Co-replication of a reovirus and a polydnavirus in the ichneumonid parasitoid Hyposoter exiguae

Insect-specific RNA virus affects the stylet penetration activity of brown citrus aphid (Aphis citricidus) to facilitate its transmission

Sap-sucking insects often transmit plant viruses but also carry insect viruses, which infect insects but not plants. The impact of such insect viruses on insect host biology and ecology is largely unknown. Here, we identified a novel insect-specific virus carried by brown citrus aphid (Aphis citricidus), which we tentatively named Aphis citricidus picornavirus (AcPV). Phylogenetic analysis discovered a monophyletic cluster with AcPV and other unassigned viruses, suggesting that these viruses represent a new family in order Picornavirales. Systemic infection with AcPV triggered aphid antiviral immunity mediated by RNA interference, resulting in asymptomatic tolerance. Importantly, we found that AcPV was transmitted horizontally by secretion of the salivary gland into the feeding sites of plants. AcPV influenced aphid stylet behavior during feeding and increased the time required for intercellular penetration, thus promoting its transmission among aphids with plants as an intermediate site. The gene expression results suggested that this mechanism was linked with transcription of salivary protein genes and plant defense hormone signaling. Together, our results show that the horizontal transmission of AcPV in brown citrus aphids evolved in a manner similar to that of the circulative transmission of plant viruses by insect vectors, thus providing a new ecological perspective on the activity of insect-specific viruses found in aphids and improving the understanding of insect virus ecology.

Nonenveloped Avian Reoviruses Released with Small Extracellular Vesicles Are Highly Infectious

Vesicle-encapsulated nonenveloped viruses are a recently recognized alternate form of nonenveloped viruses that can avoid immune detection and potentially increase systemic transmission. Avian orthoreoviruses (ARVs) are the leading cause of various disease conditions among birds and poultry. However, whether ARVs use cellular vesicle trafficking routes for egress and cell-to-cell transmission is still poorly understood. We demonstrated that fusogenic ARV-infected quail cells generated small (~100 nm diameter) extracellular vesicles (EVs) that contained electron-dense material when observed by transmission electron microscope. Cryo-EM tomography indicated that these vesicles did not contain ARV virions or core particles, but the EV fractions of OptiPrep gradients did contain a small percent of the ARV virions released from cells. Western blotting of detergent-treated EVs revealed that soluble virus proteins and the fusogenic p10 FAST protein were contained within the EVs. Notably, virus particles mixed with the EVs were up to 50 times more infectious than virions alone. These results suggest that EVs and perhaps fusogenic FAST-EVs could contribute to ARV virulence.

A Novel RNA Virus in the Parasitoid Wasp Lysiphlebus fabarum: Genomic Structure, Prevalence, and Transmission

We report on a novel RNA virus infecting the wasp Lysiphlebus fabarum, a parasitoid of aphids. This virus, tentatively named "Lysiphlebus fabarum virus" (LysV), was discovered in transcriptome sequences of wasps from an experimental evolution study in which the parasitoids were allowed to adapt to aphid hosts (Aphis fabae) with or without resistance-conferring endosymbionts. Based on phylogenetic analyses of the viral RNA-dependent RNA polymerase (RdRp), LysV belongs to the Iflaviridae family in the order of the Picornavirales, with the closest known relatives all being parasitoid wasp-infecting viruses. We developed an endpoint PCR and a more sensitive qPCR assay to screen for LysV in field samples and laboratory lines. These screens verified the occurrence of LysV in wild parasitoids and identified the likely wild-source population for lab infections in Western Switzerland. Three viral haplotypes could be distinguished in wild populations, of which two were found in the laboratory. Both vertical and horizontal transmission of LysV were demonstrated experimentally, and repeated sampling of laboratory populations suggests that the virus can form persistent infections without obvious symptoms in infected wasps.

Naked Viruses That Aren't Always Naked: Quasi-Enveloped Agents of Acute Hepatitis

Historically, viruses were considered to be either enveloped or nonenveloped. However, recent work on hepatitis A virus and hepatitis E virus challenges this long-held tenet. Whereas these human pathogens are shed in feces as naked nonenveloped virions, recent studies indicate that both circulate in the blood completely masked in membranes during acute infection. These membrane-wrapped virions are as infectious as their naked counterparts, although they do not express a virally encoded protein on their surface, thus distinguishing them from conventional enveloped viruses. The absence of a viral fusion protein implies that these quasi-enveloped virions have unique mechanisms for entry into cells. Like true enveloped viruses, however, these phylogenetically distinct viruses usurp components of the host ESCRT system to hijack host cell membranes and noncytolytically exit infected cells. The membrane protects these viruses from neutralizing antibodies, facilitating dissemination within the host, whereas nonenveloped virions shed in feces are stable in the environment, allowing for epidemic transmission.

Reovirus forms neo-organelles for progeny particle assembly within reorganized cell membranes

Most viruses that replicate in the cytoplasm of host cells form neo-organelles that serve as sites of viral genome replication and particle assembly. These highly specialized structures concentrate viral replication proteins and nucleic acids, prevent the activation of cell-intrinsic defenses, and coordinate the release of progeny particles. Despite the importance of inclusion complexes in viral replication, there are key gaps in the knowledge of how these organelles form and mediate their functions. Reoviruses are nonenveloped, double-stranded RNA (dsRNA) viruses that serve as tractable experimental models for studies of dsRNA virus replication and pathogenesis. Following reovirus entry into cells, replication occurs in large cytoplasmic structures termed inclusions that fill with progeny virions. Reovirus inclusions are nucleated by viral nonstructural proteins, which in turn recruit viral structural proteins for genome replication and particle assembly. Components of reovirus inclusions are poorly understood, but these structures are generally thought to be devoid of membranes. We used transmission electron microscopy and three-dimensional image reconstructions to visualize reovirus inclusions in infected cells. These studies revealed that reovirus inclusions form within a membranous network. Viral inclusions contain filled and empty viral particles and microtubules and appose mitochondria and rough endoplasmic reticulum (RER). Immunofluorescence confocal microscopy analysis demonstrated that markers of the ER and ER-Golgi intermediate compartment (ERGIC) codistribute with inclusions during infection, as does dsRNA. dsRNA colocalizes with the viral protein σNS and an ERGIC marker inside inclusions. These findings suggest that cell membranes within reovirus inclusions form a scaffold to coordinate viral replication and assembly.

Viruses alter the architecture of host cells to form an intracellular environment conducive to viral replication. This step in viral infection requires the concerted action of viral and host components and is potentially vulnerable to pharmacological intervention. Reoviruses form large cytoplasmic replication sites called inclusions, which have been described as membrane-free structures. Despite the importance of inclusions in the reovirus replication cycle, little is known about their formation and composition. We used light and electron microscopy to demonstrate that reovirus inclusions are membrane-containing structures and that the endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment interact closely with these viral organelles. These findings enhance our understanding of the cellular machinery usurped by viruses to form inclusion organelles and complete an infectious cycle. This information, in turn, may foster the development of antiviral drugs that impede this essential viral replication step.

RNA Viruses in Parasitoid Wasps

This chapter describes the different RNA viruses that have been detected at least once in parasitoid wasps. It could be wondered whether the other families of parasitoids are really absent, or if this is due to the fact that inadequate techniques were used to detect viruses. In fact, several different methods have been used: TEM of the venom glands of females, extraction of nucleic acids, followed by DNAse digestion to eliminate the polydnavirus or ascovirus genome, RT-PCR with primers specific of RdRp or data mining in an EST library. To resolve this problem of detecting RNA viruses, a systematic search for viruses in parasitoids should be carried out using a combination of these different methods. Only a few hymenopteran species are known to be infected by RNA viruses, although thousands of species are known to carry polydnaviruses or VLPs. This could suggest that the presence of polydnaviruses or VLPs may block infections with other viruses.

Oogenesis of Diadegma semiclausum (Hymenoptera: Ichneumonidae) and its associated polydnavirus

We present a detailed ultrastructural analysis of ovary and calyx cell differentiation of Diadegma semiclausum. Numerous gametangia in various developmental stages were examined with electron microscopy to characterize the ultrastructure features of oogenesis, the most important of which is the development of nurse cells. In the germarium, the undifferentiated germ cells diversify, and one of the central cells of the cluster differentiates into an oocyte while the remaining become nurse cells. Germ cells continue developing in the vitellarium until mature and then enter into the calyx region. The calyx epithelium contains typical ichneumonid polydnaviruses with the following characteristics: (1) the virus particles assemble in the nuclei of calyx cells where they acquire an initial (inner) membrane, then migrate through the cytoplasm and budd out into the lumen of the ovary, at which time they acquire a second envelope (outer membrane); (2) the particle has a genome comprising several DNA segments. However, this new polydnavirus (Diadegma semiclausum polydnavirus) in the genus ichnovirus was not attached to the surface of the egg chorion.

Sequence analysis of a reovirus isolated from the winter moth Operophtera brumata (Lepidoptera: Geometridae) and its parasitoid wasp Phobocampe tempestiva (Hymenoptera: Ichneumonidae)

A reovirus was isolated from Operophtera brumata (ObRV) and its parasitoid wasp Phobocampe tempestiva. Each of the 10 dsRNA genome segments of ObRV was sequenced and shown to contain a single open reading frame (ORF). Conserved motifs ([+ve] 5'-AAATAAA ...(G)/(T)AGGTT-3') were found at the termini of each segment, with the exception of Seg-6 and Seg-8, where the 5' termini were 5'-AACAAA...-3'. The putative proteins encoded by each segment were compared with those of other members of the family Reoviridae. Phylogenetic comparisons to published sequences for the RNA-dependent RNA polymerase genes from other reoviruses indicated that ObRV is most closely related to members of the genus Cypovirus. However, unlike the cypoviruses, ObRV has a double-layered capsid structure. When the protein encoded by ObRV Seg-10 was expressed (by inserting the open reading frame into a baculovirus expression vector) no 'occlusion bodies' were observed in the recombinant baculovirus infected insect cell cultures. This suggests that unlike the cypoviruses, Seg-10 of ObRV does not contain a polyhedrin gene. Further phylogenetic comparisons also identified relationships between Seg-2 and Seg-10 of ObRV, and genes of Diadromus pulchellus Idnoreovirus 1 (DpIRV1), suggesting that ObRV represents a new species from the genus Idnoreovirus.

Exposure of ichnovirus particles to digitonin leads to enhanced infectivity and induces fusion from without in an in vitro model system

Unlike most viruses, the mature ichnovirus particle possesses two unit membrane envelopes. Following loss of the outer membrane in vivo, nucleocapsids are believed to gain entry into the cytosol via a membrane fusion event involving the inner membrane and the plasma membrane of susceptible host cells; accordingly, experimentally induced damage to the outer membrane might be expected to increase infectivity. Here, in an attempt to develop an in vitro model system for studying ichnovirus infection, we show that digitonin-induced disruption of the virion outer membrane not only increases infectivity, but also uncovers an activity not previously associated with any polydnavirus: fusion from without.

Artifical transfer and morphological description of virus particles associated with superparasitism behaviour in a parasitoid wasp

In parasitoids, the adaptive significance of superparasitism (laying of egg(s) in already parasitized hosts) has been the subject of strong controversy. The current view is to interpret this behaviour as an adaptation to increased competition for hosts, because the supernumerary egg still has a chance to win possession for the host. However, we recently discovered that in the solitary parasitoid Leptopilina boulardi, superparasitism is rather caused by an unknown infectious element: stable non superparasitizing lineages (NS) are transformed into stable superparasitizing lineages (S) after eggs from both lineages have competed inside the same host (superparasitism). In this report, we investigate the nature and location of the causative agent. Involvement of bacteria is unlikely because antibiotic treatments do not affect wasp phenotype and because bacterial 16S ribosomal DNA was not detected using PCR. We report successful injection experiments showing that the causative agents are located in wasp poison gland and ovaries and are stably inherited. Electron microscopic studies demonstrate that long filamentous virus particles located in wasp oviducts are strongly associated with superparasitism behaviour, leading to reconsider the adaptive significance of this behaviour in parasitoids. Interestingly, parasitoids are often infected with similar viruses for which no phenotypic effect has been documented. This raises the possibility that they could induce the same behavioural manipulation.

Detection and characterisation of three novel species of reovirus (Reoviridae), isolated from geographically separate populations of the winter moth Operophtera brumata (Lepidoptera: Geometridae) on Orkney

Geographically separate populations of winter moth (Operophtera brumata L.) were sampled in heather habitats on the Orkney Isles in order to investigate the prevalence of virus pathogens. Reoviruses were isolated in 11 of the 13 winter moth populations sampled, with 3 novel species being detected. Two species of Cypoviridae (CPV) were isolated, Operophtera brumata CPV18 and O. brumata CPV19, with one host population suffering 46% infection prevalence of OpbuCPV19. A third virus, O. brumata Reovirus (OpbuRV), was isolated from both winter moth and a hymenopteran parasitoid wasp, Phobocampe tempestiva, which is abundant in these populations. This was identified as a non-occluded reovirus, which was clearly able to infect and persist in both the lepidopteran and the hymenopteran host. The genomes of the three viruses were characterised using gel electrophoresis and the virus structure was investigated using transmission electron microscopy. The relationship of these viruses with a baculovirus that also infects winter moth, OpbuNPV, was investigated, as well as the association of OpbuRV with P. tempestiva. The detection of such viruses is discussed with reference to studies of similar viruses in other lepidopteran and hymenopteran host systems.

Superparasitism Evolution: Adaptation or Manipulation?

Superparasitism refers to the oviposition behavior of parasitoid females who lay their eggs in an already parasitized host. This often yields intense competition among larvae that are sharing the same host. Why would a female oviposit in such hostile habitat instead of looking for a better quality, unparasitized host? Here we present a continuous-time model of host-parasitoid interaction and discuss alternative scenarios. This model is first used to analyze the evolution of the superparasitism behavior of a solitary proovigenic parasitoid under both time and egg limitation. Then, following the recent discovery by Varaldi et al., we allow the parasitoid to be infected by a virus that alters the superparasitism behavior of its host to enhance its own horizontal transmission. The analysis of the coevolution of this manipulative behavior with the oviposition behavior of uninfected females clarifies and quantifies the conflict that emerges between the parasitoid and its virus. The model also yields new testable predictions. For example, we expect that uninfected parasitoids should superparasite less after coevolving with the manipulative virus. More generally, this model provides a theoretical framework for analyzing the evolution of the manipulation of parasitoid life-history traits by microparasites.

Transmission of the Diachasmimorpha longicaudata rhabdovirus (DlRhV) to wasp offspring: an ultrastructural analysis

During oviposition, the parasitic wasp Diachasmimorpha longicaudata introduces an entomopoxvirus (DlEPV) and a rhabdovirus (DlRhV) into larvae of its tephritid fruit fly host Anastrepha suspensa. DlEPV and DlRhV replicate, respectively, in host hemocytes and epidermal cells. Both viruses, like many beneficial viruses of parasitic wasps, are retained in all wasp generations but their avenue(s) of transmission are unknown. This study tests the hypothesis that DlRhV is transmitted transovarially or through larval feeding on infected host hemolymph. Transmission electron microscopy (TEM) revealed no virions in pre-vitellogenic or vitellogenic ova, or in the lateral oviduct of D. longicaudata females. However, numerous virions occurred in subchorionic regions of 33-36-h-old oviposited eggs. This suggests that DlRhV is introduced into the egg either as (a) intact virions after chorionogenesis but prior to oviposition and/or as (b) unencapsidated RNA molecules, undetectable by TEM in pre-vitellogenic ova, that subsequently replicate and assemble into mature virions. DlRhV particles also occurred in the midgut lumen of 20-24-h-old wasp first instars, suggesting that they were ingested. These virions may have been released from the egg into the hemolymph during hatching or may have come from virions introduced by the female wasp directly into the host, separate from the egg. DlRhV particles were also evident in the intracellular vesicles and intercellular spaces of the larval midgut. Taken together, these data support the hypothesis that DlRhV is transovarially transmitted as virions and/or as unencapsidated RNA. Further studies are needed to determine whether the DlRhV that ultimately resides within the female wasp's accessory gland filaments is the progeny of the virus from the egg and/or larval midgut cells.

Commensal and mutualistic relationships of reoviruses with their parasitoid wasp hosts

During evolution, certain endoparasitoid wasps have developed mechanisms to suppress the defence systems of their hosts. For this purpose, these species, all of which belong to the families Ichneumonidae and Braconidae, inject various kinds of virus-like particles. The most studied of these particles are classified as polydnaviruses (family Polydnaviridae) which are symbiotic viruses. Over the past decade, it has also been shown that several wasp species harbour reoviruses (family Reoviridae), and that two of these suppress host defence, allowing the development of the parasitoid eggs. In this paper, we summarize the key features of these viruses and their relationships with their wasp hosts. Five reoviruses are known that appear to be non-pathogenic for the wasps. Three of these, McRVLP, HeRV, OpRVLP, use their wasp hosts as vectors, and do not appear to be involved in host defence suppression. The fourth, DpRV-1, is a commensal reovirus detected in most field populations of the wasp, Diadromus pulchellus. This reovirus is always found associated with an ascovirus, DpAV-4a, which is indispensable for host immune suppression. Although DpRV-1 has not been shown to directly increase D. pulchellus parasitic success, it may contribute to this success by retarding DpAV-4a replication in the wasp. The fifth reovirus, DpRV-2, occurs in a specific population of D. pulchellus in which DpRV-1 and DpAV-4 are absent. This virus has a mutualistic relationship with its wasp host, as its injection by females during oviposition is essential for host immunosuppression. Interestingly, these viruses belong to several different reovirus genera.

Comparative analysis of selected genes from Diachasmimorpha longicaudata entomopoxvirus and other poxviruses

The Diachasmimorpha longicaudata entomopoxvirus (DlEPV) is the first symbiotic EPV described from a parasitic wasp. The DlEPV is introduced into the tephritid fruit fly larval host along with the wasp egg at oviposition. We sequenced a shotgun genomic library of the DlEPV DNA and analyzed and compared the predicted protein sequences of eight ORFs with those of selected poxviruses and other organisms. BlastP searches showed that five of these are homologous to poxvirus putative proteins such as metalloprotease, a putative membrane protein, late transcription factor-3, virion surface protein, and poly (A) polymerase (PAP) regulatory small subunit. Three of these are similar to those of other organisms such as the gamma-glutamyltransferase (GGT) of Arabidopsis thaliana, eukaryotic initiation factor 4A (eIF4A) of Caenorhabditis briggsae and lambda phage integrase (lambda-Int) of Enterococcus faecium. Transcription motifs for early (TGA,A/T,XXXXA) or late (TAAATG, TAAT, or TAAAT) gene expression conserved in poxviruses were identified with those ORFs. Phylogenetic analysis of multiple alignments of five ORFs and 20 poxvirus homologous sequences and of a concatenate of multiple alignments suggested that DlEPV probably diverged from the ancestral node between the fowlpox virus and the genus B, lepidopteran and orthopteran EPVs, to which Amsacta moorei and Melanoplus sanguinipes EPV, respectively, belong. The DlEPV putative GGT, eIF4A, and lambda-Int contained many conserved domains that typified these proteins. These homologues may be involved in either viral pathogenicity or enhancing parasitism via the gamma-glutamyl cycle and compensation of eIF4A levels in the parasitized fly, or via the integration of a portion of the viral genome into the wasp and/or parasitized fly.

Presence of a novel small RNA-containing virus in a laboratory culture of the endoparasitic wasp Venturia canescens (Hymenoptera: Ichneumonidae)

Parasitoid wasps use a variety of mechanisms to alter their host's physiology to the benefit of the developing endoparasite inside the host larva. Association of certain wasps with viruses and virus-like particles (VLPs) that contribute to their success in parasitism is one of the fascinating evolutionary adaptations conferring active or passive protection for the endoparasite from the host immune system. Venturia canescens has been shown to produce VLPs that provide protection for the developing parasitoid egg inside the host, Ephestia kuehniella. Here, we report on the presence of a novel small RNA-containing virus from V. canescens, designated as VcSRV, occurring in the ovaries of the wasp. The virus particles are found together with VcVLPs in the lumen of the calyx region of the ovaries and are injected together with the egg and VcVLPs into E. kuehniella larvae where they enter hemocytes. Alignment of the RNA-dependent RNA polymerase gene of VcSRV indicates that the virus most likely belongs to the recently described genus Iflavirus.

Morphogenesis and cytopathic effects of the Diachasmimorpha longicaudata entomopoxvirus in host haemocytes

The Diachasmimorpha longicaudata entomopoxvirus (DlEPV), the first reported symbiotic entomopoxvirus, occurs in the venom apparatus of D. longicaudata female wasps and is introduced into Anastrepha suspensa larvae during parasitism. The DlEPV 250-300 kb double stranded DNA genome encodes putative proteins having 30 to >60% amino acid identity with poxvirus homologs such as DNA helicase, DNA-dependent RNA polymerase, and the poxvirus-specific rifampicin resistance protein. Although the molecular characterization of DlEPV is progressing, little is known about its morphogenesis in and effects on host haemocytes. This paper describes (1) haemocytes of third instar A. suspensa, (2) DlEPV infection and morphogenesis, and (3) DlEPV-induced changes in haemocytes. A. suspensa third instars have 3-4 haemocyte morphotypes. Dot blots of DNA from infected haemocytes hybridized with a digoxigenin-labeled DlEPV genomic probe as early as 4 h post-parasitism (hpp) and the intensity of the signal increased with time through 40 hpp. Immunofluorescence microscopy localized DlEPV proteins in cytoplasmic (but not nuclear) sites of infected haemocytes, within 24-36 hpp. Electron microscopy confirmed the presence of viral envelopes, immature spheroids with centric nucleoids, budding virus, and extracellular enveloped virus in three haemocyte types, 24-84 hpp and later. Infected haemocytes exhibited blebbing, DNA concatenation, and inability to encapsulate sephadex beads in vitro. These data indicate that DlEPV disrupts the normal function of host haemocytes, thereby insuring the successful development of D. longicaudata offspring and as such should be regarded as a symbiont of the wasp.

Reptilian reovirus: a new fusogenic orthoreovirus species

The fusogenic subgroup of orthoreoviruses contains most of the few known examples of non-enveloped viruses capable of inducing syncytium formation. The only unclassified orthoreoviruses at the species level represent several fusogenic reptilian isolates. To clarify the relationship of reptilian reoviruses (RRV) to the existing fusogenic and nonfusogenic orthoreovirus species, we undertook a characterization of a python reovirus isolate. Biochemical, biophysical, and biological analyses confirmed the designation of this reptilian reovirus (RRV) isolate as an unclassified fusogenic orthoreovirus. Sequence analysis revealed that the RRV S1 and S3 genome segments contain a novel conserved 5'-terminal sequence not found in other orthoreovirus species. In addition, the gene arrangement and the coding potential of the bicistronic RRV S1 genome segment differ from that of established orthoreovirus species, encoding a predicted homologue of the reovirus cell attachment protein and a unique 125 residue p14 protein. The RRV S3 genome segment encodes a homologue of the reovirus sigma-class major outer capsid protein, although it is highly diverged from that of other orthoreovirus species (amino acid identities of only 16-25%). Based on sequence analysis, biological properties, and phylogenetic analysis, we propose this python reovirus be designated as the prototype strain of a fifth species of orthoreoviruses, the reptilian reoviruses.

The cypovirus Diadromus pulchellus RV-2 is sporadically associated with the endoparasitoid wasp D. pulchellus and modulates the defence mechanisms of pupae of the parasitized leek-moth, Acrolepiopsis assectella

Diadromus pulchellus is a solitary endoparasitoid wasp that parasitizes the pupae of the leek-moth, Acrolepiosis assectella (Lepidoptera). Hitherto, every individual D. pulchellus from France that has been investigated was infected by an orthoreovirus, DpRV-1, and an ascovirus, DpAV-4. Recently, a new strain of D. pulchellus, established from a French field population, was found to be able to develop on leek-moth pupae, but lacked both DpRV-1 and DpAV-4. However, all these wasps were infected with a new cypovirus, DpRV-2. This cypovirus is transmitted to the A. assectella pupae at each wasp oviposition and is replicated mainly in the gut cells of the parasitized pupae. DpRV-2, like the ascovirus DpAV-4, is able to inhibit the defence reaction of A. assectella pupae and so contributes to the parasitic success of D. pulchellus wasps.

Overt viral diseases induced from apparent latency following parasitization by the ichneumonid wasp, Hyposoter exiguae

Latent and/or asymptomatic viral infections are commonplace in insects, but factors inducing overt disease are poorly understood. Here we show that in Trichoplusia ni larvae parasitized by the ichneumonid wasp, Hyposoter exiguae, overt, lethal disease may on occasion be observed; however, disease has been consistently absent in control (non-parasitized) larvae. Thus far, we have detected two such diseases, one of which is caused by a granulosis virus affecting primarily fat body tissue. The other is associated with the presence of two viruses replicating together in larval midgut epithelial cells; of these, one has been identified as a non-occluded form of TnCPV. Since H. exiguae carries a polydnavirus, which is delivered to host larvae during oviposition, it is tempting to speculate that viral latency may in some cases be broken through immunosuppressive activity resulting from insect parasitism.

Virus or not? Phylogenetics of polydnaviruses and their wasp carriers

Our current, still limited, understanding of the comparative biology and evolution of polydnaviruses (PDVs) is reviewed, especially in the context of the possible origins of these parasitoid viruses and of their coevolution with carrier wasps. A hypothetical scenario of evolution of PDVs from ascovirus (or ascovirus-like) ancestors is presented, with examples of apparent extant transitional forms. PDVs appear, in the case of bracoviruses, to show phylogenetic relationships that mirror those of their wasp carriers: with ichnoviruses, the picture is less clear. Ongoing sequencing studies of entire PDV genomes from diverse wasp species are likely to greatly contribute to our understanding of PDV evolution.