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

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.

The complete genome sequence of a second alphabaculovirus from the true armyworm, Mythimna unipuncta: implications for baculovirus phylogeny and host specificity

The Mythimna unipuncta nucleopolyhedrovirus isolate KY310 (MyunNPV-KY310) is an alphabaculovirus isolated from a true armyworm (Mythimna unipuncta) population in Kentucky, USA. Occlusion bodies of this virus were examined by electron microscopy and the genome sequence was determined by 454 pyrosequencing. MyunNPV-KY310 occlusion bodies consisted of irregular polyhedra measuring 0.8-1.8 µm in diameter and containing multiple virions, with one to six nucleocapsids per virion. The genome sequence was determined to be 156,647 bp with a nucleotide distribution of 43.9% G+C. 152 ORFs and six homologous repeat (hr) regions were annotated for the sequence, including the 38 core genes of family Baculoviridae and an additional group of 26 conserved alphabaculovirus genes. BLAST queries and phylogenetic inference confirmed that MyunNPV-KY310 is most closely related to the alphabaculovirus Leucania separata nucleopolyhedrovirus isolate AH1, which infects Mythimna separata. In contrast, MyunNPV-KY310 did not exhibit a close relationship with Mythimna unipuncta nucleopolyhedrovirus isolate #7, an alphabaculovirus from the same host species. MyunNPV-KY310 lacks the gp64 envelope glycoprotein, which is a characteristic of group II alphabaculoviruses. However, this virus and five other alphabaculoviruses lacking gp64 are placed outside the group I and group II clades in core gene phylogenies, further demonstrating that viruses of genus Alphabaculovirus do not occur in two monophyletic clades. Potential instances of MyunNPV-KY310 ORFs arising by horizontal transfer were detected. Although there are now genome sequences of four different baculoviruses from M. unipuncta, comparison of their genome sequences provides little insight into the genetic basis for their host specificity.

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

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

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.

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.

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

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

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.

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.

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

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

Exposure of ichnovirus particles to digitonin leads to enhanced infectivity and induces fusion from without in an in vitro model system

Unlike most viruses, the mature ichnovirus particle possesses two unit membrane envelopes. Following loss of the outer membrane in vivo, nucleocapsids are believed to gain entry into the cytosol via a membrane fusion event involving the inner membrane and the plasma membrane of susceptible host cells; accordingly, experimentally induced damage to the outer membrane might be expected to increase infectivity. Here, in an attempt to develop an in vitro model system for studying ichnovirus infection, we show that digitonin-induced disruption of the virion outer membrane not only increases infectivity, but also uncovers an activity not previously associated with any polydnavirus: fusion from without.

Transcriptional analysis of polydnaviral genes in the course of parasitization reveals segment-specific patterns

Polydnaviruses are symbiotic viruses of endoparasitic wasps, which are formed in their ovary and injected along with the eggs into the host. They manipulate the host in a way to allow successful parasitoid development. A hallmark of polydnaviruses is their segmented genome consisting of several circles of double-stranded DNA. We are studying the solitary egg-larval parasitoid Chelonus inanitus (Braconidae) parasitizing Spodoptera littoralis (Noctuidae). The polydnavirus of Chelonus inanitus (CiV) protects the parasitoid larva from encapsulation by the host's immune system, slightly modifies host nutritional physiology, and induces a developmental arrest of the host in the prepupal stage. Here we present data on newly identified CiV genes and their expression patterns in the course of parasitization. None of these genes has similarity to other genes and so far no gene families could be found. A rough estimation of transcript quantities revealed that even the most highly expressed CiV genes reach maximal values, which are 250 times lower than actin. This indicates that the CiV-induced alterations of the host are brought about by a concerted action of low levels of transcripts. In an overview, we show the expression patterns of all CiV genes analysed up to now; they indicate that several genes with similar expression patterns (early, persistent, intermediate, or late) are grouped together on the same segment. This is the first observation of this type. It suggests that one function of the segmentation of the polydnavirus genome may be the grouping together of genes, which are regulated in a similar manner.

Shared and species-specific features among ichnovirus genomes

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

In vitro integration of an ichnovirus genome segment into the genomic DNA of lepidopteran cells

Polydnaviruses (PDVs) are dsDNA viruses transmitted by ichneumonid and braconid endoparasitoids to their lepidopteran hosts during oviposition. Wasp carriers are asymptomatic and transmit the virus to their progeny through the germ line; replication is confined to the calyx region of the wasp ovary, where the virus accumulates in the fluid bathing the eggs. In the lepidopteran host, however, no virus replication takes place, but PDV gene expression is essential for successful parasitism. Sustained gene expression in the absence of virus replication thus requires that the circular PDV genome segments persist for days within host cells. Available evidence suggests that most genome segments persist as episomes, but recent studies have indicated that some genome segments may undergo integration within lepidopteran genomic DNA, at least in vitro. In the present study, an integrated form of a Tranosema rostrale ichnovirus (TrIV) genome segment was cloned from genomic DNA extracted from infected Choristoneura fumiferana CF-124T cells and junction regions on either side of the viral DNA sequence were sequenced. This is the first proven example of integration of an ichnovirus genome segment in infected lepidopteran cells. Interestingly, circular forms of this genome segment do not appear to persist in these cells; none the less, a gene (TrFrep1) carried by this genome segment displays long-term transcription in infected cultured cells.

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

Background

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

Results

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

Conclusion

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

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

Background

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

Results

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

Conclusion

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

Characterization and transcriptional analysis of protein tyrosine phosphatase genes and an ankyrin repeat gene of the parasitoid Glyptapanteles indiensis polydnavirus in the parasitized host

Glyptapanteles indiensis (Braconidae, Hymenoptera) is an endoparasitoid of Lymantria dispar, the gypsy moth. Expression of G. indiensis polydnavirus (GiBV)-encoded genes within the pest host results in inhibition of immune response and development and alteration of physiology, enabling successful development of the parasitoid. Here, GiBV genome segment F (segF), an 18·6 kb segment shown to encode nine protein tyrosine phosphatase (PTP) genes and a single ankyrin repeat gene (ank), is analysed. PTPs have presumed function as regulators of signal transduction, while ankyrin repeat genes are hypothesized to function in inhibition of NF-κB signalling in the parasitized host. In this study, transcription of each gene was mapped by 5′- and 3′-RACE (rapid amplification of cDNA ends) and temporal and tissue-specific expression was examined in the parasitized host. For polydnavirus gene prediction in the parasitized host, no available gene prediction parameters were entirely precise. The mRNAs for each GiBV segF gene initiated between 30 and 112 bp upstream of the translation initiation codon. All were encoded in single open reading frames (ORFs), with the exception of PTP9, which was transcribed as a bicistronic message with the adjacent ank gene. RT-PCR indicated that all GiBV segF PTPs were expressed early in parasitization and, for most, expression was sustained over the course of at least 7 days after parasitization, suggesting importance in both early and sustained virus-induced immunosuppression and alteration of physiology. Tissue-specific patterns of PTP expression of GiBV segF genes were variable, suggesting differing roles in facilitating parasitism.

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

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

Evolution of polydnaviruses as insect immune suppressors

Polydnaviruses (PDVs) are endogenous particles that are used by some endoparasitic hymenoptera to disrupt host immunity and development. Recent analyses of encapsidated PDV genes have increased the number of known PDV gene families, which are often closely related to insect genes. Several PDV proteins inactivate host haemocytes by damaging their actin cytoskeleton. These proteins share no significant sequence homology and occur in polyphyletic PDV genera, possibly indicating that convergent evolution has produced functionally similar immune-suppressive molecules causing a haemocyte phenotype characterised by damaged cytoskeleton and inactivation. These phenomena provide further insights into the immune-suppressive activity of PDVs and raise interesting questions about PDV evolution, a topic that has puzzled researchers ever since the discovery of PDVs.

Bracoviruses contain a large multigene family coding for protein tyrosine phosphatases

The relationship between parasitic wasps and bracoviruses constitutes one of the few known mutualisms between viruses and eukaryotes. The virions produced in the wasp ovaries are injected into host lepidopteran larvae, where virus genes are expressed, allowing successful development of the parasite by inducing host immune suppression and developmental arrest. Bracovirus-bearing wasps have a common phylogenetic origin, and contemporary bracoviruses are hypothesized to have been inherited by chromosomal transmission from a virus that originally integrated into the genome of the common ancestor wasp living 73.7 +/- 10 million years ago. However, so far no conserved genes have been described among different braconid wasp subfamilies. Here we show that a gene family is present in bracoviruses of different braconid wasp subfamilies (Cotesia congregata, Microgastrinae, and Toxoneuron nigriceps, Cardiochilinae) which likely corresponds to an ancient component of the bracovirus genome that might have been present in the ancestral virus. The genes encode proteins belonging to the protein tyrosine phosphatase family, known to play a key role in the control of signal transduction pathways. Bracovirus protein tyrosine phosphatase genes were shown to be expressed in different tissues of parasitized hosts, and two protein tyrosine phosphatases were produced with recombinant baculoviruses and tested for their biochemical activity. One protein tyrosine phosphatase is a functional phosphatase. These results strengthen the hypothesis that protein tyrosine phosphatases are involved in virally induced alterations of host physiology during parasitism.

Characterization and transcriptional profiles of three Spodoptera frugiperda genes encoding cysteine-rich peptides. A new class of defensin-like genes from lepidopteran insects?

The present work describes sequence and transcription of three Spodoptera frugiperda genes encoding 6-cysteine-rich peptides. Sequence alignments indicate that the predicted peptides belong to the insect defensin family, although phylogenetic analyses suggest they form a cluster distinct from that of other neopteran insect defensins. The three genes were identified in a non-immune-challenged Sf9 cells cDNA (DNA complementary to RNA) library (Landais et al., Bioinformatics, in press) and were named spodoptericin, Sf-gallerimycin and Sf-cobatoxin. Spodoptericin is a novel defensin-like gene that appears to be weakly up-regulated following injection of bacteria and fungi. Interestingly, no sequence motif clearly homologous to cis regulatory element involved in the regulation of antimicrobial genes was found. An homologue of the spodoptericin gene was identified in the SilkBase Bombyx mori cDNA library. Sf-gallerimycin is related to the Galleria mellonella gallerimycin gene and is induced after immune challenge by injection of bacteria in the larval fat body as well as in hemocytes. In silico analysis of the sequence upstream from the cDNA reveals the presence of at least one motif homologous to a nuclear factor kappaB (NF-kappaB) binding site. Finally, Sf-cobatoxin is related to the G. mellonella cobatoxin-like gene. Despite high levels of constitutive expression compared to the two previous genes, transcription of Sf-cobatoxin is increased after immune, in particular, bacterial challenge. We therefore confirm that these three genes encode potential candidate molecules involved in S. frugiperda innate humoral response.

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.

Three related TrIV genes: comparative sequence analysis and expression in host larvae and Cf-124T cells

We report on the cloning and sequencing of two Tranosema rostrale ichnovirus (TrIV) genes, and assess their relatedness to TrV1, the gene encoding the most abundant TrIV transcript in last-instar Choristoneura fumiferana larvae parasitized by T. rostrale. One of the two newly isolated genes, TrV2, features an organization similar to that of TrV1, with one intron flanked by two exons; it encodes a 102 amino acid protein showing 79% similarity to TrV1. The third gene, TrV4, encodes a larger protein (143 aa) displaying similarity to the other two only over the first approximately 50 amino acid residues of its sequence; the remaining portion contains an imperfect octad repeat. Although the TrV4 gene contains only one exon, it has an intron similar in size and sequence to that of TrV1 and TrV2; in fact, the non-coding regions of all three genes show higher sequence identity than the coding regions, pointing to their common origin. Southern analysis suggests that each gene maps to a different TrIV genome segment, with homologous sequences apparently present on other segments. TrV1 and TrV4 transcription in penultimate (5th) instar hosts, parasitized shortly after the molt, was strong for both genes 1 and 2 days p.p., with transcript abundance decreasing after the final molt; thus, neither of these genes is upregulated during induction of developmental arrest in last-instar hosts. Cf-124T cells inoculated with T. rostrale calyx fluid showed significant levels of apoptosis 24-72 h p.i.; TrV1 was detected in the culture medium, suggesting that this and/or other TrIV-encoded proteins may be responsible for the observed cytopathic effect. Southern and Northern analyses, using DNA and RNA extracted from infected Cf-124T cells, revealed the presence of both TrV1- and TrV4-carrying genome segments and transcripts, but neither DNA, at least in episomal form, nor mRNA persisted for more than a few days p.i. This in vitro system may provide a suitable starting point for the study of TrIV gene functions.