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

Genomic characterization of viruses associated with the parasitoid Anagyrus vladimiri (Hymenoptera: Encyrtidae)

Knowledge on symbiotic microorganisms of insects has increased dramatically in recent years, yet relatively little data are available regarding non-pathogenic viruses. Here we studied the virome of the parasitoid wasp Anagyrus vladimiri Triapitsyn (Hymenoptera: Encyrtidae), a biocontrol agent of mealybugs. By high-throughput sequencing of viral nucleic acids, we revealed three novel viruses, belonging to the families Reoviridae [provisionally termed AnvRV (Anagyrus vladimiri reovirus)], Iflaviridae (AnvIFV) and Dicistroviridae (AnvDV). Phylogenetic analysis further classified AnvRV in the genus Idnoreovirus, and AnvDV in the genus Triatovirus. The genome of AnvRV comprises 10 distinct genomic segments ranging in length from 1.5 to 4.2 kb, but only two out of the 10 ORFs have a known function. AnvIFV and AnvDV each have one polypeptide ORF, which is typical of iflaviruses but very un-common among dicistroviruses. Five conserved domains were found along both the ORFs of those two viruses. AnvRV was found to be fixed in an A. vladimiri population that was obtained from a mass rearing facility, whereas its prevalence in field-collected A. vladimiri was ~15 %. Similarly, the prevalence of AnvIFV and AnvDV was much higher in the mass rearing population than in the field population. The presence of AnvDV was positively correlated with the presence of Wolbachia in the same individuals. Transmission electron micrographs of females' ovaries revealed clusters and viroplasms of reovirus-like particles in follicle cells, suggesting that AnvRV is vertically transmitted from mother to offspring. AnvRV was not detected in the mealybugs, supporting the assumption that this virus is truly associated with the wasps. The possible effects of these viruses on A. vladimiri's biology, and on biocontrol agents in general, are discussed. Our findings identify RNA viruses as potentially involved in the multitrophic system of mealybugs, their parasitoids and other members of the holobiont.

A novel cripavirus of an ectoparasitoid wasp increases pupal duration and fecundity of the wasp's Drosophila melanogaster host

We identified a 9332-nucleotide-long novel picornaviral genome sequence in the transcriptome of an agriculturally important parasitoid wasp (Pachycrepoideus vindemmiae (Rondani, 1875)). The genome of the novel virus, Rondani's wasp virus 1 (RoWV-1), contains two long open reading frames encoding a nonstructural and a structural protein, respectively, and is 3'-polyadenylated. Phylogenetic analyses firmly place RoWV-1 into the dicistrovirid genus Cripavirus. We detected RoWV-1 in various tissues and life stages of the parasitoid wasp, with the highest virus load measured in the larval digestive tract. We demonstrate that RoWV-1 is transmitted horizontally from infected to uninfected wasps but not vertically to wasp offspring. Comparison of several important biological parameters between the infected and uninfected wasps indicates that RoWV-1 does not have obvious detrimental effects on wasps. We further demonstrate that RoWV-1 also infects Drosophila melanogaster (Meigen, 1830), the hosts of the pupal ectoparasitoid wasps, and thereby increases its pupal developmental duration and fecundity, but decreases the eclosion rate. Together, these results suggest that RoWV-1 may have a potential benefit to the wasp by increasing not only the number of potential wasp hosts but also the developmental time of the hosts to ensure proper development of wasp offspring.

Influence of parasitoid-associated viral symbionts on plant-insect interactions and biological control

Insect parasitoids have evolved symbiotic interactions with several viruses and thousands of parasitoid species have established mutualistic associations with polydnaviruses (PDVs). While PDVs have often been described as virulence factors allowing development of immature parasitoids inside their herbivore hosts, there is increasing awareness that PDVs can affect plant-insect interactions. We review recent literature showing that PDVs alter not only host physiology, but also feeding patterns and composition of herbivore's oral secretions. In turn PDV-induced changes in herbivore phenotype affect plant responses to herbivory with consequences ranging from differential expression of plant defense-related genes to wider ecological effects across multiple trophic levels. In this opinion paper we also highlight important missing gaps to fully understand the role of PDVs and other parasitoid-associated viral symbionts in a plant-insect interaction perspective. Because PDVs negatively impact performance and survival of herbivore pests, we conclude arguing that PDV genomes offer potential opportunities for biological control.

Facilitator roles of viruses in enhanced insect resistance to biotic stress

Virus-insect interactions are primarily parasitic, yet diverse mutualistic interactions, some of which are symbiogenic, also occur. These viruses can modify insect physiology and behavior so that hosts can gain resistance against various biotic challenges like pathogen and parasites. In the recent past, many insect mutualistic viruses have been reported. Viruses can show symbiogenic interactions with some insects, which have been explored at the molecular level. However, understanding about molecular mechanisms for many of the mutualistic viruses is still enigmatic. Exploration of these interactions and its mechanism can shed light on phenomenon of virus mediated biotic stress resistance in insects.

Characterization of Venom and Oviduct Components of Parasitoid Wasp Asobara japonica

During natural parasitization, Asobara japonica wasps introduce lateral oviduct (LO) components into their Drosophila hosts soon after the venom injection to neutralize its strong toxicity; otherwise, the host will die. Although the orchestrated relationship between the venom and LO components necessary for successful parasitism has attracted the attention of many researchers in this field, the molecular natures of both factors remain ambiguous. We here showed that precipitation of the venom components by ultracentrifugation yielded a toxic fraction that was inactivated by ultraviolet light irradiation, boiling, and sonication, suggesting that it is a virus-like entity. Morphological observation of the precipitate after ultracentrifugation showed small spherical heterogeneous virus-like particles 20-40 nm in diameter. The venom's detrimental effect on D. melanogaster larvae was not directly neutralized by the LO components but blocked by a hemolymphal neutralizing factor activated by the LO factor. Furthermore, we found that A. japonica venom and LO components acted similarly on the larvae of the common cutworm Spodoptera litura: the venom injection caused mortality but coinjection of the LO factor protected S. litura larvae from the venom's toxicity. In contrast, D. ficusphila and D. bipectinata, which are closely related to D. melanogaster but non-habitual host species of A. japonica, were not negatively affected by A. japonica venom due to an intrinsic neutralizing activity in their hemolymph, indicating that these species must have acquired a neutralizer of A. japonica venom during evolution. These results give new insights into the characteristics of both the venom and LO components: A. japonica females have utilized the virus-like toxic venom factor to exploit a wider range of host species after the evolutionary process enabled them to use the LO factor for activation of the host hemolymph neutralizer precursor, although the non-habitual host Drosophila species possess an active intrinsic neutralizer in their hemolymph.

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.

Molecular detection, penetrance, and transmission of an inherited virus responsible for behavioral manipulation of an insect parasitoid

For insects, the prevalence of numerous vertically transmitted viruses can be high in their host populations. These viruses often have few, if any, pathological effects on their hosts, and consequently, many of them can remain unnoticed for long periods, despite their potential role in the evolution of the host phenotype. Some females of Leptopilina boulardi, a solitary parasitoid of Drosophila larvae, are infected by an inherited virus (LbFV) that manipulates the behavior of the wasp by increasing its tendency to lay eggs in a host that is already parasitized (superparasitism). This behavioral alteration allows horizontal transmission of the virus within superparasitized Drosophila larvae. Using suppressive subtractive hybridization with infected and uninfected lines, we identified one putative viral sequence. Based on this sequence, we developed a simple PCR test. We tested the correlation between the superparasitism phenotype and PCR amplification of the putative viral marker using several experimental conditions (including horizontal transfers) and several parasitoid genotypes. All of the results revealed that there was a perfect match between the superparasitism phenotype and the amplification profile, which validated use of the molecular marker as a tool to track the presence of the virus and provided the first genomic data for this fascinating virus. The results also show that there was very efficient horizontal and vertical transmission of LbFV, which probably explains its high prevalence in the French populations that we sampled (67 and 70% of infected females). This manipulative virus is likely to play a major role in the ecology and evolution of its parasitoid host.

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.

Virus-like particles in venom of Meteorus pulchricornis induce host hemocyte apoptosis

Ultrastructural studies on the reproductive tract and venom apparatus of a female braconid, Meteorus pulchricornis, revealed that the parasitoid lacks the calyx region in its oviduct, but possesses a venom gland with two venom gland filaments and a venom reservoir filled with white and cloudy fluid. Its venom gland cell is concaved and has a lumen filled with numerous granules. Transmisson electron microscopic (TEM) observation revealed that virus-like particles (VLPs) were produced in venom gland cells. The virus-like particle observed in M. pulchricornis (MpVLP) is composed of membranous envelopes with two different parts: a high-density core and a whitish low-density part. The VLPs of M. pulchricornis is also found assembling ultimately in the lumen of venom gland cell. Microvilli were found thrusting into the lumen of the venom gland cell and seem to aid in driving the matured MpVLPs to the common duct of the venom gland filament. Injection of MpVLPs into non-parasitized Pseudaletia separata hosts induced apoptosis in hemocytes, particularly granulocytes (GRs). Rate of apoptosis induced in GRs peaked 48h after VLP injection. While a large part of the GR population collapsed due to apoptosis caused by MpVLPs, the plasmatocyte population was minimally affected. The capacity of MpVLPs to cause apoptosis in host's hemocytes was further demonstrated by a decrease ( approximately 10-fold) in ability of host hemocytes to encapsulate fluorescent latex beads when MpVLPs were present. Apparently, the reduced encapsulation ability was due to a decrease in the GR population resulting from MpVLP-induced apoptosis.

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.

Virus-like particles in the ovaries of Microctonus aethiopoides Loan (Hymenoptera: Braconidae): comparison of biotypes from Morocco and Europe

Virus-like particles (MaVLP) have been discovered in the ovarial epithelial cells of the solitary, koinobiont, endoparasitoid, Microctonus aethiopoides Loan (Hymenoptera: Braconidae) introduced to New Zealand originally from Morocco to control the lucerne pest Sitona discoideus Gyllenhal (Coleoptera: Curculionidae). MaVLP have been found in all females examined. It has been suggested, although not demonstrated, that like many other such VLP found in parasitoids, MaVLP might play a role in host immunosuppression. Since another biotype of M. aethiopoides from Ireland has been proposed for introduction to control the white clover pest, Sitona lepidus Gyllenhal, in New Zealand, it was considered that females from this biotype warranted transmission electron microscope examination for VLP. No VLP were observed in ovarian tissues of specimens collected from three different locations in Ireland. Similarly, none were found in M. aethiopoides sourced from France, Wales, and Norway. These observations are discussed in relation to quarantine host specificity tests with the Irish biotype, which found that the host range of the Irish biotype is likely to be less extensive than that of the Moroccan biotype already in New Zealand.

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.

Characteristics of pathogenic and mutualistic relationships of ascoviruses in field populations of parasitoid wasps

Ascoviruses are disseminated among larvae in lepidopteran populations by parasitic wasps during oviposition. Ascovirus relationships with these wasps vary from pathogenic to mutualistic, and experimentally can be shown possibly to be commensal non-pathogenic virus having little or no effect. Most ascoviruses are pathogens that female wasps vector mechanically. Other ascoviruses have a more intimate relationship with their wasp vectors in that their genome is stably maintained in all wasp nuclei through several generations by vertical transmission. In this relationship, these viruses are mutualistic, enhancing the successful development of the wasp larvae by suppressing lepidopteran defence mechanisms. The DpAV4 ascovirus is a mutualist in certain Diadromus wasps but is pathogenic or not when vectored by other species of this genus. These various biologies suggest that ascovirus/wasp relationships depend on wasp regulatory factors that control virus replication. Thus, certain ascoviruses can potentially have either a pathogenic, mutualistic, or non-pathogenic relationship with a specific wasp vector, the type of relationship being dependent upon the species system in which the relationship evolved. Finally, because ascoviruses appear to be related to ichnoviruses (Polydnaviridae), the DpAV4/Diadromus system constitutes a possible interesting intermediate between the pathogenic ascoviruses and symbiotic viruses that evolved to be ichnoviruses.

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.