Evidence for integration of Glyptapanteles indiensis polydnavirus DNA into the chromosome of Lymantria dispar in vitro

Bracoviruses recruit host integrases for their integration into caterpillar's genome

Some DNA viruses infect host animals usually by integrating their DNAs into the host genome. However, the mechanisms for integration remain largely unknown. Here, we find that Cotesia vestalis bracovirus (CvBV), a polydnavirus of the parasitic wasp C. vestalis (Haliday), integrates its DNA circles into host Plutella xylostella (L.) genome by two distinct strategies, conservatively and randomly, through high-throughput sequencing analysis. We confirmed that the conservatively integrating circles contain an essential "8+5" nucleotides motif which is required for integration. Then we find CvBV circles are integrated into the caterpillar's genome in three temporal patterns, the early, mid and late stage-integration. We further identify that three CvBV-encoded integrases are responsible for some, but not all of the virus circle integrations, indeed they mainly participate in the processes of early stage-integration. Strikingly, we find two P. xylostella retroviral integrases (PxIN1 and PxIN2) are highly induced upon wasp parasitism, and PxIN1 is crucial for integration of some other early-integrated CvBV circles, such as CvBV_04, CvBV_12 and CvBV_24, while PxIN2 is important for integration of a late-integrated CvBV circle, CvBV_21. Our data uncover a novel mechanism in which CvBV integrates into the infected host genome, not only by utilizing its own integrases, but also by recruiting host enzymes. These findings will strongly deepen our understanding of how bracoviruses regulate and integrate into their hosts.

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.

Cotesia congregata Bracovirus Circles Encoding PTP and Ankyrin Genes Integrate into the DNA of Parasitized Manduca sexta Hemocytes

Polydnaviruses (PDVs) are essential for the parasitism success of tens of thousands of species of parasitoid wasps. PDVs are present in wasp genomes as proviruses, which serve as the template for the production of double-stranded circular viral DNA carrying virulence genes that are injected into lepidopteran hosts. PDV circles do not contain genes coding for particle production, thereby impeding viral replication in caterpillar hosts during parasitism. Here, we investigated the fate of PDV circles of Cotesia congregata bracovirus during parasitism of the tobacco hornworm, Manduca sexta, by the wasp Cotesia congregata Sequences sharing similarities with host integration motifs (HIMs) of Microplitis demolitor bracovirus (MdBV) circles involved in integration into DNA could be identified in 12 CcBV circles, which encode PTP and VANK gene families involved in host immune disruption. A PCR approach performed on a subset of these circles indicated that they persisted in parasitized M. sexta hemocytes as linear forms, possibly integrated in host DNA. Furthermore, by using a primer extension capture method based on these HIMs and high-throughput sequencing, we could show that 8 out of 9 circles tested were integrated in M. sexta hemocyte genomic DNA and that integration had occurred specifically using the HIM, indicating that an HIM-mediated specific mechanism was involved in their integration. Investigation of BV circle insertion sites at the genome scale revealed that certain genomic regions appeared to be enriched in BV insertions, but no specific M. sexta target site could be identified.IMPORTANCE The identification of a specific and efficient integration mechanism shared by several bracovirus species opens the question of its role in braconid parasitoid wasp parasitism success. Indeed, results obtained here show massive integration of bracovirus DNA in somatic immune cells at each parasitism event of a caterpillar host. Given that bracoviruses do not replicate in infected cells, integration of viral sequences in host DNA might allow the production of PTP and VANK virulence proteins within newly dividing cells of caterpillar hosts that continue to develop during parasitism. Furthermore, this integration process could serve as a basis to understand how PDVs mediate the recently identified gene flux between parasitoid wasps and Lepidoptera and the frequency of these horizontal transfer events in nature.

Baculoviral p94 homologs encoded in Cotesia plutellae bracovirus suppress both immunity and development of the diamondback moth, Plutellae xylostella

Polydnaviruses (PDVs) are a group of insect DNA viruses, which exhibit a mutual symbiotic relationship with their specific host wasps. Moreover, most encapsidated genes identified so far in PDVs share homologies with insect-originated genes, but not with virus-originated genes. In the meantime, PDVs associated with 2 wasp genera Cotesia and Glytapanteles encode some genes presumably originated from other viruses. Cotesia plutellae bracovirus (CpBV) encodes 4 genes homologous to baculoviral p94: CpBV-E94k1, CpBV-E94k2, CpBV-E94k3, and CpBV-E94k4. This study was conducted to predict the origin of CpBV-E94ks by comparing their sequences with those of baculoviral orthologs and to determine the physiological functions by their transient expressions in nonparasitized larvae and subsequent specific RNA interference. Our phylogenetic analysis indicated that CpBV-E94ks were clustered with other E94ks originated from different PDVs and shared high similarity with betabaculoviral p94s. These 4 CpBV genes were expressed during most developmental stages of the larvae of Plutella xylostella parasitized by C. plutellae. Expression of these 4 E94ks was mainly detected in hemocytes and fat body. Subsequent functional analysis by in vivo transient expression showed that all 4 viral genes significantly inhibited both host immune and developmental processes. These results suggest that CpBV-E94ks share an origin with betabaculoviral p94s and play parasitic roles in suppressing host immune and developmental processes.

Polydnaviruses: Nature's Genetic Engineers

Virus-host associations are usually viewed as parasitic, but several studies in recent years have reported examples of viruses that benefit host organisms. The Polydnaviridae are of particular interest because these viruses are all obligate mutualists of insects called parasitoid wasps. Parasitoids develop during their immature stages by feeding inside the body of other insects, which serve as their hosts. Polydnaviruses are vertically transmitted as proviruses through the germ line of wasps but also function as gene delivery vectors that wasps rely upon to genetically manipulate the hosts they parasitize. Here we review the evolutionary origin of polydnaviruses, the organization and function of their genomes, and some of their roles in parasitism.

Insect Biotechnology

For the purpose of this work, insect biotechnology, which is also known as yellow biotechnology, is the use of insects as well as insect-derived cells or molecules in medical (red biotechnology), agricultural (green biotechnology), and industrial (white) biotechnology. It is based on the application of biotechnological techniques on insects or their cells to develop products or services for human use. Such products are then applied in agriculture, medicine, and industrial biotechnology. Insect biotechnology has proven to be a useful resource in diverse industries, especially for the production of industrial enzymes including chitinases and cellulases, pharmaceuticals, microbial insecticides, insect genes, and many other substances. Insect cells (ICs), and particularly lepidopteran cells, constitute a competitive strategy to mammalian cells for the manufacturing of biotechnology products. Among the wide range of methods and expression hosts available for the production of biotech products, ICs are ideal for the production of complex proteins requiring extensive posttranslational modification. The progress so far made in insect biotechnology essentially derives from scientific breakthroughs in molecular biology, especially with the advances in techniques that allow genetic manipulation of organisms and cells. Insect biotechnology has grown tremendously in the last 30 years.

Accidental genetic engineers: horizontal sequence transfer from parasitoid wasps to their Lepidopteran hosts

We show here that 105 regions in two Lepidoptera genomes appear to derive from horizontally transferred wasp DNA. We experimentally verified the presence of two of these sequences in a diverse set of silkworm (Bombyx mori) genomes. We hypothesize that these horizontal transfers are made possible by the unusual strategy many parasitoid wasps employ of injecting hosts with endosymbiotic polydnaviruses to minimize the host's defense response. Because these virus-like particles deliver wasp DNA to the cells of the host, there has been much interest in whether genetic information can be permanently transferred from the wasp to the host. Two transferred sequences code for a BEN domain, known to be associated with polydnaviruses and transcriptional regulation. These findings represent the first documented cases of horizontal transfer of genes between two organisms by a polydnavirus. This presents an interesting evolutionary paradigm in which host species can acquire new sequences from parasitoid wasps that attack them. Hymenoptera and Lepidoptera diverged ∼300 MYA, making this type of event a source of novel sequences for recipient species. Unlike many other cases of horizontal transfer between two eukaryote species, these sequence transfers can be explained without the need to invoke the sequences 'hitchhiking' on a third organism (e.g. retrovirus) capable of independent reproduction. The cellular machinery necessary for the transfer is contained entirely in the wasp genome. The work presented here is the first such discovery of what is likely to be a broader phenomenon among species affected by these wasps.

Evolutionary mechanisms driving the evolution of a large polydnavirus gene family coding for protein tyrosine phosphatases

Background

Gene duplications have been proposed to be the main mechanism involved in genome evolution and in acquisition of new functions. Polydnaviruses (PDVs), symbiotic viruses associated with parasitoid wasps, are ideal model systems to study mechanisms of gene duplications given that PDV genomes consist of virulence genes organized into multigene families. In these systems the viral genome is integrated in a wasp chromosome as a provirus and virus particles containing circular double-stranded DNA are injected into the parasitoids’ hosts and are essential for parasitism success. The viral virulence factors, organized in gene families, are required collectively to induce host immune suppression and developmental arrest. The gene family which encodes protein tyrosine phosphatases (PTPs) has undergone spectacular expansion in several PDV genomes with up to 42 genes.

Results

Here, we present strong indications that PTP gene family expansion occurred via classical mechanisms: by duplication of large segments of the chromosomally integrated form of the virus sequences (segmental duplication), by tandem duplications within this form and by dispersed duplications. We also propose a novel duplication mechanism specific to PDVs that involves viral circle reintegration into the wasp genome. The PTP copies produced were shown to undergo conservative evolution along with episodes of adaptive evolution. In particular recently produced copies have undergone positive selection in sites most likely involved in defining substrate selectivity.

Conclusion

The results provide evidence about the dynamic nature of polydnavirus proviral genomes. Classical and PDV-specific duplication mechanisms have been involved in the production of new gene copies. Selection pressures associated with antagonistic interactions with parasitized hosts have shaped these genes used to manipulate lepidopteran physiology with evidence for positive selection involved in adaptation to host targets.

The encapsidated genome of Microplitis demolitor bracovirus integrates into the host Pseudoplusia includens

Polydnaviruses (PDVs) are symbionts of parasitoid wasps that function as gene delivery vehicles in the insects (hosts) that the wasps parasitize. PDVs persist in wasps as integrated proviruses but are packaged as circularized and segmented double-stranded DNAs into the virions that wasps inject into hosts. In contrast, little is known about how PDV genomic DNAs persist in host cells. Microplitis demolitor carries Microplitis demolitor bracovirus (MdBV) and parasitizes the host Pseudoplusia includens. MdBV infects primarily host hemocytes and also infects a hemocyte-derived cell line from P. includens called CiE1 cells. Here we report that all 15 genomic segments of the MdBV encapsidated genome exhibited long-term persistence in CiE1 cells. Most MdBV genes expressed in hemocytes were persistently expressed in CiE1 cells, including members of the glc gene family whose products transformed CiE1 cells into a suspension culture. PCR-based integration assays combined with cloning and sequencing of host-virus junctions confirmed that genomic segments J and C persisted in CiE1 cells by integration. These genomic DNAs also rapidly integrated into parasitized P. includens. Sequence analysis of wasp-viral junction clones showed that the integration of proviral segments in M. demolitor was associated with a wasp excision/integration motif (WIM) known from other bracoviruses. However, integration into host cells occurred in association with a previously unknown domain that we named the host integration motif (HIM). The presence of HIMs in most MdBV genomic DNAs suggests that the integration of each genomic segment into host cells occurs through a shared mechanism.

Analysis of promoter activity of selected Cotesia plutellae bracovirus genes

In a previous study, we cloned 27 discrete genome segments of Cotesia plutellae bracovirus (CpBV) and provided the complete nucleotide sequences and annotation. Seven putative coding regions were predicted from one of the largest segments, CpBV-S30. The activity of promoters associated with six predicted ORFs from this segment were investigated using both transient and baculovirus expression assays with enhanced green fluorescent protein as a reporter gene. CpBV promoters showed activity earlier than the polyhedrin promoter and the activity of some of these promoters was superior to that of the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ie-1 promoter in the baculovirus expression assays. The promoter of ORF3004 showed the highest level of activity in insect cells, exhibiting 24 % of the activity obtained with the AcMNPV polyhedrin promoter in Sf9 cells. In Spodoptera exigua larvae, the ORF3006 promoter showed the highest activity, with about 35 % of the activity measured with the polyhedrin promoter. In addition, analysis of the ORF3006 promoter revealed that the region between -382 and -422 from the translation start point was critical for activity of this promoter. These results suggest that the CpBV-S30 promoters characterized here could be useful tools in a variety of biotechnological applications, such as gene expression analyses and insecticide development.

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.

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.

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.

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 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.

Wasp parasitoid disruption of host development: implications for new biologically based strategies for insect control

Wasp parasitoids use a variety of methods to commandeer their insect hosts in order to create an environment that will support and promote their own development, usually to the detriment of the host insect. Parasitized insects typically undergo developmental arrest and die sometime after the parasitoid has become independent of its host. Parasitoids can deactivate their host's immune system and effect changes in host hormone titers and behavior. Often, host tissues or organs become refractory to stimulation by tropic hormones. Here we present an overview of the manipulative capabilities of wasp-injected calyx fluid containing polydnaviruses and venom, as well as the parasitoid larva and the teratocytes that originate from the serosal membrane that surrounds the developing embryo of the parasitoid. Possibilities for using regulatory molecules produced by the parasitoid or its products that would be potentially useful in developing new, environmentally safe insect control agents are discussed.

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.

Quantitative expression analysis of a Glyptapanteles indiensis polydnavirus protein tyrosine phosphatase gene in its natural lepidopteran host, Lymantria dispar

In the present study, expression of a newly identified Glyptapanteles indiensis polydnavirus (GiPDV) gene encoding a putative protein tyrosine phosphatase (PDVPTP) was monitored in vivo in the parasitized host, L. dispar, using one step RT-PCR. Expression levels of the PDVPTP transcript were also evaluated in various host tissues at different times post parasitization (pp) using RT quantitative competitive PCR (RT-qcPCR). Expression levels varied, with the most abundant transcript detected in host haemolymph 2 h pp. The high expression level in host haemolymph at an early stage of parasitization suggested a potential role for viral PDVPTP in disruption of the host immune system and protection of the endoparasitoid egg from encapsulation. Additionally, the PDVPTP gene or its homolog(s) mapped to more than one GiPDV genomic DNA segment, which may account for its increased level of expression in the absence of virus replication.

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.

Persistence and expression of Cotesia congregata polydnavirus in host larvae of the tobacco hornworm, Manduca sexta

The gregarious braconid wasp Cotesia congregata parasitizes host larvae of Manduca sexta, and several other sphingid species. Parasitism induces host immunosuppression due to the disruptive action of the wasp's polydnavirus (PDV) on host blood cells. During the initial stages of parasitism, these cells undergo apoptosis followed by cell clumping, which clears the hemolymph of a large number of cells. In this study, the persistence and expression of Cotesia congregata PDV (CcPDV) were examined using Southern and Northern blots, respectively. Digoxygenin-labelled total polydnaviral DNA was used to probe genomic DNA isolated from fat body and brains of hosts with emerged wasps taken 6 days following egress of the parasitoids, and significant cross-hybridization between the host fat body genomic DNA with viral DNA was seen. Thus, the virus persists in the host for the duration of parasitism, even during the post-emergence period, and may even be integrated in the host caterpillar DNA. Viral gene expression was examined using Northern blots and probes to the Cotesia rubecula CrV1 homolog, and the CrV1-like mRNAs were expressed as early as 4 h post-parasitization for at least 72 h and faint hybrization is even seen at the time the wasps eclose. In contrast, in Pieris rapae larvae the CrV1 transcript is expressed only for a brief time, during which time hemocyte function is disrupted. The effect is transitory, and hemocytes regain their normal functions after the parasites emerge as first instars.The genome of CcPDV contains one copy of the CrV1-like homolog as shown on Southern blots of viral genomic DNA. In conjunction with our earlier studies of the PDV-encoded early protein 1, the current work suggests multiple viral transcripts are produced following parasitization of the host, and likely target host hemocytes to induce their apoptosis, thereby preventing encapsulation of the parasitoid's eggs. Whether viral DNAs are integrated in the host's genomic DNA remains to be proven, but our results provide preliminary evidence that viral DNAs are detected in the host's fat body cells examined at the time of wasp emergence and several days later.

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.

Polydnavirus of Campoletis chlorideae: characterization and temporal effect on host Helicoverpa armigera cellular immune response

Polydnavirus was isolated from oviduct calyx in the parasitoid wasp Campoletis chlorideae (Hymenoptera: Ichneumonidae), and termed CcIV. The virus particles consist of fusiform nucleocapsids surrounded by two unit membrane envelopes. The DNAs purified from these viruses were found diversified in molecular weight and existed in nonequimolar concentrations. At least 20 different-sized bands were present after electrophoresis, and they ranged from approximately 3 to 26 kb. Persistence and gene expression of CcIV were examined in parasitized and virus-injected Helicoverpa armigera larvae. Viral DNA could be detected in the hemocyte of H. armigera at 30 min post-parasitization (p.p.), and persisted for 6 days. While no viral DNA increase was found, CcIV transcripts were first detected in host hemocytes at day 1 p.p. and continued for 5 days. Similar transcripts were observed in hemocytes from larvae that had been injected with calyx fluid or CcIV 24 h earlier. CcIV viral DNAs hybridized only with certain first-strand cDNAs from hemocytes, suggesting that only part of the CcIV genome was expressed in H. armigera. The pattern of CcIV gene expression was consistent with that of the inhibition of encapsulation for Sephadex G-10 and parasitoid eggs by host larvae. The recovery of host immune response at day 4 p.p. indicated that CcIV exhibited a partial and temporal effect on the host immune system and the developing parasitoid appeared to avoid encapsulation via different mechanisms.

Persistent expression of a newly characterized Hyposoter didymator polydnavirus gene in long-term infected lepidopteran cell lines

An Hyposoter didymator ichnovirus (HdIV) gene was stably maintained and efficiently transcribed in lepidopteran cell lines more than 3 years after HdIV infection. This K-gene had two introns and the fully spliced cDNA, named K19, comprised a short open reading frame and a long 3'-untranslated region with 13 imperfectly repeated sequences (44 to 102 nt). Transcripts related to the K-gene were detected in several long-term infected cell lines (Sf9, Spodoptera littoralis haemocytes, Trichoplusia ni). Conversely, no transcripts related to seven other viral cDNAs were detected, suggesting that the K-related DNA is selectively retained in long-term infected Sf9 cells. The function of the K-gene product and its association with stably transformed insect cell lines remains to be investigated.