N-terminal tail of a viral histone H4 encoded in Cotesia plutellae bracovirus is essential to suppress gene expression of host histone H4

Proteomic analyses of venom from a Spider Hawk, Pepsis decorata

Background

The composition of the venom from solitary wasps is poorly known, although these animals are considered sources of bioactive substances. Until the present moment, there is only one proteomic characterization of the venom of wasps of the family Pompilidae and this is the first proteomic characterization for the genus Pepsis.

Methods

To elucidate the components of Pepsis decorata venom, the present work sought to identify proteins using four different experimental conditions, namely: (A) crude venom; (B) reduced and alkylated venom; (C) trypsin-digested reduced and alkylated venom, and; (D) chymotrypsin-digested reduced and alkylated venom. Furthermore, three different mass spectrometers were used (Ion Trap-Time of Flight, Quadrupole-Time of Flight, and Linear Triple Quadruple).

Results

Proteomics analysis revealed the existence of different enzymes related to the insect's physiology in the venom composition. Besides toxins, angiotensin-converting enzyme (ACE), hyaluronidase, and Kunitz-type inhibitors were also identified.

Conclusion

The data showed that the venom of Pepsis decorata is mostly composed of proteins involved in the metabolism of arthropods, as occurs in parasitic wasps, although some classical toxins were recorded, and among them, for the first time, ACE was found in the venom of solitary wasps. This integrative approach expanded the range of compounds identified in protein analyses, proving to be efficient in the proteomic characterization of little-known species. It is our understanding that the current work will provide a solid base for future studies dealing with other Hymenoptera venoms.

Genomic Analysis of Amphioxus Reveals a Wide Range of Fragments Homologous to Viral Sequences

Amphioxus species are considered living fossils and are important in the evolutionary study of chordates and vertebrates. To explore viral homologous sequences, a high-quality annotated genome of the Beihai amphioxus (Branchiostoma belcheri beihai) was examined using virus sequence queries. In this study, 347 homologous fragments (HFs) of viruses were identified in the genome of B. belcheri beihai, of which most were observed on 21 genome assembly scaffolds. HFs were preferentially located within protein-coding genes, particularly in their CDS regions and promoters. A range of amphioxus genes with a high frequency of HFs is proposed, including histone-related genes that are homologous to the Histone H4 or Histone H2B domains of viruses. Together, this comprehensive analysis of viral HFs provides insights into the neglected role of viral integration in the evolution of amphioxus.

Viral histones: pickpocket's prize or primordial progenitor?

The common histones H2A, H2B, H3, and H4 are the characteristic components of eukaryotic nucleosomes, which function to wrap DNA and compact the genome as well as to regulate access to DNA for transcription and replication in all eukaryotes. In the past two decades, histones have also been found to be encoded in some DNA viruses, where their functions and properties are largely unknown, though recently histones from two related viruses have been shown to form nucleosome-like structures in vitro. Viral histones can be highly similar to eukaryotic histones in primary sequence, suggesting they have been recently picked up from eukaryotic hosts, or they can be radically divergent in primary sequence and may occur as conjoined histone doublets, triplets, or quadruplets, suggesting ancient origins prior to the divergence of modern eukaryotes. Here, we review what is known of viral histones and discuss their possible origins and functions. We consider how the viral life cycle may affect their properties and histories, and reflect on the possible roles of viruses in the origin of the nucleus of modern eukaryotic cells.

MicroRNAs from Snellenius manilae bracovirus regulate innate and cellular immune responses of its host Spodoptera litura

To avoid inducing immune and physiological responses in insect hosts, parasitoid wasps have developed several mechanisms to inhibit them during parasitism, including the production of venom, specialized wasp cells, and symbioses with polydnaviruses (PDVs). These mechanisms alter the host physiology to give the wasp offspring a greater chance of survival. However, the molecular mechanisms for most of these alterations remain unclear. In the present study, we applied next-generation sequencing analysis and identified several miRNAs that were encoded in the genome of Snellenius manilae bracovirus (SmBV), and expressed in the host larvae, Spodoptera litura, during parasitism. Among these miRNAs, SmBV-miR-199b-5p and SmBV-miR-2989 were found to target domeless and toll-7 in the host, which are involved in the host innate immune responses. Microinjecting the inhibitors of these two miRNAs into parasitized S. litura larvae not only severely decreased the pupation rate of Snellenius manilae, but also restored the phagocytosis and encapsulation activity of the hemocytes. The results demonstrate that these two SmBV-encoded miRNAs play an important role in suppressing the immune responses of parasitized hosts. Overall, our study uncovers the functions of two SmBV-encoded miRNAs in regulating the host innate immune responses upon wasp parasitism.

Epigenetic Molecular Mechanisms in Insects

Insects are the largest animal group on Earth both in biomass and diversity. Their outstanding success has inspired genetics and developmental research, allowing the discovery of dynamic process explaining extreme phenotypic plasticity and canalization. Epigenetic molecular mechanisms (EMMs) are vital for several housekeeping functions in multicellular organisms, regulating developmental, ontogenetic trajectories and environmental adaptations. In Insecta, EMMs are involved in the development of extreme phenotypic divergences such as polyphenisms and eusocial castes. Here, we review the history of this research field and how the main EMMs found in insects help to understand their biological processes and diversity. EMMs in insects confer them rapid response capacity allowing insect either to change with plastic divergence or to keep constant when facing different stressors or stimuli. EMMs function both at intra as well as transgenerational scales, playing important roles in insect ecology and evolution. We discuss on how EMMs pervasive influences in Insecta require not only the control of gene expression but also the dynamic interplay of EMMs with further regulatory levels, including genetic, physiological, behavioral, and environmental among others, as was earlier proposed by the Probabilistic Epigenesis model and Developmental System Theory.

Characterization of joining sites of a viral histone H4 on host insect chromosomes

A viral histone H4 (CpBV-H4) is encoded in a polydnavirus, Cotesia plutellae bracovirus (CpBV). It plays a crucial role in parasitism of an endoparasitoid wasp, C. plutellae, against diamondback moth, Plutella xylostella, by altering host gene expression in an epigenetic mode by its N-terminal tail after joining host nucleosomes. Comparative transcriptomic analysis between parasitized and nonparasitized P. xylostella by RNA-Seq indicated that 1,858 genes were altered at more than two folds in expression levels at late parasitic stage, including 877 up-regulated genes and 981 down-regulated genes. Among parasitic factors altering host gene expression, CpBV-H4 alone explained 16.3% of these expressional changes. To characterize the joining sites of CpBV-H4 on host chromosomes, ChIP-Seq (chromatin immunoprecipitation followed by deep sequencing) was applied to chromatins extracted from parasitized larvae. It identified specific 538 ChIP targets. Joining sites were rich (60.2%) in AT sequence. Almost 40% of ChIP targets included short nucleotide repeat sequences presumably recognizable by transcriptional factors and chromatin remodeling factors. To further validate these CpBV-H4 targets, CpBV-H4 was transiently expressed in nonparasitized host at late larval stage and subjected to ChIP-Seq. Two kinds of ChIP-Seqs shared 51 core joining sites. Common targets were close (within 1 kb) to genes regulated at expression levels by CpBV-H4. However, other host genes not close to CpBV-H4 joining sites were also regulated by CpBV-H4. These results indicate that CpBV-H4 joins specific chromatin regions of P. xylostella and controls about one sixth of the total host genes that were regulated by C. plutellae parasitism in an epigenetic mode.

Parasitised caterpillars suffer reduced predation: potential implications for intra-guild predation

Intra-guild predation (IGP) is an important phenomenon structuring ecological communities and affects the success of biological control. Here we show that parasitism by the koinobiont wasp Cotesia vestalis is associated with behavioural changes in its larval host (diamondback moth, Plutella xylostella) that reduce risk of IGP. Compared with unparasitised caterpillars, parasitised P. xylostella moved less frequently to new feeding patches on plants and were less likely to fall from the plant. Wolf spiders killed significantly fewer parasitised larvae. Reflecting their reduced movement and capacity to select plant tissue of optimal quality, parasitised caterpillars fed at a lower rate and exhibited delayed development suggesting a trade-off between IGP avoidance and nutrient intake by the host. This change in behaviour to reduce risk may cascade to the first trophic level and help explain the stability of IGP systems.

A viral histone H4 suppresses insect insulin signal and delays host development

Parasitization by an endoparasitoid wasp, Cotesia plutellae, alters host development of Plutella xylostella by extending larval period and preventing metamorphosis. Insulin signal plays a crucial role in mediating insect development and controlling blood sugar level in insects. In this study, three insulin-like peptide genes (PxILP1-3) were predicted from the genome of P. xylostella. However, only PxILP1 was confirmed to be expressed in P. xylostella. Starvation suppressed the expression level of PxILP1 and up-regulated plasma trehalose level. RNA interference against PxILP1 mimicked starvation effect and extended the larval period of P. xylostella. Parasitized larvae exhibited significantly lower levels of PxILP1 expression compared to nonparasitized larvae. Injection of wasp-symbiotic polydnavirus C. plutellae bracovirus (CpBV) also suppressed PxILP1 expression and extended the larval period. Injection of a viral segment (CpBV-S30) containing a viral histone H4 (CpBV-H4) also suppressed PxILP1 expression. Co-injection of CpBV-S30 and double-stranded RNA (dsCpBV-H4) specific to CpBV-H4 rescued the suppression of PxILP1 expression. Injection of CpBV-S30 significantly extended larval development. Co-injection of CpBV-S30 with dsCpBV-H4 rescued the delay of larval development. Injection of a bovine insulin to parasitized larvae prevented parasitoid development. These results indicate that parasitism of C. plutellae can down-regulate host insulin signaling with the help of parasitic factor CpBV-H4.

Suppressive activity of a viral histone H4 against two host chromatin remodelling factors: lysine demethylase and SWI/SNF

Histone H4, a nucleosome subunit in eukaryotes, plays crucial roles in DNA package and regulation of gene expression through covalent modification. A viral histone H4 encoded in Cotesia plutellae bracovirus (CpBV), a polydnavirus, is called CpBV-H4. It is highly homologous to other histone H4 proteins excepting 38 extra amino acid residues in the N terminus. CpBV-H4 can form octamer with other histone subunits and alter host gene expression. In this study, CpBV-H4 was transiently expressed in a natural host (Plutella xylostella) and its suppressive activity on host gene expression was evaluated by the suppressive subtractive hybridization (SSH) technique. The SSH targets down-regulated by CpBV-H4 were read with the 454 pyrosequencing platform and annotated using the genome of P. xylostella. The down-regulated genes (610 contigs) were annotated in most functional categories based on gene ontology. Among these SSH targets, 115 genes were functionally distinct, including two chromatin remodelling factors: a lysine-specific demethylase (Px-KDM) and a chromatin remodelling complex [Px-SWI/SNF (SWItch/Sucrose Non-Fermentable)]. Px-KDM was highly expressed in all tested tissues during the entire larval period. Suppression of Px-KDM expression by specific RNA interference (RNAi) significantly (P<0.05) reduced haemocyte nodule formation in response to immune challenge and impaired both larval and pupal development. Px-SWI/SNF was expressed in all developmental stages. Suppression of Px-SWI/SNF expression by RNAi reduced cellular immune response and interfered with adult metamorphosis. These results suggest that CpBV-H4 can alter host gene expression by interfering with chromatin modification and remodelling factors in addition to its direct epigenetic control activity.

Environmental responses mediated by histone variants

Fluctuations in the ambient environment can trigger chromatin disruptions, involving replacement of nucleosomes or exchange of their histone subunits. Unlike canonical histones, which are available only during S-phase, replication-independent histone variants are present throughout the cell cycle and are adapted for chromatin repair. The H2A.Z variant mediates responses to environmental perturbations including fluctuations in temperature and seasonal variation. Phosphorylation of histone H2A.X rapidly marks double-strand DNA breaks for chromatin repair, which is mediated by both H2A and H3 histone variants. Other histones are used as weapons in conflicts between parasites and their hosts, which suggests broad involvement of histone variants in environmental responses beyond chromatin repair.

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

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

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.

Adaptive selection on bracovirus genomes drives the specialization of Cotesia parasitoid wasps

The geographic mosaic of coevolution predicts parasite virulence should be locally adapted to the host community. Cotesia parasitoid wasps adapt to local lepidopteran species possibly through their symbiotic bracovirus. The virus, essential for the parasitism success, is at the heart of the complex coevolutionary relationship linking the wasps and their hosts. The large segmented genome contained in the virus particles encodes virulence genes involved in host immune and developmental suppression. Coevolutionary arms race should result in the positive selection of particular beneficial alleles. To understand the global role of bracoviruses in the local adaptation or specialization of parasitoid wasps to their hosts, we studied the molecular evolution of four bracoviruses associated with wasps of the genus Cotesia, including C congregata, C vestalis and new data and annotation on two ecologically differentiated populations of C sesamie, Kitale and Mombasa. Paired orthologs analyses revealed more genes under positive selection when comparing the two C sesamiae bracoviruses belonging to the same species, and more genes under strong evolutionary constraint between species. Furthermore branch-site evolutionary models showed that 17 genes, out of the 54 currently available shared by the four bracoviruses, harboured sites under positive selection including: the histone H4-like, a C-type lectin, two ep1-like, ep2, a viral ankyrin, CrV1, a ben-domain, a Serine-rich, and eight unknown genes. Lastly the phylogenetic analyses of the histone, ep2 and CrV1 genes in different African C sesamiae populations showed that each gene described differently the individual relationships. In particular we found recombination had happened between the ep2 and CrV1 genes, which are localized 37.5 kb apart on the wasp chromosomes. Involved in multidirectional coevolutionary interactions, C sesamiae wasps rely on different bracovirus mediated molecular pathways to overcome local host resistance.

In vivo transient expression for the functional analysis of polydnaviral genes

Transient expression of a foreign gene in an organism is useful to determine its physiological function. This study introduces an efficient expression technique in the insect system using a recombinant eukaryotic expression vector. A recombinant construct expressing an enhanced green fluorescence protein (EGFP) gene under an immediately early promoter was injected into the larval hemocoel of Spodoptera exigua along with a cell transfection reagent. The expression of EGFP occurred earlier, and persisted for longer period with increasing injection dose. However, there was significant variation in expression efficiency among different cell transfection reagents. In addition, the transfection efficiency measured by RT-PCR varied among tissues with high expression of EGFP in hemocytes and fat body, but not in epidermis, gut, and nerve tissues. Two functional genes (CpBV15α and CpBV15β) derived from a polydnavirus were inserted into the eukaryotic expression vector and injected into S. exigua larvae. Expression levels in hemocytes and fat body were measured by RT-PCR and immunofluorescence assay. Both mRNAs and proteins were detected in the two tissues, in which expression signals depended on the amount of injected DNA. These immunosuppressive factors significantly inhibited hemocyte behavior, such as hemocyte-spreading, nodule formation, and phagocytosis. These results demonstrate the use of in vivo transient expression of polydnaviral genes for direct analysis of biological function in the host insect.

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

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

Host translational control of a polydnavirus, Cotesia plutellae bracovirus, by sequestering host eIF4A to prevent formation of a translation initiation complex

Host translational control is a viral strategy to exploit host cellular resources. Parasitization by some endoparasitoids containing polydnaviruses inhibits the synthesis of specific host proteins at post-transcriptional level. Two host translation inhibitory factors (HTIFs) have been proposed in Cotesia plutellae bracovirus (CpBV). Parasitization by C. plutellae inhibited storage protein 1 (SP1) synthesis of Plutella xylostella at post-transcriptional level. One HTIF, CpBV15β, inhibited the translation of SP1 mRNA in an in vitro translation assay using rabbit reticulocyte lysate, but did not inhibit its own mRNA. To further analyse the discrimination of target and nontarget mRNAs of the inhibitory effect of HTIF, 5' untranslated regions (UTRs) of SP1 and CpBV15β mRNA were reciprocally exchanged. In the presence of HTIFs, the chimeric CpBV15β mRNA that contained SP1 5' UTR was not translated, whereas the chimeric SP1 mRNA that contained CpBV15β 5' UTR was translated. There was a difference in the 5' UTR secondary structures between target (SP1) and nontarget (CpBV15α and CpBV15β) mRNAs in terms of thermal stability. Different mutant 5' UTRs of SP1 mRNA were prepared by point mutations to modify their secondary structures. The constructs containing 5' UTRs of high thermal stability in their secondary structures were inhibited by HTIF, but those of low thermal stability were not. Immunoprecipitation with CpBV15β antibody coprecipitated eIF4A, which would be required for unwinding the secondary structure of the 5' UTR. These results indicate that the viral HTIF discriminates between host mRNAs according to their dependency on eIF4A to form a functional initiation complex for translation.

A viral histone H4 suppresses expression of a transferrin that plays a role in the immune response of the diamondback moth, Plutella xylostella

A transferrin (Tf) gene has been predicted from an expressed sequence tag of the diamondback moth, Plutella xylostella. It encodes 681 amino acid residues that share 80-90% sequence homologies with other lepidopteran Tfs. The gene was constitutively expressed in all developmental stages of P. xylostella. Double-stranded RNA (dsRNA) specific to the Tf gene was prepared and microinjected into the larvae. We hypothesize that the dsRNA treatment suppressed the Tf gene expression level and it significantly inhibited haemocyte nodule formation in response to bacterial challenge. The larvae treated with dsRNA also showed a significantly enhanced susceptibility to an entomopathogenic bacterium, Bacillus thuringiensis. An endoparasitoid wasp, Cotesia plutellae, parasitized the larvae of P. xylostella, which showed significant reduction of Tf expression. The suppression of Tf expression was mimicked by transient expression of a viral gene CpBV-H4, encoded in the symbiotic virus of C. plutellae. A truncated form of CpBV-H4 prepared by deleting an extended N-terminal 38 amino acid residue lost its inhibitory activity against the Tf gene expression. These results suggest that Tf of P. xylostella plays an immunological role in P. xylostella and that the suppression of its expression in the parasitized larvae is caused by a viral histone H4 in an epigenetic mode.

Cotesia plutellae bracovirus suppresses expression of an antimicrobial peptide, cecropin, in the diamondback moth, Plutella xylostella, challenged by bacteria

An endoparasitoid wasp, Cotesia plutellae, induces significant immunosuppression of host insect, Plutella xylostella. This study was focused on suppression in humoral immune response of P. xylostella parasitized by C. plutellae. An EST database of P. xylostella provided a putative cecropin gene (PxCec) which is 627 bp long and encodes 66 amino acids. A signal peptide (22 amino acids) is predicted and two putative O-glycosylation sites in threonine are located at positions 58 and 64. Without bacterial infection, PxCec was expressed in pupa and adult stages but not in the egg and larval stages. Upon bacterial challenge, however, the larvae expressed PxCec as early as 3 h post infection (PI) and maintained high expression levels at 12-24 h PI. By 48 h PI, its expression noticeably diminished. All tested tissues of bacteria-infected P. xylostella showed PxCec expression. However, other microbes, such as virus and fungus, did not induce the PxCec expression. Parasitization by C. plutellae suppressed the expression of PxCec in response to bacterial challenge. Among the parasitic factors of C. plutellae, its symbiotic virus (C. plutellae bracovirus: CpBV) alone was able to inhibit the expression of PxCec of P. xylostella challenged by bacteria. These results indicate that PxCec expression is regulated by both immune and developmental processes in P. xylostella. The parasitization by C. plutellae inhibited the expression of PxCec by the wasp's symbiotic virus.