Ikappabeta-related vankyrin genes in the Campoletis sonorensis ichnovirus: temporal and tissue-specific patterns of expression in parasitized Heliothis virescens lepidopteran hosts

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.

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

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

Phylogenomics of Ichneumonoidea (Hymenoptera) and implications for evolution of mode of parasitism and viral endogenization

Ichneumonoidea is one of the most diverse lineages of animals on the planet with >48,000 described species and many more undescribed. Parasitoid wasps of this superfamily are mostly beneficial insects that attack and kill other arthropods and are important for understanding diversification and the evolution of life history strategies related to parasitoidism. Further, some lineages of parasitoids within Ichneumonoidea have acquired endogenous virus elements (EVEs) that are permanently a part of the wasp's genome and benefit the wasp through host immune disruption and behavioral control. Unfortunately, understanding the evolution of viral acquisition, parasitism strategies, diversification, and host immune disruption mechanisms, is deeply limited by the lack of a robust phylogenetic framework for Ichneumonoidea. Here we design probes targeting 541 genes across 91 taxa to test phylogenetic relationships, the evolution of parasitoid strategies, and the utility of probes to capture polydnavirus genes across a diverse array of taxa. Phylogenetic relationships among Ichneumonoidea were largely well resolved with most higher-level relationships maximally supported. We noted codon use biases between the outgroups, Braconidae, and Ichneumonidae and within Pimplinae, which were largely solved through analyses of amino acids rather than nucleotide data. These biases may impact phylogenetic reconstruction and caution for outgroup selection is recommended. Ancestral state reconstructions were variable for Braconidae across analyses, but consistent for reconstruction of idiobiosis/koinobiosis in Ichneumonidae. The data suggest many transitions between parasitoid life history traits across the whole superfamily. The two subfamilies within Ichneumonidae that have polydnaviruses are supported as distantly related, providing strong evidence for two independent acquisitions of ichnoviruses. Polydnavirus capture using our designed probes was only partially successful and suggests that more targeted approaches would be needed for this strategy to be effective for surveying taxa for these viral genes. In total, these data provide a robust framework for the evolution of Ichneumonoidea.

Intracellular dynamics of polydnavirus innexin homologues

Polydnaviruses associated with ichneumonid parasitoid wasps (Ichnoviruses) encode large numbers of genes, often in multigene families. The Ichnovirus Vinnexin gene family, which is expressed in parasitized lepidopteran larvae, encodes homologues of Innexins, the structural components of insect gap junctions. Here, we have examined intracellular behaviours of the Campoletis sonorensis Ichnovirus (CsIV) Vinnexins, alone and in combination with a host Innexin orthologue, Innexin2 (Inx2). QRT-PCR verified that transcription of CsIV vinnexins occurs contemporaneously with inx2, implying co-occurrence of Vinnexin and Inx2 proteins. Confocal microscopy demonstrated that epitope-tagged VinnexinG (VnxG) and VinnexinQ2 (VnxQ2) exhibit similar subcellular localization as Spodoptera frugiperda Inx2 (Sf-Inx2). Surface biotinylation assays verified that all three proteins localize to the cell surface, and cytochalasin B and nocodazole that they rely on actin and microtubule cytoskeletal networks for localization. Immunomicroscopy following co-transfection of constructs indicates extensive co-localization of Vinnexins with each other and Sf-Inx2, and live-cell imaging of mCherry-labelled Inx2 supports that Vinnexins may affect Sf-Inx2 distribution in a Vinnexin-specific fashion. Our findings support that the Vinnexins may disrupt host cell physiology in a protein-specific manner through altering gap junctional intercellular channel communication, as well as indirectly by affecting multicellular junction characteristics.

Detoxification of plant defensive glucosinolates by an herbivorous caterpillar is beneficial to its endoparasitic wasp

Plant chemical defences impact not only herbivores, but also organisms in higher trophic levels that prey on or parasitize herbivores. While herbivorous insects can often detoxify plant chemicals ingested from suitable host plants, how such detoxification affects endoparasitoids that use these herbivores as hosts is largely unknown. Here, we used transformed plants to experimentally manipulate the major detoxification reaction used by Plutella xylostella (diamondback moth) to deactivate the glucosinolate defences of its Brassicaceae host plants. We then assessed the developmental, metabolic, immune, and reproductive consequences of this genetic manipulation on the herbivore as well as its hymenopteran endoparasitoid Diadegma semiclausum. Inhibition of P. xylostella glucosinolate metabolism by plant-mediated RNA interference increased the accumulation of the principal glucosinolate activation products, the toxic isothiocyanates, in the herbivore, with negative effects on its growth. Although the endoparasitoid manipulated the excretion of toxins by its insect host to its own advantage, the inhibition of herbivore glucosinolate detoxification slowed endoparasitoid development, impaired its reproduction, and suppressed the expression of genes of a parasitoid-symbiotic polydnavirus that aids parasitism. Therefore, the detoxification of plant glucosinolates by an herbivore lowers its toxicity as a host and benefits the parasitoid D. semiclausum at multiple levels.

Facilitator roles of viruses in enhanced insect resistance to biotic stress

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

Insights into the venom protein components of Microplitis mediator, an endoparasitoid wasp

Endoparasitoid wasps deliver a variety of maternal factors, such as venom proteins, viruses, and virus-like particles, from their venom and calyx fluid into hosts and thereby regulate the hosts' immune response, metabolism and development. The endoparasitoid, Microplitis mediator, is used as an important biological agent for controlling the devastating pest Helicoverpa armigera. In this study, using an integrated transcriptomic and proteomic analysis approach, we identified 75 putative venom proteins in M. mediator. The identified venom components were consistent with other known parasitoid wasps' venom proteins, including metalloproteases, serine protease inhibitors, and glycoside hydrolase family 18 enzymes. The metalloprotease and serpin family showed extensive gene duplications in venom apparatus. Isobaric tags for relative and absolute quantitation (iTRAQ) based quantitative proteomics revealed 521 proteins that were differentially expressed at 6 h and 24 h post-parasitism, including 10 wasp venom proteins that were released into the host hemolymph. Further analysis indicated that 511 differentially expressed proteins (DEP) from the host are primarily involved in the immune response, material metabolism, and extracellular matrix receptor interaction. Taken together, our results on parasitoid wasp venoms have the potential to enhance the application of endoparasitoid wasps for controlling insect pest.

Parasitoid polydnaviruses and immune interaction with secondary hosts

Polydnaviruses (PDVs) are obligatory symbionts with parasitoid wasps. The PDV virions are produced solely in wasp (the primary host) calyx cells. They are injected into caterpillar hosts (the secondary host) during parasitoid oviposition, where they express irreplaceable actions to ensure survival and development of wasp larvae. Some of PDV gene products suppress host immune responses while others alter host growth, metabolism or endocrine system. Here, we treat new findings on PDV gene products and their action on immunity within secondary hosts.

Improving the baculovirus expression vector system with vankyrin-enhanced technology

The baculovirus expression vector system (BEVS) is a widely used platform for the production of recombinant eukaryotic proteins. However, the BEVS has limitations in comparison to other higher eukaryotic expression systems. First, the insect cell lines used in the BEVS cannot produce glycoproteins with complex‐type N‐glycosylation patterns. Second, protein production is limited as cells die and lyse in response to baculovirus infection. To delay cell death and lysis, we transformed several insect cell lines with an expression plasmid harboring a vankyrin gene (P‐vank‐1), which encodes an anti‐apoptotic protein. Specifically, we transformed Sf9 cells, Trichoplusia ni High Five^TM cells, and SfSWT‐4 cells, which can produce glycoproteins with complex‐type N‐glycosylation patterns. The latter was included with the aim to increase production of glycoproteins with complex N‐glycans, thereby overcoming the two aforementioned limitations of the BEVS. To further increase vankyrin expression levels and further delay cell death, we also modified baculovirus vectors with the P‐vank‐1 gene. We found that cell lysis was delayed and recombinant glycoprotein yield increased when SfSWT‐4 cells were infected with a vankyrin‐encoding baculovirus. A synergistic effect in elevated levels of recombinant protein production was observed when vankyrin‐expressing cells were combined with a vankyrin‐encoding baculovirus. These effects were observed with various model proteins including medically relevant therapeutic proteins. In summary, we found that cell lysis could be delayed and recombinant protein yields could be increased by using cell lines constitutively expressing vankyrin or vankyrin‐encoding baculovirus vectors. © 2017 The Authors Biotechnology Progress published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers Biotechnol. Prog., 33:1496–1507, 2017

High temperature induces downregulation of polydnavirus gene transcription in lepidopteran host and enhances accumulation of host immunity gene transcripts

Endoparasitoids face the challenge of overcoming the immune reaction of their hosts, which typically consists of encapsulation and melanisation of parasitoid eggs or larvae. Some endoparasitic wasps such as the solitary Tranosema rostrale (Hymenoptera: Ichneumonidae) that lay their eggs in larvae of the spruce budworm, Choristoneura fumiferana (Lepidoptera: Tortricidae), have evolved a symbiotic relationship with a polydnavirus (PDV), which in turn helps them suppress the host's immune response. We observed an increase in mortality of immature T. rostrale with increasing temperature, and we tested two hypotheses about the mechanisms involved: high temperatures (1) hamper the expression of T. rostrale PDV genes and (2) enhance the expression of spruce budworm immunity-related genes. Dissections of parasitized spruce budworm larvae reared at 30°C revealed that most parasitoid eggs or larvae had died as a result of encapsulation and melanisation by the host. A qPCR analysis of T. rostrale PDV (TrIV) gene expression showed that the transcription of several TrIV genes in host larvae was downregulated at high temperature. On the other hand, encapsulation, but not melanisation, of foreign bodies in spruce budworm larvae was enhanced at high temperatures, as shown by the injection of Sephadex™ beads into larvae. However, at the molecular level, the transcription of genes related to spruce budworm's melanisation process (prophenoloxidase 1 and 2) was upregulated. Our results support the hypothesis that a temperature-dependent increase of encapsulation response is due to the combined effects of reduced expression of TrIV genes and enhanced expression of host immune genes.

Virus Innexins induce alterations in insect cell and tissue function

Polydnaviruses are dsDNA viruses that induce immune and developmental alterations in their caterpillar hosts. Characterization of polydnavirus gene families and family members is necessary to understand mechanisms of pathology and evolution of these viruses, and may aid to elucidate the role of host homologues if present. For example, the polydnavirus vinnexin gene family encodes homologues of insect gap junction genes (innexins) that are expressed in host immune cells (hemocytes). While the roles of Innexin proteins and gap junctions in insect immunity are largely unclear, we previously demonstrated that Vinnexins form functional gap junctions and alter the junctional characteristics of a host Innexin when co-expressed in paired Xenopus oocytes. Here, we test the effect of ectopic vinnexin expression on host cell physiology using both a lepidopteran cell culture model and a dipteran whole organism model. Vinnexin expression in the cell culture system resulted in gene-specific alterations in cell morphology and a slight, but non-statistically significant, reduction in gap junction activity as measured by dye transfer, while ectopic expression of a lepidopteran innexin2 gene led to morphological alterations and increase in gap junction activity. Global ectopic expression in the model dipteran, Drosophila melanogaster, of one vinnexin (vinnexinG) or D. melanogaster innexin2 (Dm-inx2) resulted in embryonic lethality, while expression of the other vinnexin genes had no effect. Furthermore, ectopic expression of vinnexinG, but not other vinnexin genes or Dm-inx2, in D. melanogaster larval gut resulted in developmental arrest in the pupal stage. These data indicate the vinnexins likely have gene-specific roles in host manipulation. They also support the use of Drosophila in further analysis of the role of Vinnexins and other polydnavirus genes in modifying host physiological processes. Finally, our findings suggest the vinnexin genes may be useful to perturb and characterize the physiological functions of insect Innexins.

Cytokine Diedel and a viral homologue suppress the IMD pathway in Drosophila

Viruses are obligatory intracellular parasites that suffer strong evolutionary pressure from the host immune system. Rapidly evolving viral genomes can adapt to this pressure by acquiring genes that counteract host defense mechanisms. For example, many vertebrate DNA viruses have hijacked cellular genes encoding cytokines or cytokine receptors to disrupt host cell communication. Insect viruses express suppressors of RNA interference or apoptosis, highlighting the importance of these cell intrinsic antiviral mechanisms in invertebrates. Here, we report the identification and characterization of a family of proteins encoded by insect DNA viruses that are homologous to a 12-kDa circulating protein encoded by the virus-induced Drosophila gene diedel (die). We show that die mutant flies have shortened lifespan and succumb more rapidly than controls when infected with Sindbis virus. This reduced viability is associated with deregulated activation of the immune deficiency (IMD) pathway of host defense and can be rescued by mutations in the genes encoding the homolog of IKKγ or IMD itself. Our results reveal an endogenous pathway that is exploited by insect viruses to modulate NF-κB signaling and promote fly survival during the antiviral response.

Parasitism and survival rate of Diadegma fenestrale (Hymenoptera: Ichneumonidae) and DfIV gene expression patterns in two lepidopteran hosts

The genus Diadegma is a well-known parasitoid group and some are known to have symbiotic virus, polydnavirus (PDV). A novel IV was discovered from the calyx of Diadegma fenestrale female and sequenced its genome. D. fenestrale has more than two hosts, including potato tuber moth (PTM) and diamondback moth (DBM). D. fenestrale preferred PTM to DBM as hosts based on the oviposition and survival rate. Nevertheless, the developmental period and morphology of D. fenestrale were not significantly different between PTM and DBM. We compared DfIV gene expression patterns between PTM and DBM under various conditions to understand the phenomena. DfIV genes were more widely expressed in PTM with large numbers than in DBM after parasitized by D. fenestrale, particularly at the initial point. They showed differential expression patterns between two lepidopteran hosts. This DfIV gene expression plasticity showed a dependency on the lepidopteran host species and parasitization time, suggesting that it may contribute to increase the parasitoid survival rate. This might be one of the key elements that determine the symbiotic relationship between PDV and parasitoid.

Specificities of ichnoviruses associated with campoplegine wasps: genome, genes and role in host-parasitoid interaction

Ichnoviruses (IVs), unique symbiotic viruses carried by ichneumonid campoplegine wasps, derive from integration of a paleo-ichnovirus into an ancestral wasp genome. The modern 'genome' is composed of both regions that are amplified, circularized and encapsidated into viral particles and non-encapsidated viral genomic regions involved in particle morphogenesis. Packaged genomes include multiple circular dsDNAs encoding many genes mostly organized in gene families. Virus particles are assembled in specialized ovarian cells from which they exit into the oviduct lumen; mature virions are injected during oviposition into the insect host. Expression of viral proteins in infected cells correlates with physiological alterations of the host enabling success of parasitism.

Extensive transcription analysis of the Hyposoter didymator Ichnovirus genome in permissive and non-permissive lepidopteran host species

Ichnoviruses are large dsDNA viruses that belong to the Polydnaviridae family. They are specifically associated with endoparasitic wasps of the family Ichneumonidae and essential for host parasitization by these wasps. We sequenced the Hyposoter didymator Ichnovirus (HdIV) encapsidated genome for further analysis of the transcription pattern of the entire set of HdIV genes following the parasitization of four different lepidopteran host species. The HdIV genome was found to consist of at least 50 circular dsDNA molecules, carrying 135 genes, 98 of which formed 18 gene families. The HdIV genome had general features typical of Ichnovirus (IV) genomes and closely resembled that of the IV carried by Hyposoter fugitivus. Subsequent transcriptomic analysis with Illumina technology during the course of Spodoptera frugiperda parasitization led to the identification of a small subset of less than 30 genes with high RPKM values in permissive hosts, consisting with these genes encoding crucial virulence proteins. Comparisons of HdIV expression profiles between host species revealed differences in transcript levels for given HdIV genes between two permissive hosts, S. frugiperda and Pseudoplusia includens. However, we found no evident intrafamily gene-specific transcription pattern consistent with the presence of multigenic families within IV genomes reflecting an ability of the wasps concerned to exploit different host species. Interestingly, in two non-permissive hosts, Mamestra brassiccae and Anticarsia gemmatalis (most of the parasitoid eggs were eliminated by the host cellular immune response), HdIV genes were generally less strongly transcribed than in permissive hosts. This suggests that successful parasitism is dependent on the expression of given HdIV genes exceeding a particular threshold value. These results raise questions about the mecanisms involved in regulating IV gene expression according to the nature of the lepidopteran host species encountered.

How the venom from the ectoparasitoid Wasp nasonia vitripennis exhibits anti-inflammatory properties on mammalian cell lines

With more than 150,000 species, parasitoids are a large group of hymenopteran insects that inject venom into and then lay their eggs in or on other insects, eventually killing the hosts. Their venoms have evolved into different mechanisms for manipulating host immunity, physiology and behavior in such a way that enhance development of the parasitoid young. The venom from the ectoparasitoid Nasonia vitripennis inhibits the immune system in its host organism in order to protect their offspring from elimination. Since the major innate immune pathways in insects, the Toll and Imd pathways, are homologous to the NF-κB pathway in mammals, we were interested in whether a similar immune suppression seen in insects could be elicited in a mammalian cell system. A well characterized NF-κB reporter gene assay in fibrosarcoma cells showed a dose-dependent inhibition of NF-κB signaling caused by the venom. In line with this NF-κB inhibitory action, N. vitripennis venom dampened the expression of IL-6, a prototypical proinflammatory cytokine, from LPS-treated macrophages. The venom also inhibited the expression of two NF-κB target genes, IκBα and A20, that act in a negative feedback loop to prevent excessive NF-κB activity. Surprisingly, we did not detect any effect of the venom on the early events in the canonical NF-κB activation pathway, leading to NF-κB nuclear translocation, which was unaltered in venom-treated cells. The MAP kinases ERK, p38 and JNK are other crucial regulators of immune responses. We observed that venom treatment did not affect p38 and ERK activation, but induced a prolonged JNK activation. In summary, our data indicate that venom from N. vitripennis inhibits NF-κB signaling in mammalian cells. We identify venom-induced up regulation of the glucocorticoid receptor-regulated GILZ as a most likely molecular mediator for this inhibition.

A polydnavirus ANK protein acts as virulence factor by disrupting the function of prothoracic gland steroidogenic cells

Polydnaviruses are obligate symbionts integrated as proviruses in the genome of some ichneumonoid wasps that parasitize lepidopteran larvae. Polydnavirus free viral particles, which are injected into the host at oviposition, express virulence factors that impair immunity and development. To date, most studies have focused on the molecular mechanisms underpinning immunosuppression, whereas how viral genes disrupt the endocrine balance remains largely uninvestigated. Using Drosophila as a model system, the present report analyzes the function of a member of the ankyrin gene family of the bracovirus associated with Toxoneuron nigriceps, a larval parasitoid of the noctuid moth Heliothis virescens. We found that the TnBVank1 expression in the Drosophila prothoracic gland blocks the larval-pupal molt. This phenotype can be rescued by feeding the larvae with 20-hydroxyecdysone. The localization of the TnBVANK1 is restricted to the cytoplasm where it interacts with Hrs and Alix marked endosomes. Collectively, our data demonstrate that the TnBVANK1 protein acts as a virulence factor that causes the disruption of ecdysone biosynthesis and developmental arrest by impairing the vesicular traffic of ecdysteroid precursors in the prothoracic gland steroidogenic cells.

Polydnaviral ankyrin proteins aid parasitic wasp survival by coordinate and selective inhibition of hematopoietic and immune NF-kappa B signaling in insect hosts

Polydnaviruses are mutualists of their parasitoid wasps and express genes in immune cells of their Lepidopteran hosts. Polydnaviral genomes carry multiple copies of viral ankyrins or vankyrins. Vankyrin proteins are homologous to IκB proteins, but lack sequences for regulated degradation. We tested if Ichnoviral Vankyrins differentially impede Toll-NF-κB-dependent hematopoietic and immune signaling in a heterologous in vivo Drosophila, system. We first show that hematopoiesis and the cellular encapsulation response against parasitoid wasps are tightly-linked via NF-κB signaling. The niche, which neighbors the larval hematopoietic progenitors, responds to parasite infection. Drosophila NF-κB proteins are expressed in the niche, and non cell-autonomously influence fate choice in basal and parasite-activated hematopoiesis. These effects are blocked by the Vankyrin I²-vank-3, but not by P-vank-1, as is the expression of a NF-κB target transgene. I²-vank-3 and P-vank-1 differentially obstruct cellular and humoral inflammation. Additionally, their maternal expression weakens ventral embryonic patterning. We propose that selective perturbation of NF-κB-IκB interactions in natural hosts of parasitic wasps negatively impacts the outcome of hematopoietic and immune signaling and this immune deficit contributes to parasite survival and species success in nature.

Parasitoid wasps are insects whose development takes place within the body of other insects. To survive, wasp larvae must overcome the immune defenses of their hosts. How parasitic wasps overcome host immunity is not fully understood even though we know that different strategies using venoms, virus-like particles, or viruses are involved. A unique class of viruses, called polydnaviruses is found in two families of wasps that comprise more than 20,000 wasp species. The genomes of polydnaviruses encode proteins with ankyrin repeats. Ankyrin repeats are also found in Cactus, the inhibitor protein of NF-κB signaling in Drosophila. Viral ankyrin proteins, or Vankyrins, however, lack the amino acid sequences necessary for turnover found in Cactus and mammalian IκB family members. We show that Vankyrins produced by polydnaviruses of a parasitic wasp that attacks caterpillars of many common agricultural pests can block NF-κB signaling in fruit fly larvae. This inhibition supports parasite success. Our work highlights the crucial role of NF-κB signaling across insect taxa in insect-insect and insect-virus interactions. Studies of polydnaviral ankyrin proteins in Drosophila reveal that immune-suppressive viruses may block both cellular and humoral immunity in insects to win the biological ‘arms race’.

Analysis of gene transcription and relative abundance of the cys-motif gene family from Campoletis sonorensis ichnovirus (CsIV) and further characterization of the most abundant cys-motif protein, WHv1.6

The cys-motif gene family associated with Campoletis sonorensis ichnovirus contains 10 members, WHv1.6, WHv1.0, VHv1.1, VHv1.4, AHv1.0, A'Hv0.8, FHv1.4, LHv2.8, UHv0.8, and UHv0.8a. The results of this study indicated that, within the encapsidated virion, WHv1.6 is the most abundant cys-motif gene, while the combined AHv genes are the least abundant. During parasitization of Heliothis virescens by Campoletis sonorenis, WHv1.6 transcripts were the mostly highly expressed, while the combined UHv genes had the lowest expression. Further proteomic analysis of WHv1.6 showed that it accumulates at high levels in parasitized plasma by 6 h, and is detectable in the haemocytes, fat body, malpighian tubules, nerve cord and epidermis by 2 days after parasitization. Localization experiments led us to conclude that WHv1.6 interacts with the cell membrane along with other organelles within a virus-infected cell and prevents immunocytes from spreading or adhering to a foreign surface. Similarly to VHv1.4 and VHv1.1, WHv1.6 is able to inhibit the translation of haemocyte and Malpighian tubule RNAs. Our results showed that the expression of cys-motif genes during parasitization is related to the gene copy number of each gene within the encapsidated virion and may also be dependent upon cis-regulatory element activity in different target tissues. In addition, WHv1.6 plays a major role in inhibiting the cellular encapsulation response by H. virescens.

Characterization of the polydnaviral ‘T. rostrale virus’ (TrV) gene family: TrV1 expression inhibits in vitro cell proliferation

Tranosema rostrale ichnovirus (TrIV) is a polydnavirus (PDV) transmitted by the endoparasitic wasp T. rostrale to its host Choristoneura fumiferana during oviposition. PDV genes are expressed in infected caterpillars, causing physiological disturbances that promote the survival of the developing endoparasite. The previously sequenced genome of TrIV contains ~86 genes organized in multigene families and distributed on multiple segments of circular dsDNA. Among these, the ‘T. rostrale virus’ (TrV) family comprises seven genes that are absent in other PDV genomes examined to date and whose function(s) remain(s) unknown. Here, we initiated a functional analysis of the TrV family using qPCR, transfection and RNAi approaches. TrV family genes were weakly expressed in wasp ovaries, but some displayed high transcript abundance in parasitized caterpillars. Whilst TrV1 was the most highly transcribed TrV gene in infected caterpillars, transcript levels for TrV5 and TrV6 were nearly undetectable, indicating that they may be pseudogenes. Temporal and tissue-specific patterns of transcript abundance were similar for all expressed TrV family genes, indicative of an apparent lack of difference in function or tissue specificity. Infection of Cf-203 and Sf-21 insect cells with TrIV led to a dose-dependent inhibition of cell proliferation with no sign of apoptosis. Whilst similar inhibition was observed following transfection of cells with a cloned genome segment carrying the TrV1 gene, RNA interference targeting TrV1 largely restored cell growth in TrIV-infected cells, indicating that TrV1 expression was responsible for the observed inhibition. We suggest that TrV genes may contribute to host developmental disruption by interfering with host-cell proliferation during parasitism.

Intrinsic inter- and intraspecific competition in parasitoid wasps

Immature development of parasitoid wasps is restricted to resources found in a single host that is often similar in size to the adult parasitoid. When two or more parasitoids of the same or different species attack the same host, there is competition for monopolization of host resources. The success of intrinsic competition differs between parasitoids attacking growing hosts and parasitoids attacking paralyzed hosts. Furthermore, the evolution of gregarious development in parasitoids reflects differences in various developmental and behavioral traits, as these influence antagonistic encounters among immature parasitoids. Fitness-related costs (or benefits) of competition for the winning parasitoid reveal that time lags between successive attacks influence the outcome of competition. Physiological mechanisms used to exclude competitors include physical and biochemical factors that originate with the ovipositing female wasp or her progeny. In a broader multitrophic framework, indirect factors, such as plant quality, may affect parasitoids through effects on immunity and nutrition.

NF-κB: where did it come from and why?

The vast majority of research on nuclear factor κB (NF-κB) signaling in the past 25 years has focused on its roles in normal and disease-related processes in vertebrates, especially mice and humans. Recent genome and transcriptome sequencing efforts have shown that homologs of NF-κB transcription factors, inhibitor of NF-κB (IκB) proteins, and IκB kinases are present in a variety of invertebrates, including several in phyla simpler than Arthropoda, the phylum containing insects such Drosophila. Moreover, many invertebrates also contain genes encoding homologs of upstream signaling proteins in the Toll-like receptor signaling pathway, which is well-known for its downstream activation of NF-κB for innate immunity. This review describes what we now know or can infer and speculate about the evolution of the core elements of NF-κB signaling as well as the biological processes controlled by NF-κB in invertebrates. Further research on NF-κB in invertebrates is likely to uncover information about the evolutionary origins of this key human signaling pathway and may have relevance to our management of the responses of ecologically and economically important organisms to environmental and adaptive pressures.

Polydnavirus Ank proteins bind NF-κB homodimers and inhibit processing of Relish

Recent studies have greatly increased understanding of how the immune system of insects responds to infection, whereas much less is known about how pathogens subvert immune defenses. Key regulators of the insect immune system are Rel proteins that form Nuclear Factor-κB (NF-κB) transcription factors, and inhibitor κB (IκB) proteins that complex with and regulate NF-κBs. Major mortality agents of insects are parasitoid wasps that carry immunosuppressive polydnaviruses (PDVs). Most PDVs encode ank genes that share features with IκBs, while our own prior studies suggested that two ank family members from Microplitis demolitor bracovirus (MdBV) (Ank-H4 and Ank-N5) behave as IκB mimics. However, the binding affinities of these viral mimics for Rel proteins relative to endogenous IκBs remained unclear. Surface plasmon resonance (SPR) and co-immunoprecipitation assays showed that the IκB Cactus from Drosophila bound Dif and Dorsal homodimers more strongly than Relish homodimers. Ank-H4 and –N5 bound Dif, Dorsal and Relish homodimers with higher affinity than the IκB domain of Relish (Rel-49), and also bound Relish homodimers more strongly than Cactus. Ank-H4 and –N5 inhibited processing of compound Relish and reduced the expression of several antimicrobial peptide genes regulated by the Imd signaling pathway in Drosophila mbn2 cells. Studies conducted in the natural host Pseudoplusia includens suggested that parasitism by M. demolitor also activates NF-κB signaling and that MdBV inhibits this response. Overall, our data provide the first quantitative measures of insect and viral IκB binding affinities, while also showing that viral mimics disable Relish processing.

Central to the study of host-pathogen interactions is understanding how the immune system of hosts responds to infection, and reciprocally how pathogens subvert host defenses. In the case of insects, understanding of how the immune system responds to infection greatly exceeds understanding of pathogen counterstrategies. Parasitoid wasps are key mortality agents of insects. Thousands of wasp species have also evolved a symbiotic relationship with large DNA viruses in the family Polydnaviridae whose primary function is to deliver immunosuppressive virulence genes to the insect hosts that wasps parasitize. The function of most PDV-encoded virulence genes, however, remains unknown. In this article, we investigated the function of two ank gene family members from Microplitis demolitor bracovirus (MdBV). Our results indicate that Ank-H4 and Ank-N5 function as mimics of IκB proteins, which regulate a family of transcription factors called NF-κBs that control many genes of the insect immune system. IκBs and NF-κBs also function as key regulators of the mammalian immune system. Our results thus suggest that viral Ank proteins subvert the immune system of host insects by targeting conserved signaling pathways used by a diversity of organisms.

Multigenic families in Ichnovirus: a tissue and host specificity study through expression analysis of vankyrins from Hyposoter didymator Ichnovirus

The viral ankyrin (vankyrin) gene family is represented in all polydnavirus (PDVs) genomes and encodes proteins homologous to I-kappaBs, inhibitors of NF-kappaB transcription factors. The structural similarities led to the hypothesis that vankyrins mimic eukaryotic factors to subvert important physiological pathways in the infected host. Here, we identified nine vankyrin genes in the genome of the Hyposoter didymator Ichnovirus (HdIV). Time-course gene expression experiments indicate that all members are expressed throughout parasitism of Spodoptera frugiperda, as assessed using RNA extracted from whole larvae. To study tissue and/or species specificity transcriptions, the expression of HdIV vankyrin genes were compared between HdIV-injected larvae of S. frugiperda and S. littoralis. The transcriptional profiles were similar in the two species, including the largely predominant expression of Hd27-vank1 in all tissues examined. However, in various insect cell lines, the expression patterns of HdIV vankyrins differed according to species. No clear relationship between vankyrin expression patterns and abundance of vankyrin-bearing genomic segments were found in the lepidopteran cell lines. Moreover, in these cells, the amount of vankyrin-bearing genomic segments differed substantially between cytosol and nuclei of infected cells, implying the existence of an unexpected step regulating the copy number of HdIV segments in cell nuclei. Our in vitro results reveal a host-specific transcriptional profile of vankyrins that may be related to the success of parasitism in different hosts. In Spodoptera hosts, the predominant expression of Hd27-vank1 suggests that this protein might have pleiotropic functions during parasitism of these insect species.

Functional interactions between polydnavirus and host cellular innexins

Polydnaviruses are double-stranded DNA viruses associated with some subfamilies of ichneumonoid parasitoid wasps. Polydnavirus virions are delivered during wasp parasitization of a host, and virus gene expression in the host induces alterations of host physiology. Infection of susceptible host caterpillars by the polydnavirus Campoletis sonorensis ichnovirus (CsIV) leads to expression of virus genes, resulting in immune and developmental disruptions. CsIV carries four homologues of insect gap junction genes (innexins) termed vinnexins, which are expressed in multiple tissues of infected caterpillars. Previously, we demonstrated that two of these, VinnexinD and VinnexinG, form functional gap junctions in paired Xenopus oocytes. Here we show that VinnexinQ1 and VinnexinQ2, likewise, form junctions in this heterologous system. Moreover, we demonstrate that the vinnexins interact differentially with the Innexin2 orthologue of an ichnovirus host, Spodoptera frugiperda. Cell pairs coexpressing a vinnexin and Innexin2 or pairs in which one cell expresses a vinnexin and the neighboring cell Innexin2 assemble functional junctions with properties that differ from those of junctions composed of Innexin2 alone. These data suggest that altered gap junctional intercellular communication may underlie certain cellular pathologies associated with ichnovirus infection of caterpillar hosts.

Phylogenetic analysis of the large family of poxvirus ankyrin-repeat proteins reveals orthologue groups within and across chordopoxvirus genera

Ankyrin-repeat (ANK) protein-interaction domains are common in cellular proteins but are relatively rare in viruses. Chordopoxviruses, however, encode a large number of ANK domain-containing ORFs of largely unknown function. Recently, a second protein-interaction domain, an F-box-like motif, was identified in several poxvirus ANK proteins. Cellular F-box proteins recruit substrates to the ubiquitination machinery of the cell, a putative function for ANK/poxviral F-box proteins. Using publicly available genome sequence data we examined all 328 predicted ANK proteins encoded by 27 chordopoxviruses that represented the eight vertebrate poxvirus genera whose members encode ANK proteins. Within these we identified 15 putative ANK protein orthologue groups within orthopoxviruses, five within parapoxviruses, 23 within avipoxviruses and seven across members of the genera Leporipoxvirus, Capripoxvirus, Yatapoxvirus, Suipoxvirus and Cervidpoxvirus. Sequence comparisons showed that members of each of these four clusters of orthologues were not closely related to members of any of the other clusters. Of these ORFs, 67% encoded a C-terminal poxviral F-box-like motif, whose absence could largely be attributed to fragmentation of ORFs. Our findings suggest that the large family of poxvirus ANK proteins arose by extensive gene duplication and divergence that occurred independently in four major genus-based groups after the groups diverged from each other. It seems likely that the ancestor ANK proteins of poxviruses contained both the N-terminal ANK repeats and a C-terminal F-box-like domain, with the latter domain subsequently being lost in a small subset of these proteins.

The impact on microtubule network of a bracovirus IkappaB-like protein

Polydnavirus-encoded IkappaB-like proteins are similar to insect and mammalian IkappaB, and an immunosuppressive function in the host cells has been inferred to these proteins. Here we show that the expression of one of these IkappaB-like viral genes, the TnBVank1, in the Drosophila germline affects the localization of gurken, bicoid, and oskar mRNAs whose gene products are relevant for proper embryonic patterning. The altered localization of these mRNAs is suggestive of general defects in the intracellular, microtubule-based, trafficking routes. Analysis of microtubule motor proteins components such as the dynein heavy chain and the kinesin heavy chain revealed defects in the polarized microtubule network. Interestingly, the TnBVANK1 viral protein is uniformly distributed over the entire oocyte cortex, and appears to be anchored to the microtubule ends. Our data open up a very interesting issue on novel function(s) played by the ank gene family by interfering with cytoskeleton organization.

The Campoletis sonorensis ichnovirus vankyrin protein P-vank-1 inhibits apoptosis in insect Sf9 cells

The Campoletis sonorensis ichnovirus (CsIV) vankyrin genes encode proteins containing truncated ankyrin repeat domains with sequence homology to the inhibitory domains of NF-kappaB transcription factor inhibitors, IkappaBs. The CsIV vankyrin proteins are thought to be involved in the suppression of NF-kappaB activity during immune response and/or developmental events in the parasitized host. Here we report that when P-vank-1 was expressed stably from Sf9 cells, prolonged survival of these cells was observed after baculovirus infection, UV irradiation, and treatment with the apoptosis-inducing chemical camptothecin compared to untransformed Sf9 cells. Furthermore, P-vank-1 inhibited nuclear and internucleosomal degradation and caspase activity after induction of apoptosis in Sf9 cells stably expressing P-vank-1. This is the first report of a polydnavirus protein with anti-apoptotic function.

Tranosema rostrale ichnovirus repeat element genes display distinct transcriptional patterns in caterpillar and wasp hosts

The endoparasitic wasp Tranosema rostrale transmits an ichnovirus to its lepidopteran host, Choristoneura fumiferana, during parasitization. As shown for other ichnoviruses, the segmented dsDNA genome of the T. rostrale ichnovirus (TrIV) features several multi-gene families, including the repeat element (rep) family, whose products display no known similarity to non-ichnovirus proteins, except for a homologue encoded by the genome of the Helicoverpa armigera granulovirus; their functions remain unknown. This study applied linear regression of efficiency analysis to real-time PCR quantification of transcript abundance for all 17 TrIV rep open reading frames (ORFs) in parasitized and virus-injected C. fumiferana larvae, as well as in T. rostrale ovaries and head-thorax complexes. Although transcripts were detected for most rep ORFs in infected caterpillars, two of them clearly outnumbered the others in whole larvae, with a tendency for levels to drop over time after infection. The genome segments bearing the three most highly expressed rep genes in parasitized caterpillars were present in higher proportions than other rep-bearing genome segments in TrIV DNA, suggesting a possible role for gene dosage in the regulation of transcription level. TrIV rep genes also showed important differences in the relative abundance of their transcripts in specific tissues (cuticular epithelium, the fat body, haemocytes and the midgut), implying tissue-specific roles for individual members of this gene family. Significantly, no rep transcripts were detected in T. rostrale head-thorax complexes, whereas some were abundant in ovaries. There, the transcription pattern was completely different from that observed in infected caterpillars, suggesting that some rep genes have wasp-specific functions.

Characterization of an IkappaB-like gene in Cotesia vestalis polydnavirus

Cotesia vestalis (Braconidae, Hymenoptera) depends mainly on 3 regulatory factors to manipulate its host's development and immune response, including polydnavirus, venom, and teratocytes, among which polydnavirus plays a key role in suppressing the host immune system. In the present work, we cloned the full sequence of gene CvBV-ank2, encoding an IkappaB-like protein in C. vestalis polydnavirus (CvBV). The full sequence of CvBV-ank2 is 955 bp, encoding 162 amino acids with a calculated molecular mass of 18,355 Da. CvBV-ank2 shares high similarity with the exon I and exon II of CvBV-ank1, which is on the same segment with CvBV-ank2. This result suggests that gene duplication might occur in CvBV-ank1 and CvBV-ank2. Phylogenetic analysis indicated that CvBV-ank2 and CvBV-ank1, both on segment CvBV-S2, are, respectively, closely related with CcBV-26.3 and CcBV-26.2, both on segment Circle26 of C. congregata polydnavirus (CcBV). BLAST search using the sequence of segment CvBV-S2 as a query revealed that segment CvBV-S2 shares 90% max identity with segment Circle26 of CcBV over 67% query coverage. These results demonstrate that there is not only gene similarity, but also segment similarity between CvBV and CcBV. Transcripts of CvBV-ank2 were detected as early as 0.5 h post-parasitization and continued to be detected for 6 days, indicating that CvBV-ank2 might be involved in the early protection of the parasitoid egg.

Bracovirus gene products are highly divergent from insect proteins

Recently, several polydnavirus (PDV) genomes have been completely sequenced. The dsDNA circles enclosed in virus particles and injected by wasps into caterpillars appear to mainly encode virulence factors potentially involved in altering host immunity and/or development, thereby allowing the survival of the parasitoid larvae within the host tissues. Parasitoid wasps generally inject virulence factors produced in the venom gland. As PDV genomes are inherited vertically by wasps through a proviral form, wasp virulence genes may have been transferred to this chromosomal form, leading to their incorporation into virus particles. Indeed, many gene products from Cotesia congregata bracovirus (CcBV), such as PTPs, IkappaB-like, and cystatins, contain protein domains conserved in metazoans. Surprisingly however, CcBV virulence gene products are not more closely related to insect proteins than to human proteins. To determine whether the distance between CcBV and insect proteins is a specific feature of BV proteins or simply reflects a general high divergence of parasitoid wasp products, which might be due to parasitic lifestyle, we have analyzed the sequences of wasp genes obtained from a cDNA library. Wasp sequences having a high similarity with Apis mellifera genes involved in a variety of biological functions could be identified indicating that the high level of divergence observed for BV products is a hallmark of these viral proteins. We discuss how this divergence might be explained in the context of the current hypotheses on the origin and evolution of wasp-bracovirus associations.

Cloning and characterization of two Campoletis chlorideae ichnovirus vankyrin genes expressed in parasitized host Helicoverpa armigera

Polydnaviruses, symbionts of parasitic ichneumonid (ichnoviruses, IVs) and braconid (bracoviruses, BVs), are injected into hosts along with wasp eggs. Within the host, PDV genes are expressed and their products function to alter lepidopteran host physiology and enable endoparasitoid development. In the present study, we describe two Campoletis chlorideae ichnovirus (CcIV) viral ankyrin (vankyrin) genes and their transcription. The CcIV vankyrin genes possess ankyrin repeat domains that resemble the inhibitory domains of the Drosophila melanogaster NF-kappaB transcription factor inhibitor (IkappaB) cactus. The expression of CcIV vankyrin genes could be detected in Helicoverpa armigera during the whole course of parasitization with two expression peaks, 30 min post-parasitization (p.p.) and 2 days p.p. Our data indicate that the CcIV vankyrin genes are differentially expressed in the tissues of parasitized hosts and both are mainly expressed in hemocytes. The temporal and spatial variation in expression of the two CcIV vankyrin genes suggests that CcIV vankyrin genes could be involved in early protection of parasitoid eggs from host cellular immune response by suppressing NF-kappaB signaling cascades, thereby altering development and immune responses of parasitized lepidopteran hosts.

Shared and species-specific features among ichnovirus genomes

During egg-laying, some endoparasitic wasps transmit a polydnavirus to their caterpillar host, causing physiological disturbances that benefit the wasp larva. Members of the two recognized polydnavirus taxa, ichnovirus (IV) and bracovirus (BV), have large, segmented, dsDNA genomes containing virulence genes expanded into families. A recent comparison of IV and BV genomes revealed taxon-specific features, but the IV database consisted primarily of the genome sequence of a single species, the Campoletis sonorensis IV (CsIV). Here we describe analyses of two additional IV genomes, the Hyposoter fugitivus IV (HfIV) and the Tranosema rostrale IV (TrIV), which we compare to the sequence previously reported for CsIV. The three IV genomes share several features including a low coding density, a strong A+T bias, similar estimated aggregate genome sizes ( approximately 250 kb) and the presence of nested genome segments. In addition, all three IV genomes contain members of six conserved gene families: repeat element, cysteine motif, viral innexin, viral ankyrin, N-family, and a newly defined putative family, the polar-residue-rich proteins. The three genomes, however, differ in their degree of segmentation, in within-family gene frequency and in the presence, in TrIV, of a unique gene family (TrV). These interspecific variations may reflect differences in parasite/host biology, including virus-induced pathologies in the latter.

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.

Microplitis demolitor bracovirus genome segments vary in abundance and are individually packaged in virions

Polydnaviruses (PDVs) are distinguished by their unique association with parasitoid wasps and their segmented, double-stranded (ds) DNA genomes that are non-equimolar in abundance. Relatively little is actually known, however, about genome packaging or segment abundance of these viruses. Here, we conducted electron microscopy (EM) and real-time polymerase chain reaction (PCR) studies to characterize packaging and segment abundance of Microplitis demolitor bracovirus (MdBV). Like other PDVs, MdBV replicates in the ovaries of females where virions accumulate to form a suspension called calyx fluid. Wasps then inject a quantity of calyx fluid when ovipositing into hosts. The MdBV genome consists of 15 segments that range from 3.6 (segment A) to 34.3 kb (segment O). EM analysis indicated that MdBV virions contain a single nucleocapsid that encapsidates one circular DNA of variable size. We developed a semi-quantitative real-time PCR assay using SYBR Green I. This assay indicated that five (J, O, H, N and B) segments of the MdBV genome accounted for more than 60% of the viral DNAs in calyx fluid. Estimates of relative segment abundance using our real-time PCR assay were also very similar to DNA size distributions determined from micrographs. Analysis of parasitized Pseudoplusia includens larvae indicated that copy number of MdBV segments C, B and J varied between hosts but their relative abundance within a host was virtually identical to their abundance in calyx fluid. Among-tissue assays indicated that each viral segment was most abundant in hemocytes and least abundant in salivary glands. However, the relative abundance of each segment to one another was similar in all tissues. We also found no clear relationship between MdBV segment and transcript abundance in hemocytes and fat body.

Characterization of the IkappaB-like gene family in polydnaviruses associated with wasps belonging to different Braconid subfamilies

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.

Divergences in protein activity and cellular localization within the Campoletis sonorensis Ichnovirus Vankyrin family

Ichnoviruses (IVs) occur in obligate symbiotic associations with endoparasitic ichneumonid wasps. IVs are injected with eggs during parasitization, where viral infection and gene expression alter host physiology to ensure endoparasitoid survival. The seven Campoletis sonorensis IV (CsIV) vankyrin genes encode proteins that possess ankyrin repeat domains resembling the inhibitory domains of NF-kappaB transcription factor inhibitors (IkappaBs). The CsIV vankyrins are divided into two subclasses: those expressed primarily in the host fat body (three genes) and those expressed in host hemocytes (four genes). CsIV vankyrin proteins showed limited antigenic similarity when analyzed by Western blotting. Cellular localization and expression patterns of recombinant vankyrin proteins in High Five and Sf9 insect cells differed within and between the subclasses and in cells exposed to lipopolysaccharide, laminarin, or viral immune challenge. In unstimulated Sf9 cells, five vankyrins were detected in cell nuclei. The remaining two proteins localized predominantly to cytoplasmic granules. Immune stimulation of cells resulted in a nuclear-to-cytoplasmic shift of three vankyrins but did not affect localization of other variants. When expressed from recombinant Autographa californica multiple nucleopolyhedroviruses (AcMNPVs), all vankyrins showed a nuclear localization during early stages of infection with patterns resembling those of immune-challenged cells as the infection progressed. Two fat body vankyrins also produced unique biological effects when expressed from recombinant AcMNPV. Insect cells infected with these viruses exhibited enhanced longevity compared to those infected with viruses expressing other vankyrins. Together, these data suggest that vankyrin proteins in CsIV have divergent physiological functions.

Response of immunocompetent and immunosuppressed Spodoptera littoralis larvae to baculovirus infection

The Mediterranean lepidopteran pest Spodoptera littoralis is highly resistant to infection with the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) via the oral route, but highly sensitive to infection with budded virus (BV) via the intrahaemocoelic route. To study the fate of AcMNPV infection in S. littoralis, vHSGFP, an AcMNPV recombinant that expresses the reporter green fluorescent protein gene under the control of the Drosophila heat-shock promoter, and high-resolution fluorescence microscopy were utilized. S. littoralis fourth-instar larvae infected orally with vHSGFP showed melanization and encapsulation of virus-infected tracheoblast cells serving the midgut columnar cells. At 72 h post-infection, the viral foci were removed during the moult clearing the infection. Thus, oral infection was restricted by immune responses to the midgut and midgut-associated tracheal cells. By contrast, injection of BV into the haemocoel resulted in successful infection of tracheoblasts, followed by spread of the virus through the tracheal epidermis to other tissues. However, in contrast to fully permissive infections where tracheoblasts and haemocytes are equally susceptible to infection, a severe limitation to vHSGFP infection of haemocytes was observed. To investigate the resistance of S. littoralis haemocytes to BV infection with AcMNPV, the larval immune system was suppressed with the Chelonus inanitus polydnavirus or a putatively immunosuppressive polydnavirus gene, P-vank-1. Both treatments increased the susceptibility of S. littoralis larvae to AcMNPV. It is concluded that the resistance of S. littoralis to AcMNPV infection involves both humoral and cellular immune responses that act at the gut and haemocyte levels. The results also support the hypothesis that tracheolar cells mediate establishment of systemic baculovirus infections in lepidopteran larvae. The finding that polydnaviruses and their encoded genes synergize baculovirus infection also provides an approach to dissecting the responses of the lepidopteran immune system to viruses by using specific polydnavirus immunosuppressive genes.

Gene expression profiling of Spodoptera frugiperda hemocytes and fat body using cDNA microarray reveals polydnavirus-associated variations in lepidopteran host genes transcript levels

Background

Genomic approaches provide unique opportunities to study interactions of insects with their pathogens. We developed a cDNA microarray to analyze the gene transcription profile of the lepidopteran pest Spodoptera frugiperda in response to injection of the polydnavirus HdIV associated with the ichneumonid wasp Hyposoter didymator. Polydnaviruses are associated with parasitic ichneumonoid wasps and are required for their development within the lepidopteran host, in which they act as potent immunosuppressive pathogens. In this study, we analyzed transcriptional variations in the two main effectors of the insect immune response, the hemocytes and the fat body, after injection of filter-purified HdIV.

Results

Results show that 24 hours post-injection, about 4% of the 1750 arrayed host genes display changes in their transcript levels with a large proportion (76%) showing a decrease. As a comparison, in S. frugiperda fat body, after injection of the pathogenic JcDNV densovirus, 8 genes display significant changes in their transcript level. They differ from the 7 affected by HdIV and, as opposed to HdIV injection, are all up-regulated. Interestingly, several of the genes that are modulated by HdIV injection have been shown to be involved in lepidopteran innate immunity. Levels of transcripts related to calreticulin, prophenoloxidase-activating enzyme, immulectin-2 and a novel lepidopteran scavenger receptor are decreased in hemocytes of HdIV-injected caterpillars. This was confirmed by quantitative RT-PCR analysis but not observed after injection of heat-inactivated HdIV. Conversely, an increased level of transcripts was found for a galactose-binding lectin and, surprisingly, for the prophenoloxidase subunits. The results obtained suggest that HdIV injection affects transcript levels of genes encoding different components of the host immune response (non-self recognition, humoral and cellular responses).

Conclusion

This analysis of the host-polydnavirus interactions by a microarray approach indicates that the presence of HdIV induces, directly or indirectly, variations in transcript levels of specific host genes, changes that could be responsible in part for the alterations observed in the parasitized host physiology. Development of such global approaches will allow a better understanding of the strategies employed by parasites to manipulate their host physiology, and will permit the identification of potential targets of the immunosuppressive polydnaviruses.

Members of the Hyposoter didymator Ichnovirus repeat element gene family are differentially expressed in Spodoptera frugiperda

Background

The abundance and the conservation of the repeated element (rep) genes in Ichnoviruses genomes suggest that this gene family plays an important role in viral cycles. In the Ichnovirus associated with the wasp Hyposoter didymator, named HdIV, 10 rep genes were identified to date. In this work, we report a relative quantitative transcription study of these HdIV rep genes in several tissues of the lepidopteran host Spodoptera frugiperda as well as in the H. didymator wasps.

Results

The data obtained in this work indicate that, in the early phases of infection (24 hours), HdIV rep genes each display different levels of transcripts in parasitized 2^nd instar or HdIV-injected last instar S. frugiperda larvae. Only one, rep1, is significantly transcribed in female wasps. Transcript levels of the HdIV rep genes were found as not correlated to their copy number in HdIV genome. Our results also show that HdIV rep genes display different tissue specificity, and that they are primarily transcribed in S. frugiperda fat body and cuticular epithelium.

Conclusion

This work is the first quantitative analysis of transcription of the ichnovirus rep gene family, and the first investigation on a correlation between transcript levels and gene copy numbers in Ichnoviruses. Our data indicate that, despite similar gene copy numbers, not all the members of this gene family are significantly transcribed 24 hours after infection in lepidopteran larvae. Additionally, our data show that, as opposed to other described HdIV genes, rep genes are little transcribed in hemocytes, thus suggesting that they are not directly associated with the disruption of the immune response but rather involved in other physiological alterations of the infected lepidopteran larva.

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.

Polydnavirus Genes that Enhance the Baculovirus Expression Vector System

The baculovirus expression vector system (BEVS) is a powerful and versatile system for protein expression, which has many advantages. However, a limitation of any lytic viral expression system, including BEVS, is that death and lysis of infected insect cells terminates protein production. This results in interruption of protein production and higher production costs due to the need to set up new infections, maintain uninfected cells, and produce pure viral stocks. Genetic methods to slow or prevent cell death while maintaining high-level, virus-driven protein production could dramatically increase protein yields. Several approaches have been used to improve the BEVS and increase the synthesis of functional proteins. Successful enhancement of the BEVS was obtained when various gene elements were added to the virus, secretion and posttranslational processing were modified, or protein integrity was improved. A gene family from the insect virus Campoletis sonorensis ichnovirus (CsIV) was discovered that delays lysis of baculovirus-infected cells, thereby significantly enhancing recombinant protein production in the BEVS system. By using the CsIV vankyrin gene family, protein production in the vankyrin-enhanced BEVS (VE-BEVS) was increased by a factor of 4- to 15-fold by either coexpressing the vankyrin protein from a dual BEVS or by providing its activity in trans by expressing the vankyrin protein from a stably transformed cell line. In sum, VE-BEVS is an enhancement of the existing BEVS technology that markedly improves protein expression levels while reducing the cost of labor and materials.

Potential Uses of Cys‐Motif and Other Polydnavirus Genes in Biotechnology

Exploiting the ability of insect pathogens, parasites, and predators to control natural and damaging insect populations is a cornerstone of biological control. Here we focus on an unusual group of viruses, the polydnaviruses (PDV), which are obligate symbionts of some hymenopteran insect parasitoids. PDVs have a variety of important pathogenic effects on their parasitized hosts. The genes controlling some of these pathogenic effects, such as inhibition of host development, induction of precocious metamorphosis, slowed or reduced feeding, and immune suppression, may have use for biotechnological applications. In this chapter, we consider the physiological functions of both wasp and viral genes with emphasis on the Cys-motif gene family and their potential use for insect pest control.

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.

Inhibitor kappaB-like proteins from a polydnavirus inhibit NF-kappaB activation and suppress the insect immune response

Complex signaling pathways regulate the innate immune system of insects, with NF-kappaB transcription factors playing a central role in the activation of antimicrobial peptides and other immune genes. Although numerous studies have characterized the immune responses of insects to pathogens, comparatively little is known about the counter-strategies pathogens have evolved to circumvent host defenses. Among the most potent immunosuppressive pathogens of insects are polydnaviruses that are symbiotically associated with parasitoid wasps. Here, we report that the Microplitis demolitor bracovirus encodes a family of genes with homology to inhibitor kappaB (IkappaB) proteins from insects and mammals. Functional analysis of two of these genes, H4 and N5, were conducted in Drosophila S2 cells. Recombinant H4 and N5 greatly reduced the expression of drosomycin and attacin reporter constructs, which are under NF-kappaB regulation through the Toll and Imd pathways. Coimmunoprecipitation experiments indicated that H4 and N5 bound to the Rel proteins Dif and Relish, and N5 also weakly bound to Dorsal. H4 and N5 also inhibited binding of Dif and Relish to kappaB sites in the promoters of the drosomycin and cecropin A1 genes. Collectively, these results indicate that H4 and N5 function as IkappaBs and, circumstantially, suggest that other IkappaB-like gene family members are involved in the suppression of the insect immune system.