Characterization of Chelonus inanitus polydnavirus segments: sequences and analysis, excision site and demonstration of clustering

A new gene family (BAPs) of Cotesia bracovirus induces apoptosis of host hemocytes

Polydnaviruses (PDVs), obligatory symbionts with parasitoid wasps, function as host immune suppressors and growth and development regulator. PDVs can induce host haemocyte apoptosis, but the underlying mechanism remains largely unknown. Here, we provided evidence that, during the early stages of parasitism, the activated Cotesia vestalis bracovirus (CvBV) reduced the overall number of host haemocytes by inducing apoptosis. We found that one haemocyte-highly expressed CvBV gene, CvBV-26-4, could induce haemocyte apoptosis. Further analyses showed that CvBV-26-4 has four homologs from other Cotesia bracoviruses and BV from wasps in the genus Glyptapanteles, and all four of them possessed a similar structure containing 3 copies of a well-conserved motif (Gly-Tyr-Pro-Tyr, GYPY). Mass spectrometry analysis revealed that CvBV-26-4 was secreted into plasma by haemocytes and then degraded into peptides that induced the apoptosis of haemocytes. Moreover, ectopic expression of CvBV-26-4 caused fly haemocyte apoptosis and increased the susceptibility of flies to bacteria. Based on this research, a new family of bracovirus genes, Bracovirus apoptosis-inducing proteins (BAPs), was proposed. Furthermore, it was discovered that the development of wasp larvae was affected when the function of CvBV BAP was obstructed in the parasitized hosts. The results of our study indicate that the BAP gene family from the bracoviruses group is crucial for immunosuppression during the early stages of parasitism.

Chelonus inanitus bracovirus encodes lineage-specific proteins and truncated immune IκB-like factors

Bracoviruses and ichnoviruses are endogenous viruses of parasitic wasps that produce particles containing virulence genes expressed in host tissues and necessary for parasitism success. In the case of bracoviruses the particles are produced by conserved genes of nudiviral origin integrated permanently in the wasp genome, whereas the virulence genes can strikingly differ depending on the wasp lineage. To date most data obtained on bracoviruses concerned species from the braconid subfamily of Microgastrinae. To gain a broader view on the diversity of virulence genes we sequenced the genome packaged in the particles of Chelonus inanitus bracovirus (CiBV) produced by a wasp belonging to a different subfamily: the Cheloninae. These are egg-larval parasitoids, which means that they oviposit into the host egg and the wasp larvae then develop within the larval stages of the host. We found that most of CiBV virulence genes belong to families that are specific to Cheloninae. As other bracoviruses and ichnoviruses however, CiBV encode v-ank genes encoding truncated versions of the immune cactus/IκB factor, which suggests these proteins might play a key role in host–parasite interactions involving domesticated endogenous viruses. We found that the structures of CiBV V-ANKs are different from those previously reported. Phylogenetic analysis supports the hypothesis that they may originate from a cactus/IκB immune gene from the wasp genome acquired by the bracovirus. However, their evolutionary history is different from that shared by other V-ANKs, whose common origin probably reflects horizontal gene transfer events of virus sequences between braconid and ichneumonid wasps.

The Complete Genome of Chelonus insularis Reveals Dynamic Arrangement of Genome Components in Parasitoid Wasps That Produce Bracoviruses

Bracoviruses (BVs) are endogenized nudiviruses in parasitoid wasps of the microgastroid complex (family Braconidae). Microgastroid wasps have coopted nudivirus genes to produce replication-defective virions that females use to transfer virulence genes to parasitized hosts. The microgastroid complex further consists of six subfamilies and ∼50,000 species but current understanding of BV gene inventories and organization primarily derives from analysis of two wasp species in the subfamily Microgastrinae (Microplitis demolitor and Cotesia congregata) that produce M. demolitor BV (MdBV) and C. congregata BV (CcBV). Notably, several genomic features of MdBV and CcBV remain conserved since divergence of M. demolitor and C. congregata ∼53 million years ago (MYA). However, it is unknown whether these conserved traits more broadly reflect BV evolution, because no complete genomes exist for any microgastroid wasps outside the Microgastrinae. In this regard, the subfamily Cheloninae is of greatest interest because it diverged earliest from the Microgastrinae (∼85 MYA) after endogenization of the nudivirus ancestor. Here, we present the complete genome of Chelonus insularis, which is an egg-larval parasitoid in the Cheloninae that produces C. insularis BV (CinsBV). We report that the inventory of nudivirus genes in C. insularis is conserved but are dissimilarly organized compared to M. demolitor and C. congregata. Reciprocally, CinsBV proviral segments share organizational features with MdBV and CcBV but virulence gene inventories exhibit almost no overlap. Altogether, our results point to the functional importance of a conserved inventory of nudivirus genes and a dynamic set of virulence genes for the successful parasitism of hosts. Our results also suggest organizational features previously identified in MdBV and CcBV are likely not essential for BV virion formation. IMPORTANCE Bracoviruses are a remarkable example of virus endogenization, because large sets of genes from a nudivirus ancestor continue to produce virions that thousands of wasp species rely upon to parasitize hosts. Understanding how these genes interact and have been coopted by wasps for novel functions is of broad interest in the study of virus evolution. This work characterizes bracovirus genome components in the parasitoid wasp Chelonus insularis, which together with existing wasp genomes captures a large portion of the diversity among wasp species that produce bracoviruses. Results provide new information about how bracovirus genome components are organized in different wasps while also providing additional insights on key features required for function.

CLP gene family, a new gene family of Cotesia vestalis bracovirus inhibits melanization of Plutella xylostella hemolymph

Polydnaviruses (PDVs) are obligatory symbionts of parasitoid wasps and play an important role in suppressing host immune defenses. Although PDV genes that inhibit host melanization are known in Microplitis bracovirus, the functional homologs in Cotesia bracoviruses remain unknown. Here, we find that Cotesia vestalis bracovirus (CvBV) can inhibit hemolymph melanization of its host, Plutella xylostella larvae, during the early stages of parasitization, and that overexpression of highly expressed CvBV genes reduced host phenoloxidase activity. Furthermore, CvBV-7-1 in particular reduced host phenoloxidase activity within 12 h, and the injection of anti-CvBV-7-1 antibody increased the melanization of parasitized host larvae. Further analyses showed that CvBV-7-1 and three homologs from other Cotesia bracoviruses possessed a C-terminal leucine/isoleucine-rich region and had a similar function in inhibiting melanization. Therefore, a new family of bracovirus genes was proposed and named as C-terminal Leucine/isoleucine-rich Protein (CLP). Ectopic expression of CvBV-7-1 in Drosophila hemocytes increased susceptibility to bacterial repression of melanization and reduced the melanotic encapsulation of parasitized D. melanogaster by the parasitoid Leptopilina boulardi. The formation rate of wasp pupae and the eclosion rate of C. vestalis were affected when the function of CvBV-7-1 was blocked. Our findings suggest that CLP genes from Cotesia bracoviruses encoded proteins that contain a C-terminal leucine/isoleucine-rich region and function as melanization inhibitors during the early stage of parasitization, which is important for successful parasitization.

Genome-Wide Profiling of Diadegma semiclausum Ichnovirus Integration in Parasitized Plutella xylostella Hemocytes Identifies Host Integration Motifs and Insertion Sites

Polydnaviruses (PDVs), classified into two genera, bracoviruses (BVs) and ichnoviruses (IVs), are large, double-stranded DNA viruses, which are beneficial symbionts of parasitoid wasps. PDVs do not replicate in their infected lepidopteran hosts. BV circles have been demonstrated to be integrated into host genomic DNA after natural parasitization. However, the integrations of IV circles in vivo remain largely unknown. Here, we analyzed the integration of Diadegma semiclausum ichnovirus (DsIV) in the genomic DNA of parasitized Plutella xylostella hemocytes. We found that DsIV circles are present in host hemocytes with non-integrated and integrated forms. Moreover, DsIV integrates its DNA circles into the host genome by two distinct strategies, conservatively, and randomly. We also found that four conserved-broken circles share similar motifs containing two reverse complementary repeats at their breaking sites, which were host integration motifs (HIMs). We also predicted HIMs of eight circles from other ichnoviruses, indicating that a HIM-mediated specific mechanism was conserved in IV integrations. Investigation of DsIV circle insertion sites of the host genome revealed the enrichment of microhomologies between the host genome and the DsIV circles at integration breakpoints. These findings will deepen our understanding of the infections of PDVs, especially IVs.

A viral histone h4 joins to eukaryotic nucleosomes and alters host gene expression

A viral histone H4 (CpBV-H4) is encoded in a polydnavirus, Cotesia plutellae bracovirus. Its predicted amino acid sequence is highly homologous to that of host insect histone H4 except for an extended N-terminal tail containing 38 amino acids with nine lysine residues. Its expression induces an immunosuppression of target insects by suppressing immune-associated genes, presumably through an epigenetic control. This study analyzed its molecular interaction with eukaryotic host nucleosomes and subsequent regulation of host gene expression. Purified recombinant CpBV-H4 could associate with nucleosomal components (H2A, H2B, H3, and H4) and form an octamer. Transient expression of CpBV-H4 in an insect, Tribolium castaneum, was performed by microinjection of a recombinant expression vector and confirmed by both reverse transcriptase PCR (RT-PCR) and immunoblotting assays. Under this transient expression condition, total RNAs were extracted and read by a deep-sequencing technique. Annotated transcripts were classified into different gene ontology (GO) categories and compared with those of control insects injected with a truncated CpBV-H4. Target genes manipulated by CpBV-H4 expression showing significant differences (fold changes > 10(9)) included all GO categories, including development and immune-associated genes. When the target genes were physically mapped, they were found to be scattered on entire chromosomes of T. castaneum. In addition, chromatin immunoprecipitation against CpBV-H4 determined 16 nucleosome sites (P < 10(-5)) of the viral histone incorporation, which were noncoding regions near DNA-binding and inducible genes. These findings suggest that the viral histone H4 alters host gene expression by a direct molecular interaction with insect nucleosomes.

Polydnaviruses of Parasitic Wasps: Domestication of Viruses To Act as Gene Delivery Vectors

Symbiosis is a common phenomenon in which associated organisms can cooperate in ways that increase their ability to survive, reproduce, or utilize hostile environments. Here, we discuss polydnavirus symbionts of parasitic wasps. These viruses are novel in two ways: (1) they have become non-autonomous domesticated entities that cannot replicate outside of wasps; and (2) they function as a delivery vector of genes that ensure successful parasitism of host insects that wasps parasitize. In this review we discuss how these novelties may have arisen, which genes are potentially involved, and what the consequences have been for genome evolution.

Deep sequencing identifies viral and wasp genes with potential roles in replication of Microplitis demolitor Bracovirus

Viruses in the genus Bracovirus (BV) (Polydnaviridae) are symbionts of parasitoid wasps that specifically replicate in the ovaries of females. Recent analysis of expressed sequence tags from two wasp species, Cotesia congregata and Chelonus inanitus, identified transcripts related to 24 different nudivirus genes. These results together with other data strongly indicate that BVs evolved from a nudivirus ancestor. However, it remains unclear whether BV-carrying wasps contain other nudivirus-like genes and what types of wasp genes may also be required for BV replication. Microplitis demolitor carries Microplitis demolitor bracovirus (MdBV). Here we characterized MdBV replication and performed massively parallel sequencing of M. demolitor ovary transcripts. Our results indicated that MdBV replication begins in stage 2 pupae and continues in adults. Analysis of prereplication- and active-replication-stage ovary RNAs yielded 22 Gb of sequence that assembled into 66,425 transcripts. This breadth of sampling indicated that a large percentage of genes in the M. demolitor genome were sequenced. A total of 41 nudivirus-like transcripts were identified, of which a majority were highly expressed during MdBV replication. Our results also identified a suite of wasp genes that were highly expressed during MdBV replication. Among these products were several transcripts with conserved roles in regulating locus-specific DNA amplification by eukaryotes. Overall, our data set together with prior results likely identify the majority of nudivirus-related genes that are transcriptionally functional during BV replication. Our results also suggest that amplification of proviral DNAs for packaging into BV virions may depend upon the replication machinery of wasps.

Transient expression of a viral histone H4 inhibits expression of cellular and humoral immune-associated genes in Tribolium castaneum

A viral histone H4 is encoded in a polydnavirus called Cotesia plutellae bracovirus (CpBV), which is symbiotic to an endoparasitoid wasp, C. plutellae. Compared to general histone H4s, the viral H4 possesses an extra N-terminal tail containing 38 amino acid residues, which has been presumed to control host gene expression in an epigenetic mode. To analyze the epigenetic control activity of CpBV-H4 on expression of immune-associated genes, it was transiently expressed in larvae of Tribolium castaneum that had been annotated in the immune genes from a full genome sequence. Subsequent alteration of gene expression pattern was compared with that of its mutant form deleting N-terminal tail (truncated CpBV-H4). In response to bacterial challenge, T. castaneum induces expression of 13 antimicrobial peptide (AMP) genes. When CpBV-H4 was expressed, the larvae failed to express 12 inducible AMP genes. By contrast, when truncated CpBV-H4 was transiently expressed, all AMP genes were expressed. Hemocyte nodule formation was significantly impaired by expression of CpBV-H4, in which expressions of tyrosine hydroxylase and dihydroxyphenylalanine decarboxylase were suppressed. However, expression of truncated CpBV-H4 did not give any significant adverse effect on the cellular immunity. The immunosuppression of CpBV-H4 was further supported by its activity of enhancing bacterial pathogenicity of an entomopathogenic bacterium, Xenorhabdus nematophila, against larvae transiently expressing CpBV-H4. These results suggest that CpBV-H4 suppresses both humoral and cellular immune responses of T. castaneum by altering a normal epigenetic control of immune-associated gene expression.

Exogenous JH and ecdysteroid applications alter initiation of polydnaviral replication in an endoparasitoid wasp, Cotesia plutellae (Braconidae: Hymenoptera)

Polydnaviruses are a group of double-stranded DNA viruses and are symbiotically associated with some ichneumonoid wasps. As proviruses, the replication of polydnaviruses occurs in the female reproductive organ at the pupal stage. This study analyzed the effects of two developmental hormones, juvenile hormone (JH) and ecdysteroid, on the viral replication of Cotesia plutellae bracovirus (CpBV). All 23 CpBV segments identified contained a conserved excision/rejoining site ('AGCTTT') from their proviral segments. Using quantitative real-time PCR based on this excision/rejoining site marker, initiation of CpBV replication was determined to have occurred on day 4 on the pupal stage. Pyriproxyfen, a JH agonist, significantly inhibited adult emergence of C. plutellae, whereas RH5992, an ecdysteroid agonist, had no inhibitory effect. Although RH5992 had no effect dose on adult development, it significantly accelerated viral replication. The results of immunoblotting assays against viral coat proteins support the effects of the hormone agonists on viral replication.

Comparative genomics of mutualistic viruses of Glyptapanteles parasitic wasps

Comparative genome analysis of two endosymbiotic polydnaviruses from Glyptapanteles parasitic wasps reveals new insights into the evolutionary arms race between host and parasite.

Background

Polydnaviruses, double-stranded DNA viruses with segmented genomes, have evolved as obligate endosymbionts of parasitoid wasps. Virus particles are replication deficient and produced by female wasps from proviral sequences integrated into the wasp genome. These particles are co-injected with eggs into caterpillar hosts, where viral gene expression facilitates parasitoid survival and, thereby, survival of proviral DNA. Here we characterize and compare the encapsidated viral genome sequences of bracoviruses in the family Polydnaviridae associated with Glyptapanteles gypsy moth parasitoids, along with near complete proviral sequences from which both viral genomes are derived.

Results

The encapsidated Glyptapanteles indiensis and Glyptapanteles flavicoxis bracoviral genomes, each composed of 29 different size segments, total approximately 517 and 594 kbp, respectively. They are generated from a minimum of seven distinct loci in the wasp genome. Annotation of these sequences revealed numerous novel features for polydnaviruses, including insect-like sugar transporter genes and transposable elements. Evolutionary analyses suggest that positive selection is widespread among bracoviral genes.

Conclusions

The structure and organization of G. indiensis and G. flavicoxis bracovirus proviral segments as multiple loci containing one to many viral segments, flanked and separated by wasp gene-encoding DNA, is confirmed. Rapid evolution of bracovirus genes supports the hypothesis of bracovirus genes in an 'arms race' between bracovirus and caterpillar. Phylogenetic analyses of the bracoviral genes encoding sugar transporters provides the first robust evidence of a wasp origin for some polydnavirus genes. We hypothesize transposable elements, such as those described here, could facilitate transfer of genes between proviral segments and host DNA.

Transcriptional analysis of polydnaviral genes in the course of parasitization reveals segment-specific patterns

Polydnaviruses are symbiotic viruses of endoparasitic wasps, which are formed in their ovary and injected along with the eggs into the host. They manipulate the host in a way to allow successful parasitoid development. A hallmark of polydnaviruses is their segmented genome consisting of several circles of double-stranded DNA. We are studying the solitary egg-larval parasitoid Chelonus inanitus (Braconidae) parasitizing Spodoptera littoralis (Noctuidae). The polydnavirus of Chelonus inanitus (CiV) protects the parasitoid larva from encapsulation by the host's immune system, slightly modifies host nutritional physiology, and induces a developmental arrest of the host in the prepupal stage. Here we present data on newly identified CiV genes and their expression patterns in the course of parasitization. None of these genes has similarity to other genes and so far no gene families could be found. A rough estimation of transcript quantities revealed that even the most highly expressed CiV genes reach maximal values, which are 250 times lower than actin. This indicates that the CiV-induced alterations of the host are brought about by a concerted action of low levels of transcripts. In an overview, we show the expression patterns of all CiV genes analysed up to now; they indicate that several genes with similar expression patterns (early, persistent, intermediate, or late) are grouped together on the same segment. This is the first observation of this type. It suggests that one function of the segmentation of the polydnavirus genome may be the grouping together of genes, which are regulated in a similar manner.

Structure and evolution of a proviral locus of Glyptapanteles indiensis bracovirus

Background

Bracoviruses (BVs), a group of double-stranded DNA viruses with segmented genomes, are mutualistic endosymbionts of parasitoid wasps. Virus particles are replication deficient and are produced only by female wasps from proviral sequences integrated into the wasp genome. Virus particles are injected along with eggs into caterpillar hosts, where viral gene expression facilitates parasitoid survival and therefore perpetuation of proviral DNA. Here we describe a 223 kbp region of Glyptapanteles indiensis genomic DNA which contains a part of the G. indiensis bracovirus (GiBV) proviral genome.

Results

Eighteen of ~24 GiBV viral segment sequences are encoded by 7 non-overlapping sets of BAC clones, revealing that some proviral segment sequences are separated by long stretches of intervening DNA. Two overlapping BACs, which contain a locus of 8 tandemly arrayed proviral segments flanked on either side by ~35 kbp of non-packaged DNA, were sequenced and annotated. Structural and compositional analyses of this cluster revealed it exhibits a G+C and nucleotide composition distinct from the flanking DNA. By analyzing sequence polymorphisms in the 8 GiBV viral segment sequences, we found evidence for widespread selection acting on both protein-coding and non-coding DNA. Comparative analysis of viral and proviral segment sequences revealed a sequence motif involved in the excision of proviral genome segments which is highly conserved in two other bracoviruses.

Conclusion

Contrary to current concepts of bracovirus proviral genome organization our results demonstrate that some but not all GiBV proviral segment sequences exist in a tandem array. Unexpectedly, non-coding DNA in the 8 proviral genome segments which typically occupies ~70% of BV viral genomes is under selection pressure suggesting it serves some function(s). We hypothesize that selection acting on GiBV proviral sequences maintains the genetic island-like nature of the cluster of proviral genome segments described herein. In contrast to large differences in the predicted gene composition of BV genomes, sequences that appear to mediate processes of viral segment formation, such as proviral segment excision and circularization, appear to be highly conserved, supporting the hypothesis of a single origin for BVs.

Genomic and morphological features of a banchine polydnavirus: comparison with bracoviruses and ichnoviruses

Many ichneumonid and braconid endoparasitoids inject a polydnavirus (PDV) into their caterpillar hosts during oviposition. The viral entities carried by wasps of these families are referred to as "ichnoviruses" (IVs) and "bracoviruses" (BVs), respectively. All IV genomes characterized to date are found in wasps of the subfamily Campopleginae; consequently, little is known about PDVs found in wasps of the subfamily Banchinae, the only other ichneumonid taxon thus far shown to carry these viruses. Here we report on the genome sequence and virion morphology of a PDV carried by the banchine parasitoid Glypta fumiferanae. With an aggregate genome size of approximately 290 kb and 105 genome segments, this virus displays a degree of genome segmentation far greater than that reported for BVs or IVs. The size range of its genome segments is also lower than those in the latter two groups. As reported for other PDVs, the predicted open reading frames of this virus cluster into gene families, including the protein tyrosine phosphatase (PTP) and viral ankyrin (ank) families, but phylogenetic analysis indicates that ank genes of the G. fumiferanae virus are not embedded within the IV lineage, while its PTPs and those of BVs form distinct clusters. The banchine PDV genome also encodes a novel family of NTPase-like proteins displaying a pox-D5 domain. The unique genomic features of the first banchine virus examined, along with the morphological singularities of its virions (IV-like nucleocapsids, but enveloped in groups like some of the BVs), suggest that they could have an origin distinct from those of IVs and BVs.

Genome organization of the Chelonus inanitus polydnavirus: excision sites, spacers and abundance of proviral and excised segments

Polydnaviruses are only found in symbiotic association with parasitic wasps within the families Ichneumonidae and Braconidae (ichnoviruses and bracoviruses). They have a segmented genome consisting of circular double-stranded DNA. In the proviral linear form they are integrated in the wasp's genome; in two bracoviruses, segments were found to be clustered. Proviral segments have direct terminal repeats. Segment excision has been proposed to occur through juxtaposition of these repeats by formation of a loop and recombination; one copy of the repeat then ends up in the circular segment and one in the rejoined DNA. Here we analysed the excision/circularization site of four segments of the Chelonus inanitus bracovirus (CiV) and found that they are similar to the two already known sites; on the basis of the combined data an extended excision site motif was found. Analyses of segment flanking sequences led to the first identification of one complete and several partial spacers between proviral segments in a polydnavirus. The spacer between the proviral segments CiV14 and CiV22.5 has a length of 2065 bp; the terminal repeats of CiV14 and CiV22.5 were seen to have an opposite orientation and from this a model on the spacial organization of the loops of the proviral cluster is proposed. Through various approaches it was shown that spacers are not excised or injected into the host. Measurement of relative abundances of various segments in proviral and excised form indicates for the first time that abundant segments are present in multiple copies in the proviral form.

In vitro integration of an ichnovirus genome segment into the genomic DNA of lepidopteran cells

Polydnaviruses (PDVs) are dsDNA viruses transmitted by ichneumonid and braconid endoparasitoids to their lepidopteran hosts during oviposition. Wasp carriers are asymptomatic and transmit the virus to their progeny through the germ line; replication is confined to the calyx region of the wasp ovary, where the virus accumulates in the fluid bathing the eggs. In the lepidopteran host, however, no virus replication takes place, but PDV gene expression is essential for successful parasitism. Sustained gene expression in the absence of virus replication thus requires that the circular PDV genome segments persist for days within host cells. Available evidence suggests that most genome segments persist as episomes, but recent studies have indicated that some genome segments may undergo integration within lepidopteran genomic DNA, at least in vitro. In the present study, an integrated form of a Tranosema rostrale ichnovirus (TrIV) genome segment was cloned from genomic DNA extracted from infected Choristoneura fumiferana CF-124T cells and junction regions on either side of the viral DNA sequence were sequenced. This is the first proven example of integration of an ichnovirus genome segment in infected lepidopteran cells. Interestingly, circular forms of this genome segment do not appear to persist in these cells; none the less, a gene (TrFrep1) carried by this genome segment displays long-term transcription in infected cultured cells.

Spodoptera litura multicapsid nucleopolyhedrovirus inhibits Microplitis bicoloratus polydnavirus-induced host granulocytes apoptosis

Baculoviruses and parasitoids are critically important biological control agents in integrated pest management (IPM). They have been simultaneously and sequentially used to target insect pests. In this study, we examined the impacts of both baculovirus and polydnavirus (PDV) infection on the host cellular immune response. Larvae of the lepidopteran Spodoptera litura were infected by Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV) and then the animals were parasitized by the braconid wasp Microplitis bicoloratus. The fate of the parasitoids in the dually infected hosts was followed and encapsulation of M. bicoloratus first instar larvae was observed. Hemocytes of S. litura larvae underwent apoptosis in naturally parasitized hosts and in non-parasitized larvae after injection of M. bicoloratus ovarian calyx fluid (containing MbPDV) plus venom (CFPV). However, assessments of the percentages of cells undergoing apoptosis under different treatments indicated that SpltMNPV could inhibit MbPDV-induced apoptosis in hemocytes when hosts were first injected with SpltMNPV budded virus (BV) followed by injection with M. bicoloratus CFPV. As the time of injection with SpltMNPV BV increased, the percentages of apoptosis in hemocytes population declined. Furthermore, in vitro, the percentages of apoptosis showed that SpltMNPV BV could inhibit MbPDV-induced granulocytes apoptosis. The occurrence of MbPDV-induced host granulocytes apoptosis was inhibited in the dually infected hosts. As hemocytes apoptosis causes host immunosuppression, the parasitoids are normally protected from the host immune system. However, in larvae infected with both baculovirus and PDV, the parasitoids underwent encapsulation in the host hemocoel.

Unfolding the evolutionary story of polydnaviruses

Polydnaviruses (PDVs) are fascinating viruses. Described in thousands of parasitoid wasp species they are unique viruses having both a segmented DNA genome in viral particles and an integrated form that persists as a provirus in the wasp genome. Parasitoid wasps inject their eggs in another insect host typically a lepidopteran. In these host-parasitoid interactions, the virus particles are co-injected along with the eggs and are essential to ensure wasp parasitism success. PDVs do not replicate in the lepidopteran host, but expression of viral gene products confers protection from the host immune defence response. Two genera of PDVs phylogenetically unrelated exist, the bracoviruses (BVs) and the ichnoviruses (IVs), associated with braconid and ichneumonid wasps, respectively. New data on the genomes of two bracoviruses (Microplitis demolitor BV and Cotesia congregata BV) and an ichnovirus associated with Campoletis sonorensis (CsIV) offers us new elements to discuss the central questions concerning the origin of these viral entities and how they have evolved. The results of sequencing approaches indicate that the tens of millions of years of mutualistic associations between PDVs and wasps have had a strong impact on PDV genomes that now ressemble eukaryotic regions both in organization and gene content.

Characterization and transcriptional analysis of protein tyrosine phosphatase genes and an ankyrin repeat gene of the parasitoid Glyptapanteles indiensis polydnavirus in the parasitized host

Glyptapanteles indiensis (Braconidae, Hymenoptera) is an endoparasitoid of Lymantria dispar, the gypsy moth. Expression of G. indiensis polydnavirus (GiBV)-encoded genes within the pest host results in inhibition of immune response and development and alteration of physiology, enabling successful development of the parasitoid. Here, GiBV genome segment F (segF), an 18·6 kb segment shown to encode nine protein tyrosine phosphatase (PTP) genes and a single ankyrin repeat gene (ank), is analysed. PTPs have presumed function as regulators of signal transduction, while ankyrin repeat genes are hypothesized to function in inhibition of NF-κB signalling in the parasitized host. In this study, transcription of each gene was mapped by 5′- and 3′-RACE (rapid amplification of cDNA ends) and temporal and tissue-specific expression was examined in the parasitized host. For polydnavirus gene prediction in the parasitized host, no available gene prediction parameters were entirely precise. The mRNAs for each GiBV segF gene initiated between 30 and 112 bp upstream of the translation initiation codon. All were encoded in single open reading frames (ORFs), with the exception of PTP9, which was transcribed as a bicistronic message with the adjacent ank gene. RT-PCR indicated that all GiBV segF PTPs were expressed early in parasitization and, for most, expression was sustained over the course of at least 7 days after parasitization, suggesting importance in both early and sustained virus-induced immunosuppression and alteration of physiology. Tissue-specific patterns of PTP expression of GiBV segF genes were variable, suggesting differing roles in facilitating parasitism.

Polydnavirus genomes reflect their dual roles as mutualists and pathogens

Symbionts often exhibit significant reductions in genome complexity while pathogens often exhibit increased complexity through acquisition and diversification of virulence determinants. A few organisms have evolved complex life cycles in which they interact as symbionts with one host and pathogens with another. How the predicted and opposing influences of symbiosis and pathogenesis affect genome evolution in such instances, however, is unclear. The Polydnaviridae is a family of double-stranded (ds) DNA viruses associated with parasitoid wasps that parasitize other insects. Polydnaviruses (PDVs) only replicate in wasps but infect and cause severe disease in parasitized hosts. This disease is essential for survival of the parasitoid's offspring. Thus, a true mutualism exists between PDVs and wasps as viral transmission depends on parasitoid survival and parasitoid survival depends on viral infection of the wasp's host. To investigate how life cycle and ancestry affect PDVs, we compared the genomes of Campoletis sonorensis ichnovirus (CsIV) and Microplitis demolitor bracovirus (MdBV). CsIV and MdBV have no direct common ancestor, yet their encapsidated genomes share several features including segmentation, diversification of virulence genes into families, and the absence of genes required for replication. In contrast, CsIV and MdBV share few genes expressed in parasitized hosts. We conclude that the similar organizational features of PDV genomes reflect their shared life cycle but that PDVs associated with ichneumonid and braconid wasps have likely evolved different strategies to cause disease in the wasp's host and promote parasitoid survival.

Genomic segments cloning and analysis of Cotesia plutellae polydnavirus using plasmid capture system

Cotesia plutellae polydnaviruses (CpBV) has a segmented genome consisting of multiple circular double stranded DNAs. Recently, we have developed an easy, simple, and convenient system based on Tn7 transposition in order to clone genomic segments of CpBV in Escherichia coli cell and designated plasmid capture system (PCS). The PCS donor-S transferred a pUC19 origin of replication and an ampicillin resistance marker into CpBV genomic DNA by in vitro transposition. Through PCS system, we were able to clone 53 genomic clones ranging from 0.1 to 25.5 kb and further they were classified into 29 segments by their sizes and restriction endonuclease patterns. Among them, a complete nucleotide sequence of CpBV-S28 segment was determined and 10 putative genes were predicted from this segment. Interestingly, 9 of 10 putative ORFs had high level of similarities with catalytic domain of protein tyrosine phosphatase. Also, ORF2807 showed similarity with EP1-like proteins of C. congregata polydnavirus.

Cloning, characterization and analysis by RNA interference of various genes of the Chelonus inanitus polydnavirus

Successful parasitism of some endoparasitic wasps depends on an obligately symbiotic association with polydnaviruses. These unique viruses have a segmented genome consisting of circles of double-stranded (ds) DNA and do not replicate in the parasitized host. They are produced in the wasp's ovary and injected into the host along with the egg. Chelonus inanitus is an egg–larval parasitoid; its polydnavirus (CiV) has been shown to protect the parasitoid larva from the host's immune system and to induce developmental arrest in the prepupal stage. The genome of CiV consists of at least 10–12 segments and five have been sequenced up to now. Here, the complete (CiV12g2) or partial (CiV12g1, CiV16.8g1) cloning of three new CiV genes is reported. All three occur only on one viral segment and have no similarity to other known polydnavirus genes, with the exception of a high similarity of CiV12g1 to CiV14g1 and CiV12g2 to CiV14g2. Furthermore, the first attempt of in vivo application of RNA interference to study the function of polydnavirus genes is shown. Injection of dsRNA of two late- and one early- and late-expressed CiV genes into CiV/venom-containing host eggs partially rescued last-instar larvae from developmental arrest. Injection of the same dsRNAs into parasitized eggs partially reduced parasitoid survival, mainly by preventing the successful emergence of the parasitoid from the host. These viral genes thus seem to be involved in inducing developmental arrest and in keeping the cuticle soft, which appears to be necessary for parasitoid emergence and host feeding.

Bracoviruses contain a large multigene family coding for protein tyrosine phosphatases

The relationship between parasitic wasps and bracoviruses constitutes one of the few known mutualisms between viruses and eukaryotes. The virions produced in the wasp ovaries are injected into host lepidopteran larvae, where virus genes are expressed, allowing successful development of the parasite by inducing host immune suppression and developmental arrest. Bracovirus-bearing wasps have a common phylogenetic origin, and contemporary bracoviruses are hypothesized to have been inherited by chromosomal transmission from a virus that originally integrated into the genome of the common ancestor wasp living 73.7 +/- 10 million years ago. However, so far no conserved genes have been described among different braconid wasp subfamilies. Here we show that a gene family is present in bracoviruses of different braconid wasp subfamilies (Cotesia congregata, Microgastrinae, and Toxoneuron nigriceps, Cardiochilinae) which likely corresponds to an ancient component of the bracovirus genome that might have been present in the ancestral virus. The genes encode proteins belonging to the protein tyrosine phosphatase family, known to play a key role in the control of signal transduction pathways. Bracovirus protein tyrosine phosphatase genes were shown to be expressed in different tissues of parasitized hosts, and two protein tyrosine phosphatases were produced with recombinant baculoviruses and tested for their biochemical activity. One protein tyrosine phosphatase is a functional phosphatase. These results strengthen the hypothesis that protein tyrosine phosphatases are involved in virally induced alterations of host physiology during parasitism.

Stage-dependent expression of Chelonus inanitus polydnavirus genes in the host and the parasitoid

Chelonus inanitus (Braconidae) is a solitary egg-larval parasitoid of Spodoptera littoralis (Noctuidae). Along with the egg it also injects polydnaviruses (CiV) and venom, which are prerequisites for successful parasitoid development. CiV protects the parasitoid from encapsulation by the host's immune system and induces a developmental arrest in the prepupal stage. The polydnavirus genome consists of several double-stranded circular DNA segments. Proviral DNA is integrated in the wasp's genome and virus replication is restricted to the wasp's ovary. Here, the analysis of eight CiV genes located on five different segments revealed four patterns of expression in the course of parasitization: early, late, persistent but variable, and early and late. The comparison between parasitized and CiV/venom only containing hosts indicated that the presence of the parasitoid larva modulates transcript levels. Haemocytes, fat body and nervous tissue contained viral transcripts, values being highest in haemocytes. Small amounts of CiV transcripts were also observed in parasitoid larvae and pupae, suggesting transcription from the proviral integrated form of viral DNA. This is the first comparative analysis of the expression patterns of several viral genes in both parasitized and CiV/venom only containing hosts over the entire period of parasitization, and it reveals intricate interactions between the parasitoid, the polydnavirus and the host.

Polydnavirus genes and genomes: emerging gene families and new insights into polydnavirus replication

Polydnavirus genome sequencing is providing new insights into viral genome organization and viral gene function. Sequence analyses demonstrate that the genomes of these viral mutualists are largely noncoding but maintain genes and gene families that are unrelated to other viral genes. Interestingly, these organizational patterns in polydnavirus genomes are evident in both the bracovirus and ichnovirus genera, even though these two genera are evolutionarily unrelated. The identity and function of some polydnavirus gene families are considered with some functions experimentally supported and others implied by homology relationships with known insect genes. The evidence relative to polydnavirus origins and evolution is considered but remains an area of speculation. However, sequencing of these viral genomes has been informative and provides opportunities for productive investigation of these unusual mutualistic insect viruses.

Morphological and genomic characterization of the polydnavirus associated with the parasitoid wasp Glyptapanteles indiensis (Hymenoptera: Braconidae)

Glyptapanteles indiensis polydnavirus (GiPDV) is essential for successful parasitization of the larval stage of the lepidopteran Lymantria dispar (gypsy moth) by the endoparasitic wasp Glyptapanteles indiensis. This virus has not been characterized previously. Ultrastructural studies of GiPDV showed that virions had a rod-like or rectangular form and each contained as many as ten nucleocapsids enclosed by a single unit membrane envelope. Field inversion gel electrophoresis (FIGE) analysis of the virus genomic DNA revealed that GiPDV had a segmented genome composed of 13 dsDNA segments, ranging in size from approximately 11 kb to more than 30 kb. Four genomic segments were present in higher molar concentration than the others. Further characterization of the GiPDV genome yielded several cDNA clones which derived from GiPDV-specific mRNAs, and Northern blot analysis confirmed expression of isolated cDNA clones in the parasitized host. Each was present on more than one GiPDV genomic DNA segment, suggesting the existence of related sequences among DNA segments. It has been proposed previously that in polydnavirus systems, genome segmentation, hypermolar ratio segments and segment nesting may function to increase the copy number of essential genes and to increase the levels of gene expression in the absence of virus replication. The present data support this notion and suggest that GiPDV morphology and genomic organization may be intrinsically linked to the function and evolutionary strategies of the virus.

Polydnavirus integration in lepidopteran host cells in vitro

The long-term persistence of polydnavirus (PDV) DNA in infected lepidopteran cell cultures has suggested that at least some of the virus sequences become integrated permanently into the cell genome. In the current study, we provide supportive evidence of this event. Cloned libraries were prepared from two different Lymantria dispar (gypsy moth) cell lines that had been maintained in continuous culture for more than five years after infection with Glyptapanteles indiensis PDV (GiPDV). Junction clones containing both insect chromosomal and polydnaviral sequences were isolated. Precise integration junction sites were identified by sequence comparison of linear (integrated) and circular forms of the GiPDV genome segment F, from which viral sequences originated. Host chromosomal sequences at the site of integration varied between the two L. dispar cell lines but virus sequence junctions were identical and contained a 4-base pair CATG palindromic repeat. The GiPDV segment F does not encode any self-replication or self-insertion proteins, suggesting a host-derived mechanism is responsible for its in vitro integration. The chromosomal site of one junction clone contained sequences indicative of a new L. dispar retrotransposon, including a putative reverse transcriptase and integrase located upstream of the site of viral integration. A potential mechanism is proposed for the integration of PDV DNA in vitro. It remains to be seen if integration of the virus also occurs in the lepidopteran host in vivo.

Characterization of Campoletis sonorensis ichnovirus unique segment B and excision locus structure

Polydnaviruses (PDVs) are segmented, symbiotic, double-stranded DNA viruses that are vertically transmitted as proviruses within the genomes of some parasitoid Hymenoptera. The PDV associated with the ichneumonid wasp Campoletis sonorensis (CsIV) consists of 24 non-redundant DNA segments varying in size from approximately 6 to 20 kbp. CsIV segment B, one of the smallest genome segments, was sequenced and the excision sites of the proviral segment were characterized. The segment B sequence was 83.2% non-coding with only two open reading frames (ORFs). Some non-coding sequences have similarities to database sequences and were likely pseudogenic, but most were unrelated to known nucleic acid or predicted protein sequences. One ORF, BHv0.9, encodes a member of the rep gene family and was expressed only in parasitized insects while transcription of the other ORF could not be detected. Previously, a third region of the segment was shown to hybridize to 0.6 and 1.2 kb poly A+ RNAs from female wasps during virus replication (Theilmann and Summers, 1988) but this region did not have an identifiable ORF in the determined sequence. In contrast to CsIV segment W, segment B had little repetitive sequence. The segment B proviral integration locus contains a 59 bp direct imperfect repeat. Further analyses of this integration locus demonstrated that segment B was excised from wasp genomic DNA with flanking sequences at the integration site rejoined after segment excision. The segment B "excision locus" retained one of the two copies of the 59 bp repeat sequence with the other repeat present in the excised segment. The data indicate that Ichnovirus segments have distinctive characteristics possibly reflecting functional co-evolution between the wasp and individual types of polydnavirus segments.

Ovary development and polydnavirus morphogenesis in the parasitic wasp Chelonus inanitus. I. Ovary morphogenesis, amplification of viral DNA and ecdysteroid titres

Polydnaviruses are unique symbiotic viruses that are replicated in the calyx cells of the ovary of some parasitic wasps. They have a segmented genome of circular double-stranded DNA and are injected along with the wasp's egg into the host, where they are essential for successful parasitism. Polydnaviruses replicate from integrated proviral DNA, and after excision of viral segments, flanking DNA is rejoined. Little is known about ovarian morphogenesis, the mode of amplification of the viral DNA and the involvement of ecdysteroids. Here we have analysed these parameters in the course of pupal-adult development in the braconid wasp Chelonus inanitus. Immediately after pupation, ovarian cells proliferated and calyx cells began to differentiate; at this stage ecdysteroids, in particular 20-hydroxyecdysone, were highest. Thereafter, calyx cells began to increase in size and DNA content and eventually became gigantic. Amplification of non-viral DNA (actin) and viral DNA in its integrated and excised form and of corresponding rejoined flanking regions was measured by quantitative real-time PCR. In the early phase of calyx cell differentiation, copy numbers of actin and integrated viral DNA increased to a similar extent. This, along with the increase in nuclear volume and DNA content in the absence of extensive cell proliferation, suggested polyploidization of the early stage calyx cells. In the following phase, integrated viral DNA was selectively and intensively amplified and eventually excised and circularized. As copy numbers of excised circular viral DNA and rejoined flanking DNA reached similarly high levels, excised viral DNA appeared not to replicate. After adult eclosion, amplification of viral DNA declined.

Polydnavirus genome: integrated vs. free virus

Polydnaviruses are unique because of their obligatory association with thousands of parasitoid wasp species from the braconid and ichneumonid families of hymenopterans. PDVs are injected into the parasitized hosts and are essential for parasitism success. However, polydnaviruses are also unique because of their genome composed of multiple dsDNA segments. Cytological evidence has recently confirmed the results of genetic and molecular analyses indicating that PDV segments were integrated in the wasp genome. Moreover a phylogenetic study performed using the age of available fossils to calibrate the molecular clock indicated that the polydnaviruses harboured by braconid wasps have resided within the wasp genome for approximately 70 million years. In the absence of horizontal transmission, the evolution of the PDV genomes has been driven exclusively by the reproductive success they have offered the wasps. The consequences of this particular selection pressure can be observed in the gene content of certain PDV genomes from which increasing sequence data are available. Molecular mechanisms already identified could be involved in the acquisition and loss of genes by the PDV genomes and lead us to speculate on the definition of the virus genome.

Ovary development and polydnavirus morphogenesis in the parasitic wasp Chelonus inanitus. II. Ultrastructural analysis of calyx cell development, virion formation and release

Polydnaviruses are unique symbiotic viruses that are formed only in calyx cells in the ovary of parasitic wasps in the families Braconidae and Ichneumonidae; accordingly, two genera, Bracovirus and Ichnovirus are recognized. We have presented a detailed ultrastructural analysis of ovary and calyx cell differentiation and virion morphogenesis, together with the first data on virion release in a bracovirus. Differentiation of the ovary into germarium/vitellarium and the calyx region begins immediately after pupation. In the periphery and central part of the calyx, some cells and their nuclei begin to enlarge and the DNA content increases. The calyx cell nuclei then further increase and become highly lobulated, nuclear pores become very abundant and the cytoplasm is rich in ribosomes. This suggests synthesis and import of viral envelope proteins as viral envelopes appear in the nuclei shortly later. The appearance of viral envelopes is accompanied by a swelling of the nucleus and a change in electron density. Thereafter, the calyx cells reach the final stage with a highly swollen nucleus containing virogenic stroma and mature virions with nucleocapsids. Up to this stage, the DNA content of nuclei increases 120-fold and the volume 45-fold. The mature calyx cells are positioned in the vicinity of the oviduct lumen; for release of virions first the nuclear and then the plasma membrane disintegrate. On the border of the oviduct lumen, cells of an epithelial layer become phagocytic and remove debris, leading to a calyx fluid that contains only densely packed virions.

Fate of polydnavirus DNA of the egg-larval parasitoid Chelonus inanitus in the host Spodoptera littoralis

In situ hybridizations show that 5 min after parasitization, polydnavirus DNA is in close vicinity of the parasitoid egg, but 5 h later also in the yolk and partially in the host embryo. Fifteen hours after parasitization, the viral DNA is seen all over the host embryo and hardly in the yolk. The tissue distribution of the viral DNA was analysed and quantified by dot blots in the fifth instar parasitized larvae. On a per host basis, haemocytes and fat body contained the highest amount of viral DNA, while nervous tissue, intestinal tract and carcass contained less. Of the three viral segments tested, all were found in all tissues. Relative to the quantity of host DNA, viral DNA was most abundant in haemocytes, about five times less abundant in fat body and nervous tissue and about 25 times less abundant in intestinal tract. The total quantity of viral DNA per host was 444+/-145 pg which is similar to the quantity injected by the wasp; thus, the viral DNA persists throughout parasitization. The parasitoid larva contains 820+/-80 pg viral DNA integrated in the genome. This illustrates that the dose of viral DNA injected in virions represents approximately one third of the total viral genomic information present in a host at a late stage of parasitism.

Characterization of two genes of the polydnavirus of Chelonus inanitus and their stage-specific expression in the host Spodoptera littoralis

Chelonus inanitus (Braconidae, Hymenoptera) is a solitary egg-larval parasitoid of Spodoptera littoralis. Along with the egg the female wasp injects polydnaviruses, which are prerequisites for successful parasitoid development. The polydnavirus genome is segmented and consists of double-stranded circular DNA. Proviral DNA is integrated in the wasp's genome; virus replication is restricted to the wasp's ovary and does not occur in the parasitized host. The polydnavirus of C. inanitus (CiV) protects the parasitoid larva from encapsulation by the host's immune system and causes a developmental arrest of the host in the prepupal stage. Here we report on the first two cloned CiV genes, which are named CiV14g1 and CiV14g2 because of their localization on segment CiV14. The cDNA of CiV14g1 has a size of 2036 bp; the gene contains seven exons interrupted by six introns of similar size and encodes a putative polypeptide of 548 amino acids. The cDNA of CiV14g2 has a size of 618 bp; the gene consists of three exons and encodes a putative peptide of 77 amino acids. Transcript quantities of both genes are very low up to the penultimate larval instar of the host. In the last instar, at the stage of pupal cell formation, CiV14g1 expression increases about 5-fold and CiV14g2 expression about a 1000-fold. These are the first data to show strong upregulation of polydnavirus genes towards the end of parasitization. These two genes might be involved in the reduction of host ecdysteroids observed at this stage.