Jean-Michel Drezen

Polydnaviruses of braconid wasps derive from an ancestral nudivirus

Many species of parasitoid wasps inject polydnavirus particles in order to manipulate host defenses and development. Because the DNA packaged in these particles encodes almost no viral structural proteins, their relation to viruses has been debated. Characterization of complementary DNAs derived from braconid wasp ovaries identified genes encoding subunits of a viral RNA polymerase and structural components of polydnavirus particles related most closely to those of nudiviruses—a sister group of baculoviruses. The conservation of this viral machinery in different braconid wasp lineages sharing polydnaviruses suggests that parasitoid wasps incorporated a nudivirus-related genome into their own genetic material. We found that the nudiviral genes themselves are no longer packaged but are actively transcribed and produce particles used to deliver genes essential for successful parasitism in lepidopteran hosts.

Analysis of virion structural components reveals vestiges of the ancestral ichnovirus genome.

Many thousands of endoparasitic wasp species are known to inject polydnavirus (PDV) particles into their caterpillar host during oviposition, causing immune and developmental dysfunctions that benefit the wasp larva. PDVs associated with braconid and ichneumonid wasps, bracoviruses and ichnoviruses respectively, both deliver multiple circular dsDNA molecules to the caterpillar. These molecules contain virulence genes but lack core genes typically involved in particle production. This is not completely unexpected given that no PDV replication takes place in the caterpillar. Particle production is confined to the wasp ovary where viral DNAs are generated from proviral copies maintained within the wasp genome. We recently showed that the genes involved in bracovirus particle production reside within the wasp genome and are related to nudiviruses. In the present work we characterized genes involved in ichnovirus particle production by analyzing the components of purified Hyposoter didymator Ichnovirus particles by LC-MS/MS and studying their organization in the wasp genome. Their products are conserved among ichnovirus-associated wasps and constitute a specific set of proteins in the virosphere. Strikingly, these genes are clustered in specialized regions of the wasp genome which are amplified along with proviral DNA during virus particle replication, but are not packaged in the particles. Clearly our results show that ichnoviruses and bracoviruses particles originated from different viral entities, thus providing an example of convergent evolution where two groups of wasps have independently domesticated viruses to deliver genes into their hosts.

Visualization of Polydnavirus Sequences in a Parasitoid Wasp Chromosome

ABSTRACT Polydnaviruses, obligatorily associated with endoparasitoid wasps, are unique in that their segmented genome is composed of multiple double-stranded DNA circles. We present here the first cytological evidence that virus segments are integrated in the wasp genome, obtained by using in situ hybridization of virus probes with viral sequences in the chromosomes of a wasp from the braconid family of hymenopterans.

Paleozoic origin of insect large dsDNA viruses.

To understand how extant viruses interact with their hosts, we need a historical framework of their evolutionary association. Akin to retrovirus or hepadnavirus viral fossils present in eukaryotic genomes, bracoviruses are integrated in braconid wasp genomes and are transmitted by Mendelian inheritance. However, unlike viral genomic fossils, they have retained functional machineries homologous to those of large dsDNA viruses pathogenic to arthropods. Using a phylogenomic approach, we resolved the relationships between bracoviruses and their closest free relatives: baculoviruses and nudiviruses. The phylogeny showed that bracoviruses are nested within the nudivirus clade. Bracoviruses establish a bridge between the virus and animal worlds. Their inclusion in a virus phylogeny allowed us to relate free viruses to fossils. The ages of the wasps were used to calibrate the virus phylogeny. Bayesian analyses revealed that insect dsDNA viruses first evolved at ∼310 Mya in the Paleozoic Era during the Carboniferous Period with the first insects. Furthermore the virus diversification time frame during the Mesozoic Era appears linked to the diversification of insect orders; baculoviruses that infect larvae evolved at the same period as holometabolous insects. These results imply ancient coevolution by resource tracking between several insect dsDNA virus families and their hosts, dating back to 310 Mya.

A serpin from the parasitoid wasp Leptopilina boulardi targets the Drosophila phenoloxidase cascade.

The insect phenoloxidase (PO) cascade is known to be tightly regulated by serine proteases and serine protease inhibitors of the serpin family. As a key component of the insect immune system, it is also suspected to be inhibited by several endoparasitoid wasps, insects that develop inside other arthropods as hosts. However, the underlying mechanisms of this inhibition are largely undescribed. Here, we report the characterization of a gene encoding a serpin, LbSPNy, highly expressed in the venom of the wasp Leptopilina boulardi (IS(y) type), and we show that either the venom or the recombinant LbSPNy inhibit the PO cascade in the hemolymph of Drosophila yakuba host larva. Altogether, our results identify the first serpin used as a virulence factor by a parasitoid wasp and show that it disrupts the activation pathway of the PO in the Drosophila host.

Bracoviruses Contain a Large Multigene Family Coding for Protein Tyrosine Phosphatases

ABSTRACT 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.

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

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

The venom composition of the parasitic wasp Chelonus inanitus resolved by combined expressed sequence tags analysis and proteomic approach

BackgroundParasitic wasps constitute one of the largest group of venomous animals. Although some physiological effects of their venoms are well documented, relatively little is known at the molecular level on the protein composition of these secretions. To identify the majority of the venom proteins of the endoparasitoid wasp Chelonus inanitus (Hymenoptera: Braconidae), we have randomly sequenced 2111 expressed sequence tags (ESTs) from a cDNA library of venom gland. In parallel, proteins from pure venom were separated by gel electrophoresis and individually submitted to a nano-LC-MS/MS analysis allowing comparison of peptides and ESTs sequences.ResultsAbout 60% of sequenced ESTs encoded proteins whose presence in venom was attested by mass spectrometry. Most of the remaining ESTs corresponded to gene products likely involved in the transcriptional and translational machinery of venom gland cells. In addition, a small number of transcripts were found to encode proteins that share sequence similarity with well-known venom constituents of social hymenopteran species, such as hyaluronidase-like proteins and an Allergen-5 protein.An overall number of 29 venom proteins could be identified through the combination of ESTs sequencing and proteomic analyses. The most highly redundant set of ESTs encoded a protein that shared sequence similarity with a venom protein of unknown function potentially specific of the Chelonus lineage. Venom components specific to C. inanitus included a C-type lectin domain containing protein, a chemosensory protein-like protein, a protein related to yellow-e3 and ten new proteins which shared no significant sequence similarity with known sequences. In addition, several venom proteins potentially able to interact with chitin were also identified including a chitinase, an imaginal disc growth factor-like protein and two putative mucin-like peritrophins.ConclusionsThe use of the combined approaches has allowed to discriminate between cellular and truly venom proteins. The venom of C. inanitus appears as a mixture of conserved venom components and of potentially lineage-specific proteins. These new molecular data enrich our knowledge on parasitoid venoms and more generally, might contribute to a better understanding of the evolution and functional diversity of venom proteins within Hymenoptera.

Active suppression of D. melanogaster immune response by long gland products of the parasitic wasp Leptopilina boulardi

To develop inside their insect hosts, endoparasitoid wasps must either evade or overcome the hosts immune system. Several ichneumonid and braconid wasps inject polydnaviruses that display well-studied immune suppressive effects. However, little is known about the strategies of immunoevasion used by other parasitoid families, such as figitid wasps. The present study provides experimental evidence, based on superparasitism and injection experiments, that the figitid species Leptopilina boulardi uses an active mechanism to suppress the Drosophila melanogaster host immune response, i.e. the encapsulation of the parasitoid eggs. The immune suppressive factors are localised in the long gland and reservoir of the female genital tractus, where virus-like particles (VLPs) have been observed. Parasitism experiments using a host tumorous strain indicate that these factors do not destroy host lamellocytes but that they impair the melanisation pathway. Interestingly, they are not susceptible to heating and are not depleted with prolonged oviposition experience, in contrast to observations reported for L. heterotoma, another figitid species. The mechanisms that prevent encapsulation of eggs from L. boulardi and L. heterotoma differ in several respects, suggesting that different physiological strategies of immunosuppression might be used by specialised and generalist parasitoids.

Polydnavirus hidden face: the genes producing virus particles of parasitic wasps.

Very few obligatory relationships involve viruses to the remarkable exception of polydnaviruses (PDVs) associated with tens of thousands species of parasitic wasps that develop within the body of lepidopteran larvae. PDV particles, injected along with parasite eggs into the host body, act by manipulating host immune defences, development and physiology, thereby enabling wasp larvae to survive in a potentially harmful environment. Particle production does not occur in infected tissues of parasitized caterpillars, but is restricted to specialized cells of the wasp ovaries. Moreover, the genome enclosed in the particles encodes almost no viral structural protein, but mostly factors used to manipulate the physiology of the parasitized host. We recently unravelled the viral nature of PDVs associated with braconid wasps by characterizing a large set of nudivirus genes residing permanently in the wasp chromosome(s). Many of these genes encode structural components of the bracovirus particles and their expression pattern correlates with particle production. They constitute a viral machinery comprising a large number of core genes shared by nudiviruses and baculoviruses. Thus bracoviruses do not appear to be nudiviruses remnants, but instead complex nudiviral devices carrying DNA for the delivery of virulence genes into lepidopteran hosts. This highlights the fact that viruses should no longer be exclusively considered obligatory parasites, and that in certain cases they are obligatory symbionts.