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

An Overview of Ovarian Calyx Fluid Proteins of Toxoneuron nigriceps (Viereck) (Hymenoptera: Braconidae): An Integrated Transcriptomic and Proteomic Approach

The larval stages of the tobacco budworm, Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae), are parasitized by the endophagous parasitoid wasp, Toxoneuron nigriceps (Viereck) (Hymenoptera: Braconidae). During the injections of eggs, this parasitoid wasp also injects into the host body the secretion of the venom gland and the calyx fluid, which contains a polydnavirus (T. nigriceps BracoVirus: TnBV) and the Ovarian calyx fluid Proteins (OPs). The effects of the OPs on the host immune system have recently been described. In particular, it has been demonstrated that the OPs cause hemocytes to undergo a number of changes, such as cellular oxidative stress, actin cytoskeleton modifications, vacuolization, and the inhibition of hemocyte encapsulation capacity, which results in both a loss of hemocyte functionality and cell death. In this study, by using a combined transcriptomic and proteomic analysis, the main components of T. nigriceps ovarian calyx fluid proteins were identified and their possible role in the parasitic syndrome was discussed. This study provides useful information to support the analysis of the function of ovarian calyx fluid proteins, to better understand T. nigriceps parasitization success and for a more thorough understanding of the components of ovarian calyx fluid proteins and their potential function in combination with other parasitoid factors.

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.

Microplitis bicoloratus bracovirus regulates cyclophilin A-apoptosis-inducing factor interaction to induce cell apoptosis in the insect immunosuppressive process

Microplitis bicoloratus bracovirus (MbBV) induces apoptosis in hemocytes of the host (Spodoptera litura) via the cyclophilin A (CypA)-mediated signaling pathway. However, the mechanisms underlying CypA-mediated signaling during apoptosis remain largely unknown. Therefore, in this study, we investigated how CypA and apoptosis-inducing factor (AIF) interact during MbBV-mediated apoptosis. Our findings showed that MbBV induces apoptosis through the CypA-AIF axis of insect immune suppression. In MbBV-infected Spli221 cells, both the expression of the cypa gene and the release of AIF from the mitochondria increased the number of apoptotic cells. CypA and AIF underwent concurrent cytoplasm-nuclear translocation. Conversely, blocking of AIF release from mitochondria not only inhibited the CypA-AIF interaction but also inhibited the cytoplasmic-nuclear translocation of AIF and CypA. Importantly, the survival of the apoptotic phenotype was significantly rescued in MbBV-infected Spli221 cells. In addition, we found that the cyclosporine A-mediated inhibition of CypA did not prevent the formation of the CypA and AIF complex; rather, this only suppressed genomic DNA fragmentation. In vitro experiments revealed direct molecular interactions between recombinant CypA and AIF. Taken together, our results demonstrate that the CypA-AIF interaction plays an important role in MbBV-induced innate immune suppression. This study will help to clarify aspects of insect immunological mechanisms and will be relevant to biological pest control.

Adaptation of Fig Wasps (Agaodinae) to Their Host Revealed by Large-Scale Transcriptomic Data

Simple Summary

Research on fig wasps has made a considerable contribution to the understanding of insect–plant interactions. However, the molecular mechanisms underlying fig wasp host specificity are poorly understood. This study reports on a relatively large-scale transcriptomic dataset of 25 fig wasp species. We outline potential genetic mechanisms underlying the specific host adaptation by investigating changes in a gene family, in evolutionary rates, and in genes under positive selection. The transcriptome datasets reported here (1) provide new insights into the evolutionary diversification and host specificity of fig wasps and (2) contribute to a growing dataset on fig wasp genomics.

Abstract

Figs and fig wasps are highly species-specific and comprise a model system for studying co-evolution and co-speciation. The evolutionary relationships and molecular adaptations of fig wasps to their fig hosts are poorly understood, and this is in part due to limited sequence data. Here, we present large-scale transcriptomic datasets of 25 fig wasp species with the aim of uncovering the genetic basis for host specificity. Our phylogenetic results support the monophyly of all genera associated with dioecious figs, and two genera associated with monoecious figs, Eupristina and Platyscapa, were revealed to be close relatives. We identified gene loss and gain, potentially rapidly evolving genes, and genes under positive selection. Potentially functional changes were documented and we hypothesize as to how these may determine host specificity. Overall, our study provides new insights into the evolutionary diversification of fig wasps and contributes to our understanding of adaptation in this group.

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.

Role of Ovarian Proteins Secreted by Toxoneuron nigriceps (Viereck) (Hymenoptera, Braconidae) in the Early Suppression of Host Immune Response

Simple Summary

Toxoneuron nigriceps is an endoparasitoid of the tobacco budworm Heliothis virescens. Parasitoid strategies to survive involve different regulating factors that are injected into the host body together with the egg: the venom and the calyx fluid, containing a Polydnavirus (PDV) and Ovarian Proteins (OPs). The combination of these factors increases the success of parasitism. Although many studies have been reported on venom protein components and the knowledge on PDVs is increasing, little is known on OPs. These secretions are able to interfere early with the host cellular immune response, acting specifically on host haemocytes, cells involved in immune response. Our results show that OPs induce several alterations on haemocytes, including cellular oxidative stress condition and modifications of actin cytoskeleton, so inducing both a loss of haemocyte functionality and cell death. Overall, in synergy with PDV and venom, OPs positively contribute to the evasion of the host immune response by T. nigriceps.

Abstract

Toxoneuron nigriceps (Viereck) (Hymenoptera, Braconidae) is an endophagous parasitoid of the larval stages of the tobacco budworm, Heliothis virescens (Fabricius) (Lepidoptera, Noctuidae). During oviposition, T. nigriceps injects into the host body, along with the egg, the venom, the calyx fluid, which contains a Polydnavirus (T. nigriceps BracoVirus: TnBV), and the Ovarian Proteins (OPs). Although viral gene expression in the host reaches detectable levels after a few hours, a precocious disruption of the host metabolism and immune system is observed right after parasitization. This alteration appears to be induced by female secretions including TnBV venom and OPs. OPs, originating from the ovarian calyx cells, are involved in the induction of precocious symptoms in the host immune system alteration. It is known that OPs in braconid and ichneumonid wasps can interfere with the cellular immune response before Polydnavirus infects and expresses its genes in the host tissues. Here we show that T. nigriceps OPs induce several alterations on host haemocytes that trigger cell death. The OP injection induces an extensive oxidative stress and a disorganization of actin cytoskeleton and these alterations can explain the high-level of haemocyte mortality, the loss of haemocyte functionality, and so the reduction in encapsulation ability by the host.

A nidovirus perspective on SARS-CoV-2

Two pandemics of respiratory distress diseases associated with zoonotic introductions of the species Severe acute respiratory syndrome-related coronavirus in the human population during 21st century raised unprecedented interest in coronavirus research and assigned it unseen urgency. The two viruses responsible for the outbreaks, SARS-CoV and SARS-CoV-2, respectively, are in the spotlight, and SARS-CoV-2 is the focus of the current fast-paced research. Its foundation was laid down by studies of many corona- and related viruses that collectively form the vast order Nidovirales. Comparative genomics of nidoviruses played a key role in this advancement over more than 30 years. It facilitated the transfer of knowledge from characterized to newly identified viruses, including SARS-CoV and SARS-CoV-2, as well as contributed to the dissection of the nidovirus proteome and identification of patterns of variations between different taxonomic groups, from species to families. This review revisits selected cases of protein conservation and variation that define nidoviruses, illustrates the remarkable plasticity of the proteome during nidovirus adaptation, and asks questions at the interface of the proteome and processes that are vital for nidovirus reproduction and could inform the ongoing research of SARS-CoV-2.

Quantitative trait loci involved in the reproductive success of a parasitoid wasp

Dissecting the genetic basis of intraspecific variations in life history traits is essential to understand their evolution, notably for potential biocontrol agents. Such variations are observed in the endoparasitoid Cotesia typhae (Hymenoptera: Braconidae), specialized on the pest Sesamia nonagrioides (Lepidoptera: Noctuidae). Previously, we identified two strains of C. typhae that differed significantly for life history traits on an allopatric host population. To investigate the genetic basis underlying these phenotypic differences, we used a quantitative trait locus (QTL) approach based on restriction site-associated DNA markers. The characteristic of C. typhae reproduction allowed us generating sisters sharing almost the same genetic content, named clonal sibship. Crosses between individuals from the two strains were performed to generate F2 and F8 recombinant CSS. The genotypes of 181 clonal sibships were determined as well as the phenotypes of the corresponding 4,000 females. Informative markers were then used to build a high-quality genetic map. These 465 markers spanned a total length of 1,300 cM and were organized in 10 linkage groups which corresponded to the number of C. typhae chromosomes. Three QTLs were detected for parasitism success and two for offspring number, while none were identified for sex ratio. The QTLs explained, respectively, 27.7% and 24.5% of the phenotypic variation observed. The gene content of the genomic intervals was investigated based on the genome of C. congregata and revealed 67 interesting candidates, as potentially involved in the studied traits, including components of the venom and of the symbiotic virus (bracovirus) shown to be necessary for parasitism success in related wasps.

The white spot syndrome virus hijacks the expression of the Penaeus vannamei Toll signaling pathway to evade host immunity and facilitate its replication

The white spot syndrome virus (WSSV), the most lethal pathogen of shrimp, is a dsDNA virus with approximately a 300,000 base pairs and contains approximately 180-500 predicted open reading frames (ORFs), of which only 6% show homology to any known protein from other viruses or organisms. Although most of its ORFs encode enzymes for nucleotide metabolism, DNA replication, and protein modification, the WSSV uses some of its encoded proteins successfully to take control of the metabolism of the host and avoid immune responses. The contribution of the shrimp innate immune response to prevent viral invasions is recognized but yet not fully understood. Thus, the role of several components of Toll pathway of the shrimp Penaeus vannamei against WSSV has been previously described, and the consequential effects occurring through the cascade remain unknown. In the current study the effects of WSSV over various components of the shrimp Toll pathway were studied. The gene expression of Spätzle, Toll, Tube, Cactus and Dorsal was altered after 6-12 h post inoculation. The expression of LvToll3, LvCactus, LvDorsal, decreased ~4.4-, ~3.7- and ~7.3-fold at 48, 24 and 48 hpi, respectively. Furthermore, a remarkable reduction (~18-fold) in the expression of the gene encoding LvCactus in WSSV infected specimens was observed at 6 hpi. This may be a sophisticated strategy exploited by WSSV to evade the Toll-mediated immune action, and to promote its replication, thereby contributing to viral fitness.

The dual life of ichnoviruses

Ichnoviruses (IVs) are mutualistic, double-stranded DNA viruses playing a key role in the successful parasitism of thousands of endoparasitoid wasp species. IV particles are produced exclusively in the female wasp reproductive tract. They are co-injected along with the parasitoid egg into caterpillar hosts upon parasitization. The expression of viral genes by infected host cells leads to an immunosuppressive state and delayed development of the host, two pathologies that are critical to the successful development of the wasp egg and larva. Ichnovirus is one of the two recognized genera within the family Polydnaviridae (polydnaviruses or PDVs), the other genus being Bracovirus (BV), associated with braconid wasps. IVs are associated with ichneumonid wasps belonging to the subfamilies Campopleginae and Banchinae; attempts to identify IV particles in other ichneumonid subfamilies have so far been unsuccessful. Functional studies targeting IV genes expressed in parasitized hosts, along with investigations of the molecular mechanisms responsible for viral morphogenesis in the female wasp, have resulted in a better understanding of the biology of these atypical viruses.

Spodoptera litura cyclophilin A is required for Microplitis bicoloratus bracovirus-induced apoptosis during insect cellular immune response

Microplitis bicoloratus bracovirus (MbBV) is a polydnavirus found in the parasitic wasp M. bicoloratus. Although MbBV is a known inducer of apoptosis in host hemocytes, the mechanism by which this occurs remains elusive. In this study, we found that expression of cyclophilin A (CypA) was significantly upregulated in Spodoptera litura hemocytes at 6-day post-parasitization. Similar results were reported in High Five cells (Hi5 cells) infected by MbBV, suggesting that the upregulation of CypA is linked to MbBV infection in insect cells. cDNA encoding CypA was cloned from parasitized hemocytes of S. litura, and bioinformatic analyses showed that S. litura CypA belongs to the cyclophilin family of proteins. Overexpression of S. litura CypA in Hi5 cells revealed that the protein promotes MbBV-induced apoptosis in vitro. Conversely, suppression of the expression and activity of CypA protein significantly rescued the apoptotic phenotype observed in MbBV-infected Hi5 cells, suggesting that it plays a key role in this process. MbBV infection also promoted the cytoplasmic-nuclear translocation of CypA in Hi5 cells. Taken together, these results suggest that MbBV infection upregulates the expression of CypA, which is required for MbBV-mediated apoptosis. Our findings provide insight into the role that CypA plays in insect cellular immune response.

HIV-1 Vpu is a potent transcriptional suppressor of NF-κB-elicited antiviral immune responses

Many viral pathogens target innate sensing cascades and/or cellular transcription factors to suppress antiviral immune responses. Here, we show that the accessory viral protein U (Vpu) of HIV-1 exerts broad immunosuppressive effects by inhibiting activation of the transcription factor NF-κB. Global transcriptional profiling of infected CD4 +T cells revealed that vpu-deficient HIV-1 strains induce substantially stronger immune responses than the respective wild type viruses. Gene set enrichment analyses and cytokine arrays showed that Vpu suppresses the expression of NF-κB targets including interferons and restriction factors. Mutational analyses demonstrated that this immunosuppressive activity of Vpu is independent of its ability to counteract the restriction factor and innate sensor tetherin. However, Vpu-mediated inhibition of immune activation required an arginine residue in the cytoplasmic domain that is critical for blocking NF-κB signaling downstream of tetherin. In summary, our findings demonstrate that HIV-1 Vpu potently suppresses NF-κB-elicited antiviral immune responses at the transcriptional level.

The Human Immunodeficiency Virus (or HIV for short) has infected more than 70 million people worldwide. Although effective therapies exist to prevent the replication of the virus and the development to AIDS, there is no cure or vaccine, and the virus still spreads efficiently in human populations, infecting about 1.8 million new people every year.

The unfortunate success of HIV can in part be explained by several viral proteins that trick our immune system and enable the virus to persist at high levels in the human body. For example, an HIV protein called viral protein U (Vpu) prevents infected cells from producing alarm signals such as interferons, which usually help healthy, uninfected cells to defend themselves against viruses. However, the extent to which Vpu interferes with interferons and other proteins involved in immune responses has remained unclear.

To address this question, Langer, Hammer, Hopfensperger et al. compared how different variants of HIV affect immune responses in human cells. The experiments showed that cells infected with HIV variants lacking Vpu released larger amounts of interferons and other cellular proteins that are involved in immune responses compared to HIV variants with Vpu. Further experiments showed that Vpu works by inhibiting the activation of a protein called NF-κB, which usually switches on genes that encode interferons and many other proteins involved in immune responses.

These findings demonstrate that Vpu has a broader impact on the human immune response than previously thought. In order to multiply efficiently, HIV initially requires the NF-κB protein to be active. Therefore, when NF-κB is inactive, HIV may adopt a dormant state that prevents current antiviral drug treatments from eradicating the virus in the human body. In the future, developing new drugs that can activate dormant HIV particles may therefore have the potential to help cure HIV infections.

Novel Factors of Viral Origin Inhibit TOR Pathway Gene Expression

Polydnaviruses (PDVs) are obligate symbionts of endoparasitoid wasps, which exclusively attack the larval stages of their lepidopteran hosts. The Polydnavirus is injected by the parasitoid female during oviposition to selectively infect host tissues by the expression of viral genes without undergoing replication. Toxoneuron nigriceps bracovirus (TnBV) is associated with Toxoneuron nigriceps (Hymenoptera: Braconidae) wasp, an endoparasitoid of the tobacco budworm larval stages, Heliothis virescens (Lepidoptera: Noctuidae). Previous studies showed that TnBV is responsible for alterations in host physiology. The arrest of ecdysteroidogenesis is the main alteration which occurs in last (fifth) instar larvae and, as a consequence, prevents pupation. TnBV induces the functional inactivation of H. virescens prothoracic glands (PGs), resulting in decreased protein synthesis and phosphorylation. Previous work showed the involvement of the PI3K/Akt/TOR pathway in H. virescens PG ecdysteroidogenesis. Here, we demonstrate that this cellular signaling is one of the targets of TnBV infection. Western blot analysis and enzyme immunoassay (EIA) showed that parasitism inhibits ecdysteroidogenesis and the phosphorylation of the two targets of TOR (4E-BP and S6K), despite the stimulation of PTTH contained in the brain extract. Using a transcriptomic approach, we identified viral genes selectively expressed in last instar H. virescens PGs, 48 h after parasitization, and evaluated expression levels of PI3K/Akt/TOR pathway genes in these tissues. The relative expression of selected genes belonging to the TOR pathway (tor, 4e-bp, and s6k) in PGs of parasitized larvae was further confirmed by qRT-PCR. The down-regulation of these genes in PGs of parasitized larvae supports the hypothesis of TnBV involvement in blocking ecdysteroidogenesis, through alterations of the PI3K/Akt/TOR pathway at the transcriptional level.

A planarian nidovirus expands the limits of RNA genome size

RNA viruses are the only known RNA-protein (RNP) entities capable of autonomous replication (albeit within a permissive host environment). A 33.5 kilobase (kb) nidovirus has been considered close to the upper size limit for such entities; conversely, the minimal cellular DNA genome is in the 100–300 kb range. This large difference presents a daunting gap for the transition from primordial RNP to contemporary DNA-RNP-based life. Whether or not RNA viruses represent transitional steps towards DNA-based life, studies of larger RNA viruses advance our understanding of the size constraints on RNP entities and the role of genome size in virus adaptation. For example, emergence of the largest previously known RNA genomes (20–34 kb in positive-stranded nidoviruses, including coronaviruses) is associated with the acquisition of a proofreading exoribonuclease (ExoN) encoded in the open reading frame 1b (ORF1b) in a monophyletic subset of nidoviruses. However, apparent constraints on the size of ORF1b, which encodes this and other key replicative enzymes, have been hypothesized to limit further expansion of these viral RNA genomes. Here, we characterize a novel nidovirus (planarian secretory cell nidovirus; PSCNV) whose disproportionately large ORF1b-like region including unannotated domains, and overall 41.1-kb genome, substantially extend the presumed limits on RNA genome size. This genome encodes a predicted 13,556-aa polyprotein in an unconventional single ORF, yet retains canonical nidoviral genome organization and expression, as well as key replicative domains. These domains may include functionally relevant substitutions rarely or never before observed in highly conserved sites of RdRp, NiRAN, ExoN and 3CLpro. Our evolutionary analysis suggests that PSCNV diverged early from multi-ORF nidoviruses, and acquired additional genes, including those typical of large DNA viruses or hosts, e.g. Ankyrin and Fibronectin type II, which might modulate virus-host interactions. PSCNV's greatly expanded genome, proteomic complexity, and unique features–impressive in themselves–attest to the likelihood of still-larger RNA genomes awaiting discovery.

RNA viruses are the only known RNA-protein (RNP) entities capable of autonomous replication. The upper genome size for such entities was assumed to be <35 kb; conversely, the minimal cellular DNA genome is in the 100–300 kilobase (kb) range. This large difference presents a daunting gap for the proposed evolution of contemporary DNA-RNP-based life from primordial RNP entities. Here, we describe a nidovirus from planarians, named planarian secretory cell nidovirus (PSCNV), whose 41.1 kb genome is 23% larger than any riboviral genome yet discovered. This increase is nearly equivalent in size to the entire poliovirus genome, and it equips PSCNV with an unprecedented extra coding capacity to adapt. PSCNV has broken apparent constraints on the size of the genomic subregion that encodes core replication machinery in other nidoviruses, including coronaviruses, and has acquired genes not previously observed in RNA viruses. This virus challenges and advances our understanding of the limits to RNA genome size.

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.

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.

A polydnavirus-encoded ANK protein has a negative impact on steroidogenesis and development

Polydnaviruses (PDV) are viral symbionts associated with ichneumonid and braconid wasps parasitizing moth larvae, which are able to disrupt the host immune response and development, as well as a number of other physiological pathways. The immunosuppressive role of PDV has been more intensely investigated, while very little is known about the PDV-encoded factors disrupting host development. Here we address this research issue by further expanding the functional analysis of ankyrin genes encoded by the bracovirus associated with Toxoneuron nigriceps (Hymenoptera, Braconidae). In a previous study, using Drosophila melanogaster as experimental model system, we demonstrated the negative impact of TnBVank1 impairing the ecdysone biosynthesis by altering endocytic traffic in prothoracic gland cells. With a similar approach here we demonstrate that another member of the viral ank gene family, TnBVank3, does also contribute to the disruption of ecdysone biosynthesis, but with a completely different mechanism. We show that its expression in Drosophila prothoracic gland (PG) blocks the larval-pupal transition by impairing the expression of steroidogenic genes. Furthermore, we found that TnBVank3 affects the expression of genes involved in the insulin/TOR signaling and the constitutive activation of the insulin pathway in the PG rescues the pupariation impairment. Collectively, our data demonstrate that TnBVANK3 acts as a virulence factor by exerting a synergistic and non-overlapping function with TnBVANK1 to disrupt the ecdysone biosynthesis.

Inhibition of translation initiation factor eIF4A is required for apoptosis mediated by Microplitis bicoloratus bracovirus

Apoptotic hemocytes induced by Microplitis bicoloratus parasitism have been reported, and M. bicoloratus bracovirus (MbBV) is known to be the apoptosis inducer. However, the mechanism how MbBV regulates apoptosis remains unclear. eIF4A, one of translation initiation factors, was found from a Spodoptera litura transcriptome, the expression of which in the parasitized hemocytes of S. litura was inhibited in RT-qPCR analysis. The western blot also illustrated eIF4A at 6-day post-parasitization was inhibited in hemocytes. For testing interaction of MbBV-eIF4A-apoptosis, a cDNA clone encoding 1,266 bp of eIF4A was obtained from S. litura hemocytes and sequenced. Then, a 48 kDa V5-fusion protein of the eIF4A was detected by using the anti-V5 antibody at 72-h post-transfection in the High Five cells, which is located in the cell cytoplasm. In vitro, overexpression of eIF4A rescued the apoptotic High Five cells induced by MbBV. Conversely, in vivo, loss of eIF4A proteins by dsRNA feeding increased apoptosis of hemocytes. Furthermore, RNAi and parasitism significantly increased apoptosis of hemocytes in S. litura. These findings suggested that MbBV inhibited the expression of eIF4A, which was required for apoptosis mediated by MbBV. This study will contribute to biological pest control and enhance our understanding of molecular mechanisms underlying polydnavirus-parasitoid-host interaction.

The multifunctional polydnavirus TnBVANK1 protein: impact on host apoptotic pathway

Toxoneuron nigriceps (Hymenoptera, Braconidae) is an endophagous parasitoid of the larval stages of the tobacco budworm, Heliothis virescens (Lepidoptera, Noctuidae). The bracovirus associated with this wasp (TnBV) is currently being studied. Several genes expressed in parasitised host larvae have been isolated and their possible roles partly elucidated. TnBVank1 encodes an ankyrin motif protein similar to insect and mammalian IκB, an inhibitor of the transcription nuclear factor κB (NF-κB). Here we show that, when TnBVank1 was stably expressed in polyclonal Drosophila S2 cells, apoptosis is induced. Furthermore, we observed the same effects in haemocytes of H. virescens larvae, after TnBVank1 in vivo transient transfection, and in haemocytes of parasitised larvae. Coimmunoprecipitation experiments showed that TnBVANK1 binds to ALG-2 interacting protein X (Alix/AIP1), an interactor of apoptosis-linked gene protein 2 (ALG-2). Using double-immunofluorescence labeling, we observed the potential colocalization of TnBVANK1 and Alix proteins in the cytoplasm of polyclonal S2 cells. When Alix was silenced by RNA interference, TnBVANK1 was no longer able to cause apoptosis in both S2 cells and H. virescens haemocytes. Collectively, these results indicate that TnBVANK1 induces apoptosis by interacting with Alix, suggesting a role of TnBVANK1 in the suppression of host immune response observed after parasitisation by T. nigriceps.

The recurrent domestication of viruses: major evolutionary transitions in parasitic wasps

Several lineages of endoparasitoid wasps, which develop inside the body of other insects, have domesticated viruses, used as delivery tools of essential virulence factors for the successful development of their progeny. Virus domestications are major evolutionary transitions in highly diverse parasitoid wasps. Much progress has recently been made to characterize the nature of these ancestrally captured endogenous viruses that have evolved within the wasp genomes. Virus domestication from different viral families occurred at least three times in parasitoid wasps. This evolutionary convergence led to different strategies. Polydnaviruses (PDVs) are viral gene transfer agents and virus-like particles of the wasp Venturia canescens deliver proteins. Here, we take the standpoint of parasitoid wasps to review current knowledge on virus domestications by different parasitoid lineages. Then, based on genomic data from parasitoid wasps, PDVs and exogenous viruses, we discuss the different evolutionary steps required to transform viruses into vehicles for the delivery of the virulence molecules that we observe today. Finally, we discuss how endoparasitoid wasps manipulate host physiology and ensure parasitism success, to highlight the possible advantages of viral domestication as compared with other virulence strategies.

Identification of major Toxoneuron nigriceps venom proteins using an integrated transcriptomic/proteomic approach

Endoparasitoids in the order Hymenoptera are natural enemies of several herbivorous insect pest species. During oviposition they inject a mixture of factors, which include venom, into the host, ensuring the successful parasitism and the development of their progeny. Although these parasitoid factors are known to be responsible for host manipulation, such as immune system suppression, little is known about both identity and function of the majority of their venom components. To identify the major proteins of Toxoneuron nigriceps (Hymenoptera: Braconidae) venom, we used an integrated transcriptomic and proteomic approach. The tandem-mass spectrometric (LC-MS/MS) data combined with T. nigriceps venom gland transcriptome used as a reference database resulted in the identification of a total of thirty one different proteins. While some of the identified proteins have been described in venom from several parasitoids, others were identified for the first time. Among the identified proteins, hydrolases constituted the most abundant family followed by transferases, oxidoreductases, ligases, lyases and isomerases. The hydrolases identified in the T. nigriceps venom glands included proteases, peptidases and glycosidases, reported as common components of venom from several parasitoid species. Taken together, the identified proteins included factors that could potentially inhibit the host immune system, manipulate host physiological processes and host development, as well as provide nutrients to the parasitoid progeny, degrading host tissues by specific hydrolytic enzymes. The venom decoding provides us with information about the identity of candidate venom factors which could contribute to the success of parasitism, together with other maternal and embryonic factors.

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.

Transcriptomic response of Manduca sexta immune tissues to parasitization by the bracovirus associated wasp Cotesia congregata

During oviposition, Cotesia congregata parasitoid wasps inject into their host, Manduca sexta, some biological factors such as venom, ovarian fluid and a symbiotic polydnavirus (PDV) named Cotesia congregata bracovirus (CcBV). During parasitism, complex interactions occur between wasp-derived factors and host targets that lead to important modifications in host physiology. In particular, the immune response leading to wasp egg encapsulation is inhibited allowing wasp survival. To date, the regulation of host genes during the interaction had only been studied for a limited number of genes. In this study, we analysed the global impact of parasitism on host gene regulation 24 h post oviposition by high throughput 454 transcriptomic analyses of two tissues known to be involved in the host immune response (hemocytes and fat body). To identify specific effects of parasitism on host transcription at this time point, transcriptomes were obtained from non-treated and parasitized larvae, and also from larvae injected with heat-killed bacteria and double stimulated larvae that were parasitized prior to bacterial challenge. Results showed that, immune challenge by bacteria leads to induction of certain antimicrobial peptide (AMP) genes in M. sexta larvae whether they were parasitized or not prior to bacterial challenge. These results show that at 24 h post oviposition pathways leading to expression of AMP genes are not all inactivated suggesting wasps are in an antiseptic environment. In contrast, at this time point genes involved in phenoloxidase activation and cellular immune responses were globally down-regulated after parasitism in accordance with the observed inhibition of wasp egg encapsulation.

Origin and evolution of symbiotic viruses associated with parasitoid wasps

The Polydnaviridae (PDV), including the Bracovirus (BV) and Ichnovirus (IV) genera, originated from the integration of viruses in the genomes of two parasitoid wasp lineages. In a remarkable example of convergent evolution BVs evolved from the domestication of a nudivirus, while IVs originate from a different ancestral virus belonging to a new virus entity. In both cases the ancestor genomes have been maintained in wasp genomes as endogenous viral elements involved in production of particles containing DNA encoding virulence genes that are injected into lepidopteran hosts. However many PDV virulence genes appear to be of eukaryotic origin, and expansion and diversification of these genes have led to the production of novel PDVs in different wasp species that promote survival of offspring in particular hosts.

Functional annotation of Cotesia congregata bracovirus: identification of viral genes expressed in parasitized host immune tissues

Bracoviruses (BVs) from the Polydnaviridae family are symbiotic viruses used as biological weapons by parasitoid wasps to manipulate lepidopteran host physiology and induce parasitism success. BV particles are produced by wasp ovaries and injected along with the eggs into the caterpillar host body, where viral gene expression is necessary for wasp development. Recent sequencing of the proviral genome of Cotesia congregata BV (CcBV) identified 222 predicted virulence genes present on 35 proviral segments integrated into the wasp genome. To date, the expressions of only a few selected candidate virulence genes have been studied in the caterpillar host, and we lacked a global vision of viral gene expression. In this study, a large-scale transcriptomic analysis by 454 sequencing of two immune tissues (fat body and hemocytes) of parasitized Manduca sexta caterpillar hosts allowed the detection of expression of 88 CcBV genes expressed 24 h after the onset of parasitism. We linked the expression profiles of these genes to several factors, showing that different regulatory mechanisms control viral gene expression in the host. These factors include the presence of signal peptides in encoded proteins, diversification of promoter regions, and, more surprisingly, gene position on the proviral genome. Indeed, most genes for which expression could be detected are localized in particular proviral regions globally producing higher numbers of circles. Moreover, this polydnavirus (PDV) transcriptomic analysis also reveals that a majority of CcBV genes possess at least one intron and an arthropod transcription start site, consistent with an insect origin of these virulence genes.

IMPORTANCE

Bracoviruses (BVs) are symbiotic polydnaviruses used by parasitoid wasps to manipulate lepidopteran host physiology, ensuring wasp offspring survival. To date, the expressions of only a few selected candidate BV virulence genes have been studied in caterpillar hosts. We performed a large-scale analysis of BV gene expression in two immune tissues of Manduca sexta caterpillars parasitized by Cotesia congregata wasps. Genes for which expression could be detected corresponded to genes localized in particular regions of the viral genome globally producing higher numbers of circles. Our study thus brings an original global vision of viral gene expression and paves the way to the determination of the regulatory mechanisms enabling the expression of BV genes in targeted organisms, such as major insect pests. In addition, we identify sequence features suggesting that most BV virulence genes were acquired from insect genomes.

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.

When parasitic wasps hijacked viruses: genomic and functional evolution of polydnaviruses

The Polydnaviridae (PDV), including the Bracovirus (BV) and Ichnovirus genera, originated from the integration of unrelated viruses in the genomes of two parasitoid wasp lineages, in a remarkable example of convergent evolution. Functionally active PDVs represent the most compelling evolutionary success among endogenous viral elements (EVEs). BV evolved from the domestication by braconid wasps of a nudivirus 100 Ma. The nudivirus genome has become an EVE involved in BV particle production but is not encapsidated. Instead, BV genomes have co-opted virulence genes, used by the wasps to control the immunity and development of their hosts. Gene transfers and duplications have shaped BV genomes, now encoding hundreds of genes. Phylogenomic studies suggest that BVs contribute largely to wasp diversification and adaptation to their hosts. A genome evolution model explains how multidirectional wasp adaptation to different host species could have fostered PDV genome extension. Integrative studies linking ecological data on the wasp to genomic analyses should provide new insights into the adaptive role of particular BV genes. Forthcoming genomic advances should also indicate if the associations between endoparasitoid wasps and symbiotic viruses evolved because of their particularly intimate interactions with their hosts, or if similar domesticated EVEs could be uncovered in other parasites.

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’.

Ultrastructural and genomic characterization of a second banchine polydnavirus confirms the existence of shared features within this ichnovirus lineage

Polydnaviruses (PDVs) are symbiotic viruses carried by endoparasitic wasps and transmitted to caterpillar hosts during parasitization. Although they share several features, including a segmented dsDNA genome, a unique life cycle where replication is restricted to the wasp host, and immunodepressive/developmental effects on the caterpillar host, PDVs carried by ichneumonid and braconid wasps (referred to as ichnoviruses and bracoviruses, respectively) have different evolutionary origins. In addition, ichnoviruses (IVs) form two distinct lineages, with viral entities found in wasps belonging to the subfamilies Campopleginae and Banchinae displaying strikingly different virion morphologies and genomic features. However, the current description for banchine IVs is based on the characterization of a single species, namely that of the Glypta fumiferanae IV (GfIV). Here we provide an ultrastructural and genomic analysis of a second banchine IV isolated from the wasp Apophua simplicipes, and we show that this virus shares many features with GfIV, including a multi-nucleocapsid virion, an aggregate genome size of ~300 kb, genome segments <5 kb, an impressively high degree of genome segmentation and a very similar gene content (same gene families in both viruses). Altogether, the data presented here confirm the existence of shared characteristics within this banchine IV lineage.

Beyond RNAi: antiviral defense strategies in Drosophila and mosquito

Virus transmission and spread by arthropods is a major economic and public health concern. The ongoing dissemination of arthropod-borne viruses by blood-feeding insects is an important incentive to study antiviral immunity in these animals. RNA interference is a major mechanism for antiviral defense in insects, including the fruit fly Drosophila melanogaster and several vector mosquitoes. However, recent data suggest that the evolutionary conserved Toll, Imd and Jak-Stat signaling pathways also contribute to antiviral immunity. Moreover, symbionts, such as the intracellular bacterium Wolbachia and the gut microflora, influence the course of virus infection in insects. These results add an additional level of complexity to antiviral immunity, but also provide novel opportunities to control the spread of arboviruses. In this review, we provide an overview of the current knowledge and recent developments in antiviral immunity in Dipteran insects, with a focus on non-RNAi mediated inducible responses.

Synergistic parasite-pathogen interactions mediated by host immunity can drive the collapse of honeybee colonies

The health of the honeybee and, indirectly, global crop production are threatened by several biotic and abiotic factors, which play a poorly defined role in the induction of widespread colony losses. Recent descriptive studies suggest that colony losses are often related to the interaction between pathogens and other stress factors, including parasites. Through an integrated analysis of the population and molecular changes associated with the collapse of honeybee colonies infested by the parasitic mite Varroa destructor, we show that this parasite can de-stabilise the within-host dynamics of Deformed wing virus (DWV), transforming a cryptic and vertically transmitted virus into a rapidly replicating killer, which attains lethal levels late in the season. The de-stabilisation of DWV infection is associated with an immunosuppression syndrome, characterized by a strong down-regulation of the transcription factor NF-κB. The centrality of NF-κB in host responses to a range of environmental challenges suggests that this transcription factor can act as a common currency underlying colony collapse that may be triggered by different causes. Our results offer an integrated account for the multifactorial origin of honeybee losses and a new framework for assessing, and possibly mitigating, the impact of environmental challenges on honeybee health.

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.

Differential inhibition of BmRelish1-dependent transcription in lepidopteran cells by bracovirus ankyrin-repeat proteins

In the tripartite parasitization system of the lepidopteran host Manduca sexta, the endoparasitoid wasp Cotesia congregata and its endosymbiotic virus, C. congregata Bracovirus (CcBV), the expression of viral proteins is necessary for successful parasitization. Here we have examined the in vitro effects of six members of the ankyrin-repeat protein family (Ank) of CcBV, which are thought to interfere with the host's induced innate immune responses, on the transcriptional activity of a heterologous lepidopteran Rel/NFκB transcription factor, Relish1 of Bombyx mori. Using as transcriptional activator BmRelish1-d2 (R1d2), a constitutively active mutant of the major regulator of the Imd pathway, BmRelish1, in conjunction with a reporter gene controlled by a B. mori antimicrobial peptide gene promoter, we have found that 5 of the 6 examined Anks suppress R1d2-dependent transcriptional activity to various degrees. Immunofluorescence studies have also revealed that while some of the Ank proteins have a rather strict cytoplasmic localization, others are detected both in the cytoplasm and the nucleus of the expressing cells and that colocalization with R1d2 occurs exclusively in the nucleus. Thus, our results suggest that functional and spatial differences among the various CcBV Ank family members may be responsible for the observed differential inhibition of R1d2 activity.

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.

The shrimp NF-κB pathway is activated by white spot syndrome virus (WSSV) 449 to facilitate the expression of WSSV069 (ie1), WSSV303 and WSSV371

The Toll-like receptor (TLR)-mediated NF-κB pathway is essential for defending against viruses in insects and mammals. Viruses also develop strategies to utilize this pathway to benefit their infection and replication in mammal hosts. In invertebrates, the TLR-mediated NF-κB pathway has only been well-studied in insects and has been demonstrated to be important in antiviral responses. However, there are few reports of interactions between viruses and the TLR-mediated NF-κB pathway in invertebrate hosts. Here, we studied Litopenaeus vannamei Pelle, which is the central regulator of the Toll pathway, and proposed that a similar TLR/MyD88/Tube/Pelle/TRAF6/NF-κB cascade may exist in shrimp for immune gene regulation. After performing genome-wild analysis of white spot syndrome virus (WSSV) encoded proteins, we found that WSSV449 shows 15.7-19.4% identity to Tube, which is an important component of the insect Toll pathway. We further found that WSSV449 activated promoters of Toll pathway-controlled antimicrobial peptide genes, indicating WSSV449 has a similar function to host Tube in activating the NF-κB pathway. We suspected that WSSV449 activated the Toll-mediated NF-κB pathway for regulating viral gene expression. To test this hypothesis, we analyzed the promoters of viral genes and found 40 promoters that possess NF-κB binding sites. A promoter screen showed that the promoter activities of WSSV069 (ie1), WSSV303 and WSSV371 can be highly induced by the shrimp NF-κB family protein LvDorsal. WSSV449 also induced these three viral promoter activities by activating the NF-κB pathway. To our knowledge, this is the first report of a virus that encodes a protein similar to the Toll pathway component Tube to upregulate gene expression in the invertebrate host.

Transcriptomic profiling of Microplitis demolitor bracovirus reveals host, tissue and stage-specific patterns of activity

The polydnaviruses (PDVs) are a family of DNA viruses that are symbiotically associated with parasitoid wasps. The transcription of particular genes or gene-family members have been reported for several PDVs, but no studies have characterized the spatio-temporal patterns of expression for the entire complement of predicted genes in the encapsidated genome of any PDV isolate. The braconid wasp Microplitis demolitor carries the PDV Microplitis demolitor bracovirus (MdBV) and parasitizes larval stage Pseudoplusia (Chrysodeixis) includens. The encapsidated genome consists of 15 genomic segments with 51 predicted ORFs encoding proteins ≥100 aa. A majority of these ORFs form four multimember gene families (ptp, ank, glc and egf) while the remaining ORFs consist of single copy (orph) genes. Here we used RT-PCR and quantitative real-time PCR methods to profile the encapsidated transcriptome of MdBV in P. includens and M. demolitor. Our results indicate that most predicted genes are expressed in P. includens. Spatial patterns of expression in P. includens differed among genes, but temporal patterns of expression were generally similar, with transcript abundance progressively declining between 24 and 120 h. A subset of ptp, ank and orph genes were also expressed in adult female but not male M. demolitor. Only one encapsidated gene (ank-H4) was expressed in all life stages of M. demolitor, albeit at much lower levels than in P. includens. However, another encapsidated gene (orph-B1) was expressed in adult M. demolitor at similar levels to those detected in P. includens.

Deep sequencing of Cotesia vestalis bracovirus reveals the complexity of a polydnavirus genome

Here we completed the whole genome sequence of Cotesia vestalis bracovirus (CvBV) by deep sequencing and compared the genome features of CvBV to those of other polydnaviruses (PDVs). The genome is 540,215 base pairs divided into 35 genomic segments that range from 2.6 to 39.2kb. Comparison of CvBV with other PDVs shows that more segments are found, including new segments that have no corresponding segments in other phylogenetically related PDVs, which suggests that there might be still more segments not being sequenced in the present known PDVs. We identified eight gene families and five genes in CvBV, including new genes which were first found in PDVs. Strikingly, we identified a putative helicase protein displaying similarity to human Pif1 helicase, which has never been reported for other PDVs. This finding will bring new insights in research of these special viruses.

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.

Functional diversity of ankyrin repeats in microbial proteins

The ankyrin repeat (ANK) is the most common protein-protein interaction motif in nature, and is predominantly found in eukaryotic proteins. Genome sequencing of various pathogenic or symbiotic bacteria and eukaryotic viruses has identified numerous genes encoding ANK-containing proteins that are proposed to have been acquired from eukaryotes by horizontal gene transfer. However, the recent discovery of additional ANK-containing proteins encoded in the genomes of archaea and free-living bacteria suggests either a more ancient origin of the ANK motif or multiple convergent evolution events. Many bacterial pathogens employ various types of secretion systems to deliver ANK-containing proteins into eukaryotic cells, where they mimic or manipulate various host functions. Studying the molecular and biochemical functions of this family of proteins will enhance our understanding of important host-microbe interactions.

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.

IkB genes encoded in Cotesia plutellae bracovirus suppress an antiviral response and enhance baculovirus pathogenicity against the diamondback moth, Plutella xylostella

An endoparasitoid wasp, Cotesia plutellae, parasitizes larvae of the diamondback moth, Plutella xylostella, with its symbiotic polydnavirus, C. plutellae bracovirus (CpBV). This study analyzed the role of Inhibitor-kB (IkB)-like genes encoded in CpBV in suppressing host antiviral response. Identified eight CpBV-IkBs are scattered on different viral genome segments and showed high homologies with other bracoviral IkBs in their amino acid sequences. Compared to an insect ortholog (e.g., Cactus of Drosophila melanogaster), they possessed a shorter ankyrin repeat domain without any regulatory domains. The eight CpBV-IkBs are, however, different in their promoter components and expression patterns in the parasitized host. To test their inhibitory activity on host antiviral response, a midgut response of P. xylostella against baculovirus infection was used as a model reaction. When the larvae were orally fed the virus, they exhibited melanotic responses of midgut epithelium, which increased with baculovirus dose and incubation time. Parasitized larvae exhibited a significant reduction in the midgut melanotic response, compared to nonparasitized larvae. Micro-injection of each of the four CpBV genome segments containing CpBV-IkBs into the hemocoel of nonparasitized larvae showed the gene expressions of the encoded IkBs and suppressed the midgut melanotic response in response to the baculovirus treatment. When nonparasitized larvae were orally administered with a recombinant baculovirus containing CpBV-IkB, they showed a significant reduction in midgut melanotic response and an enhanced susceptibility to the baculovirus infectivity.

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.

Comparison of three methods of parasitoid polydnavirus genomic DNA isolation to facilitate polydnavirus genomic sequencing

A major long-term goal of polydnavirus (PDV) genome research is to identify novel virally encoded molecules that may serve as biopesticides to target insect pests that threaten agriculture and human health. As PDV viral replication in cell culture in vitro has not yet been achieved, several thousands of wasps must be dissected to yield enough viral DNA from the adult ovaries to carry out PDV genomic sequencing. This study compares three methods of PDV genomic DNA isolation for the PDV of Cotesia flavipes, which parasitizes the sugarcane borer, Diatraea saccharalis, preparatory to sequencing the C. flavipes bracovirus genome. Two of these protocols incorporate phenol-chloroform DNA extraction steps in the procedure and the third protocol uses a modified Qiagen DNA kit method to extract viral DNA. The latter method proved significantly less time-consuming and more cost-effective. Efforts are currently underway to bioengineer insect pathogenic viruses with PDV genes, so that their gene products will enhance baculovirus virulence for agricultural insect pests, either via suppression of the immune system of the host or by PDV-mediated induction of its developmental arrest. Sequencing a growing number of complete PDV genomes will enhance those efforts, which will be facilitated by the study reported here.