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

Genome-wide identification of lncRNAs associated with viral infection in Spodoptera frugiperda

The role of lncRNAs in immune defence has been demonstrated in many multicellular and unicellular organisms. However, investigation of the identification and characterization of long non-coding RNAs (lncRNAs) involved in the insect immune response is still limited. In this study, we used RNA sequencing (RNA-seq) to investigate the expression profiles of lncRNAs and mRNAs in the fall armyworm Spodoptera frugiperda in response to virus infection. To assess the tissue- and virus-specificity of lncRNAs, we analysed and compared their expression profiles in haemocytes and fat body of larvae infected with two entomopathogenic viruses with different lifestyles, i.e. the polydnavirus HdIV (Hyposoter didymator IchnoVirus) and the densovirus JcDV (Junonia coenia densovirus). We identified 1883 candidate lncRNAs, of which 529 showed differential expression following viral infection. Expression profiles differed considerably between samples, indicating that many differentially expressed (DE) lncRNAs showed virus- and tissue-specific expression patterns. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment and target prediction analyses indicated that DE-LncRNAs were mainly enriched in metabolic process, DNA replication and repair, immune response, metabolism of insect hormone and cell adhesion. In addition, we identified three DE-lncRNAs potentially acting as microRNA host genes, suggesting that they participate in gene regulation by producing miRNAs in response to virus infection. This study provides a catalogue of lncRNAs expressed in two important immune tissues and potential insight into their roles in the antiviral defence in S. frugiperda. The results may help future in-depth functional studies to better understand the biological function of lncRNAs in interaction between viruses and the fall armyworm.

Host Transcriptome Analysis of Spodoptera frugiperda Larvae Parasitized by Microplitis manilae

It has been extensively found that parasitoids manipulate host physiology to benefit the survival and development of their offspring. However, the underlying regulatory mechanisms have not received much attention. To reveal the effects of parasitization of the larval solitary endoparasitoid Microplitis manilae (Hymenoptera: Braconidae) on host Spodoptera frugiperda (Lepidoptera: Noctuidae), one of the most destructive agricultural pests in China, deep-sequencing-based transcriptome analysis was conducted to compare the host gene expression levels after 2 h, 24 h, and 48 h parasitization. A total of 1861, 962, and 108 differentially expressed genes (DEGs) were obtained from the S. frugiperda larvae at 2 h, 24 h, and 48 h post-parasitization, respectively, compared with unparasitized controls. The changes in host gene expressions were most likely caused by the injection of wasp parasitic factors, including PDVs, that were injected along with the eggs during oviposition. Based on the functional annotations in GO and KEGG databases, we revealed that most DEGs were implicated in host metabolism and immunity. Further analysis of the common DEGs in three comparisons between the unparasitized and parasitized groups identified four genes, including one unknown and three prophenoloxidase (PPO) genes. Moreover, 46 and 7 common DEGs involved in host metabolism and immunity were identified at two or three time points after parasitization, respectively. Among these, most DEGs showed increased expressions at 2 h post-wasp parasitization while exhibiting significantly decreased expression levels at 24 h post-parasitization, demonstrating the expression regulations of M. manilae parasitization on host metabolism and immune-related genes. Further qPCR verification in 20 randomly selected DEGs confirmed the accuracy and reproducibility of the gene expression profiles generated from RNA-seq. This study reveals the molecular regulatory network about how host insects respond to wasp parasitism, laying a solid foundation for revealing the physiological manipulation of wasp parasitization on host insects, which facilitates the development of biological control practices for parasitoids.

Genome, host genome integration, and gene expression in Diadegma fenestrale ichnovirus from the perspective of coevolutionary hosts

Polydnaviruses (PDVs) exhibit species-specific mutualistic relationships with endoparasitoid wasps. PDVs can be categorized into bracoviruses and ichnoviruses, which have independent evolutionary origins. In our previous study, we identified an ichnovirus of the endoparasitoid Diadegma fenestrale and named it DfIV. Here, DfIV virions from the ovarian calyx of gravid female wasps were characterized. DfIV virion particles were ellipsoidal (246.5 nm × 109.0 nm) with a double-layered envelope. Next-generation sequencing of the DfIV genome revealed 62 non-overlapping circular DNA segments (A1-A5, B1-B9, C1-C15, D1-D23, E1-E7, and F1-F3); the aggregate genome size was approximately 240 kb, and the GC content (43%) was similar to that of other IVs (41%-43%). A total of 123 open reading frames were predicted and included typical IV gene families such as repeat element protein (41 members), cysteine motif (10 members), vankyrin (9 members), polar residue-rich protein (7 members), vinnexin (6 members), and N gene (3 members). Neuromodulin N (2 members) was found to be unique to DfIV, along with 45 hypothetical genes. Among the 62 segments, 54 showed high (76%-98%) sequence similarities to the genome of Diadegma semiclausum ichnovirus (DsIV). Three segments, namely, D22, E3, and F2, contained lepidopteran host genome integration motifs with homologous regions of about 36-46 bp between them (Diadegma fenestrale ichnovirus, DfIV and lepidopteran host, Plutella xylostella). Most of the DfIV genes were expressed in the hymenopteran host and some in the lepidopteran host (P. xylostella), parasitized by D. fenestrale. Five segments (A4, C3, C15, D5, and E4) were differentially expressed at different developmental stages of the parasitized P. xylostella, and two segments (C15 and D14) were highly expressed in the ovaries of D. fenestrale. Comparative analysis between DfIV and DsIV revealed that the genomes differed in the number of segments, composition of sequences, and internal sequence homologies.

Diverse Host Immune Responses of Different Geographical Populations of the Coconut Rhinoceros Beetle to Oryctes Rhinoceros Nudivirus (OrNV) Infection

Incursions of the coconut rhinoceros beetle (CRB), Oryctes rhinoceros, into different islands in the South Pacific have been detected in recent years. It has been suggested that this range expansion is related to an O. rhinoceros haplotype reported to show reduced susceptibility to the well-established classical biocontrol agent, Oryctes rhinoceros nudivirus (OrNV). Our understanding of the genetic characteristics which distinguish the population of O. rhinoceros that has recently established in Solomon Islands from other well-established populations across the region is very limited. Here, we hypothesized that the recently established O. rhinoceros population should have greater innate immune responses when challenged by OrNV than those of well-established and native O. rhinoceros populations. We used the RNA sequencing (RNA-Seq) approach to generate gene expression profiles of midgut tissue from OrNV-infected and noninfected individuals collected in the Solomon Islands (recent incursion), Papua New Guinea and Fiji (previously established), and the Philippines (within the native range). The collections included individuals from each of the three major mitochondrial lineages (CRB-G, CRB-PNG, and CRB-S) known to the region, allowing us to explore the specific responses of each haplotype to infection. Although insects from the Philippines and Solomon Islands that were tested belong to the same mitochondrial lineage (CRB-G), their overall responses to infection were different. The number of differentially expressed genes between OrNV-infected and noninfected wild-caught individuals from the four different locations varied from 148 to 252. Persistent OrNV infection caused a high level of induced antimicrobial activity and immune responses in O. rhinoceros, but the direction and magnitude of the responses were population specific. The insects tested from the Solomon Islands displayed extremely high expression of genes which are known to be involved in immune responses (e.g. coleoptericin, cecropin, and serpin). These variations in the host immune system among insects from different geographical regions might be driven by variations in the virulence of OrNV isolates, and this requires further investigation. Overall, our current findings support the importance of immunity in insect pest incursion and an expansion of the pest's geographic range. IMPORTANCE Oryctes rhinoceros nudivirus (OrNV) is a double-stranded DNA (dsDNA) virus which has been used as a biocontrol agent to suppress coconut rhinoceros beetle (CRB) in the Pacific Islands. Recently a new wave of CRB incursions in Oceania is thought to be related to the presence of low-virulence isolates of OrNV or virus-tolerant haplotypes of beetles (CRB-G). Our comparative analysis of OrNV-infected and noninfected CRBs revealed that specific sets of genes were induced by viral infection in the beetles. This induction was much stronger in beetles collected from the Solomon Islands, a newly invaded country, than in individuals collected from within the beetle's native range (the Philippines) or from longer-established populations in its exotic range (Fiji and Papua New Guinea [PNG]). Beetles from the Philippines and the Solomon Islands that were tested in this study all belonged to the CRB-G haplotype, but the country-specific responses of the beetles to OrNV infection were different.

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.

Partner-specific induction of Spodoptera frugiperda immune genes in response to the entomopathogenic nematobacterial complex Steinernema carpocapsae-Xenorhabdus nematophila

The Steinernema carpocapsae-Xenorhabdus nematophila association is a nematobacterial complex used in biological control of insect crop pests. The infection success of this dual pathogen strongly depends on its interactions with the host's immune system. Here, we used the lepidopteran pest Spodoptera frugiperda to analyze the respective impact of each partner in the induction of its immune responses. First, we used previously obtained RNAseq data to construct the immunome of S. frugiperda and analyze its induction. We then selected representative genes to study by RT-qPCR their induction kinetics and specificity after independent injections of each partner. We showed that both X. nematophila and S. carpocapsae participate in the induction of stable immune responses to the complex. While X. nematophila mainly induces genes classically involved in antibacterial responses, S. carpocapsae induces lectins and genes involved in melanization and encapsulation. We discuss putative relationships between these differential inductions and the pathogen immunosuppressive strategies.

Regulation of the Larval Transcriptome of Diatraea saccharalis (Lepidoptera: Crambidae) by Maternal and Other Factors of the Parasitoid Cotesia flavipes (Hymenoptera: Braconidae)

Koinobiont endoparasitoid wasps regulate the host's physiology to their own benefit during their growth and development, using maternal, immature and/or derived-tissue weaponry. The tools used to subdue the wasps' hosts interfere directly with host transcription activity. The broad range of host tissues and pathways affected impedes our overall understanding of the host-regulation process during parasitoid development. Next-generation sequencing and de novo transcriptomes are helpful approaches to broad questions, including in non-model organisms. In the present study, we used Illumina sequencing to assemble a de novo reference transcriptome of the sugarcane borer Diatraea saccharalis, to investigate the regulation of host gene expression by the larval endoparasitoid Cotesia flavipes. We obtained 174,809,358 reads and assembled 144,116 transcripts, of which 44,325 were putatively identified as lepidopteran genes and represented a substantial number of pathways that are well described in other lepidopteran species. Comparative transcriptome analyses of unparasitized versus parasitized larvae identified 1,432 transcripts of D. saccharalis that were up-regulated under parasitization by C. flavipes, while 1,027 transcripts were down-regulated. Comparison of the transcriptomes of unparasitized and pseudoparasitized D. saccharalis larvae led to the identification of 1,253 up-regulated transcripts and 972 down-regulated transcripts in the pseudoparasitized larvae. Analysis of the differentially expressed transcripts showed that C. flavipes regulated several pathways, including the Ca+2 transduction signaling pathway, glycolysis/gluconeogenesis, chitin metabolism, and hormone biosynthesis and degradation, as well as the immune system, allowing us to identify key target genes involved in the metabolism and development of D. saccharalis.

Facilitator roles of viruses in enhanced insect resistance to biotic stress

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

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.

Functional Multiplicity of an Insect Cytokine Family Assists Defense Against Environmental Stress

The widespread distribution of insects over many ecological niches owes much to evolution of multiple mechanisms to defend against environmental stress, especially because their ectothermic nature and small body size render them particularly susceptible to extremes in temperature and water availability. In this review, we will summarize the latest information describing a single, multifunctional cytokine family that is deployed by six orders of insect species to combat a diverse variety of environmental stresses. The originating member of this peptide family was identified in Mythimna (formerly called Pseudaletia) separata armyworm; the cytokine was named growth-blocking peptide (GBP), reflecting its actions in combating parasitic invasion. The peptide’s name has been retained, though the list of its regulatory activities has greatly expanded. All members of this family are small peptides, 19–25 amino acid residues, whose major source is fat body. They are now known to regulate embryonic morphogenesis, larval growth rates, feeding activities, immune responses, nutrition, and aging. In this review, we will describe recent developments in our understanding of the mechanisms of action of the GBP family, but we will also highlight remaining gaps in our knowledge.

Virus innexin expression in insect cells disrupts cell membrane potential and pH

Certain parasitoid wasps are associated with Polydnaviruses, symbiotic viruses that encode virulence factors which are essential to successful parasitization by the wasp of a caterpillar host. Members of one group of Polydnaviruses, the Ichnoviruses, encode a multi-gene family known as Vinnexins. Vinnexins are homologues of insect gap junction genes, and form functional gap junctions that may affect host cell physiology. However, the role of Vinnexins in host pathology and the mechanism by which these affect their caterpillar host are largely unknown. In this article, we generated recombinant baculoviruses to express vinnexins in Spodoptera frugiperda (Sf9) cells. To measure cell physiological changes caused by Vinnexins, cells were probed with a membrane potential-sensitive probe, DiBac4(3), and a pH indicator, carboxyfluorescein diacetate (CFDA). In addition, we utilized carbenoxolone and ouabain, respectively, to probe the role of gap junctions and hemi-channels, and Na+/K+-ATPase in establishing membrane potential in studied cells. Our results indicate that Vinnexins induce cell membrane depolarization and cytoplasmic alkalization to a degree specific to each tested Vinnexin, and that neither Vinnexin hemi-channels nor Na+/K+-ATPase appear to underlie these effects directly. These results hint that members of the Vinnexin protein family may affect host bio-electrical phenomena to disrupt host cell physiology, and that the individual proteins of the family may differentially affect host physiology.

Virus Innexins induce alterations in insect cell and tissue function

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

Venom of Parasitoid Pteromalus puparum Impairs Host Humoral Antimicrobial Activity by Decreasing Host Cecropin and Lysozyme Gene Expression

Insect host/parasitoid interactions are co-evolved systems in which host defenses are balanced by parasitoid mechanisms to disable or hide from host immune effectors. Here, we report that Pteromalus puparum venom impairs the antimicrobial activity of its host Pieris rapae. Inhibition zone results showed that bead injection induced the antimicrobial activity of the host hemolymph but that venom inhibited it. The cDNAs encoding cecropin and lysozyme were screened. Relative quantitative PCR results indicated that all of the microorganisms and bead injections up-regulated the transcript levels of the two genes but that venom down-regulated them. At 8 h post bead challenge, there was a peak in the transcript level of the cecropin gene, whereas the peak of lysozyme gene occurred at 24 h. The transcripts levels of the two genes were higher in the granulocytes and fat body than in other tissues. RNA interference decreased the transcript levels of the two genes and the antimicrobial activity of the pupal hemolymph. Venom injections similarly silenced the expression of the two genes during the first 8 h post-treatment in time- and dose-dependent manners, after which the silence effects abated. Additionally, recombinant cecropin and lysozyme had no significant effect on the emergence rate of pupae that were parasitized by P. puparum females. These findings suggest one mechanism of impairing host antimicrobial activity by parasitoid venom.

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.

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

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

Anopheles gambiae densovirus (AgDNV) has negligible effects on adult survival and transcriptome of its mosquito host

Mosquito densoviruses (DNVs) are candidate agents for paratransgenic control of malaria and other vector-borne diseases. Unlike other mosquito DNVs, the Anopheles gambiae DNV (AgDNV) is non-pathogenic to larval mosquitoes. However, the cost of infection upon adults and the molecular mechanisms underpinning infection in the mosquito host are unknown. Using life table analysis, we show that AgDNV infection has minimal effects on An. gambiae survival (no significant effect in 2 replicates and a slight 2 day survival decrease in the third replicate). Using microarrays, we show that AgDNV has very minimal effect on the adult mosquito transcriptome, with only 4-15 genes differentially regulated depending on the statistical criteria imposed. The minimal impact upon global transcription provides some mechanistic understanding of lack of virus pathogenicity, suggesting a long co-evolutionary history that has shifted towards avirulence. From an applied standpoint, lack of strong induced fitness costs makes AgDNV an attractive agent for paratransgenic malaria control.

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

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

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.

Suitability of three Ostrinia species as hosts for Macrocentrus cingulum: a comparison of their encapsulation abilities

Two cornborer species, Ostrinia furnacalis (Lepidoptera: Crambidae) and O. nubilalis, are major corn pests in Asia and Europe, respectively. In both continents, the larval endoparasitoid Macrocentrus cingulum (Hymenoptera: Braconidae) develops on another, closely related stemborer, O. scapulalis, which feeds on mugwort and other dicotyledons. M. cingulum also emerges from O. furnacalis in Asia and O. nubilalis in North America, but not from O. nubilalis in Europe. We assessed the ability of three populations of each of the three Ostrinia species to encapsulate foreign bodies of a size similar to that of a M. cingulum egg. We conclude that variations in encapsulation ability alone cannot account for the differences observed in the field between parasite emergence rates in these different host species and geographic areas.

Venom of the ectoparasitoid, Nasonia vitripennis, influences gene expression in Musca domestica hemocytes

Insect hosts have evolved potent innate immunity against invasion by parasitoid wasps. Host/parasitoids live in co-evolutionary relationships. Nasonia vitripennis females inject venom into their dipteran hosts just prior to laying eggs on the host's outer integument. The parasitoid larvae are ectoparasitoids because they feed on their hosts within the puparium, but do not enter the host body. We investigated the influence of N. vitripennis venom on the gene expression profile of hemocytes of their hosts, pupae of the housefly, Musca domestica. We prepared venom by isolating venom glands and treated experimental host pupae with venom. We used suppression subtractive hybridization (SSH) to determine the influence of venom on hemocyte gene expression. At 1 h post treatment, we recorded decreases in transcript levels of 133 EST clones derived from forward a subtractive library of host hemocytes and upregulation in transcript levels of 111 EST clones from the reverse library. These genes are related to immune and stress response, cytoskeleton, cell cycle and apoptosis, metabolism, transport, and transcription/translation regulation. We verified the reliability of our data with reverse transcription quantitative real-time PCR analysis of randomly selected genes, and with assays of enzyme activities. These analyses showed that the expression level of all selected genes were downregulated after venom treatment. Outcomes of our experiments support the hypothesis that N. vitripennis venom influences the gene expression in host hemocytes. We conclude that the actions of venom on host gene expression influence host biology in ways that benefit the development and emergence of the next generation of parasitoids.

Hyposoter didymator uses a combination of passive and active strategies to escape from the Spodoptera frugiperda cellular immune response

An endoparasitic life style is widespread among Hymenoptera, and various different strategies allowing parasitoids to escape from the host encapsulation response have been reported. Species carrying polydnaviruses (PDVs), such as the ichneumonid Hyposoter didymator, generally rely on the viral symbionts to evade host immune responses. In this work, we show that H. didymator eggs can evade encapsulation by the host in the absence of calyx fluid (containing the viral particles), whereas protection of the larvae requires the presence of calyx fluid. This evasion by the eggs depends on proteins associated with the exochorion. This type of local passive strategy has been described for a few species carrying PDVs. Immune evasion by braconid eggs appears to be related to PDVs or proteins synthesized in the oviducts being associated with the egg. We report that in H. didymator, by contrast, proteins already present in the ovarian follicles are responsible for the eggs avoiding encapsulation. Mass spectrometry analysis of the egg surface proteins revealed the presence of host immune-related proteins, including one with similarities with apolipophorin-III, and also the presence of three viral proteins encoded by IVSPERs (Ichnovirus Structural Protein Encoding Regions).

Transcriptomic immune response of Tenebrio molitor pupae to parasitization by Scleroderma guani

BACKGROUND

Host and parasitoid interaction is one of the most fascinating relationships of insects, which is currently receiving an increasing interest. Understanding the mechanisms evolved by the parasitoids to evade or suppress the host immune system is important for dissecting this interaction, while it was still poorly known. In order to gain insight into the immune response of Tenebrio molitor to parasitization by Scleroderma guani, the transcriptome of T. molitor pupae was sequenced with focus on immune-related gene, and the non-parasitized and parasitized T. molitor pupae were analyzed by digital gene expression (DGE) analysis with special emphasis on parasitoid-induced immune-related genes using Illumina sequencing.

METHODOLOGY/PRINCIPAL FINDINGS

In a single run, 264,698 raw reads were obtained. De novo assembly generated 71,514 unigenes with mean length of 424 bp. Of those unigenes, 37,373 (52.26%) showed similarity to the known proteins in the NCBI nr database. Via analysis of the transcriptome data in depth, 430 unigenes related to immunity were identified. DGE analysis revealed that parasitization by S. guani had considerable impacts on the transcriptome profile of T. molitor pupae, as indicated by the significant up- or down-regulation of 3,431 parasitism-responsive transcripts. The expression of a total of 74 unigenes involved in immune response of T. molitor was significantly altered after parasitization.

CONCLUSIONS/SIGNIFICANCE

obtained T. molitor transcriptome, in addition to establishing a fundamental resource for further research on functional genomics, has allowed the discovery of a large group of immune genes that might provide a meaningful framework to better understand the immune response in this species and other beetles. The DGE profiling data provides comprehensive T. molitor immune gene expression information at the transcriptional level following parasitization, and sheds valuable light on the molecular understanding of the host-parasitoid interaction.

Differentially expressed genes in the cuticle and hemolymph of the silkworm, Bombyx mori, injected with the fungus Beauveria bassiana

The most important pathogenic fungus of the silkworm, Bombyx mori L. (Lepidoptera: Bombycidae), is Beauveria bassiana (Balsamo-Crivelli ) Vuillemin (Hypocreales: Clavicipitaceae), which causes significant damage to sericulture production. Therefore, diagnosing fungal disease and developing new control measures are crucial to silk production. To better understand the responsive and interactive mechanisms between the host silkworm and this fungus, variations in silkworm gene expression were investigated using the suppression subtractive hybridization method following the injection of B. bassiana conidia. Two cDNA libraries were constructed, and 140 cDNA clones were isolated. Of the 50 differentially expressed genes identified, 45 (112 clones) were identified in the forward library, and 5 (28 clones) were identified in the reverse library. Expression profiling of six of these genes by quantitative polymerase chain reaction (qPCR) verified that they were induced by the fungal challenge. The present study provides insight into the interaction between lepidopteran insects and pathogenic fungi.

Suppression of scavenger receptors transcription by parasitoid factors

Scavenger receptors (SR) are a group of membrane proteins that play central roles in various functions, such as immune responses in insects. Members of different SR classes were identified from Plutella xylostella larval transcriptome. SR B1 and B3 were found to be differentially expressed in larvae and pupae. Expression of P. xylostella SR genes was significantly altered during immune challenge induced in P. xylostella cells (Px) and parasitized larvae. Maternal factors injected into the larvae by the endoparasitoid wasp Diadegma semiclausum at oviposition include venom and ichnovirus (DsIV) genes to suppress the host immune system. Transient expression of two DsIV genes, Vankyrin1 and Repeat element 4 (Rep4), in Px cells led to significant down-regulation of both SR B1 and B3 transcript levels, while DsIV Rep4 expression did not change the relative transcription levels of SR B3. In conclusion, it appears that the two members of the SR family play important roles in innate immune responses in P. xylostella and that each member of this group may play different roles in the host-parasitoid interaction.

Molecular cloning and functional study of calreticulin from a lepidopteran pest, Pieris rapae

Insects have an effective innate immune system to protect themselves from exogenous invaders. Calreticulin is a multifunctional protein mainly involved in directing proper conformation of proteins, controlling calcium level, and participating in immune responses. Previous suppression subtractive hybridization assay showed that the expression of Pieris rapae calreticulin (PrCRT) was suppressed after injection of Pteromalus puparum venom. In this study, we obtained a full length cDNA of PrCRT and expressed recombinant wild type and the N-domain deleted mutant PrCRT in bacteria. Real time quantitative PCR and western blot analyses showed that PrCRT mRNA and protein were expressed in hemocytes, Malpighian tubule, midgut, epidermis and fat body, with a higher level in hemocytes. PrCRT was probably located in endoplasmic reticulum distributing in the cytoplasm of hemocytes. Recombinant PrCRT was first able to attach and then enter the hemocytes by endocytosis. PrCRT mRNA in hemocytes was significantly induced after injection of yeast or beads, but did not change noticeably after injection of Escherichia coli or Micrococcus lysodeikticus. Recombinant PrCRT enhanced cellular encapsulation by P. rapae hemocytes in vitro, and the N-domain of PrCRT was required for encapsulation. RNAi of PrCRT by dsRNA injection impaired the ability of hemocytes to encapsulate beads. After parasitization by P. puparum, PrCRT mRNA and protein levels in P. rapae pupal hemocytes were significantly suppressed compared to non-parasitized control. Our results suggest that PrCRT is involved in cellular encapsulation and the pupal parasitoid P. puparum can decrease PrCRT expression to impair host cellular immune response.

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.

Pteromalus puparum venom impairs host cellular immune responses by decreasing expression of its scavenger receptor gene

Insect host/parasitoid interactions are co-evolved systems in which host defenses are balanced by parasitoid mechanisms to disable or hide from host immune effectors. Although there is a rich literature on these systems, parasitoid immune-disabling mechanisms have not been fully elucidated. Here we report on a newly discovered immune-disabling mechanism in the Pieris rapae/Pteromalus puparum host/parasitoid system. Because venom injections and parasitization suppresses host phagocytosis, we turned attention to the P. rapae scavenger receptor (Pr-SR), posing the hypothesis that P. puparum venom suppresses expression of the host Pr-SR gene. To test our hypothesis, we cloned a full-length cDNA of the Pr-SR. Multiple sequences alignment showed the deduced amino acid sequence of Pr-SR is similar to scavenger receptors of other lepidopterans. Bacterial and bead injections induced Pr-SR mRNA and protein expression, which peaked at 4h post-bead injection. Venom injection inhibited Pr-SR expression. Pr-SR was specifically expressed in granulocytes compared to plasmatocytes. We localized the Pr-SR protein in cytoplasm and cellular membrane, with no evidence of secretion into host plasma. Double-strand RNA designed to Pr-SR mRNA silenced expression of Pr-SR and significantly impaired host phagocytosis and encapsulation reactions. Venom injections similarly silenced Pr-SR expression during the first 8h post-treatment, after which the silencing effects gradually abated. We infer from these findings that one mechanism of impairing P. rapae hemocytic immune reactions is by silencing expression of Pr-SR.

Venom of parasitoid, Pteromalus puparum, suppresses host, Pieris rapae, immune promotion by decreasing host C-type lectin gene expression

Background

Insect hosts have evolved immunity against invasion by parasitoids, and in co-evolutionary response parasitoids have also developed strategies to overcome host immune systems. The mechanisms through which parasitoid venoms disrupt the promotion of host immunity are still unclear. We report here a new mechanism evolved by parasitoid Pteromalus puparum, whose venom inhibited the promotion of immunity in its host Pieris rapae (cabbage white butterfly).

Methodology/Principal Findings

A full-length cDNA encoding a C-type lectin (Pr-CTL) was isolated from P. rapae. Quantitative PCR and immunoblotting showed that injection of bacterial and inert beads induced expression of Pr-CTL, with peaks of mRNA and Pr-CTL protein levels at 4 and 8 h post beads challenge, respectively. In contrast, parasitoid venom suppressed Pr-CTL expression when co-injected with beads, in a time and dose-dependent manner. Immunolocalization and immunoblotting results showed that Pr-CTL was first detectable in vesicles present in cytoplasm of granulocytes in host hemolymph, and was then secreted from cells into circulatory fluid. Finally, the secreted Pr-CTL bound to cellular membranes of both granulocytes and plasmatocytes. Injection of double-stranded RNA specific for target gene decreased expression of Pr-CTL, and a few other host immune-related genes. Suppression of Pr-CTL expression also down-regulated antimicrobial and phenoloxidase activities, and reducing phagocytotic and encapsulation rates in host. The inhibitory effect of parasitoid venom on host encapsulation is consistent with its effect in suppressing Pr-CTL expression. Binding assay results showed that recombinant Pr-CTL directly attached to the surface of P. puparum egges. We infer that Pr-CTL may serve as an immune signalling co-effector, first binding to parasitoid eggs, regulating expression of a set of immune-related genes and promoting host immunity.

Conclusions/Significance

P. puparum venom inhibits promotion of host immune responses by silencing expression of host C-type lectin gene Pr-CTL, whose expression affected transcription of other host immune-related genes.

Deep sequencing-based transcriptome analysis of Plutella xylostella larvae parasitized by Diadegma semiclausum

Background

Parasitoid insects manipulate their hosts' physiology by injecting various factors into their host upon parasitization. Transcriptomic approaches provide a powerful approach to study insect host-parasitoid interactions at the molecular level. In order to investigate the effects of parasitization by an ichneumonid wasp (Diadegma semiclausum) on the host (Plutella xylostella), the larval transcriptome profile was analyzed using a short-read deep sequencing method (Illumina). Symbiotic polydnaviruses (PDVs) associated with ichneumonid parasitoids, known as ichnoviruses, play significant roles in host immune suppression and developmental regulation. In the current study, D. semiclausum ichnovirus (DsIV) genes expressed in P. xylostella were identified and their sequences compared with other reported PDVs. Five of these genes encode proteins of unknown identity, that have not previously been reported.

Results

De novo assembly of cDNA sequence data generated 172,660 contigs between 100 and 10000 bp in length; with 35% of > 200 bp in length. Parasitization had significant impacts on expression levels of 928 identified insect host transcripts. Gene ontology data illustrated that the majority of the differentially expressed genes are involved in binding, catalytic activity, and metabolic and cellular processes. In addition, the results show that transcription levels of antimicrobial peptides, such as gloverin, cecropin E and lysozyme, were up-regulated after parasitism. Expression of ichnovirus genes were detected in parasitized larvae with 19 unique sequences identified from five PDV gene families including vankyrin, viral innexin, repeat elements, a cysteine-rich motif, and polar residue rich protein. Vankyrin 1 and repeat element 1 genes showed the highest transcription levels among the DsIV genes.

Conclusion

This study provides detailed information on differential expression of P. xylostella larval genes following parasitization, DsIV genes expressed in the host and also improves our current understanding of this host-parasitoid interaction.

Lepidopteran transcriptome analysis following infection by phylogenetically unrelated polydnaviruses highlights differential and common responses

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

Neuropeptide physiology in insects

In a search for more environmentally benign alternatives to chemical pesticides, insect neuropeptides have been suggested as ideal candidates. Neuropeptides are neuromodulators and/or neurohormones that regulate most major physiological and behavioral processes in insects. The major neuropeptide structures have been identified through peptide purification in insects (peptidomics) and insect genome projects. Neuropeptide receptors have been identified and characterized in Drosophila and similar receptors are being targeted in other insects considered to be economically detrimental pests in agriculture and forestry. Defining neuropeptide action in different insect systems has been more challenging and as a consequence, identifying unique targets for potential pest control is also a challenge. In this chapter, neuropeptide biosynthesis as well as select physiological processes are examined with a view to pest control targets. The application of molecular techniques to transform insects with neuropeptide or neuropeptide receptor genes, or knockout genes to identify potential pest control targets, is a relatively new area that offers promise to insect control. Insect immune systems may also be manipulated through neuropeptides which may aid in compromising the insects ability to defend against foreign invasion.

Expression of immune-response genes in lepidopteran host is suppressed by venom from an endoparasitoid, Pteromalus puparum

BACKGROUND

The relationships between parasitoids and their insect hosts have attracted attention at two levels. First, the basic biology of host-parasitoid interactions is of fundamental interest. Second, parasitoids are widely used as biological control agents in sustainable agricultural programs. Females of the gregarious endoparasitoid Pteromalus puparum (Hymenoptera: Pteromalidae) inject venom along with eggs into their hosts. P. puparum does not inject polydnaviruses during oviposition. For this reason, P. puparum and its pupal host, the small white butterfly Pieris rapae (Lepidoptera: Pieridae), comprise an excellent model system for studying the influence of an endoparasitoid venom on the biology of the pupal host. P. puparum venom suppresses the immunity of its host, although the suppressive mechanisms are not fully understood. In this study, we tested our hypothesis that P. puparum venom influences host gene expression in the two main immunity-conferring tissues, hemocytes and fat body.

RESULTS

At 1 h post-venom injection, we recorded significant decreases in transcript levels of 217 EST clones (revealing 113 genes identified in silico, including 62 unknown contigs) derived from forward subtractive libraries of host hemocytes and in transcript levels of 288 EST clones (221 genes identified in silico, including 123 unknown contigs) from libraries of host fat body. These genes are related to insect immune response, cytoskeleton, cell cycle and apoptosis, metabolism, transport, stress response and transcriptional and translational regulation. We verified the reliability of the suppression subtractive hybridization (SSH) data with semi-quantitative RT-PCR analysis of a set of randomly selected genes. This analysis showed that most of the selected genes were down-regulated after venom injection.

CONCLUSIONS

Our findings support our hypothesis that P. puparum venom influences gene expression in host hemocytes and fat body. Specifically, the venom treatments led to reductions in expression of a large number of genes. Many of the down-regulated genes act in immunity, although others act in non-immune areas of host biology. We conclude that the actions of venom on host gene expression influence immunity as well as other aspects of host biology in ways that benefit the development and emergence of the next generation of parasitoids.

Comparative analysis of Bombyx mori nucleopolyhedrovirus responsive genes in fat body and haemocyte of B. mori resistant and susceptible strains

The infection profiles of the Bombyx mori nucleopolyhedrovirus (BmNPV) in B. mori larvae revealed that the virus invaded the fat body and haemocyte of both KN and 306 strains, which are highly resistant and susceptible, respectively, to BmNPV infection. However, viral proliferation was notably slowed in the resistant B. mori strain. Using suppression subtractive hybridization, two fat body cDNA libraries were constructed to compare BmNPV responsive gene expression levels between the two silkworm lines. In total, 96 differentially expressed genes were obtained. Real-time quantitative PCR (qPCR) analysis confirmed that eight genes were significantly up-regulated in the fat body and haemocyte of the KN strain following BmNPV injection. Our results suggest that these genes may have potential roles in B. mori antiviral infection mechanisms.

Pathogenesis of Junonia coenia densovirus in Spodoptera frugiperda: a route of infection that leads to hypoxia

To evaluate densovirus potential against lepidopteran pests and their capacity to invade new hosts, we have characterised in vivo the infection and pathogenesis of the Junonia coenia densovirus (JcDNV) in the noctuid pest Spodoptera frugiperda. Here we show that infection starts with the ingestion of viral particles that cross the midgut epithelium without replicating. By quantitative PCR we established the kinetic and the route of infection, from virus ingestion to replication in visceral tracheae and hemocytes. JcDNV has a high particle-to-infection ratio mostly due to the barrier function of the midgut. Pathology and cytopathology suggested that infection of tracheal cells impairs oxygen delivery to demanding tissues leading to cytopathic effects in all the tissues. Finally, larval death results from several physiological shocks, including molting arrest and anoxia.

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.

Time series analysis of the transcriptional responses of Biomphalaria glabrata throughout the course of intramolluscan development of Schistosoma mansoni and Echinostoma paraensei

Successful colonization of a compatible snail host by a digenetic trematode miracidium initiates a complex, proliferative development program requiring weeks to reach culmination in the form of production of cercariae which, once started, may persist for the remainder of the life span of the infected snail. How are such proliferative and invasive parasites able to circumvent host defenses and establish chronic infections? Using a microarray designed to monitor the internal defense and stress-related responses of the freshwater snail Biomphalaria glabrata, we have undertaken a time course study to monitor snail responses following exposure to two different trematode species to which the snail is susceptible: the medically important Schistosoma mansoni, exemplifying sporocyst production in its larval development, or Echinostoma paraensei, representing an emphasis on rediae production in its larval development. We sampled eight time points (0.5, 1, 2, 4, 8, 16 and 32 days p.i.) that cover the period required for cercariae to be produced. Following exposure to S. mansoni, there was a preponderance of up-regulated over down-regulated array features through 2 days p.i. but by 4 days p.i. and thereafter, this pattern was strongly reversed. For E. paraensei, there was a preponderance of down-regulated array features over up-regulated features at even 0.5 days p.i., a pattern that persists throughout the course of infection except for 1 day p.i., when up-regulated array features slightly outnumbered down-regulated features. Examination of particular array features revealed several that were up-regulated by both parasites early in the course of infection and one, fibrinogen related protein 4 (FREP 4), that remained significantly elevated throughout the course of infection with either parasite, effectively serving as a marker of infection. Many defense-related transcripts were persistently down-regulated, including several fibrinogen-containing lectins and homologs of molecules best known from vertebrate phagocytic cells. Our results are consistent with earlier studies suggesting that both parasites are able to interfere with host defense responses, including a tendency for E. paraensei to do so more rapidly and strongly than S. mansoni. They further suggest mechanisms for how trematodes are able to establish the chronic infections necessary for their continued success.

Bacterial, but not baculoviral infections stimulate Hemolin expression in noctuid moths

Lepidopteran larvae are regularly infected by baculoviruses during feeding on infected plants. The differences in sensitivity to these infections can be substantial, even among closely related species. For example, the noctuids Cotton bollworm (Helicoverpa zea) and Tobacco budworm (Heliothis virescens), have a 1000-fold difference in sensitivity to Autographa californica multiple nucleopolyhedrovirus (AcMNPV) infection. Recent data were interpreted to indicate that the lepidopteran immunoglobulin protein, Hemolin, is synthesized upon viral injection and therefore to participate in anti-viral responses. To investigate whether Hemolin synthesis is affected by a natural viral infection, specific transcription in fat bodies and hemocytes of H. zea and H. virescens larvae was monitored following per os infection with the baculovirus HzSNPV (H. zea single nucleopolyhedrovirus). Both moths showed the same expression pattern as seen in uninfected animals and coincided with ecdysone responses, previously known to induce Hemolin expression. In contrast, injection of lyophilized Micrococcus luteus resulted in increased Hemolin expression supporting a role for Hemolin as an immuno-responsive protein in these species. The combined data are consistent with the suggestion that while Hemolin seems to participate in the response to virus infection in the superfamily Bombycoidea, this is not true in the Noctuoidea.

Lysozymes and lysozyme-like proteins from the fall armyworm, Spodoptera frugiperda

Lysozyme is an important component of the insect non-specific immune response against bacteria that is characterized by its ability to break down bacterial cell-walls. By searching an EST database from the fall armyworm, Spodoptera frugiperda (Negre et al., 2006), we identified five sequences encoding proteins of the lysozyme family. The deduced protein sequences corresponded to three classical c-type lysozymes Sf-Lys1, Sf-Lys2 and Sf-Lys3, and two lysozyme-like proteins, Sf-LLP1 and Sf-LLP2. Sf-Lys1 was purified from the hemolymph of Escherichia coli-challenged S. frugiperda larvae. The mature protein had a molecular mass of 13.975 Da with an isoelectric point of 8.77 and showed 98.3% and 96.7% identity with lysozymes from Spodoptera litura and Spodoptera exigua, respectively. As the other insect lysozymes, Sf-Lys1 was active against gram positive bacteria such as Micrococcus luteus but also induced a slight permeabilization of the inner membrane of E. coli. Genes encoding these five Sf-Lys or Sf-LLPs were differentially up-regulated in three immune-competent tissues (hemocytes, fat body and gut) after challenges with non-pathogenic bacteria, E. coli and M. luteus, or entomopathogenic bacterium, Photorhabdus luminescens. Sf-Lys1 and Sf-Lys2 were mainly induced in fat body in the presence of E. coli or P. luminescens. Sf-Lys3, which had an acidic isoelectric point, was found to be the most up-regulated of all five Sf-Lys or Sf-LLPs in hemocytes and gut after challenge with P. luminescens. More molecular data are now available to investigate differences in physiological functions of these different members of the lysozyme superfamily.

Gene expression profiling during early response to injury and microbial challenges in the silkworm, Bombyx mori

To identify Bombyx mori genes involved in the early response to injury and microbial challenge, we performed genome-wide gene expression-profiling experiments using oligonucleotide DNA microarrays. Of approximately 23,000 genes examined, 465 displayed changes in mRNA expression levels. Of these, 306 were induced and 159 were repressed in response to injury (injection with phosphate buffer saline) or challenges by Gram-negative (Serratia marcescens), Gram-positive bacteria (Staphylococcus aureus), or fungus (Beauveria bassiana). Many of these differentially expressed genes can be assigned to specific functional groups of the innate immune response, including recognition, signaling, melanization and coagulation, and antimicrobial peptides. Seventeen percent of differentially expressed genes encode proteins with no obvious similarity to known functional domains. Of particular interest is a member of the juvenile hormone-binding protein family, which was highly induced by both injury and microbial challenges. The possible role of juvenile hormone in innate immunity is discussed.

Proteome changes in the plasma of Papilio xuthus (Lepidoptera: Papilionidae): effect of parasitization by the endoparasitic wasp Pteromalus puparum (Hymenoptera: Pteromalidae)

Although the biochemical dissection of parasitoid-host interactions is becoming well characterized, the molecular knowledge concerning them is minimal. In order to understand the molecular bases of the host immune response to parasitoid attack, we explored the response of Papilio xuthus parasitized by the endoparasitic wasp Pteromalus puparum using proteomic approach. By examining the differential expression of plasma proteins in the parasitized and unparasitized host pupae by two-dimensional (2D) electrophoresis, 16 proteins were found to vary in relation to parasitization compared with unparasitized control samples. All of them were submitted to identification by mass spectrometry coupled with a database search. The modulated proteins were found to fall into the following functional groups: humoral or cellular immunity, detoxification, energy metabolism, and others. This study contributes insights into the molecular mechanism of the relationships between parasitoids and their host insects.

Gene expression profiling of resistant and susceptible Bombyx mori strains reveals nucleopolyhedrovirus-associated variations in host gene transcript levels

We investigated variations in the gene expression of Bombyx mori following infection with a nucleopolyhedrovirus (BmNPV). Two B. mori strains, KN and 306, which are highly resistant and susceptible to BmNPV infection, respectively, were used in this study. The infection profiles of BmNPV in the B. mori KN and 306 larvae revealed that the virus invaded the midguts of both these strains. However, its proliferation was notably inhibited in the midgut of the resistant strain. By using the suppression subtractive hybridization method, two cDNA libraries were constructed in order to compare the BmNPV responsive gene expressions between the two silkworm lines. In total, 62 differentially expressed genes were obtained. Real-time qPCR analysis confirmed that eight genes were significantly up-regulated in the midgut of the KN strain following BmNPV infection. Our results imply that these up-regulated genes may be involved in the B. mori immune response against BmNPV infection.

Cloning and characterization of the secreted hemocytic prophenoloxidases of Heliothis virescens

The plasma enzyme phenoloxidase plays an important role in host resistance against viral, bacterial, fungal, filarial, and parasitoid challenge. Two Heliothis virescens prophenoloxidase transcripts, HvPPO-1 and HvPPO-2, were assembled from ESTs derived from a hemocyte cDNA library. The 2,363-bp HvPPO-1 contig encoded a 696-amino acid protein. The 3,255-bp HvPPO-2 contig encoded a 684-amino acid protein. Hemocyte and fat body transcript levels of HvPPO-1 were slightly elevated by bacterial infection in 5th instar larvae; however, HvPPO-2 expression was not significantly elevated above controls by bacterial infection. Per os infection of 4th instar larvae with the baculovirus Helicoverpa zea SNPV (HzSNPV) had a mild but significant suppressive effect upon fat body and hemocytic HvPPO-1 expression when compared to expression in same-aged controls. HvPPO-2 expression levels in fat bodies and hemocytes from 4th instar larvae was not significantly altered by HzSNPV infection. HzSNPV infection of 5th instar larvae caused no significant alteration of HvPPO-1 or of HvPPO-2 expression in either fat bodies or hemocytes. Thus, even though prophenoloxidase subunits are constitutively expressed at high levels in larval H. virescens hemocytes and fat bodies, the subunit HvPPO-1 is differentially regulated by bacterial and baculoviral infection.

Differentially expressed genes in resistant and susceptible Bombyx mori strains infected with a densonucleosis virus

We investigated variations in the gene expression of Bombyx mori following infection with a densonucleosis virus (BmDNV-Z). Two B. mori near-isogenic lines, Jingsong and Jingsong.nsd-Z.NIL, which are highly susceptible and completely resistant to BmDNV-Z, respectively, were used in this study. The infection profiles of BmDNV-Z in the midguts of the B. mori Jingsong and Jingsong.nsd-Z.NIL larvae revealed that the virus invaded the midguts of both of these strains. However, its proliferation was notably inhibited in the midgut of the resistant strain. By using the suppression subtractive hybridization method, three cDNA libraries were constructed to compare BmDNV-Z responsive gene expression between the two silkworm lines. In total, 151 differentially expressed genes were obtained. Real-time qPCR analysis confirmed that 11 genes were significantly up-regulated in the midgut of the Jingsong.nsd-Z.NIL strain following BmDNV-Z infection. Our results imply that these up-regulated genes might be involved in B. mori immune responses against BmDNV infection.

Parasitization by the wasp Eretmocerus mundus induces transcription of genes related to immune response and symbiotic bacteria proliferation in the whitefly Bemisia tabaci

Background

The whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), and the viruses it transmits, are a major constraint to growing vegetable crops worldwide. Although the whitefly is often controlled using chemical pesticides, biological control agents constitute an important component in integrated pest management programs, especially in protected agriculture. One of these agents is the wasp Eretmocerus mundus (Mercet) (Hymenoptera: Aphelinidae). E. mundus lays its egg on the leaf underneath the second-third instar nymph of B. tabaci. First instars of the wasp hatch and penetrate the whitefly nymphs. Initiation of parasitization induces the host to form a capsule composed of epidermal cells around the parasitoid. The physiological and molecular processes underlying B. tabaci-E. mundus interactions have never been investigated.

Results

We used a cDNA microarray containing 6,000 expressed sequence tags (ESTs) from the whitefly genome to study the parasitoid-whitefly interaction. We compared RNA samples collected at two time points of the parasitization process: when the parasitoid first instar starts the penetration process and once it has fully penetrated the host. The results clearly indicated that genes known to be part of the defense pathways described in other insects are also involved in the response of B. tabaci to parasitization by E. mundus. Some of these responses included repression of a serine protease inhibitor (serpin) and induction of a melanization cascade. A second set of genes that responded strongly to parasitization were bacterial, encoded by whitefly symbionts. Quantitative real-time PCR and FISH analyses showed that proliferation of Rickettsia, a facultative secondary symbiont, is strongly induced upon initiation of the parasitization process, a result that supported previous reports suggesting that endosymbionts might be involved in the insect host's resistance to various environmental stresses.

Conclusion

This is the first study to examine the transcriptional response of a hemipteran insect to attack by a biological control agent (hymenopterous parasitoid), using a new genomic approach developed for this insect pest. The defense response in B. tabaci involves genes related to the immune response as described in model organisms such as Drosophila melanogaster. Moreover, endosymbionts of B. tabaci appear to play a role in the response to parasitization, as supported by previously published results from aphids.

Differential gene expression of the honey bee Apis mellifera associated with Varroa destructor infection

Background

The parasitic mite, Varroa destructor, is the most serious pest of the western honey bee, Apis mellifera, and has caused the death of millions of colonies worldwide. This mite reproduces in brood cells and parasitizes immature and adult bees. We investigated whether Varroa infestation induces changes in Apis mellifera gene expression, and whether there are genotypic differences that affect gene expression relevant to the bee's tolerance, as first steps toward unravelling mechanisms of host response and differences in susceptibility to Varroa parasitism.

Results

We explored the transcriptional response to mite parasitism in two genetic stocks of A. mellifera which differ in susceptibility to Varroa, comparing parasitized and non-parasitized full-sister pupae from both stocks. Bee expression profiles were analyzed using microarrays derived from honey bee ESTs whose annotation has recently been enhanced by results from the honey bee genome sequence. We measured differences in gene expression in two colonies of Varroa-susceptible and two colonies of Varroa-tolerant bees. We identified a set of 148 genes with significantly different patterns of expression: 32 varied with the presence of Varroa, 116 varied with bee genotype, and 2 with both. Varroa parasitism caused changes in the expression of genes related to embryonic development, cell metabolism and immunity. Bees tolerant to Varroa were mainly characterized by differences in the expression of genes regulating neuronal development, neuronal sensitivity and olfaction. Differences in olfaction and sensitivity to stimuli are two parameters that could, at least in part, account for bee tolerance to Varroa; differences in olfaction may be related to increased grooming and hygienic behavior, important behaviors known to be involved in Varroa tolerance.

Conclusion

These results suggest that differences in behavior, rather than in the immune system, underlie Varroa tolerance in honey bees, and give an indication of the specific physiological changes found in parasitized bees. They provide a first step toward better understanding molecular pathways involved in this important host-parasite relationship.

Two homologous parasitism-specific proteins encoded in Cotesia plutellae bracovirus and their expression profiles in parasitized Plutella xylostella

A wasp, Cotesia plutellae, parasitizes the diamondback moth, Plutella xylostella, and interrupts host physiology for wasp survival and development. Identification of parasitism-specific factors would be helpful to understand the host-parasitoid interaction. This study focused on identification of a 15-kDa protein found only in plasma of the parasitized P. xylostella. Degenerate primers were designed after N-terminal amino acid sequencing of the parasitism-specific protein and used to clone the corresponding gene from the parasitized P. xylostella by a nested reverse transcriptase-polymerase chain reaction (RT-PCR). Two homologous genes were cloned and identified as "CpBV15alpha" and "CpBV15beta," respectively, due to the identical size (158 amino acid residues) of the predicted open reading frames, in which they shared amino acid sequences in both terminal regions, but varied in internal sequences. Southern hybridization analysis indicated that both genes were located on C. plutellae bracovirus genome. Real-time quantitative RT-PCR revealed that both genes were mostly expressed at the late parasitization period, which was further confirmed by an immunoblotting assay using CpBV15 antibody. A recombinant CpBV15 protein was produced from Sf9 cells via a baculovirus expression system. The purified CpBV15 protein could enter hemocytes of P. xylostella and were localized in the cytosol. Along with the sequence similarities of CpBV15s with eukaryotic initiation factors, their putative biological role has been discussed in terms of the host translation inhibitory factor.