Relationships between polydnavirus gene expression and host range of the parasitoid wasp Campoletis sonorensis

Multiparasitism enables a specialist endoparasitoid to complete parasitism in an unsuitable host caterpillar

Parasitoid wasps serve as natural enemies of numerous insect species; therefore, knowledge of host-parasitoid interactions is fundamental for understanding ecosystems. Each endoparasitoid wasp taxon exhibits a specific host range. Female parasitoids, however, occasionally oviposit into non-host species. Since the survival probability of eggs in non-host species is virtually zero, these behaviors have long been considered maladaptive. However, in the present study, we found that eggs of a specialist parasitoid, Cotesia kariyai (Hymenoptera: Braconidae), oviposited in unsuitable host caterpillars, Mythimna loreyi (Lepidoptera: Noctuidae), successfully complete larval development in the non-host when these caterpillars are simultaneously oviposited by another naturally sympatric parasitoid wasp, Meteorus pulchricornis (Hymenoptera: Braconidae), for which My. loreyi is the usual host. This observation suggests that the seemingly maladaptive behavior of ovipositing in unsuitable host insects can be adaptive, allowing them to maintain reproductive potential in environments where their ordinary hosts are absent. We propose a new term, "pirate parasitism", for this type of obligatory multiparasitism. Understanding detailed mechanisms of this phenomenon may provide deeper insights into parasitoid-host dynamics and evolution of host use strategies by parasitoids.

Host range of a parasitoid wasp is linked to host susceptibility to its mutualistic viral symbiont

Parasitoid wasps are one of the most species-rich groups of animals on Earth, due to their ability to successfully develop as parasites of nearly all types of insects. Unlike most known parasitoid wasps that specialize towards one or a few host species, Diachasmimorpha longicaudata is a generalist that can survive within multiple genera of tephritid fruit fly hosts, including many globally important pest species. Diachasmimorpha longicaudata has therefore been widely released to suppress pest populations as part of biological control efforts in tropical and subtropical agricultural ecosystems. In this study, we investigated the role of a mutualistic poxvirus in shaping the host range of D. longicaudata across three genera of agricultural pest species: two of which are permissive hosts for D. longicaudata parasitism and one that is a nonpermissive host. We found that permissive hosts Ceratitis capitata and Bactrocera dorsalis were highly susceptible to manual virus injection, displaying rapid virus replication and abundant fly mortality. However, the nonpermissive host Zeugodacus cucurbitae largely overcame virus infection, exhibiting substantially lower mortality and no virus replication. Investigation of transcriptional dynamics during virus infection demonstrated hindered viral gene expression and limited changes in fly gene expression within the nonpermissive host compared with the permissive species, indicating that the host range of the viral symbiont may influence the host range of D. longicaudata wasps. These findings also reveal that viral symbiont activity may be a major contributor to the success of D. longicaudata as a generalist parasitoid species and a globally successful biological control agent.

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.

Polydnavirus Innexins Disrupt Host Cellular Encapsulation and Larval Maturation

Polydnaviruses are dsDNA viruses associated with endoparasitoid wasps. Delivery of the virus during parasitization of a caterpillar and subsequent virus gene expression is required for production of an amenable environment for parasitoid offspring development. Consequently, understanding of Polydnavirus gene function provides insight into mechanisms of host susceptibility and parasitoid wasp host range. Polydnavirus genes predominantly are arranged in multimember gene families, one of which is the vinnexins, which are virus homologues of insect gap junction genes, the innexins. Previous studies of Campoletis sonorensis Ichnovirus Vinnexins using various heterologous systems have suggested the four encoded members may provide different functionality in the infected caterpillar host. Here, we expressed two of the members, vnxG and vnxQ2, using recombinant baculoviruses in susceptible host, the caterpillar Heliothis virescens. Following intrahemocoelic injections, we observed that >90% of hemocytes (blood cells) were infected, producing recombinant protein. Larvae infected with a vinnexin-recombinant baculovirus exhibited significantly reduced molting rates relative to larvae infected with a control recombinant baculovirus and mock-infected larvae. Similarly, larvae infected with vinnexin-recombinant baculoviruses were less likely to survive relative to controls and showed reduced ability to encapsulate chromatography beads in an immune assay. In most assays, the VnxG protein was associated with more severe pathology than VnxQ2. Our findings support a role for Vinnexins in CsIV and more broadly Ichnovirus pathology in infected lepidopteran hosts, particularly in disrupting multicellular developmental and immune physiology.

Intracellular dynamics of polydnavirus innexin homologues

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

Generalism in Nature…The Great Misnomer: Aphids and Wasp Parasitoids as Examples

In the present article we discuss why, in our view, the term ‘generalism’ to define the dietary breadth of a species is a misnomer and should be revised by entomologists/ecologists with the more exact title relating to the animal in question’s level of phagy—mono-, oligo, or polyphagy. We discard generalism as a concept because of the indisputable fact that all living organisms fill a unique ecological niche, and that entry and exit from such niches are the acknowledged routes and mechanisms driving ecological divergence and ultimately speciation. The term specialist is probably still useful and we support its continuing usage simply because all species and lower levels of evolutionary diverge are indeed specialists to a large degree. Using aphids and parasitoid wasps as examples, we provide evidence from the literature that even some apparently highly polyphagous agricultural aphid pest species and their wasp parasitoids are probably not as polyphagous as formerly assumed. We suggest that the shifting of plant hosts by herbivorous insects like aphids, whilst having positive benefits in reducing competition, and reducing antagonists by moving the target organism into ‘enemy free space’, produces trade-offs in survival, involving relaxed selection in the case of the manicured agro-ecosystem.

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.

Potential Host Range of the Larval Endoparasitoid Cotesia vestalis (=plutellae) (Hymenoptera: Braconidae)

Many parasitoid wasps are highly specialized in nature, attacking only one or a few species of hosts. Host range is often determined by a range of biological and ecological characteristics of the host including diet, growth potential, immunity, and phylogeny. The solitary koinobiont endoparasitoid wasp, Cotesia vestalis, mainly parasitizes diamondback moth (DBM) larvae in the field, although it has been reported that to possess a relatively wide lepidopteran host range. To better understand the biology of C vestalis as a potential biological control of hosts other than the DBM, it is necessary to determine suitability for potential hosts. In this study, the potential host range of the wasp and its developmental capacity in each host larva were examined under laboratory conditions using 27 lepidopteran species from 10 families. The wasp was able to parasitize 15 of the 27 species successfully. Some host species were not able to exclude C vestalis via their internal physiological defenses. When parasitization was unsuccessful, most hosts killed the parasitoid at the egg stage or early first-instar stage using encapsulation, but some host species disturbed the development of the parasitoid at various stages. No phylogenetic relationships were found among suitable and unsuitable hosts, revealing that host range in some endoparasitoids is not constrained by relatedness among hosts based on immunity.

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.

Encapsulation and Self-Superparasitism of Pseudapanteles dignus (Muesebeck) (Hymenoptera: Braconidae), a Parasitoid of Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

Endoparasitoids can be killed by host encapsulation, a cellular-mediated host immunological response against parasitism that involves hemocytes aggregation. As a counteracting strategy, many parasitoids can evade this host response through self-superparasitism. The objectives of this study were: 1) to describe the parasitoid Pseudapanteles dignus (Hymenoptera: Braconidae) early immature stages (egg and larva) encapsulation by the host Tuta absoluta (Lepidoptera: Gelechiidae), and 2) to determine the occurrence of self-superparasitism and the rate of escaping to encapsulation of this parasitoid. Knowledge of host-parasitoid immunological interaction is crucial when evaluating the potential of an endoparasitoid as a biological control candidate. Parasitoid-exposed T. absoluta larvae were dissected in vivo under light stereoscope microscope at 24-h intervals, for five days after exposition to detect encapsulation. The preimaginal stages of P. dignus and numbers of healthy and encapsulated immature parasitoids per host were recorded. Samples of parasitoid eggs and larvae were processed for SEM visualization of encapsulation. Necropsies evidenced that only the early first larval instar of P. dignus (up to 96 h-old) was partially or completely encapsulated. A non-melanized capsule, formed by layers of granulocyte-type hemocytes enveloping around the parasitoid body, was recorded. Approximately 50% of the parasitized T. absoluta larvae had significantly only one P. dignus egg, meanwhile supernumerary parasitization yielded up to seven immature parasitoids per host. The proportion of single-early first larval instar of P. dignus reached ≈ 0.5 and decreased significantly as the number of parasitoid individuals per host increased. P. dignus encapsulation and its ability to overcome with the host immune defense through self-superparasitism indicate that T. absoluta is a semi-permissive host for this parasitoid.

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

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

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 interactions between polydnavirus and host cellular innexins

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

The natural genetic engineering of polydnaviruses

The polydnaviruses represent an unusual example of a highly evolved symbiosis between some parasitic wasps, DNA containing particles or viruses, and lepidopteran larval hosts of the wasp. The viruses can no longer replicate independently, as genes that encode viral structural proteins are restricted to the wasp genome and are not encapsidated. Interestingly, the DNA that is encapsidated is more similar in terms of gene identity and gene density to eukaryotic genomes than viral genomes. We compare and relate this unusual example of natural genetic engineering to the well-known system of viral lysogeny. The similarities in the two systems may prove useful in understanding the replication strategy and genomic organization of polydnaviruses and provide some insight into how this unusual virus system may have evolved.

Odour-mediated long-range avoidance of interspecific competition by a solitary endoparasitoid: a time-saving foraging strategy

1. In studies on optimal foraging strategies, long-range decisions in the pursuit of resource are rarely considered. This is also the case for sympatric parasitoids, which may be confronted with the decision to accept or reject host larvae that are already parasitized by a competing species. They can be expected to reject already parasitized hosts if it is likely that they will lose the resulting intrinsic competition. However, examples of such interspecific host discrimination are rare. 2. We propose that parasitoids that are not egg-limited should reject inferior hosts only if it saves them time, and that this will be achieved mainly when the parasitoids are able to detect competitors from a distance. We tested this hypothesis using the sympatric parasitoids Cotesia marginiventris (Cresson) and Campoletis sonorensis (Cameron). 3. C. sonorensis was found to be the superior intrinsic competitor but, upon contact with a host larva, both wasps readily accepted hosts that had already been parasitized by the other species. However, in an olfactometer experiment, C. marginiventris females were found to strongly avoid the odour of their superior competitor. 4. These results are in accordance with a time optimization scenario, whereby the inferior competitor accepts competition if it costs only an egg, but avoids competition if it may save time that can be allocated to the search for more profitable hosts. 5. Models on host discrimination strategies in parasitoids had not yet considered discrimination from a distance. Long-range foraging decisions can also be expected for other organisms that have to choose between resources of varying suitability and profitability.

Differential expression of the CrV1 haemocyte inactivation-associated polydnavirus gene in the African maize stem borer Busseola fusca (Fuller) parasitized by two biotypes of the endoparasitoid Cotesia sesamiae (Cameron)

Polydnaviruses are rarely studied for their natural variation in immune suppressive abilities. The polydnavirus harboring braconid Cotesia sesamiae, a widespread endoparasitoid of Busseola fusca and Sesamia calamistis in sub-Saharan Africa exists as two biotypes. In Kenya, the western biotype completes development in B. fusca larvae. However, eggs of the coastal C. sesamiae are encapsulated in this host and ultimately, no parasitoids emerge from parasitized B. fusca larvae. Both biotypes develop successfully in S. calamistis larvae. Encapsulation activity by B. fusca larvae towards eggs of the avirulent C. sesamiae was detectable six hours post-parasitization. The differences in encapsulation of virulent and avirulent strains were associated with differences in nucleotide sequences and expression of a CrV1 polydnavirus (PDV) gene, which is associated with haemocyte inactivation in the Cotesia rubecula/Pieris rapae system. CrV1 expression was faint or absent in fat body and haemolymph samples from B. fusca parasitized by the avirulent C. sesamiae, which exhibited encapsulation of eggs. Expression was high in fat body and haemolymph samples from both B. fusca and S. calamistis larvae parasitized by the virulent C. sesamiae, encapsulation in the former peaking at the same time points as CrV1 expression in the latter. Non synonymous difference in CrV1 gene sequences between virulent and avirulent wasp suggests that variations in B. fusca parasitism by C. sesamiae may be due to qualitative differences in CrV1-haemocyte interactions.

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

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

Structure and evolution of a proviral locus of Glyptapanteles indiensis bracovirus

Background

Bracoviruses (BVs), a group of double-stranded DNA viruses with segmented genomes, are mutualistic endosymbionts of parasitoid wasps. Virus particles are replication deficient and are produced only by female wasps from proviral sequences integrated into the wasp genome. Virus particles are injected along with eggs into caterpillar hosts, where viral gene expression facilitates parasitoid survival and therefore perpetuation of proviral DNA. Here we describe a 223 kbp region of Glyptapanteles indiensis genomic DNA which contains a part of the G. indiensis bracovirus (GiBV) proviral genome.

Results

Eighteen of ~24 GiBV viral segment sequences are encoded by 7 non-overlapping sets of BAC clones, revealing that some proviral segment sequences are separated by long stretches of intervening DNA. Two overlapping BACs, which contain a locus of 8 tandemly arrayed proviral segments flanked on either side by ~35 kbp of non-packaged DNA, were sequenced and annotated. Structural and compositional analyses of this cluster revealed it exhibits a G+C and nucleotide composition distinct from the flanking DNA. By analyzing sequence polymorphisms in the 8 GiBV viral segment sequences, we found evidence for widespread selection acting on both protein-coding and non-coding DNA. Comparative analysis of viral and proviral segment sequences revealed a sequence motif involved in the excision of proviral genome segments which is highly conserved in two other bracoviruses.

Conclusion

Contrary to current concepts of bracovirus proviral genome organization our results demonstrate that some but not all GiBV proviral segment sequences exist in a tandem array. Unexpectedly, non-coding DNA in the 8 proviral genome segments which typically occupies ~70% of BV viral genomes is under selection pressure suggesting it serves some function(s). We hypothesize that selection acting on GiBV proviral sequences maintains the genetic island-like nature of the cluster of proviral genome segments described herein. In contrast to large differences in the predicted gene composition of BV genomes, sequences that appear to mediate processes of viral segment formation, such as proviral segment excision and circularization, appear to be highly conserved, supporting the hypothesis of a single origin for BVs.

Polydnavirus genomes reflect their dual roles as mutualists and pathogens

Symbionts often exhibit significant reductions in genome complexity while pathogens often exhibit increased complexity through acquisition and diversification of virulence determinants. A few organisms have evolved complex life cycles in which they interact as symbionts with one host and pathogens with another. How the predicted and opposing influences of symbiosis and pathogenesis affect genome evolution in such instances, however, is unclear. The Polydnaviridae is a family of double-stranded (ds) DNA viruses associated with parasitoid wasps that parasitize other insects. Polydnaviruses (PDVs) only replicate in wasps but infect and cause severe disease in parasitized hosts. This disease is essential for survival of the parasitoid's offspring. Thus, a true mutualism exists between PDVs and wasps as viral transmission depends on parasitoid survival and parasitoid survival depends on viral infection of the wasp's host. To investigate how life cycle and ancestry affect PDVs, we compared the genomes of Campoletis sonorensis ichnovirus (CsIV) and Microplitis demolitor bracovirus (MdBV). CsIV and MdBV have no direct common ancestor, yet their encapsidated genomes share several features including segmentation, diversification of virulence genes into families, and the absence of genes required for replication. In contrast, CsIV and MdBV share few genes expressed in parasitized hosts. We conclude that the similar organizational features of PDV genomes reflect their shared life cycle but that PDVs associated with ichneumonid and braconid wasps have likely evolved different strategies to cause disease in the wasp's host and promote parasitoid survival.

Virus-like particles in the ovaries of Microctonus aethiopoides Loan (Hymenoptera: Braconidae): comparison of biotypes from Morocco and Europe

Virus-like particles (MaVLP) have been discovered in the ovarial epithelial cells of the solitary, koinobiont, endoparasitoid, Microctonus aethiopoides Loan (Hymenoptera: Braconidae) introduced to New Zealand originally from Morocco to control the lucerne pest Sitona discoideus Gyllenhal (Coleoptera: Curculionidae). MaVLP have been found in all females examined. It has been suggested, although not demonstrated, that like many other such VLP found in parasitoids, MaVLP might play a role in host immunosuppression. Since another biotype of M. aethiopoides from Ireland has been proposed for introduction to control the white clover pest, Sitona lepidus Gyllenhal, in New Zealand, it was considered that females from this biotype warranted transmission electron microscope examination for VLP. No VLP were observed in ovarian tissues of specimens collected from three different locations in Ireland. Similarly, none were found in M. aethiopoides sourced from France, Wales, and Norway. These observations are discussed in relation to quarantine host specificity tests with the Irish biotype, which found that the host range of the Irish biotype is likely to be less extensive than that of the Moroccan biotype already in New Zealand.

Host refractoriness of the tobacco hornworm, Manduca sexta, to the braconid endoparasitoid Cotesia flavipes

Cotesia flavipes (Hymenoptera:Braconidae) is a gregarious endoparasitoid of several pyralid stemborer larvae of economic significance including the sugarcane borer, Diatraea saccharalis. In this study, the ability of this parasitoid to develop in a sphingid host, Manduca sexta, was tested. First, second, third, fourth, and even pharate fifth instar host tobacco hornworm larvae were readily parasitized by the female C. flavipes parasitoids but no wasp larvae hatched from the eggs in this refractory host. Instead, the parasitoid eggs were invariably encapsulated by the host's hemocytes and, ultimately, no parasitoids emerged from tobacco hornworm hosts. The first stages of encapsulation were evident at 2 h post-parasitization of the host M. sexta larvae, when the beginning stages of capsule formation were seen. The developmental fate of the host larvae with encapsulated parasitoids was variable. Most succumbed as abnormally small fifth instars or as post-wandering prepupal animals, while a few developed normally to the pupal stage. Dissection of all the larvae or pupae with encapsulated wasp eggs showed evidence of hemocytic encapsulation and melanization of the C. flavipes eggs. This report describes the association between C. flavipes and M. sexta, which appears to be an excellent model system for studying the physiological processes accompanying wasp egg encapsulation that result in death of the host as well as the parasitoid. Since the parasitoid egg never hatches, the system offers an excellent opportunity to identify and study the effects of parasitoid-injected polydnavirus and venom on host physiology.

Identification of host translation inhibitory factor of Campoletis sonorensis ichnovirus on the tobacco budworm, Heliothis virescens

Parasitization of a wasp, Campoletis sonorensis, against the larvae of Heliothis virescens depresses synthesis of specific host proteins related to growth and immunity. It has been suggested that the inhibition of host gene expression is targeted at a posttranscriptional level. This study aimed to verify the identity of host translation inhibitory factor (HTIF) derived from wasp parasitization. To identify HTIF, the proteins in the parasitized host were fractionated using different protein purification methods, and each fraction's HTIF activity was assessed. In the course of the protein purification steps, HTIF activity was highly correlated with the fractions containing VHv 1.4 protein, which has a conserved cysteine-motif and is encoded in C. sonorensis ichnovirus (CsIV). Purified VHv 1.4 protein using an immunoaffinity column exhibited a significant HTIF effect, while the heat-inactivated VHv 1.4 did not. Both recombinant VHv 1.4 and VHv 1.1 (another cys-motif protein encoded in CsIV) proteins were synthesized in Sf 9 cells through a baculovirus expression system. The purified recombinant VHv 1.4 and VHv 1.1 exhibited significant HTIF activities in a nanomolar range. However, VHv1.4 protein showed about four times higher HTIF activity than did VHv 1.1 protein. Both HTIFs acted directly on translation machinery because they inhibited a cell-free in vitro translation system using rabbit reticulocyte lysate. Both HTIFs are likely to discriminate specific target mRNAs because they inhibited translation of RNA extracts from the Tn 368 cell line, but not from Sf 9 cells. In addition, they inhibited translation of RNAs from fat body, hemocytes, and testis, but not from epidermis, gut, labial gland, and nerve tissues of H. virescens. These results indicate that both cys-motif proteins of VHv 1.4 and VHv 1.1 play a role as HTIF in C. sonorensis parasitization.

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

Polydnaviruses (PDVs) are unusual insect viruses that occur in obligate symbiotic associations with parasitic ichneumonid (ichnoviruses, or IVs) and braconid (bracoviruses, or BVs) wasps. PDVs are injected with eggs, ovarian proteins, and venom during parasitization. Following infection of cells in host tissues, viral genes are expressed and their products function to alter lepidopteran host physiology, enabling endoparasitoid development. Here we describe the Campoletis sonorensis IV viral ankyrin (vankyrin) gene family and its transcription. The seven members of this gene family possess ankyrin repeat domains that resemble the inhibitory domains of the Drosophila melanogaster NF-kappabeta transcription factor inhibitor (Ikappabeta) cactus. vankyrin gene expression is detected within 2 to 4 h postparasitization (p.p.) in Heliothis virescens hosts and reaches peak levels by 3 days p.p. Our data indicate that vankyrin genes from the C. sonorensis IV genome are differentially expressed in the tissues of parasitized hosts and can be divided into two subclasses: those that target the host fat body and those that target host hemocytes. Polyclonal antibodies raised against a fat-body targeting vankyrin detected a 19-kDa protein in crude extracts prepared from the 3 days p.p. fat body. Vankyrin-specific Abs localized to 3-day p.p. fat-body and hemocyte nuclei, suggesting a role for vankyrin proteins in the nuclei of C. sonorensis IV-infected cells. These data are evidence for divergent tissue specificities and targeting of multigene families in IVs. We hypothesize that PDV vankyrin genes may suppress NF-kappabeta activity during immune responses and developmental cascades in parasitized lepidopteran hosts of C. sonorensis.

BACULOVIRUS-BASED EXPRESSION OF AN INSECT VIRAL PROTEIN IN 12 DIFFERENT INSECT CELL LINES

The ability of 12 unique lepidopteran insect cell lines from Anticarsia gemmatalis, Heliothis virescens, Lymantria dispar (two lines), Mamestra brassica, Plutella xylostella, Spodoptera frugiperda (two lines), and Trichoplusia ni (three lines) to support production of a recombinant polydnavirus (PDV) protein (GiPDV 1.1) expressed using the Bac-to-Bac baculovirus expression system was examined. Polydnavirus gene GiPDV 1.1 was cloned into the pFastBac baculovirus vector under the control of the polyhedron promoter, followed by generation of recombinant bacmid-GiPDV 1.1 by site-specific transposition. The ability of each insect cell line to support recombinant PDV gene expression was estimated using reverse transcriptase-polymerase chain reaction and Western blot. Each insect cell line infected with recombinant bacmid-GiPDV 1.1 and tested in this study was capable of supporting and producing recombinant protein. Time course expression analysis showed that 72-96 h after transfection to be the optimal time for harvest of recombinant protein for each insect cell line.

Polydnavirus genes and genomes: emerging gene families and new insights into polydnavirus replication

Polydnavirus genome sequencing is providing new insights into viral genome organization and viral gene function. Sequence analyses demonstrate that the genomes of these viral mutualists are largely noncoding but maintain genes and gene families that are unrelated to other viral genes. Interestingly, these organizational patterns in polydnavirus genomes are evident in both the bracovirus and ichnovirus genera, even though these two genera are evolutionarily unrelated. The identity and function of some polydnavirus gene families are considered with some functions experimentally supported and others implied by homology relationships with known insect genes. The evidence relative to polydnavirus origins and evolution is considered but remains an area of speculation. However, sequencing of these viral genomes has been informative and provides opportunities for productive investigation of these unusual mutualistic insect viruses.

Similar polydnavirus genes of two parasitoids, Cotesia kariyai and Cotesia ruficrus, of the host Pseudaletia separata

Endoparasitoids can achieve successful development in suitable hosts with the help of polydnavirus (PDV), if it is injected into the host with their eggs at parasitization. It was predicted that different species of endoparasitoids that parasitize the same host successfully use a similar system to avoid the defense reaction from the host. In this study, we identified similar PDVs genes. In two endoparasitoids, Cotesia kariyai (Cky) and Cotesia ruficrus (Crf), which can parasitize the same host species, Pseudaletia separata. The electrophoretic profiles of nondigestive segments of PDV in the two parasitoids were different. However, genomic Southern hybridization using reciprocal probes between the two species indicated a similar sequence of PDV genes. Two cDNA libraries were established and screened by a Cky probe. Cky811 was 695-bp, Crf111 was 690-bp, and the predicted peptide sequences were 162aa and 157aa, respectively. The two transcripts had 72.8% similarity with each other and resembled a C-type lectin2 domain. Clones of Cky811 from C. kariyai and Crf111 from C. ruficrus were expressed in hemocytes of the hosts that were injected with PDV plus venom. The Cky811 gene was expressed in hemocytes and fat body of the Pseudaletia host. Hybridization analysis revealed that the Cky811 gene was expressed 6 hrs after injection of Cky-PDV into nonparasitized hosts with venom, and the hybridization signal declined with time. The genetic distance to the lectin of other insects was closer to P. americana than to Lepidopteran insects.

Polydnavirus of Campoletis chlorideae: characterization and temporal effect on host Helicoverpa armigera cellular immune response

Polydnavirus was isolated from oviduct calyx in the parasitoid wasp Campoletis chlorideae (Hymenoptera: Ichneumonidae), and termed CcIV. The virus particles consist of fusiform nucleocapsids surrounded by two unit membrane envelopes. The DNAs purified from these viruses were found diversified in molecular weight and existed in nonequimolar concentrations. At least 20 different-sized bands were present after electrophoresis, and they ranged from approximately 3 to 26 kb. Persistence and gene expression of CcIV were examined in parasitized and virus-injected Helicoverpa armigera larvae. Viral DNA could be detected in the hemocyte of H. armigera at 30 min post-parasitization (p.p.), and persisted for 6 days. While no viral DNA increase was found, CcIV transcripts were first detected in host hemocytes at day 1 p.p. and continued for 5 days. Similar transcripts were observed in hemocytes from larvae that had been injected with calyx fluid or CcIV 24 h earlier. CcIV viral DNAs hybridized only with certain first-strand cDNAs from hemocytes, suggesting that only part of the CcIV genome was expressed in H. armigera. The pattern of CcIV gene expression was consistent with that of the inhibition of encapsulation for Sephadex G-10 and parasitoid eggs by host larvae. The recovery of host immune response at day 4 p.p. indicated that CcIV exhibited a partial and temporal effect on the host immune system and the developing parasitoid appeared to avoid encapsulation via different mechanisms.

Differentially expressed midgut transcripts in Culicoides sonorensis (Diptera: ceratopogonidae) following Orbivirus (reoviridae) oral feeding

Understanding the vector insect's gene expression response to a virus infection may aid design of control measures for arbovirus diseases. Culicoides sonorensis is a vector of several agriculturally important pathogens, such as epizootic haemorrhagic disease virus (EHDV) that causes disease in ruminants. Two approaches, differential display and suppression subtractive hybridization, were used to identify 400+ Culicoides transcripts that were more abundant in midguts 1 day following an oral meal containing EHDV. Of these, quantitative PCR confirmed seven to be more abundant in virus-fed midguts than controls. One such transcript encodes a putative RNA editase, CsRED1, induced by dsRNA. Transcripts encoding putative receptors involved in cell differentiation included CsLAR, a protein tyrosine phosphatase, and CsFZ2, homologous to the wingless receptor in D. melanogaster. Transcripts encoding putative translation machinery components included CseIF3, CseIF5A and CsRPS6. Overall, the cDNA fragments identified in this study increased in the midgut at one day postfeeding; by 2 days postfeeding, increases in transcript levels shifted from the midgut to the remainder of the infected midge.

Evidence for a conserved polydnavirus gene family: ichnovirus homologs of the CsIV repeat element genes

In Campoletis sonorensis Ichnovirus (CsIV), the repeat element genes constitute a gene family of 28 members. In the present work, we document the presence of members of this gene family in two additional ichnoviruses, Hyposoter didymator Ichnovirus (HdIV) and Tranosema rostrale Ichnovirus (TrIV). Two repeat element genes, representing at least one functional gene, were identified in TrIV, whereas HdIV was found to contain at least three such genes. In both HdIV and TrIV, the known repeat element genes are encoded on single genome segments, with hybridization studies suggesting the presence of other, related but as yet uncharacterized genes. The HdIV and TrIV repeat element genes are all transcribed in infected caterpillars, although differences exist among genes in levels and in tissue specificity of expression. A heuristic tree was generated indicating that the repeat element genes are more similar within a species of wasp than between species, with TrIV genes being more closely related to the CsIV than to the HdIV genes. These results suggest that the most significant duplication, divergence, and expansion of the repeat element genes occurred after speciation. The finding that repeat element genes form an interspecific family within the genus Ichnovirus supports the view that the proteins they encode play an important role in ichnovirus biology.

Effects of the polydnavirus of Cotesia congregata on the immune system and development of non-habitual hosts of the parasitoid

Polydnaviruses (PDV) are obligate mutualistic symbionts found in association with some groups of parasitic Hymenoptera. In these groups, they suppress the immune response of the parasitoid's host and are required for successful parasitoid reproduction. Several PDV effects have been described in different experimental systems, but no clear picture of PDV mode of immunosuppression has emerged. No study to date has directly tested if PDV modes of action are evolutionarily conserved or divergent among parasitoid taxa within the Ichneumonoidea. We hypothesize the divergence in PDV mode of immunosuppression can be detected by identifying points of divergence in the immune response of different host species to PDV from one parasitoid species. This study tests the effects of purified PDV from Cotesia congregata on the immune response of three larval lepidopteran species that naturally are hosts of parasitoid species that differ in taxonomic relatedness to C. congregata. Here we demonstrate that despite associations with distantly related parasitoids (Ichneumonidae and Braconidae), Manduca sexta and Heliothis virescens showed similar patterns of increased glucose dehydrogenase (GLD) activity, suppressed cellular encapsulation in vitro, and increased time to pupation. In contrast, Lymantria dispar showed no response to C. congregata PDV across any of the parameters measured, even though it has an evolutionary association with several parasitoids closely related to C. congregata and within the Microgastrinae. The PDV immunosuppression in H. virescens and M. sexta does not correlate with host molecular phylogeny either. The suborganismal effects shown in M. sexta and H. virescens translated into significantly reduced pupation success in M. sexta only. Results demonstrate that while some PDV modes of immunosuppression in hosts may be divergent, others may be conserved across broad host groups.