多分DNA病毒及其在寄生蜂与寄主关系中的作用

分DNA病毒(PDV)在协调寄生蜂与寄主的相互关系中起重要作用。该文就PDV 的特征、PDV与寄生蜂的相互关系、PDV对鳞翅目寄主的免疫抑制及发育调节等方面进行综述。     

Origin and evolution of polydnaviruses by symbiogenesis of insect DNA viruses in endoparasitic wasps

During oviposition, many endoparasitic wasps inject virus-like particles into their insect hosts that enable these parasitoids to evade or directly suppress their hosts' immune system, especially encapsulation by hemocytes. These particles are defined as virions that belong to viruses of the two genera that comprise the family Polydnaviridae, bracoviruses (genus Bracovirus) transmitted by braconid wasps, and ichnoviruses (genus Ichnovirus) transmitted by ichneumonid wasps. Structurally, bracovirus virions resemble nudivirus and baculovirus virions (family Baculoviridae), and ichnovirus virions resemble those of ascoviruses (family Ascoviridae). Whereas nudiviruses, baculoviruses and ascoviruses replicate their DNA and produce progeny virions, polydnavirus DNA is integrated into and replicated from the wasp genome, which also directs virion synthesis. The structural similarity of polydnavirus virions to those of viruses that attack the wasps' lepidopteran hosts, along with polydnavirus transmission and replication biology, suggest that these viruses evolved from insect DNA viruses by symbiogenesis, the same process by which mitochondia and chloroplasts evolved from bacteria. Molecular evidence supporting this hypothesis comes from similarities among structural proteins of ascoviruses and the Campoletis sonorensis ichnovirus. Implications of this hypothesis are that polydnaviruses evolved from viruses, but are no longer viruses, and that DNA packaged into polydnavirus virions is not viral genomic DNA per se, but rather wasp genomic DNA consisting primarily of wasp genes and non-coding DNA. Thus, we suggest that a better understanding of polydnaviruses would result by viewing these not as viruses, but rather as a wasp organelle system that evolved to shuttle wasp genes and proteins into hosts to evade and suppress their immune response.     

Polydnavirus-mediated suppression of insect immunity

Polydnaviruses are symbiotic proviruses of some ichneumonid and braconid wasps that modify the physiology, growth and development of host lepidopteran larvae. Polydnavirus infection targets neuroendocrine and immune systems, altering behavior, stunting growth, and immobilizing immune responses to wasp eggs and larvae. Polydnavirus-mediated disruption of cellular and humoral immunity renders parasitized lepidopteran larvae suitable for development of wasp larvae as well as more susceptible to opportunistic infections. Evidence from the Campoletis sonorensis polydnavirus system indicates that the unique genomic organization of polydnaviruses may have evolved to amplify the synthesis of immunosuppressive viral proteins. Immunosuppressive viruses have been essential to elucidating vertebrate immunity. Polydnaviruses have similar potential to clarify insect immune responses and may also provide novel insights into the role of insect immunity in shaping polydnavirus genomes.     

Influence of the parasitoid Chelonus inanitus and its polydnavirus on host nutritional physiology and implications for parasitoid development

Chelonus inanitus is a solitary egg-larval endoparasitoid, which feeds on host haemolymph during its internal phase. Parasitization induces in the host Spodoptera littoralis a precocious onset of metamorphosis and a developmental arrest in the prepupal stage. At this stage the parasitoid larva emerges from the host and consumes it. We show here that parasitization and the co-injected polydnaviruses affect the nutritional physiology of the host mainly in the last larval instar. Polydnaviruses cause a reduced uptake of food and an increase in the concentration of free sugars in the haemolymph and of glycogen in whole body. The parasitoid larva, along with polydnaviruses, causes a reduction of proteins in the host's plasma and an accumulation of lipids in whole body. Dilution of host haemolymph led to a reduced concentration of lipid in parasitoid larvae and a reduced survival rate. Thus, a sufficient concentration of nutrients in the host's haemolymph appears to be crucial for successful parasitoid development. Altogether, the data show that the parasitoid and the polydnavirus differentially influence host nutritional physiology and that the accumulated lipids and glycogen are taken up by the parasitoid in its haematophagous stage as well as through the subsequent external host feeding.     

Virulence strategies in parasitoid Hymenoptera as an example of adaptive diversity

Parasitoids are mostly insects that develop at the expense of other arthropods, which will die as a result of the interaction. Their reproductive success thus totally depends on their ability to successfully infest their host whose reproductive success relies on its own ability to avoid or overcome parasitism. Such intense selective pressures have resulted in extremely diverse adaptations in parasitoid strategies that ensure parasitism success. For instance, wasp-specific viruses (polydnaviruses) are injected into the host by parasitoid females to modulate its physiology and immunity. This article synthesizes available physiological and molecular data on parasitoid virulence strategies and discusses the evolutionary processes at work. To cite this article: M. Poirié et al., C. R. Biologies 332 (2009).     

Variation in encapsulation sensitivity of Cotesia sesamiae biotypes to Busseola fusca

Many braconid wasp species inject polydnaviruses to overcome their host's immune system. In the species Cotesia sesamiae, two biotypes exist that differ in their ability to develop in the host Busseola fusca. The biotype from coastal Kenya is infected with Wolbachia and is not able to develop in larvae of B. fusca, whereas the uninfected inland biotype of this wasp can develop in B. fusca. The genetic transmission of the developmental ability was studied through a series of genetic crosses and superparasitization experiments. The Wolbachia infection of the coastal type did not play a role in the encapsulation response of the host. Experiments show that the polydnaviruses of the wasps could not prevent the encapsulation of the coastal parasitoid eggs. Most likely, larval characteristics such as surface proteins played a more important role in the encapsulation response of the host even in the presence of a functional polydnavirus.     

Variation in encapsulation sensitivity of Cotesia sesamiae biotypes to Busseola fusca

Many braconid wasp species inject polydnaviruses to overcome their host's immune system. In the species Cotesia sesamiae, two biotypes exist that differ in their ability to develop in the host Busseola fusca. The biotype from coastal Kenya is infected with Wolbachia and is not able to develop in larvae of B. fusca, whereas the uninfected inland biotype of this wasp can develop in B. fusca. The genetic transmission of the developmental ability was studied through a series of genetic crosses and superparasitization experiments. The Wolbachia infection of the coastal type did not play a role in the encapsulation response of the host. Experiments show that the polydnaviruses of the wasps could not prevent the encapsulation of the coastal parasitoid eggs. Most likely, larval characteristics such as surface proteins played a more important role in the encapsulation response of the host even in the presence of a functional polydnavirus.     

Characterization of a novel protein with homology to C-type lectins expressed by the Cotesia rubecula bracovirus in larvae of the lepidopteran host,Pieris rapae

Polydnaviruses are essential for the survival of many Ichneumonoid endoparasitoids, providing active immune suppression of the host in which parasitoid larvae develop. The Cotesia rubecula bracovirus is unique among polydnaviruses in that only four major genes are detected in parasitized host (Pieris rapae) tissues, and gene expression is transient. Here we describe a novel C. rubecula bracovirus gene (CrV3) encoding a lectin monomer composed of 159 amino acids, which has conserved residues consistent with invertebrate and mammalian C-type lectins. Bacterially expressed CrV3 agglutinated sheep red blood cells in a divalent ion-dependent but Ca2+-independent manner. Agglutination was inhibited by EDTA but not by biological concentrations of any saccharides tested. Two monomers of approximately 14 and approximately 17 kDa in size were identified on SDS-PAGE in parasitized P. rapae larvae. The 17-kDa monomer was found to be an N-glyscosylated form of the 14-kDa monomer. CrV3 is produced in infected hemocytes and fat body cells and subsequently secreted into hemolymph. We propose that CrV3 is a novel lectin, the first characterized from an invertebrate virus. CrV3 shows over 60% homology with hypothetical proteins isolated from polydnaviruses in two other Cotesia wasps, indicating that these proteins may also be C-type lectins and that a novel polydnavirus lectin family exists in Cotesia-associated bracoviruses. CrV3 is probably interacting with components in host hemolymph, resulting in suppression of the Pieris immune response. The high similarity of CrV3 with invertebrate lectins, as opposed to those from viruses, may indicate that some bracovirus functions were acquired from their hosts.     

Parasitoids, polydnaviruses and endosymbiosis

Symbiotic associations traditionally have been treated as evolutionary curios rather than as a major source of evolutionary innovation. Recent research on a wide variety of organisms is changing this view and is breaking down the barriers between the traditional categories of parasitism, commensalism and mutualism, to produce a more flexible view of multispecific interactions. An especially abundant, but little discussed, mutualism exists between parasitoid wasps in the superfamily Ichneumonoidea and a novel form of DNA viruses known as polydnaviruses. Mutualisms between viruses and eukaryotes are not often reported, although as many as 100 000 species of organisms may exhibit this unusual association. In this review Jim Whitfield considers what is known about the parasitoid-polydnavirus relationship and how (and from what) it might have arisen.     

Characterization of a venom gland-associated rhabdovirus in the parasitoid wasp Diachasmimorpha longicaudata

Parasitoid wasps reproduce by laying their eggs on or inside of a host insect, which triggers a defense response in the host insect that kills the developing wasp. To counteract the host’s lethal response, some parasitoid wasps are associated with symbiotic viruses that alter host metabolism and development to promote successful development of the wasp embryo. These symbiotic viruses display a number of characteristics that differ from those of pathogenic viruses, but are poorly understood with the exception of one group, the polydnaviruses. Here, we characterize the genome of a non-polydnavirus associated with parasitoid wasps, Diachasmimorpha longicaudata rhabdovirus (DlRhV), and assess its role as a potential mutualistic virus. Our results show that the DlRhV genome contains six open reading frames (ORFs). Three ORFs show sequence homology to known viral genes and one ORF encodes a previously identified protein, called parasitism-specific protein 24 (PSP24), that has been hypothesized to play a role in promoting successful parasitism by D. longicaudata. We constructed a phylogeny that shows that DlRhV is most closely related to other insect-infecting rhabdoviruses. Finally, we report that DlRhV infection does not occur in all populations of D. longicaudata, and is not required for successful parasitism.     

Parasitic wasp‐associated symbiont affects plant‐mediated species interactions between herbivores

Microbial mutualistic symbiosis is increasingly recognised as a hidden driving force in the ecology of plant–insect interactions. Although plant‐associated and herbivore‐associated symbionts clearly affect interactions between plants and herbivores, the effects of symbionts associated with higher trophic levels has been largely overlooked. At the third‐trophic level, parasitic wasps are a common group of insects that can inject symbiotic viruses (polydnaviruses) and venom into their herbivorous hosts to support parasitoid offspring development. Here, we show that such third‐trophic level symbionts act in combination with venom to affect plant‐mediated interactions by reducing colonisation of subsequent herbivore species. This ecological effect correlated with changes induced by polydnaviruses and venom in caterpillar salivary glands and in plant defence responses to herbivory. Because thousands of parasitoid species are associated with mutualistic symbiotic viruses in an intimate, specific relationship, our findings may represent a novel and widespread ecological phenomenon in plant–insect interactions.     

寄生蜂多分DNA病毒的基因表达产物及对寄主昆虫的生理作用

最近有关由多分DNA病毒介导的寄生蜂与寄主关系的研究主要集中在多分DNA病毒基因的表达和伴随寄主生理功能失常的分子机理上 ,本文介绍在寄生蜂作用于寄主时一些重要的PDV基因表达产物 ,包括CsPDV ,MdPDV ,CcPDV ,TrPDV ,HdPDV ,CrPDV和其它PDV的基因表达产物 ,并简叙这些基因表达产物对寄主的生理影响。     

Polydnaviruses: potent mediators of host insect immune dysfunction

Endoparasitic insects are used as biological control agents to kill many species of insect pest. One key to the success of parasitoids that develop in the hemocoel of their host is their ability to knock out the host's immune system, inducing a decline in the responsiveness of a variety of cellular and humoral components so that parasitoid eggs are not encapsulated. In many species parasitized by braconid and ichneumonid wasps, host immunosuppression appears to be mediated by polydnaviruses (PDVs) injected by the female parasitoid into the host hemocoel. The viruses exhibit a complex and intimate genetic relationship with the wasp, since viral sequences are integrated within the wasp's chromosomal DNA. Here Mark Lavine and Nancy Beckage summarize the current evidence for mechanisms of virally induced host immunosuppression in parasitized insects, as well as the roles of other factors including wasp ovarian proteins and venom components, in suppressing hemocyte-mediated and humoral immune responses. Interestingly, in some species, the PDV-induced host immunosuppression appears transitory, with older parasitoid larvae probably exploiting other mechanisms to protect themselves from the host's immune system during the final stages of parasitism. During the final stages of parasitism, the parasitoids likely exploit other mechanisms of immunoevasion via antigen masking, antigen mimicry, or production of active inhibitors of the hemocyte-mediated encapsulation response as well as inhibiting melanization.     

Characterization of a cDNA encoding a putative chitinase from teratocytes of the endoparasitoid Toxoneuron nigriceps

Females of insect endoparasitoids inject substances along with the egg at oviposition, which can regulate or induce changes in a number of host physiological processes to benefit the developing parasitoid. These changes can be caused by substances such as venoms, calyx fluids, or symbiotic‐associated virus particles (polydnavirus), which are injected by females along with their eggs, and by substances secreted by parasitoid‐derived tissues (teratocytes) or the developing parasitoid larvae. Teratocytes (dissociated cells of the serosal membrane after parasitoid eclosion) release substances that have roles (i) in parasitoid nutrition, (ii) in the digestion of host tissues, and (iii) in the regulation of host development. Teratocytes of Toxoneuron nigriceps (Viereck) (Hymenoptera: Braconidae) have been implicated in the arrestment of the host development and in the regulation of circulating levels of host ecdysteroids. Here we describe the cDNA of a teratocyte‐secreted chitinase and its expression during parasitoid development, and discuss its putative role in this host–parasitoid association.     

Relationships between polydnavirus genomes and viral gene expression

Polydnavirus genomes and viral gene functions are atypical for viruses. Polydnaviruses are the only group of viruses with segmented DNA genomes and have an unusual obligate mutualistic association with parasitic Hymenoptera, in which the virus is required for survival of the wasp host and vice versa. The virus replicates asymptomatically in the wasp host but severely disrupts lepidopteran host physiology in the absence of viral DNA replication. It is not surprising then that viral gene expression is divergent in its two insect hosts and that differences in viral gene expression are linked to these divergent functions. Some viral genes are expressed only in the wasp host while other viral genes are expressed only in the lepidopteran host and are presumed to be involved in the disruption of host physiological systems. Our laboratory has described the expression and regulation of a family of viral genes implicated in suppressing the lepidopteran immune system, the cys-motif genes. In conjunction with these studies we have described the physical organization of additional viral gene segments. We have cloned, mapped and begun the sequence analysis of selected viral DNA segments. We have noted that some viral DNA segments are nested and that nested viral DNA segments encode the abundantly expressed, secreted cys-motif genes. Conversely, other viral segments are not nested, encode less abundantly expressed genes and may be targeted intra-cellularly. These results suggest that nesting of segments in polydnavirus genomes may be linked to the levels of gene expression. By extension, the unique, segmented organization of polydnavirus genomes may be associated, in part, with the requirement for divergent levels of viral gene expression in lepidopteran hosts in the absence of viral DNA replication.     

The UGA-CiE1 cell line from Chrysodeixis includens exhibits characteristics of granulocytes and is permissive to infection by two viruses

The soybean looper, Chrysodeixis (Pseudoplusia) includens (Lepidoptera: Noctuidae) is an economically important insect pest and a highly permissive host for the parasitoid Microplitis demolitor and its associated polydnavirus M. demolitor bracovirus (MdBV). Here we established a cell line from C. includens embryos designated UGA-CiE1 cells. CiE1 cells morphologically resemble granulocytes, which are a subpopulation of C. includens hemocytes. Antibody and RT-PCR analyses indicated that CiE1 cells express several molecular and functional markers that identify granulocytes. We further determined that CiE1 cells are permissive to infection by MdBV, exhibiting alterations very similar to MdBV-infected granulocytes, and Autographa californica multiple nucleopolyhedrosis virus (AcMNPV). Combined with the ability to transfect CiE1 cells with high efficiency and knock down expression of viral genes by RNA interference, we conclude this cell line has several attributes of value for studying immune interactions with polydnaviruses and potentially other pathogens.     

Bracovirus gene products are highly divergent from insect proteins

Recently, several polydnavirus (PDV) genomes have been completely sequenced. The dsDNA circles enclosed in virus particles and injected by wasps into caterpillars appear to mainly encode virulence factors potentially involved in altering host immunity and/or development, thereby allowing the survival of the parasitoid larvae within the host tissues. Parasitoid wasps generally inject virulence factors produced in the venom gland. As PDV genomes are inherited vertically by wasps through a proviral form, wasp virulence genes may have been transferred to this chromosomal form, leading to their incorporation into virus particles. Indeed, many gene products from Cotesia congregata bracovirus (CcBV), such as PTPs, IkappaB-like, and cystatins, contain protein domains conserved in metazoans. Surprisingly however, CcBV virulence gene products are not more closely related to insect proteins than to human proteins. To determine whether the distance between CcBV and insect proteins is a specific feature of BV proteins or simply reflects a general high divergence of parasitoid wasp products, which might be due to parasitic lifestyle, we have analyzed the sequences of wasp genes obtained from a cDNA library. Wasp sequences having a high similarity with Apis mellifera genes involved in a variety of biological functions could be identified indicating that the high level of divergence observed for BV products is a hallmark of these viral proteins. We discuss how this divergence might be explained in the context of the current hypotheses on the origin and evolution of wasp-bracovirus associations.     

Insect endosymbionts: manipulators of insect herbivore trophic interactions?

Throughout their evolutionary history, insects have formed multiple relationships with bacteria. Although many of these bacteria are pathogenic, with deleterious effects on the fitness of infected insects, there are also numerous examples of symbiotic bacteria that are harmless or even beneficial to their insect host. Symbiotic bacteria that form obligate or facultative associations with insects and that are located intracellularly in the host insect are known as endosymbionts. Endosymbiosis can be a strong driving force for evolution when the acquisition and maintenance of a microorganism by the insect host results in the formation of novel structures or changes in physiology and metabolism. The complex evolutionary dynamics of vertically transmitted symbiotic bacteria have led to distinctive symbiont genome characteristics that have profound effects on the phenotype of the host insect. Symbiotic bacteria are key players in insect–plant interactions influencing many aspects of insect ecology and playing a key role in shaping the diversification of many insect groups. In this review, we discuss the role of endosymbionts in manipulating insect herbivore trophic interactions focussing on their impact on plant utilisation patterns and parasitoid biology.