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

Parasitoid Serpins Evolve Novel Functions to Manipulate Host Homeostasis

Parasitoids introduce various virulence factors when parasitism occurs, and some taxa generate teratocytes to manipulate the host immune system and metabolic homeostasis for the survival and development of their progeny. Host-parasitoid interactions are extremely diverse and complex, yet the evolutionary dynamics are still poorly understood. A category of serpin genes, named CvT-serpins, was discovered to be specifically expressed and secreted by the teratocytes of Cotesia vestalis, an endoparasitoid of the diamondback moth Plutella xylostella. Genomic and phylogenetic analysis indicated that the C. vestalis serpin genes are duplicated and most of them are clustered into 1 monophyletic clade. Intense positive selection was detected at the residues around the P1-P1' cleavage sites of the Cv-serpin reactive center loop domain. Functional analyses revealed that, in addition to the conserved function of melanization inhibition (CvT-serpins 1, 16, 18, and 21), CvT-serpins exhibited novel functions, i.e. bacteriostasis (CvT-serpins 3 and 5) and nutrient metabolism regulation (CvT-serpins 8 and 10). When the host-parasitoid system is challenged with foreign bacteria, CvT-serpins act as an immune regulator to reprogram the host immune system through sustained inhibition of host melanization while simultaneously functioning as immune effectors to compensate for this suppression. In addition, we provided evidence that CvT-serpin8 and 10 participate in the regulation of host trehalose and lipid levels by affecting genes involved in these metabolic pathways. These findings illustrate an exquisite tactic by which parasitoids win out in the parasite-host evolutionary arms race by manipulating host immune and nutrition homeostasis via adaptive gene evolution and neofunctionalization.

A new gene family (BAPs) of Cotesia bracovirus induces apoptosis of host hemocytes

Polydnaviruses (PDVs), obligatory symbionts with parasitoid wasps, function as host immune suppressors and growth and development regulator. PDVs can induce host haemocyte apoptosis, but the underlying mechanism remains largely unknown. Here, we provided evidence that, during the early stages of parasitism, the activated Cotesia vestalis bracovirus (CvBV) reduced the overall number of host haemocytes by inducing apoptosis. We found that one haemocyte-highly expressed CvBV gene, CvBV-26-4, could induce haemocyte apoptosis. Further analyses showed that CvBV-26-4 has four homologs from other Cotesia bracoviruses and BV from wasps in the genus Glyptapanteles, and all four of them possessed a similar structure containing 3 copies of a well-conserved motif (Gly-Tyr-Pro-Tyr, GYPY). Mass spectrometry analysis revealed that CvBV-26-4 was secreted into plasma by haemocytes and then degraded into peptides that induced the apoptosis of haemocytes. Moreover, ectopic expression of CvBV-26-4 caused fly haemocyte apoptosis and increased the susceptibility of flies to bacteria. Based on this research, a new family of bracovirus genes, Bracovirus apoptosis-inducing proteins (BAPs), was proposed. Furthermore, it was discovered that the development of wasp larvae was affected when the function of CvBV BAP was obstructed in the parasitized hosts. The results of our study indicate that the BAP gene family from the bracoviruses group is crucial for immunosuppression during the early stages of parasitism.

A Review on Digestive System of Rhynchophorus ferrugineus as Potential Target to Develop Control Strategies

Rhynchophorus ferrugineus, commonly known as red palm weevil (RPW), is a high-risk insect pest that has become a threat to many important palm species. There are several dominant factors that lead to the successful infestation of RPW, including its stealthy lifestyle, highly chitinized mouthpart, and high fecundity rate. Due to that, millions of dollars of losses have been suffered by many countries invaded by RPW. Several methods have been designed to control its invasion, including the usage of insecticides, but many cause resistance and environmental pollution. Therefore, an environmentally friendly insecticide that targets specific systems or pathways in RPW is urgently needed. One of the potential targets is the digestive system of RPW, as it is the major interface between the insect and its plant host. The related knowledge of RPW's digestive system, such as the anatomy, microflora, transcriptomic analysis, and proteomic analysis, is important to understand its effects on RPW's survival. Several data from different omics regarding the digestive systems of RPW have been published in separate reports. Some of the potential targets have been reported to be inhibited by certain potential insecticides, while other targets have not yet been tested with any inhibitors. Hence, this review may lead to a better understanding on managing infestations of RPW using the system biology approach for its digestive system.

A Polydnavirus Protein Tyrosine Phosphatase Negatively Regulates the Host Phenoloxidase Pathway

Polydnavirus (PDV) is a parasitic factor of endoparasitic wasps and contributes greatly to overcoming the immune response of parasitized hosts. Protein tyrosine phosphatases (PTPs) regulate a wide variety of biological processes at the post-transcriptional level in mammals, but knowledge of PDV PTP action during a parasitoid−host interaction is limited. In this study, we characterized a PTP gene, CvBV_12-6, derived from Cotesia vestalis bracovirus (CvBV), and explored its possible regulatory role in the immune response of the host Plutella xylostella. Our results from qPCR show that CvBV_12-6 was highly expressed in hemocytes at an early stage of parasitization. To explore CvBV_12-6 function, we specifically expressed CvBV_12-6 in Drosophila melanogaster hemocytes. The results show that Hml-Gal4 > CvBV_12-6 suppressed the phenoloxidase activity of hemolymph in D. melanogaster, but exerted no effect on the total count or the viability of the hemocytes. In addition, the Hml-Gal4 > CvBV_12-6 flies exhibited decreased antibacterial abilities against Staphylococcus aureus. Similarly, we found that CvBV_12-6 significantly suppressed the melanization of the host P. xylostella 24 h post parasitization and reduced the viability, but not the number, of hemocytes. In conclusion, CvBV_12-6 negatively regulated both cellular and humoral immunity in P. xylostella, and the related molecular mechanism may be universal to insects.

Chelonus inanitus bracovirus encodes lineage-specific proteins and truncated immune IκB-like factors

Bracoviruses and ichnoviruses are endogenous viruses of parasitic wasps that produce particles containing virulence genes expressed in host tissues and necessary for parasitism success. In the case of bracoviruses the particles are produced by conserved genes of nudiviral origin integrated permanently in the wasp genome, whereas the virulence genes can strikingly differ depending on the wasp lineage. To date most data obtained on bracoviruses concerned species from the braconid subfamily of Microgastrinae. To gain a broader view on the diversity of virulence genes we sequenced the genome packaged in the particles of Chelonus inanitus bracovirus (CiBV) produced by a wasp belonging to a different subfamily: the Cheloninae. These are egg-larval parasitoids, which means that they oviposit into the host egg and the wasp larvae then develop within the larval stages of the host. We found that most of CiBV virulence genes belong to families that are specific to Cheloninae. As other bracoviruses and ichnoviruses however, CiBV encode v-ank genes encoding truncated versions of the immune cactus/IκB factor, which suggests these proteins might play a key role in host–parasite interactions involving domesticated endogenous viruses. We found that the structures of CiBV V-ANKs are different from those previously reported. Phylogenetic analysis supports the hypothesis that they may originate from a cactus/IκB immune gene from the wasp genome acquired by the bracovirus. However, their evolutionary history is different from that shared by other V-ANKs, whose common origin probably reflects horizontal gene transfer events of virus sequences between braconid and ichneumonid wasps.

The Dual Functions of a Bracovirus C-Type Lectin in Caterpillar Immune Response Manipulation

Parasitoids are widespread in natural ecosystems and normally equipped with diverse viral factors to defeat host immune responses. On the other hand, parasitoids can enhance the antibacterial abilities and improve the hypoimmunity traits of parasitized hosts that may encounter pathogenic infections. These adaptive strategies guarantee the survival of parasitoid offspring, yet their underlying mechanisms are poorly understood. Here, we focused on Cotesia vestalis, an endoparasitoid of the diamondback moth Plutella xylostella, and found that C. vestalis parasitization decreases the number of host hemocytes, leading to disruption of the encapsulation reaction. We further found that one bracovirus C-type lectin gene, CvBV_28-1, is highly expressed in the hemocytes of parasitized hosts and participates in suppressing the proliferation rate of host hemocytes, which in turn reduces their population and represses the process of encapsulation. Moreover, CvBV_28-1 presents a classical bacterial clearance ability via the agglutination response in a Ca²⁺-dependent manner in response to gram-positive bacteria. Our study provides insights into the innovative strategy of a parasitoid-derived viral gene that has dual functions to manipulate host immunity for a successful parasitism.

Viral histones: pickpocket's prize or primordial progenitor?

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

CLP gene family, a new gene family of Cotesia vestalis bracovirus inhibits melanization of Plutella xylostella hemolymph

Polydnaviruses (PDVs) are obligatory symbionts of parasitoid wasps and play an important role in suppressing host immune defenses. Although PDV genes that inhibit host melanization are known in Microplitis bracovirus, the functional homologs in Cotesia bracoviruses remain unknown. Here, we find that Cotesia vestalis bracovirus (CvBV) can inhibit hemolymph melanization of its host, Plutella xylostella larvae, during the early stages of parasitization, and that overexpression of highly expressed CvBV genes reduced host phenoloxidase activity. Furthermore, CvBV-7-1 in particular reduced host phenoloxidase activity within 12 h, and the injection of anti-CvBV-7-1 antibody increased the melanization of parasitized host larvae. Further analyses showed that CvBV-7-1 and three homologs from other Cotesia bracoviruses possessed a C-terminal leucine/isoleucine-rich region and had a similar function in inhibiting melanization. Therefore, a new family of bracovirus genes was proposed and named as C-terminal Leucine/isoleucine-rich Protein (CLP). Ectopic expression of CvBV-7-1 in Drosophila hemocytes increased susceptibility to bacterial repression of melanization and reduced the melanotic encapsulation of parasitized D. melanogaster by the parasitoid Leptopilina boulardi. The formation rate of wasp pupae and the eclosion rate of C. vestalis were affected when the function of CvBV-7-1 was blocked. Our findings suggest that CLP genes from Cotesia bracoviruses encoded proteins that contain a C-terminal leucine/isoleucine-rich region and function as melanization inhibitors during the early stage of parasitization, which is important for successful parasitization.

Genome-Wide Patterns of Bracovirus Chromosomal Integration into Multiple Host Tissues during Parasitism

Bracoviruses are domesticated viruses found in parasitic wasp genomes. They are composed of genes of nudiviral origin that are involved in particle production and proviral segments containing virulence genes that are necessary for parasitism success. During particle production, proviral segments are amplified and individually packaged as DNA circles in nucleocapsids. These particles are injected by parasitic wasps into host larvae together with their eggs. Bracovirus circles of two wasp species were reported to undergo chromosomal integration in parasitized host hemocytes, through a conserved sequence named the host integration motif (HIM). Here, we used bulk Illumina sequencing to survey integrations of Cotesia typhae bracovirus circles in the DNA of its host, the maize corn borer (Sesamia nonagrioides), 7 days after parasitism. First, assembly and annotation of a high-quality genome for C. typhae enabled us to characterize 27 proviral segments clustered in proviral loci. Using these data, we characterized large numbers of chromosomal integrations (from 12 to 85 events per host haploid genome) for all 16 bracovirus circles containing a HIM. Integrations were found in four S. nonagrioides tissues and in the body of a caterpillar in which parasitism had failed. The 12 remaining circles do not integrate but are maintained at high levels in host tissues. Surprisingly, we found that HIM-mediated chromosomal integration in the wasp germ line has occurred accidentally at least six times during evolution. Overall, our study furthers our understanding of wasp-host genome interactions and supports HIM-mediated chromosomal integration as a possible mechanism of horizontal transfer from wasps to their hosts. IMPORTANCE Bracoviruses are endogenous domesticated viruses of parasitoid wasps that are injected together with wasp eggs into wasp host larvae during parasitism. Several studies have shown that some DNA circles packaged into bracovirus particles become integrated into host somatic genomes during parasitism, but the phenomenon has never been studied using nontargeted approaches. Here, we use bulk Illumina sequencing to systematically characterize and quantify bracovirus circle integrations that occur in four tissues of the Mediterranean corn borer (Sesamia nonagrioides) during parasitism by the Cotesia typhae wasp. Our analysis reveals that all circles containing a HIM integrate at substantial levels (from 12 to 85 integrations per host cell, in total) in all tissues, while other circles do not integrate. In addition to shedding new light on wasp-bracovirus-host interactions, our study supports HIM-mediated chromosomal integration of bracovirus as a possible source of wasp-to-host horizontal transfer, with long-term evolutionary consequences.

Draft nuclear genome and complete mitogenome of the Mediterranean corn borer, Sesamia nonagrioides, a major pest of maize

The Mediterranean corn borer (Sesamia nonagrioides, Noctuidae, Lepidoptera) is a major pest of maize in Europe and Africa. Here, we report an assembly of the nuclear and mitochondrial genome of a pool of inbred males and females third-instar larvae, based on short- and long-read sequencing. The complete mitochondrial genome is 15,330 bp and contains all expected 13 and 24 protein-coding and RNA genes, respectively. The nuclear assembly is 1021 Mb, composed of 2553 scaffolds and it has an N50 of 1105 kb. It is more than twice larger than that of all Noctuidae species sequenced to date, mainly due to a higher repeat content. A total of 17,230 protein-coding genes were predicted, including 15,776 with InterPro domains. We provide detailed annotation of genes involved in sex determination (doublesex, insulin-like growth factor 2 mRNA-binding protein, and P-element somatic inhibitor) and of alpha-amylase genes possibly involved in interaction with parasitoid wasps. We found no evidence of recent horizontal transfer of bracovirus genes from parasitoid wasps. These genome assemblies provide a solid molecular basis to study insect genome evolution and to further develop biocontrol strategies against S. nonagrioides.

Influence of parasitoid-associated viral symbionts on plant-insect interactions and biological control

Insect parasitoids have evolved symbiotic interactions with several viruses and thousands of parasitoid species have established mutualistic associations with polydnaviruses (PDVs). While PDVs have often been described as virulence factors allowing development of immature parasitoids inside their herbivore hosts, there is increasing awareness that PDVs can affect plant-insect interactions. We review recent literature showing that PDVs alter not only host physiology, but also feeding patterns and composition of herbivore's oral secretions. In turn PDV-induced changes in herbivore phenotype affect plant responses to herbivory with consequences ranging from differential expression of plant defense-related genes to wider ecological effects across multiple trophic levels. In this opinion paper we also highlight important missing gaps to fully understand the role of PDVs and other parasitoid-associated viral symbionts in a plant-insect interaction perspective. Because PDVs negatively impact performance and survival of herbivore pests, we conclude arguing that PDV genomes offer potential opportunities for biological control.

Symbiotic bracovirus of a parasite manipulates host lipid metabolism via tachykinin signaling

Parasites alter host energy homeostasis for their own development, but the mechanisms underlying this phenomenon remain largely unknown. Here, we show that Cotesia vestalis, an endoparasitic wasp of Plutella xylostella larvae, stimulates a reduction of host lipid levels. This process requires excess secretion of P. xylostella tachykinin (PxTK) peptides from enteroendocrine cells (EEs) in the midgut of the parasitized host larvae. We found that parasitization upregulates PxTK signaling to suppress lipogenesis in midgut enterocytes (ECs) in a non-cell-autonomous manner, and the reduced host lipid level benefits the development of wasp offspring and their subsequent parasitic ability. We further found that a C. vestalis bracovirus (CvBV) gene, CvBV 9–2, is responsible for PxTK induction, which in turn reduces the systemic lipid level of the host. Taken together, these findings illustrate a novel mechanism for parasite manipulation of host energy homeostasis by a symbiotic bracovirus gene to promote the development and increase the parasitic efficiency of an agriculturally important wasp species.

Parasitic wasps are ubiquitous on earth and diverse. They lay eggs in or on the bodies of their hosts, and they have evolved adaptive strategies to regulate the energy metabolism of their hosts to match their own specific nutrition requirements. Here, we found that Cotesia vestalis, a solitary endoparasitoid of Plutella xylostella, uses symbiotic bracovirus as a weapon to manipulate host systemic lipid levels. Specifically, a C. vestalis bracovirus (CvBV) gene, CvBV 9–2, is responsible for the induction of PxTK, which in turn suppresses lipogenesis in the midgut of the parasitized host, leading to a nutritional lipid level suitable for the development and subsequent parasitic efficiency of C. vestalis wasps. Our study provides innovative insights into the mechanisms by which parasitic wasps manipulate host lipid homeostasis and may help to expand our knowledge of other parasitic systems.

Genome-Wide Profiling of Diadegma semiclausum Ichnovirus Integration in Parasitized Plutella xylostella Hemocytes Identifies Host Integration Motifs and Insertion Sites

Polydnaviruses (PDVs), classified into two genera, bracoviruses (BVs) and ichnoviruses (IVs), are large, double-stranded DNA viruses, which are beneficial symbionts of parasitoid wasps. PDVs do not replicate in their infected lepidopteran hosts. BV circles have been demonstrated to be integrated into host genomic DNA after natural parasitization. However, the integrations of IV circles in vivo remain largely unknown. Here, we analyzed the integration of Diadegma semiclausum ichnovirus (DsIV) in the genomic DNA of parasitized Plutella xylostella hemocytes. We found that DsIV circles are present in host hemocytes with non-integrated and integrated forms. Moreover, DsIV integrates its DNA circles into the host genome by two distinct strategies, conservatively, and randomly. We also found that four conserved-broken circles share similar motifs containing two reverse complementary repeats at their breaking sites, which were host integration motifs (HIMs). We also predicted HIMs of eight circles from other ichnoviruses, indicating that a HIM-mediated specific mechanism was conserved in IV integrations. Investigation of DsIV circle insertion sites of the host genome revealed the enrichment of microhomologies between the host genome and the DsIV circles at integration breakpoints. These findings will deepen our understanding of the infections of PDVs, especially IVs.

Detecting bracoviral orthologs distribution in five tsetse fly species and the housefly genomes

Objective

Mutualism between endogenous viruses and eukaryotes is still poorly understood. Several endogenous double-stranded polydnaviruses, bracoviruses, homologous to those present in parasitic braconid wasp genomes were detected in the tsetse fly (Glossina morsitans morsitans). This is peculiar since tsetse flies do not share a reproductive lifestyle similar to wasps, but deliver fully developed larvae that pupate within minutes of exiting their mothers. The objective of this study is to investigate genomic distribution of bracoviral sequences in five tsetse fly species and the housefly, and examine its value as a potential vector control strategy target point. We use comparative genomics to determine the presence, distribution across Glossina species genomes, and evolutionary relationships of bracoviruses of five tsetse fly species and the housefly.

Results

We report on homologous bracoviruses in multiple Dipteran genomes. Phylogenetic reconstruction using within-species concatenated bracoviral orthologs shows great congruence with previously reconstructed insect species phylogenies. Our findings suggest that bracoviruses present in Diptera originate from a single integration event of the viral genome that occurred in an ancestor insect before the evolutionary radiation of different insect orders.

Quantitative trait loci involved in the reproductive success of a parasitoid wasp

Dissecting the genetic basis of intraspecific variations in life history traits is essential to understand their evolution, notably for potential biocontrol agents. Such variations are observed in the endoparasitoid Cotesia typhae (Hymenoptera: Braconidae), specialized on the pest Sesamia nonagrioides (Lepidoptera: Noctuidae). Previously, we identified two strains of C. typhae that differed significantly for life history traits on an allopatric host population. To investigate the genetic basis underlying these phenotypic differences, we used a quantitative trait locus (QTL) approach based on restriction site-associated DNA markers. The characteristic of C. typhae reproduction allowed us generating sisters sharing almost the same genetic content, named clonal sibship. Crosses between individuals from the two strains were performed to generate F2 and F8 recombinant CSS. The genotypes of 181 clonal sibships were determined as well as the phenotypes of the corresponding 4,000 females. Informative markers were then used to build a high-quality genetic map. These 465 markers spanned a total length of 1,300 cM and were organized in 10 linkage groups which corresponded to the number of C. typhae chromosomes. Three QTLs were detected for parasitism success and two for offspring number, while none were identified for sex ratio. The QTLs explained, respectively, 27.7% and 24.5% of the phenotypic variation observed. The gene content of the genomic intervals was investigated based on the genome of C. congregata and revealed 67 interesting candidates, as potentially involved in the studied traits, including components of the venom and of the symbiotic virus (bracovirus) shown to be necessary for parasitism success in related wasps.

Role of egg-laying behavior, virulence and local adaptation in a parasitoid's chances of reproducing in a new host

Understanding the ability of parasitoid insects to succeed in new host populations is a relevant question for biological control and adaptive mechanisms. Cotesia typhae is an African parasitoid specialized on the moth Sesamiae nonagrioides, also called the Mediterranean corn borer. Two Kenyan strains of C. typhae differ in their virulence against a new host population from France. We explored behavioral and physiological hypotheses about this differentiation. Cotesia genus belongs to a group of Hymenoptera in which females inject a domesticated virus in their host to overcome its resistance. Since viral particles are injected along with eggs and since the strain with the higher virulence injects more eggs, we hypothesized that virulence could be explained by the quantity of virus injected. To test this assumption, we measured the injected quantities of eggs and viral particles (estimated by viral DNA segments) of each parasitoid strain along several ovipositions, to vary these quantities. Unexpectedly, results showed that virulence against the French host was not correlated to the injected quantities of eggs or viral segments, indicating that virulence differentiation is explained by other causes. The virulence against the respective natural hosts of the two C. typhae strains was also measured, and results suggest that local adaptation to a more resistant natural host may explain the pre-adaptation of one strain to the new host population. We also identified a differentiation of oviposition strategy and subsequent offspring number between the parasitoid strains, which is important in a biocontrol perspective.

Parasitic insect-derived miRNAs modulate host development

Parasitic wasps produce several factors including venom, polydnaviruses (PDVs) and specialized wasp cells named teratocytes that benefit the survival of offspring by altering the physiology of hosts. However, the underlying molecular mechanisms for the alterations remain unclear. Here we find that the teratocytes of Cotesia vestalis, an endoparasitoid of the diamondback moth Plutella xylostella, and its associated bracovirus (CvBV) can produce miRNAs and deliver the products into the host via different ways. Certain miRNAs in the parasitized host are mainly produced by teratocytes, while the expression level of miRNAs encoded by CvBV can be 100-fold greater in parasitized hosts than non-parasitized ones. We further show that one teratocyte-produced miRNA (Cve-miR-281-3p) and one CvBV-produced miRNA (Cve-miR-novel22-5p-1) arrest host growth by modulating expression of the host ecdysone receptor (EcR). Altogether, our results show the first evidence of cross-species regulation by miRNAs in animal parasitism and their possible function in the alteration of host physiology during parasitism.

Parasitoid polydnaviruses and immune interaction with secondary hosts

Polydnaviruses (PDVs) are obligatory symbionts with parasitoid wasps. The PDV virions are produced solely in wasp (the primary host) calyx cells. They are injected into caterpillar hosts (the secondary host) during parasitoid oviposition, where they express irreplaceable actions to ensure survival and development of wasp larvae. Some of PDV gene products suppress host immune responses while others alter host growth, metabolism or endocrine system. Here, we treat new findings on PDV gene products and their action on immunity within secondary hosts.

Systematics, Biology, and Evolution of Microgastrine Parasitoid Wasps

The braconid parasitoid wasp subfamily Microgastrinae is perhaps the most species-rich subfamily of animals on Earth. Despite their small size, they are familiar to agriculturalists and field ecologists alike as one of the principal groups of natural enemies of caterpillars feeding on plants. Their abundance and nearly ubiquitous terrestrial distribution, their intricate interactions with host insects, and their historical association with mutualistic polydnaviruses have all contributed to Microgastrinae becoming a key group of organisms for studying parasitism, parasitoid genomics, and mating biology. However, these rich sources of data have not yet led to a robust genus-level classification of the group, and some taxonomic confusion persists as a result. We present the current status of understanding of the general biology, taxonomic history, diversity, geographical patterns, host relationships, and phylogeny of Microgastrinae as a stimulus and foundation for further study. Current progress in elucidating the biology and taxonomy of this important group is rapid and promises a revolution in the classification of these wasps in the near future.

Comparative genetics of invasive populations of walnut aphid, Chromaphis juglandicola, and its introduced parasitoid, Trioxys pallidus, in California

Coevolution may be an important component of the sustainability of importation biological control, but how frequently introduced natural enemies coevolve with their target pests is unclear. Here we explore whether comparative population genetics of the invasive walnut aphid, Chromaphis juglandicola, and its introduced parasitoid, Trioxys pallidus, provide insights into the localized breakdown of biological control services in walnut orchards in California. We found that sampled populations of C. juglandicola exhibited higher estimates of genetic differentiation (FST) than co-occurring populations of T. pallidus. In contrast, estimates of both the inbreeding coefficient (GIS) and contemporary gene flow were higher for T. pallidus than for C. juglandicola. We also found evidence of reciprocal outlier loci in some locations, but none showed significant signatures of selection. Synthesis and applications. Understanding the importance of coevolutionary interactions for the sustainability of biological control remains an important and understudied component of biological control research. Given the observed differences in gene flow and genetic differentiation among populations of T. pallidus and C. juglandicola, we suspect that temporary local disruption of biological control services may occur more frequently than expected while remaining stable at broader regional scales. Further research that combines genomewide single nucleotide polymorphism genotyping with measurements of phenotypic traits is needed to provide more conclusive evidence of whether the occurrence of outlier loci that display significant signatures of selection can be interpreted as evidence of the presence of a geographic mosaic of coevolution in this system.

Systematics and biology of Cotesia typhae sp. n. (Hymenoptera, Braconidae, Microgastrinae), a potential biological control agent against the noctuid Mediterranean corn borer, Sesamia nonagrioides

Many parasitoid species are subjected to strong selective pressures from their host, and their adaptive response may result in the formation of genetically differentiated populations, called host races. When environmental factors and reproduction traits prevent gene flow, host races become distinct species. Such a process has recently been documented within the Cotesiaflavipes species complex, all of which are larval parasitoids of moth species whose larvae are stem borers of Poales. A previous study on the African species C.sesamiae, incorporating molecular, ecological and biological data on various samples, showed that a particular population could be considered as a distinct species, because it was specialized at both host (Sesamianonagrioides) and plant (Typhadomingensis) levels, and reproductively isolated from other C.sesamiae. Due to its potential for the biological control of S.nonagrioides, a serious corn pest in Mediterranean countries and even in Iran, we describe here Cotesiatyphae Fernandez-Triana sp. n. The new species is characterized on the basis of morphological, molecular, ecological and geographical data, which proved to be useful for future collection and rapid identification of the species within the species complex. Fecundity traits and parasitism success on African and European S.nonagrioides populations, estimated by laboratory studies, are also included.

The recurrent domestication of viruses: major evolutionary transitions in parasitic wasps

Several lineages of endoparasitoid wasps, which develop inside the body of other insects, have domesticated viruses, used as delivery tools of essential virulence factors for the successful development of their progeny. Virus domestications are major evolutionary transitions in highly diverse parasitoid wasps. Much progress has recently been made to characterize the nature of these ancestrally captured endogenous viruses that have evolved within the wasp genomes. Virus domestication from different viral families occurred at least three times in parasitoid wasps. This evolutionary convergence led to different strategies. Polydnaviruses (PDVs) are viral gene transfer agents and virus-like particles of the wasp Venturia canescens deliver proteins. Here, we take the standpoint of parasitoid wasps to review current knowledge on virus domestications by different parasitoid lineages. Then, based on genomic data from parasitoid wasps, PDVs and exogenous viruses, we discuss the different evolutionary steps required to transform viruses into vehicles for the delivery of the virulence molecules that we observe today. Finally, we discuss how endoparasitoid wasps manipulate host physiology and ensure parasitism success, to highlight the possible advantages of viral domestication as compared with other virulence strategies.

Analysis of Genetic Variation across the Encapsidated Genome of Microplitis demolitor Bracovirus in Parasitoid Wasps

Insect parasitoids must complete part of their life cycle within or on another insect, ultimately resulting in the death of the host insect. One group of parasitoid wasps, the ‘microgastroid complex’ (Hymenoptera: Braconidae), engage in an association with beneficial symbiotic viruses that are essential for successful parasitism of hosts. These viruses, known as Bracoviruses, persist in an integrated form in the wasp genome, and activate to replicate in wasp ovaries during development to ultimately be delivered into host insects during parasitism. The lethal nature of host-parasitoid interactions, combined with the involvement of viruses in mediating these interactions, has led to the hypothesis that Bracoviruses are engaged in an arms race with hosts, resulting in recurrent adaptation in viral (and host) genes. Deep sequencing was employed to characterize sequence variation across the encapsidated Bracovirus genome within laboratory and field populations of the parasitoid wasp species Microplitis demolitor. Contrary to expectations, there was a paucity of evidence for positive directional selection among virulence genes, which generally exhibited signatures of purifying selection. These data suggest that the dynamics of host-parasite interactions may not result in recurrent rounds of adaptation, and that adaptation may be more variable in time than previously expected.

Polydnaviruses: Nature's Genetic Engineers

Virus-host associations are usually viewed as parasitic, but several studies in recent years have reported examples of viruses that benefit host organisms. The Polydnaviridae are of particular interest because these viruses are all obligate mutualists of insects called parasitoid wasps. Parasitoids develop during their immature stages by feeding inside the body of other insects, which serve as their hosts. Polydnaviruses are vertically transmitted as proviruses through the germ line of wasps but also function as gene delivery vectors that wasps rely upon to genetically manipulate the hosts they parasitize. Here we review the evolutionary origin of polydnaviruses, the organization and function of their genomes, and some of their roles in parasitism.

Ongoing ecological speciation in Cotesia sesamiae, a biological control agent of cereal stem borers

To develop efficient and safe biological control, we need to reliably identify natural enemy species, determine their host range, and understand the mechanisms that drive host range evolution. We investigated these points in Cotesia sesamiae, an African parasitic wasp of cereal stem borers. Phylogenetic analyses of 74 individual wasps, based on six mitochondrial and nuclear genes, revealed three lineages. We then investigated the ecological status (host plant and host insect ranges in the field, and host insect suitability tests) and the biological status (cross-mating tests) of the three lineages. We found that one highly supported lineage showed all the hallmarks of a cryptic species. It is associated with one host insect, Sesamia nonagrioides, and is reproductively isolated from the other two lineages by pre- and postmating barriers. The other two lineages had a more variable phylogenetic support, depending on the set of genes; they exhibited an overlapping and diversified range of host species and are not reproductively isolated from one another. We discuss the ecological conditions and mechanisms that likely generated this ongoing speciation and the relevance of this new specialist taxon in the genus Cotesia for biological control.

Habitat complexity reduces parasitoid foraging efficiency, but does not prevent orientation towards learned host plant odours

It is well known that many parasitic wasps use herbivore-induced plant odours (HIPVs) to locate their inconspicuous host insects, and are often able to distinguish between slight differences in plant odour composition. However, few studies have examined parasitoid foraging behaviour under (semi-)field conditions. In nature, food plants of parasitoid hosts are often embedded in non-host-plant assemblages that confer both structural and chemical complexity. By releasing both naïve and experienced Cotesia glomerata females in outdoor tents, we studied how natural vegetation surrounding Pieris brassicae-infested Sinapis arvensis and Barbarea vulgaris plants influences their foraging efficiency as well as their ability to specifically orient towards the HIPVs of the host plant species on which they previously had a positive oviposition experience. Natural background vegetation reduced the host-encounter rate of naïve C. glomerata females by 47 %. While associative learning of host plant HIPVs 1 day prior to foraging caused a 28 % increase in the overall foraging efficiency of C. glomerata, it did not reduce the negative influence of natural background vegetation. At the same time, however, females foraging in natural vegetation attacked more host patches on host-plant species on which they previously had a positive oviposition experience. We conclude that, even though the presence of natural vegetation reduces the foraging efficiency of C. glomerata, it does not prevent experienced female wasps from specifically orienting towards the host-plant species from which they had learned the HIPVs.

Transcriptome analysis of an endoparasitoid wasp Cotesia chilonis (Hymenoptera: Braconidae) reveals genes involved in successful parasitism

For successful parasitization, parasitiods usually depend on the chemosensory cues for the selection of hosts, as well as a variety of virulence factors introduced into their hosts to overcome host immunity and prevent rejection of progeny development. In bracovirus-carrying wasps, the symbiotic polydnaviruses act in manipulating development and immunity of hosts. The endoparasitoid Cotesia chilonis carrying bracovirus as a key host immunosuppressive factor is a superior endoparasitoid of rice stem borer, Chilo suppressalis. So far, genomic information for C. chilonis is not available and transcriptomic data may provide valuable resources for global studying on physiological processes of C. chilonis, including chemosensation and parasitism at molecular level. Here, we performed RNA-seq to characterize the transcriptome of C. chilonis adults. We obtained 27,717,892 reads, assembled into 38,318 unigenes with a mean size of 690 bp. Approximately, 62.1% of the unigenes were annotated using NCBI databases. A large number of chemoreception-related genes encoding proteins including odorant receptors, gustatory receptors, odorant-binding proteins, chemosensory proteins, transient receptor potential ion channels, and sensory neuron membrane proteins were identified in silico. Totally, 72 transcripts possessing high identities with the bracovirus-related genes were identified. We investigated the mRNA expression levels of several transcripts at different developmental stages (including egg, larva, pupae, and adult) by quantitative real-time PCR analysis. The results revealed that some genes had adult-specific expression, indicating their potential significance for mating and parasitism. Overall, these results provide comprehensive insights into transcriptomic data of a polydnavirus-carrying parasitoid of a rice pest.

Origin and evolution of symbiotic viruses associated with parasitoid wasps

The Polydnaviridae (PDV), including the Bracovirus (BV) and Ichnovirus (IV) genera, originated from the integration of viruses in the genomes of two parasitoid wasp lineages. In a remarkable example of convergent evolution BVs evolved from the domestication of a nudivirus, while IVs originate from a different ancestral virus belonging to a new virus entity. In both cases the ancestor genomes have been maintained in wasp genomes as endogenous viral elements involved in production of particles containing DNA encoding virulence genes that are injected into lepidopteran hosts. However many PDV virulence genes appear to be of eukaryotic origin, and expansion and diversification of these genes have led to the production of novel PDVs in different wasp species that promote survival of offspring in particular hosts.

Antiviral activity of the inducible humoral immunity and its suppression by eleven BEN family members encoded in Cotesia plutellae bracovirus

Upon parasitization by some endoparasitoids, polydnaviruses (PDVs) play a crucial role in inducing host immunosuppression. This study reports a novel immunosuppressive activity against humoral immune responses by BEN family genes encoded in Cotesia plutellae bracovirus (CpBV). A total of 11 BEN family members are encoded in 10 different CpBV DNA segments. When the CpBV segments were individually injected, specific BEN genes were expressed and suppressed the expression of antimicrobial peptide (AMP) and prophenoloxidase genes following bacterial challenge. The suppressive activities of the BEN genes were reversed by injection of the double-stranded RNA (dsRNA) specific to each BEN gene. The suppression of the AMP gene expressions by the BEN genes was also confirmed using an inhibition zone assay against Gram-positive and Gram-negative bacterial growth. The significance of the suppressive activity of BEN genes against humoral immune responses was analyzed in terms of suppression of antiviral activity by the host humoral immunity. When CpBV was incubated with the plasma obtained from the larvae challenged with bacteria, the immunized plasma severely impaired the expression activity of the viral genes. However, an expression of BEN gene significantly rescued the viral gene expression by suppressing humoral immune response. These results suggest that BEN family genes of CpBV play a crucial role in defending the antiviral response of the parasitized Plutella xylostella by inhibiting humoral immune responses.

Functional annotation of Cotesia congregata bracovirus: identification of viral genes expressed in parasitized host immune tissues

Bracoviruses (BVs) from the Polydnaviridae family are symbiotic viruses used as biological weapons by parasitoid wasps to manipulate lepidopteran host physiology and induce parasitism success. BV particles are produced by wasp ovaries and injected along with the eggs into the caterpillar host body, where viral gene expression is necessary for wasp development. Recent sequencing of the proviral genome of Cotesia congregata BV (CcBV) identified 222 predicted virulence genes present on 35 proviral segments integrated into the wasp genome. To date, the expressions of only a few selected candidate virulence genes have been studied in the caterpillar host, and we lacked a global vision of viral gene expression. In this study, a large-scale transcriptomic analysis by 454 sequencing of two immune tissues (fat body and hemocytes) of parasitized Manduca sexta caterpillar hosts allowed the detection of expression of 88 CcBV genes expressed 24 h after the onset of parasitism. We linked the expression profiles of these genes to several factors, showing that different regulatory mechanisms control viral gene expression in the host. These factors include the presence of signal peptides in encoded proteins, diversification of promoter regions, and, more surprisingly, gene position on the proviral genome. Indeed, most genes for which expression could be detected are localized in particular proviral regions globally producing higher numbers of circles. Moreover, this polydnavirus (PDV) transcriptomic analysis also reveals that a majority of CcBV genes possess at least one intron and an arthropod transcription start site, consistent with an insect origin of these virulence genes.

IMPORTANCE

Bracoviruses (BVs) are symbiotic polydnaviruses used by parasitoid wasps to manipulate lepidopteran host physiology, ensuring wasp offspring survival. To date, the expressions of only a few selected candidate BV virulence genes have been studied in caterpillar hosts. We performed a large-scale analysis of BV gene expression in two immune tissues of Manduca sexta caterpillars parasitized by Cotesia congregata wasps. Genes for which expression could be detected corresponded to genes localized in particular regions of the viral genome globally producing higher numbers of circles. Our study thus brings an original global vision of viral gene expression and paves the way to the determination of the regulatory mechanisms enabling the expression of BV genes in targeted organisms, such as major insect pests. In addition, we identify sequence features suggesting that most BV virulence genes were acquired from insect genomes.

Systematic analysis of a wasp parasitism arsenal

Parasitoid wasps are among the most diverse insects on earth with many species causing major mortality in host populations. Parasitoids introduce a variety of factors into hosts to promote parasitism, including symbiotic viruses, venom, teratocytes and wasp larvae. Polydnavirus-carrying wasps use viruses to globally suppress host immunity and prevent rejection of developing parasites. Although prior results provide detailed insights into the genes viruses deliver to hosts, little is known about other products. RNAseq and proteomics were used to characterize the proteins secreted by venom glands, teratocytes and larvae from Microplitis demolitor, which carries M. demolitor bracovirus (MdBV). These data revealed that venom glands and teratocytes secrete large amounts of a small number of products relative to ovaries and larvae. Venom and teratocyte products exhibited almost no overlap with one another or MdBV genes, which suggested that M. demolitor effector molecules are functionally partitioned according to their source. This finding was well illustrated in the case of MdBV and teratocytes. Many viral proteins have immunosuppressive functions that include disruption of antimicrobial peptide production, yet this study showed that teratocytes express high levels of the antimicrobial peptide hymenoptaecin, which likely compensates for MdBV-mediated immunosuppression. A second key finding was the prevalence of duplications among genes encoding venom and teratocyte molecules. Several of these gene families share similarities with proteins from other species, while also showing specificity of expression in venom glands or teratocytes. Overall, these results provide the first comprehensive analysis of the proteins a polydnavirus-carrying wasp introduces into its host.

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.

Functional endogenous viral elements in the genome of the parasitoid wasp Cotesia congregata: insights into the evolutionary dynamics of bracoviruses

Bracoviruses represent the most complex endogenous viral elements (EVEs) described to date. Nudiviral genes have been hosted within parasitoid wasp genomes since approximately 100 Ma. They play a crucial role in the wasp life cycle as they produce bracovirus particles, which are injected into parasitized lepidopteran hosts during wasp oviposition. Bracovirus particles encapsidate multiple dsDNA circles encoding virulence genes. Their expression in parasitized caterpillars is essential for wasp parasitism success. Here, we report on the genomic organization of the proviral segments (i.e. master sequences used to produce the encapsidated dsDNA circles) present in the Cotesia congregata parasitoid wasp genome. The provirus is composed of a macrolocus, comprising two-thirds of the proviral segments and of seven dispersed loci, each containing one to three segments. Comparative genomic analyses with closely related species gave insights into the evolutionary dynamics of bracovirus genomes. Conserved synteny in the different wasp genomes showed the orthology of the proviral macrolocus across different species. The nudiviral gene odv-e66-like1 is conserved within the macrolocus, suggesting an ancient co-localization of the nudiviral genome and bracovirus proviral segments. By contrast, the evolution of proviral segments within the macrolocus has involved a series of lineage-specific duplications.