plant.

Bacterial endosymbionts play important roles in ecological traits of aphids.In this study,we characterize the bacterial endosymbionts ofA.gossypii collected in Karaj,Iran and their role in the performance of the aphid.Our results indicated that beside Buchnera aphidicola,A.gossypii,also harbors both Hamiltonella defensa and Arsenophonus sp.Quantitative PCR (qPCR) results revealed that the populations of the endosymbionts increased throughout nymphal development up to adult emergence;thereafter,populations of Buchnera and Arsenophonus were diminished while the density ofH.defensa constantly increased.Buchnera reduction caused prolonged development and no progeny production.Furthermore,secondary symbiont reduction led to reduction of the total life span and intrinsic rate of natural increase as well as appearance of the deformed dead offspring in comparison with the control insects.Reduction of the secondary symbionts did not affect parasitism rate of the aphid by the parasitic wasp Aphidius matricariae.Together these findings showed that H.defensa and Arsenophonus contributed to the fitness ofA.gossypii by enhancing its performance,but not through parasitoid resistance.

Immunofluorescence has been widely used to localize microbes or specific molecules in insect tissues or cells.However,significant autofluorescence is frequently observed in tissues which can interfere with the fluorescent identification of target antigens,leading to inaccurate or even false positive fluorescent labeling.The alimentary canal of the potato psyllid,Bactericera cockerelli (S)ulc,exhibits intense autofluorescence,hindering the application of immunolocalization for the detection and localization of the economically important pathogen transmitted by this insect,"Candidatus Liberibacter solanacearum" (Lso).In the present study,we tested the use of irradiation,hydrogen peroxide (H2O2) and Sudan black B (SBB) treatments to reduce the autofluorescence in the B.cockerelli alimentary canal tissues.Furthermore,we assessed the compatibility of the above-mentioned treatments with Lso immunolocalization and actin staining using phalloidin.Our results showed that the autofluorescence in the alimentary canal was reduced by irradiation,H2O2,or SBB treatments.The compatibility assays indicated that irradiation and H2O2 treatment both greatly reduced the fluorescent signal associated with Lso and actin.However,the SBB incubation preserved those target signals,while efficiently eliminating autofluorescence in the psyllid alimentary canal.Therefore,herein we propose a robust method for reducing the autofluorescence in the B.cockerelli alimentary canal with SBB treatment,which may improve the use of immunofluorescence labeling in this organism.This method may also have a wide range of uses by reducing the autofluorescence in other arthropod species.

内共生菌(Endosymbionts)与其昆虫宿主的生物学特性联系非常密切.近年来,昆虫内共生菌水平传播的途径及机制已经成为昆虫学研究的热点之一.越来越多的研究表明,亲缘关系相距甚远的昆虫可以感染有相同或相似的内共生菌,说明昆虫内共生菌的水平传播在自然界中普遍存在.植物介导的昆虫内共生菌水平传播便是其中的一条途径,即同种或不同种类的昆虫可以通过取食,获得供体昆虫传入植物组织中的内共生菌,形成内共生菌从供体昆虫-寄主植物-受体昆虫传播的路径.本文主要以植物介导的昆虫内共生菌水平传播途径为对象,介绍了昆虫内共生菌的水平传播途径,以及内共生菌传入后对新宿主生物学、生态学特性的影响,以期为昆虫内共生菌水平领域的研究提供更多有价值的参考.