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The Polydnaviridae is a family of double-stranded DNA viruses that are symbionts of parasitoid wasps. The family is currently divided into two genera, the Ichnovirus (IV) and Bracovirus (BV), which are associated with wasps in the families Ichneumonidae and Braconidae, respectively. IVs and BVs have similar immunosuppressive and developmental effects on parasitized hosts but their encapsidated genomes largely encode different genes. To assess whether IV and BV infection has similar or disparate effects on the transcriptome of shared hosts, we characterized the effects of Hyposoter didymator Ichnovirus (HdIV) and Microplitis demolitor Bracovirus (MdBV) on the fat body and hemocyte transcriptome of Spodoptera frugiperda (Lepidoptera: Noctuidae). Our results indicated that HdIV and MdBV infection alters the abundance of a relatively low proportion of S. frugiperda transcripts at 24 h post-infection. A majority of the transcripts affected by infection also differed between MdBV and HdIV. However, we did identify some host transcripts that were similarly affected by both viruses. A majority of these genes were transcribed in the fat body and most belonged to functional classes with roles in immunity, detoxification, or cell structure. Particularly prominent in this suite of transcripts were genes encoding for predicted motor-related and collagen IV-like proteins. Overall, our data suggest that the broadly similar effects that HdIV and MdBV have on host growth and immunity are not due to these viruses inducing profound changes in host gene expression. Given though that IVs and BVs encode few shared genes, the host transcripts that are similarly affected by HdIV and MdBV could indicate convergence by each virus to target a few processes at the level of transcription that are important for successful parasitism of hosts by H. didymator and M. demolitor., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Many thousands of endoparasitic wasp species are known to inject polydnavirus (PDV) particles into their caterpillar host during oviposition, causing immune and developmental dysfunctions that benefit the wasp larva. PDVs associated with braconid and ichneumonid wasps, bracoviruses and ichnoviruses respectively, both deliver multiple circular dsDNA molecules to the caterpillar. These molecules contain virulence genes but lack core genes typically involved in particle production. This is not completely unexpected given that no PDV replication takes place in the caterpillar. Particle production is confined to the wasp ovary where viral DNAs are generated from proviral copies maintained within the wasp genome. We recently showed that the genes involved in bracovirus particle production reside within the wasp genome and are related to nudiviruses. In the present work we characterized genes involved in ichnovirus particle production by analyzing the components of purified Hyposoter didymator Ichnovirus particles by LC-MS/MS and studying their organization in the wasp genome. Their products are conserved among ichnovirus-associated wasps and constitute a specific set of proteins in the virosphere. Strikingly, these genes are clustered in specialized regions of the wasp genome which are amplified along with proviral DNA during virus particle replication, but are not packaged in the particles. Clearly our results show that ichnoviruses and bracoviruses particles originated from different viral entities, thus providing an example of convergent evolution where two groups of wasps have independently domesticated viruses to deliver genes into their hosts.

Polydnaviruses (PDVs) are obligate symbionts of hymenopteran parasitoids of lepidopteran larvae that induce host immunosuppression and physiological redirection. PDVs include bracoviruses (BVs) and ichnoviruses (IVs), which are associated with braconid and ichneumonid wasps, respectively. In this study, the gene family encoding IkappaB-like proteins in the BVs associated with Cotesia congregata (CcBV) and Toxoneuron nigriceps (TnBV) was analysed. PDV-encoded IkappaB-like proteins (ANK) are similar to insect and mammalian IkappaB, an inhibitor of the transcription factor nuclear factor kappaB (NF-kappaB), but display shorter ankyrin domains and lack the regulatory domains for signal-mediated degradation and turnover. Phylogenetic analysis of ANK proteins indicates that those of IVs and BVs are closely related, even though these two taxa are believed to lack a common ancestor. Starting from a few hours after parasitization, the transcripts of BV ank genes were detected, at different levels, in several host tissues. The structure of the predicted proteins suggests that they may stably bind NF-kappaB/Rel transcription factors of the tumour necrosis factor (TNF)/Toll immune pathway. Accordingly, after bacterial challenge of Heliothis virescens host larvae parasitized by T. nigriceps, NF-kappaB immunoreactive material failed to enter the nucleus of host haemocytes and fat body cells. Moreover, transfection experiments in human HeLa cells demonstrated that a TnBV ank1 gene product reduced the efficiency of the TNF-alpha-induced expression of a reporter gene under NF-kappaB transcriptional control. Altogether, these results suggest strongly that TnBV ANK proteins cause retention of NF-kappaB/Rel factors in the cytoplasm and may thus contribute to suppression of the immune response in parasitized host larvae.

Cotesia congregata is a parasitoid wasp that injects its eggs in the host caterpillar Manduca sexta. In this host-parasite interaction, successful parasitism is ensured by a third partner: a bracovirus. The relationship between parasitic wasps and bracoviruses constitutes one of the few known mutualisms between viruses and eukaryotes. The C. congregata bracovirus (CcBV) is injected at the same time as the wasp eggs in the host hemolymph. Expression of viral genes alters the caterpillar's immune defense responses and developmental program, resulting in the creation of a favorable environment for the survival and emergence of adult parasitoid wasps. Here, we describe the characterization of a CcBV multigene family which is highly expressed during parasitism and which encodes three proteins with homology to members of the cystatin superfamily. Cystatins are tightly binding, reversible inhibitors of cysteine proteases. Other cysteine protease inhibitors have been described for lepidopteran viruses; however, this is the first description of the presence of cystatins in a viral genome. The expression and purification of a recombinant form of one of the CcBV cystatins, cystatin 1, revealed that this viral cystatin is functional having potent inhibitory activity towards the cysteine proteases papain, human cathepsins L and B and Sarcophaga cathepsin B in assays in vitro. CcBV cystatins are, therefore, likely to play a role in host caterpillar physiological deregulation by inhibiting host target proteases in the course of the host-parasite interaction.

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.

Little is known of the fate of viruses involved in long-term obligatory associations with eukaryotes. For example, many species of parasitoid wasps have symbiotic viruses to manipulate host defenses and to allow development of parasitoid larvae. The complete nucleotide sequence of the DNA enclosed in the virus particles injected by a parasitoid wasp revealed a complex organization, resembling a eukaryote genomic region more than a viral genome. Although endocellular symbiont genomes have undergone a dramatic loss of genes, the evolution of symbiotic viruses appears to be characterized by extensive duplication of virulence genes coding for truncated versions of cellular proteins.

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.