A RhoGAP protein as a main immune suppressive factor in the Leptopilina boulardi (Hymenoptera, Figitidae)-Drosophila melanogaster interaction

Host JAK-STAT activity is a target of parasitoid wasp virulence strategies

Innate immune responses that allow hosts to survive infection depend on the action of multiple conserved signaling pathways. Pathogens and parasites in turn have evolved virulence factors to target these immune signaling pathways in an attempt to overcome host immunity. Consequently, the interactions between host immune molecules and pathogen virulence factors play an important role in determining the outcome of an infection. The immune responses of Drosophila melanogaster provide a valuable model to understand immune signaling and host-pathogen interactions. Flies are commonly infected by parasitoid wasps and mount a coordinated cellular immune response following infection. This response is characterized by the production of specialized blood cells called lamellocytes that form a tight capsule around wasp eggs in the host hemocoel. The conserved JAK-STAT signaling pathway has been implicated in lamellocyte proliferation and is required for successful encapsulation of wasp eggs. Here we show that activity of Stat92E, the D. melanogaster STAT ortholog, is induced in immune tissues following parasitoid infection. Virulent wasp species are able to suppress Stat92E activity during infection, suggesting they target JAK-STAT pathway activation as a virulence strategy. Furthermore, two wasp species (Leptopilina guineaensis and Ganaspis xanthopoda) suppress phenotypes associated with a gain-of-function mutation in hopscotch, the D. melanogaster JAK ortholog, indicating that they inhibit the activity of the core signaling components of the JAK-STAT pathway. Our data suggest that parasitoid wasp virulence factors block JAK-STAT signaling to overcome fly immune defenses.

Transcriptomic changes in the microsporidia proliferation and host responses in congenitally infected embryos and larvae

Congenital infection caused by vertical transmission of microsporidia N. bombycis can result in severe economic losses in the silkworm-rearing industry. Whole-transcriptome analyses have revealed non-coding RNAs and their regulatory networks in N. bombycis infected embryos and larvae. However, transcriptomic changes in the microsporidia proliferation and host responses in congenitally infected embryos and larvae remains unclear. Here, we simultaneously compared the transcriptomes of N. bombycis and its host B. mori embryos of 5-day and larvae of 1-, 5- and 10-day during congenital infection. For the transcriptome of N. bombycis, a comparison of parasite expression patterns between congenital-infected embryos and larva showed most genes related to parasite central carbon metabolism were down-regulated in larvae during infection, whereas the majority of genes involved in parasite proliferation and growth were up-regulated. Interestingly, a large number of distinct or shared differentially expressed genes (DEGs) were revealed by the Venn diagram and heat map, many of them were connected to infection related factors such as Ricin B lectin, spore wall protein, polar tube protein, and polysaccharide deacetylase. For the transcriptome of B. mori infected with N. bombycis, beyond numerous DEGs related to DNA replication and repair, mRNA surveillance pathway, RNA transport, protein biosynthesis, and proteolysis, with the progression of infection, a large number of DEGs related to immune and infection pathways, including phagocytosis, apoptosis, TNF, Toll-like receptor, NF-kappa B, Fc epsilon RI, and some diseases, were successively identified. In contrast, most genes associated with the insulin signaling pathway, 2-oxacarboxylic acid metabolism, amino acid biosynthesis, and lipid metabolisms were up-regulated in larvae compared to those in embryos. Furthermore, dozens of distinct and three shared DEGs that were involved in the epigenetic regulations, such as polycomb, histone-lysine-specific demethylases, and histone-lysine-N-methyltransferases, were identified via the Venn diagram and heat maps. Notably, many DEGs of host and parasite associated with lipid-related metabolisms were verified by RT-qPCR. Taken together, simultaneous transcriptomic analyses of both host and parasite genes lead to a better understanding of changes in the microsporidia proliferation and host responses in embryos and larvae in N. bombycis congenital infection.

Drosophila parasitoids go to space: Unexpected effects of spaceflight on hosts and their parasitoids

While fruit flies (Drosophila melanogaster) and humans exhibit immune system dysfunction in space, studies examining their immune systems' interactions with natural parasites in space are lacking. Drosophila parasitoid wasps modify blood cell function to suppress host immunity. In this study, naive and parasitized ground and space flies from a tumor-free control and a blood tumor-bearing mutant strain were examined. Inflammation-related genes were activated in space in both fly strains. Whereas control flies did not develop tumors, tumor burden increased in the space-returned tumor-bearing mutants. Surprisingly, control flies were more sensitive to spaceflight than mutant flies; many of their essential genes were downregulated. Parasitoids appeared more resilient than fly hosts, and spaceflight did not significantly impact wasp survival or the expression of their virulence genes. Previously undocumented mutant wasps with novel wing color and wing shape were isolated post-flight and will be invaluable for host-parasite studies on Earth.

Differential immune responses in new and old fruit fly-parasitoid associations: Implications for their management

The oriental fruit fly, Bactrocera dorsalis (Hendel), and marula fruit fly, Ceratitis cosyra (Walker), are major fruit-infesting tephritids across sub-Saharan Africa. Biological control of these pests using parasitic wasps has been widely adopted but with varying levels of success. Most studies investigating host-parasitoid models have focused on functional and evolutionary aspects leaving a knowledge gap about the physiological mechanisms underpinning the efficacy of parasitoids as biocontrol agents of tephritids. To better understand these physiological mechanisms, we investigated changes in the cellular immune responses of C. cosyra and B. dorsalis when exposed to the parasitic wasps, Diachasmimorpha longicaudata (Ashmaed) and Psyttalia cosyrae (Wilkinson). We found that B. dorsalis was more resistant to parasitisation, had a higher hemocyte count, and encapsulated more parasitoid eggs compared to C. cosyra, achieving up to 100% encapsulation when exposed to P. cosyrae. Exposing B. dorsalis to either parasitoid species induced the formation of a rare cell type, the giant multinucleated hemocyte, which was not observed in C. cosyra. Furthermore, compared to P. cosyrae-parasitized larvae, those of both host species parasitized by D. longicaudata had lower encapsulation rates, hemocyte counts and spreading abilities and yielded a higher number of parasitoid progeny with the highest parasitoid emergence (72.13%) recorded in C. cosyra. These results demonstrate that cellular immune responses are central to host-parasitoid interaction in tephritid fruit flies and further suggest that D. longicaudata presents greater potential as a biocontrol agent of B. dorsalis and C. cosyra in horticultural cropping systems.

Transcriptomic analysis provides insights into the immune responses and nutrition in Ostrinia furnacalis larvae parasitized by Macrocentrus cingulum

Macrocentrus cingulum is a principal endoparasite of Ostrinia furnacalis larvae. M. cingulum larvae repress host immune responses for survival and ingest host nutrients for development until emerging. However, most investigations focused on the mechanisms of how wasps repress the host immunity, the triggered immune responses and nutrient status altered by wasps in host are neglected. In this study, we found that parasitized O. furnacalis larvae activated fast recognition responses and produced some effectors such as lysozyme and antimicrobial peptides, along with more consumption of trehalose, glucose, and even lipid to defend against the invading M. cingulum. However, the expression of peroxidase 6 and superoxide dismutase 2 (SOD 2) was upregulated, and the messenger RNA (mRNA) levels of cellular immunity-related genes such as thioester-containing protein 2 (TEP 2) and hemocytin were also reduced, suggesting that some immune responses were selectively shut down by wasp parasitization. Taken together, all the results indicated that parasitized O. furnacalis larvae selectively activate the immune recognition response, and upregulate effector genes, but suppress ROS reaction and cellular immunity, and invest more energy to fuel certain immune responses to defend against the wasp invading. This study provides useful information for further identifying key components of the nutrition and innate immune repertoire which may shape host-parasitoid coevolutionary dynamics.

PGE2 mediates hemocyte-spreading behavior by activating aquaporin via cAMP and rearranging actin cytoskeleton via Ca2

Spreading behavior of hemocytes (= insect blood cells) is essential for cellular immune responses against various microbial pathogens. It is activated by prostaglandin E₂ (PGE₂) via its membrane receptor associated with secondary messenger, cAMP, in insects. This study observed an increase of calcium ion (Ca²⁺) level after an acute increase of cAMP induced by PGE₂ treatment and clarified the intracellular signals underlying the hemocyte-spreading behavior. Inhibition of Ca²⁺ flux significantly impaired the hemocyte-spreading and subsequent cellular immune response, phagocytosis. The up-regulation of intracellular Ca²⁺ in response to PGE₂ was dependent on cAMP because RNA interference (RNAi) of PGE₂ receptor expression or inhibiting adenylate cyclase prevented Ca²⁺ mobilization. The up-regulation of Ca²⁺ was induced by inositol triphosphate (IP₃) via its specific IP₃ receptor. Furthermore, inhibition of ryanodine receptor impaired Ca²⁺ mobilization, suggesting Ca²⁺-induced Ca²⁺ release. However, the effective spreading behavior of hemocytes was dependent on both secondary messengers. Ca²⁺ signal stimulated by cAMP was required for activating small G proteins because RNAi treatments of small G proteins such as Rac1, RhoA, and Cdc42 failed to stimulate hemocyte-spreading. In contrast, aquaporin was activated by cAMP. Its activity was necessary for changing cell volume during hemocyte-spreading. These results indicate that PGE₂ mediates hemocyte-spreading via cAMP signal to activate aquaporin and via Ca²⁺ signal to activate actin cytoskeletal rearrangement.

Amount of venom that Leptopilina species inject into Drosophila melanogaster larvae in relation to parasitic success

The Drosophila endoparasitoid wasps Leptopilina boulardi and L. heterotoma (Hymenoptera: Cynipidae) are pro-ovigenic species, i.e., females contain their lifetime number of mature eggs at emergence. They are therefore able to immediately parasitize many hosts when present. In response to parasitoid oviposition, the larval host D. melanogaster can mount an immune response, encapsulation, that can destroy the parasitoid eggs. This response is counteracted by the venom the wasp injects during oviposition. Here, we estimated the amount of venom injected into a D. melanogaster host larva using immunodetection of venom proteins and we attempted to correlate this amount with the number of eggs a female can lay on successive days. The venom reservoir of L. boulardi contains enough venom for at least 100 ovipositions while that of L. heterotoma contains venom for about 16 ovipositions. While a female L. boulardi may have enough venom for three days of parasitism when 20 or 40 larval hosts were presented each day, L. heterotoma certainly needs to synthesize new venom to parasitize the number of hosts offered. Interestingly, parasitism stopped (L. boulardi), egg protection (L. heterotoma) and egg hatching decreased (both species) after three days of parasitism. Thus, although venom does not appear to be a limiting factor for parasitism, our data suggest that it may have less effectiveness on the egg protection and on egg/host development after high repetitive egg laying.

Impact of Temperature on the Immune Interaction between a Parasitoid Wasp and Drosophila Host Species

Temperature is particularly important for ectotherms, including endoparasitoid wasps that develop inside another ectotherm host. In this study, we tested the impact of three temperatures (20 °C, 25 °C and 30 °C) on the host-parasitoid immune interaction using two Drosophila host species (Drosophila melanogaster and D. yakuba) and two parasitoid lines of Leptopilina boulardi. Drosophila's immune defense against parasitoids consists of the formation of a melanized capsule surrounding the parasitoid egg. To counteract this response, Leptopilina parasitoids rely on the injection of venom during oviposition. Here, we tested the effect of temperature on parasitic success and host encapsulation capacity in response to a parasitoid egg or other foreign body. Increased temperature either promoted or did not affect the parasitic success, depending on the parasitoid-host pairs considered. The mechanisms behind the higher success seemed to vary depending on whether the temperature primarily affected the host immune response or also affected the parasitoid counter-immune response. Next, we tested the effect of parasitoid rearing temperature on its success and venom composition. Venom composition varied strongly with temperature for both parasitoid lines, partially consistent with a change in their parasitic success. Overall, temperature may have a significant impact on the host-parasitoid immune interaction.

Comparative transcriptome analysis reveals a potential mechanism for host nutritional manipulation after parasitization by Leptopilina boulardi

Parasitoids have been extensively found to manipulate nutrient amounts of their hosts to benefit their own development and survival, but the underlying mechanisms are largely unknown. Leptopilina boulardi (Hymenoptera: Figitidae) is a larval-pupal endoparasitoid wasp of Drosophila melanogaster whose survival relies on the nutrients provided by its Drosophila host. Here, we used RNA-seq to compare the gene expression levels of the host midgut at 24 h and 48 h post L. boulardi parasitization. We obtained 95 and 191 differentially expressed genes (DEGs) in the parasitized host midgut at 24 h and 48 h post L. boulardi parasitization, respectively. A KEGG analysis revealed that several metabolic pathways were significantly enriched in the upregulated DEGs, and these pathways included "starch and sucrose metabolism" and "galactose metabolism". A functional annotation analysis showed that four classes of genes involved in carbohydrate digestion process had increased expression levels in the midgut post L.boulardi parasitization than nonparasitized groups: glucosidase, mannosidase, chitinase and amylase. Genes involved in protein digestion process were also found among the DEGs, and most of these genes, which belonged to the metallopeptidase and serine-type endopeptidase families, were found at higher expression levels in the parasitized host midgut comparing with nonparasitized hosts. Moreover, some immune genes, particularly those involved in the Toll and Imd pathways, also exhibited high expression levels after L.boulardi parasitization. Our study provides large-scale transcriptome data and identifies sets of DEGs between parasitized and nonparasitized host midgut tissues at 24 h and 48 h post L. boulardi parasitization. These resources help improve our understanding of how parasitoid infection affects the nutrient components in the hosts.

Immune Cell Production Is Targeted by Parasitoid Wasp Virulence in a Drosophila-Parasitoid Wasp Interaction

The interactions between Drosophila melanogaster and the parasitoid wasps that infect Drosophila species provide an important model for understanding host-parasite relationships. Following parasitoid infection, D. melanogaster larvae mount a response in which immune cells (hemocytes) form a capsule around the wasp egg, which then melanizes, leading to death of the parasitoid. Previous studies have found that host hemocyte load; the number of hemocytes available for the encapsulation response; and the production of lamellocytes, an infection induced hemocyte type, are major determinants of host resistance. Parasitoids have evolved various virulence mechanisms to overcome the immune response of the D. melanogaster host, including both active immune suppression by venom proteins and passive immune evasive mechanisms. We identified a previously undescribed parasitoid species, Asobara sp. AsDen, which utilizes an active virulence mechanism to infect D. melanogaster hosts. Asobara sp. AsDen infection inhibits host hemocyte expression of msn, a member of the JNK signaling pathway, which plays a role in lamellocyte production. Asobara sp. AsDen infection restricts the production of lamellocytes as assayed by hemocyte cell morphology and altered msn expression. Our findings suggest that Asobara sp. AsDen infection alters host signaling to suppress immunity.

Parasitoid wasp venom vesicles (venosomes) enter Drosophila melanogaster lamellocytes through a flotillin/lipid raft-dependent endocytic pathway

Venosomes are extracellular vesicles found in the venom of Leptopilina endoparasitoids wasps, which transport and target virulence factors to impair the parasitoid egg encapsulation by the lamellocytes of their Drosophila melanogaster host larva. Using the co-immunolocalization of fluorescent L. boulardi venosomes and one of the putative-transported virulence factors, LbGAP, with known markers of cellular endocytosis, we show that venosomes endocytosis by lamellocytes is not a process dependent on clathrin or macropinocytosis and internalization seems to bypass the early endosomal compartment Rab5. After internalization, LbGAP colocalizes strongly with flotillin-1 and the GPI-anchored protein Atilla/L1 (a lamellocyte surface marker) suggesting that entry occurs via a flotillin/lipid raft-dependent pathway. Once internalized, venosomes reach all intracellular compartments, including late and recycling endosomes, lysosomes, and the endoplasmic reticulum network. Venosomes therefore enter their target cells by a specific mechanism and the virulence factors are widely distributed in the lamellocytes' compartments to impair their functions.

A RhoGAP venom protein from Microplitis mediator suppresses the cellular response of its host Helicoverpa armigera

Female parasitoid wasps normally inject virulence factors together with eggs into their host to counter host immunity defenses. A newly identified RhoGAP protein in the venom of Microplitis mediator compromises the cellular immunity of its host, Helicoverpa armigera. RhoGAP1 proteins entered H. armigera hemocytes, and the host cellular cytoskeleton was disrupted. Depletion of MmGAP1 by injection of dsRNA or antibody increased the wasp egg encapsulation rate. An immunoprecipitation assay of overexpressed MmGAP1 protein in a Helicoverpa cell line showed that MmGAP1 interacts with many cellular cytoskeleton associated proteins as well as Rho GTPases. A yeast two-hybrid and a pull-down assay demonstrated that MmGAP1 interacts with H. armigera RhoA and Cdc42. These results show that the RhoGAP protein in M. mediator can destroy the H. armigera hemocyte cellular cytoskeleton, restrain host cellular immune defense, and increase the probability of successful parasitism.

Identification and Comparative Analysis of Venom Proteins in a Pupal Ectoparasitoid, Pachycrepoideus vindemmiae

Parasitoid wasps inject venom containing complex bioactive compounds to regulate the immune response and development of host arthropods and sometime paralyze host arthropods. Although extensive studies have been conducted on the identification of venom proteins in larval parasitoids, relatively few studies have examined the pupal parasitoids. In our current study, a combination of transcriptomic and proteomic methods was used to identify 64 putative venom proteins from Pachycrepoideus vindemmiae, an ectoparasitoid of Drosophila. Expression analysis revealed that 20 tested venom proteins have 419-fold higher mean expression in the venom apparatus than in other wasp tissues, indicating their specialization to venom. Comparisons of venom proteins from P. vindemmiae and other five species spanning three parasitoid families detected a core set of "ancient" orthologs in Pteromalidae. Thirty-five venom proteins of P. vindemmiae were assigned to the orthologous groups by reciprocal best matches with venoms of other pteromalids, while the remaining 29 were not. Of the 35 categories, twenty-seven have orthologous relationships with Nasonia vitripennis venom proteins and 25 with venoms of Pteromalus puparum. More distant relationships detected that five and two venom proteins of P. vindemmiae are orthologous with venoms of two Figitidae parasitoids and a Braconidae representative, respectively. Moreover, twenty-two venoms unique to P. vindemmiae were also detected, indicating considerable interspecific variation of venom proteins in parasitoids. Phylogenetic reconstruction based on a set of single-copy genes clustered P. vindemmiae with P. puparum, N. vitripennis, and other members of the family Pteromalidae. These findings provide strong evidence that P. vindemmiae venom proteins are well positioned for future functional and evolutionary studies.

Immune Suppressive Extracellular Vesicle Proteins of Leptopilina heterotoma Are Encoded in the Wasp Genome

Leptopilina heterotoma are obligate parasitoid wasps that develop in the body of their Drosophila hosts. During oviposition, female wasps introduce venom into the larval hosts' body cavity. The venom contains discrete, 300 nm-wide, mixed-strategy extracellular vesicles (MSEVs), until recently referred to as virus-like particles. While the crucial immune suppressive functions of L. heterotoma MSEVs have remained undisputed, their biotic nature and origin still remain controversial. In recent proteomics analyses of L. heterotoma MSEVs, we identified 161 proteins in three classes: conserved eukaryotic proteins, infection and immunity related proteins, and proteins without clear annotation. Here we report 246 additional proteins from the L. heterotoma MSEV proteome. An enrichment analysis of the entire proteome supports vesicular nature of these structures. Sequences for more than 90% of these proteins are present in the whole-body transcriptome. Sequencing and de novo assembly of the 460 Mb-sized L. heterotoma genome revealed 90% of MSEV proteins have coding regions within the genomic scaffolds. Altogether, these results explain the stable association of MSEVs with their wasps, and like other wasp structures, their vertical inheritance. While our results do not rule out a viral origin of MSEVs, they suggest that a similar strategy for co-opting cellular machinery for immune suppression may be shared by other wasps to gain advantage over their hosts. These results are relevant to our understanding of the evolution of figitid and related wasp species.

Proteo-Transcriptomic Characterization of the Venom from the Endoparasitoid Wasp Pimpla turionellae with Aspects on Its Biology and Evolution

Within mega-diverse Hymenoptera, non-aculeate parasitic wasps represent 75% of all hymenopteran species. Their ovipositor dual-functionally injects venom and employs eggs into (endoparasitoids) or onto (ectoparasitoids) diverse host species. Few endoparasitoid wasps such as Pimpla turionellae paralyze the host and suppress its immune responses, such as encapsulation and melanization, to guarantee their offspring's survival. Here, the venom and its possible biology and function of P. turionellae are characterized in comparison to the few existing proteo-transcriptomic analyses on parasitoid wasp venoms. Multiple transcriptome assembly and custom-tailored search and annotation strategies were applied to identify parasitoid venom proteins. To avoid false-positive hits, only transcripts were finally discussed that survived strict filter settings, including the presence in the proteome and higher expression in the venom gland. P. turionella features a venom that is mostly composed of known, typical parasitoid enzymes, cysteine-rich peptides, and other proteins and peptides. Several venom proteins were identified and named, such as pimplin2, 3, and 4. However, the specification of many novel candidates remains difficult, and annotations ambiguous. Interestingly, we do not find pimplin, a paralytic factor in Pimpla hypochondriaca, but instead a new cysteine inhibitor knot (ICK) family (pimplin2), which is highly similar to known, neurotoxic asilid1 sequences from robber flies.

Rapid and Differential Evolution of the Venom Composition of a Parasitoid Wasp Depending on the Host Strain

Parasitoid wasps rely primarily on venom to suppress the immune response and regulate the physiology of their host. Intraspecific variability of venom protein composition has been documented in some species, but its evolutionary potential is poorly understood. We performed an experimental evolution initiated with the crosses of two lines of Leptopilinaboulardi of different venom composition to generate variability and create new combinations of venom factors. The offspring were maintained for 10 generations on two strains of Drosophila melanogaster differing in resistance/susceptibility to the parental parasitoid lines. The venom composition of individuals was characterized by a semi-automatic analysis of 1D SDS-PAGE electrophoresis protein profiles whose accuracy was checked by Western blot analysis of well-characterized venom proteins. Results made evident a rapid and differential evolution of the venom composition on both hosts and showed that the proteins beneficial on one host can be costly on the other. Overall, we demonstrated the capacity of rapid evolution of the venom composition in parasitoid wasps, important regulators of arthropod populations, suggesting a potential for adaptation to new hosts. Our approach also proved relevant in identifying, among the diversity of venom proteins, those possibly involved in parasitism success and whose role deserves to be deepened.

The Pupal Ectoparasitoid Pachycrepoideus vindemmiae Regulates Cellular and Humoral Immunity of Host Drosophila melanogaster

The immunological interaction between Drosophila melanogaster and its larval parasitoids has been thoroughly investigated, however, little is known about the interaction between the host and its pupal parasitoids. Pachycrepoideus vindemmiae, a pupal ectoparasitoid of D. melanogaster, injects venom into its host while laying eggs on the puparium, which regulates host immunity and interrupts host development. To resist the invasion of parasitic wasps, various immune defense strategies have been developed in their hosts as a consequence of co-evolution. In this study, we mainly focused on the host immunomodulation by P. vindemmiae and thoroughly investigated cellular and humoral immune response, including cell adherence, cell viability, hemolymph melanization and the Toll, Imd, and JAK/STAT immune pathways. Our results indicated that venom had a significant inhibitory effect on lamellocyte adherence and induced plasmatocyte cell death. Venom injection and in vitro incubation strongly inhibited hemolymph melanization. More in-depth investigation revealed that the Toll and Imd immune pathways were immediately activated upon parasitization, followed by the JAK/STAT pathway, which was activated within the first 24 h post-parasitism. These regulatory effects were further validated by qPCR. Our present study manifested that P. vindemmiae regulated the cellular and humoral immune system of host D. melanogaster in many aspects. These findings lay the groundwork for studying the immunological interaction between D. melanogaster and its pupal parasitoid.

Venom Atypical Extracellular Vesicles as Interspecies Vehicles of Virulence Factors Involved in Host Specificity: The Case of a Drosophila Parasitoid Wasp

Endoparasitoid wasps, which lay eggs inside the bodies of other insects, use various strategies to protect their offspring from the host immune response. The hymenopteran species of the genus Leptopilina, parasites of Drosophila, rely on the injection of a venom which contains proteins and peculiar vesicles (hereafter venosomes). We show here that the injection of purified L. boulardi venosomes is sufficient to impair the function of the Drosophila melanogaster lamellocytes, a hemocyte type specialized in the defense against wasp eggs, and thus the parasitic success of the wasp. These venosomes seem to have a unique extracellular biogenesis in the wasp venom apparatus where they acquire specific secreted proteins/virulence factors and act as a transport system to deliver these compounds into host lamellocytes. The level of venosomes entry into lamellocytes of different Drosophila species was correlated with the rate of parasitism success of the wasp, suggesting that this venosome-cell interaction may represent a new evolutionary level of host-parasitoid specificity.

Bioinformatic analysis suggests potential mechanisms underlying parasitoid venom evolution and function

Hymenopteran parasitoid wasps are a diverse collection of species that infect arthropod hosts and use factors found in their venoms to manipulate host immune responses, physiology, and behaviour. Whole parasitoid venoms have been profiled using proteomic approaches, and here we present a bioinformatic characterization of the venom protein content from Ganaspis sp. 1, a parasitoid that infects flies of the genus Drosophila. We find evidence that diverse evolutionary processes including multifunctionalization, co-option, gene duplication, and horizontal gene transfer may be acting in concert to drive venom gene evolution in Ganaspis sp.1. One major role of parasitoid wasp venom is host immune evasion. We previously demonstrated that Ganaspis sp. 1 venom inhibits immune cell activation in infected Drosophila melanogaster hosts, and our current analysis has uncovered additional predicted virulence functions. Overall, this analysis represents an important step towards understanding the composition and activity of parasitoid wasp venoms.

Drosophila Cellular Immunity Against Parasitoid Wasps: A Complex and Time-Dependent Process

Host-parasitoid interactions are among the most studied interactions between invertebrates because of their fundamental interest - the evolution of original traits in parasitoids - and applied, parasitoids being widely used in biological control. Immunity, and in particular cellular immunity, is central in these interactions, the host encapsulation response being specific for large foreign bodies such as parasitoid eggs. Although already well studied in this species, recent data on Drosophila melanogaster have unquestionably improved knowledge of invertebrate cellular immunity. At the same time, the venomics of parasitoids has expanded, notably those of Drosophila. Here, we summarize and discuss these advances, with a focus on an emerging "time-dependent" view of interactions outcome at the intra- and interspecific level. We also present issues still in debate and prospects for study. Data on the Drosophila-parasitoid model paves the way to new concepts in insect immunity as well as parasitoid wasp strategies to overcome it.

Analysis of the Gynaephora qinghaiensis pupae immune transcriptome in response to parasitization by Thektogaster sp

As a pest on the Qinghai-Tibet Plateau, Gynaephora qinghaiensis causes severe damage to grassland vegetation and its pupae are also natural hosts of Thektogaster sp. To successfully parasitize, endoparasitoids generally introduce or secrete multiple parasitic factors into the host body during the spawning stage to suppress the host immune response. To study the parasitic effects of Thektogaster sp. on G. qinghaiensis, a transcriptome analysis of immune-related genes in parasitized and nonparasitized G. qinghaiensis pupae was performed. A total of 371,260,704 clean reads were assembled into 118,144 unigenes with an average length of 884.33 base pairs. Of these, 23,660 unigenes were annotated in at least one database and 94,484 unigenes were not annotated in any databases. These findings indicated that the majority of the genetic resources (79.97% of all unigenes) in Gynaephora should be further explored. Parasitization significantly affected the transcriptional profile of G. qinghaiensis pupae. The present study identified 12,322 differentially expressed genes and 57 immune-related genes were identified in parasitized G. qinghaiensis pupae. Most immune-related genes were downregulated, potentially resulting from the inhibitory effect of Thektogaster sp. on G. qinghaiensis pupae after parasitization. Overall, the transcriptome analysis sheds valuable light on the molecular mechanisms of G. qinghaiensis parasitization by Thektogaster sp. and promotes the development of novel biocontrol strategies for Gynaephora based on immune defense.

Drosophila as a Genetic Model for Hematopoiesis

In this FlyBook chapter, we present a survey of the current literature on the development of the hematopoietic system in Drosophila The Drosophila blood system consists entirely of cells that function in innate immunity, tissue integrity, wound healing, and various forms of stress response, and are therefore functionally similar to myeloid cells in mammals. The primary cell types are specialized for phagocytic, melanization, and encapsulation functions. As in mammalian systems, multiple sites of hematopoiesis are evident in Drosophila and the mechanisms involved in this process employ many of the same molecular strategies that exemplify blood development in humans. Drosophila blood progenitors respond to internal and external stress by coopting developmental pathways that involve both local and systemic signals. An important goal of these Drosophila studies is to develop the tools and mechanisms critical to further our understanding of human hematopoiesis during homeostasis and dysfunction.

Insights into the venom protein components of Microplitis mediator, an endoparasitoid wasp

Endoparasitoid wasps deliver a variety of maternal factors, such as venom proteins, viruses, and virus-like particles, from their venom and calyx fluid into hosts and thereby regulate the hosts' immune response, metabolism and development. The endoparasitoid, Microplitis mediator, is used as an important biological agent for controlling the devastating pest Helicoverpa armigera. In this study, using an integrated transcriptomic and proteomic analysis approach, we identified 75 putative venom proteins in M. mediator. The identified venom components were consistent with other known parasitoid wasps' venom proteins, including metalloproteases, serine protease inhibitors, and glycoside hydrolase family 18 enzymes. The metalloprotease and serpin family showed extensive gene duplications in venom apparatus. Isobaric tags for relative and absolute quantitation (iTRAQ) based quantitative proteomics revealed 521 proteins that were differentially expressed at 6 h and 24 h post-parasitism, including 10 wasp venom proteins that were released into the host hemolymph. Further analysis indicated that 511 differentially expressed proteins (DEP) from the host are primarily involved in the immune response, material metabolism, and extracellular matrix receptor interaction. Taken together, our results on parasitoid wasp venoms have the potential to enhance the application of endoparasitoid wasps for controlling insect pest.

The Domestication of a Large DNA Virus by the Wasp Venturia canescens Involves Targeted Genome Reduction through Pseudogenization

Polydnaviruses (PDVs) are compelling examples of viral domestication, in which wasps express a large set of genes originating from a chromosomally integrated virus to produce particles necessary for their reproductive success. Parasitoid wasps generally use PDVs as a virulence gene delivery system allowing the protection of their progeny in the body of parasitized host. However, in the wasp Venturia canescens an independent viral domestication process led to an alternative strategy as the wasp incorporates virulence proteins in viral liposomes named virus-like particles (VLPs), instead of DNA molecules. Proteomic analysis of purified VLPs and transcriptome sequencing revealed the loss of some viral functions. In particular, the genes coding for capsid components are no longer expressed, which explains why VLPs do not incorporate DNA. Here a thorough examination of V. canescens genome revealed the presence of the pseudogenes corresponding to most of the genes involved in lost functions. This strongly suggests that an accumulation of mutations that leads to gene specific pseudogenization precedes the loss of viral genes observed during virus domestication. No evidence was found for block loss of collinear genes, although extensive gene order reshuffling of the viral genome was identified from comparisons between endogenous and exogenous viruses. These results provide the first insights on the early stages of large DNA virus domestication implicating massive genome reduction through gene-specific pseudogenization, a process which differs from the large deletions described for bacterial endosymbionts.

First extensive characterization of the venom gland from an egg parasitoid: structure, transcriptome and functional role

The venom gland is a ubiquitous organ in Hymenoptera. In insect parasitoids, the venom gland has been shown to have multiple functions including regulation of host immune response, host paralysis, host castration and developmental alteration. However, the role played by the venom gland has been mainly studied in parasitoids developing in larval or pupal hosts while little is known for parasitoids developing in insect eggs. We conducted the first extensive characterization of the venom of the endoparasitoid Ooencyrtus telenomicida (Vassiliev), a species that develops in eggs of the stink bug Nezara viridula (L.). In particular we investigated the structure of the venom apparatus, its functional role and conducted a transcriptomic analysis of the venom gland. We found that injection of O. telenomicida venom induces: 1) a melanized-like process in N. viridula host eggs (host-parasitoid interaction), 2) impairment of the larval development of the competitor Trissolcus basalis (Wollaston) (parasitoid-parasitoid interaction). The O. telenomicida venom gland transcriptome reveals a majority of digestive enzymes (peptidases and glycosylases) and oxidoreductases (laccases) among the most expressed genes. The former enzymes are likely to be involved in degradation of the host resources for the specific benefit of the O. telenomicida offspring. In turn, alteration of host resources caused by these enzymes may negatively affect the larval development of the competitor T. basalis. We hypothesize that the melanization process induced by venom injection could be related to the presence of laccases, which are multicopper oxidases that belong to the phenoloxidases group. This work contributed to a better understanding of the venom in insect parasitoids and allowed to identify candidate genes whose functional role can be investigated in future studies.

Induction of stress- and immune-associated genes in the Indian meal moth Plodia interpunctella against envenomation by the ectoparasitoid Bracon hebetor

Envenomation is an important process in parasitism by parasitic wasps; it suppresses the immune and development of host insects. However, the molecular mechanisms of host responses to envenomation are not yet clear. This study aimed to determine the transcription-level responses of the Indian meal moth Plodia interpunctella against envenomation of the ectoparasitoid Bracon hebetor. Quantitative real-time reverse-transcription PCR was used to determine the transcriptional changes of 13 selected genes, which are associated with development, metabolism, stress, or immunity, in the feeding and wandering fifth instar larvae over a 4-day period after envenomation. The effects of envenomation on the feeding-stage larvae were compared with those of starvation in the transcriptional levels of the 13 genes. Most selected genes were altered in their expression by either envenomation or starvation. In particular, a heat shock protein, hsp70, was highly upregulated in envenomated larvae in both the feeding and wandering stages as well as in starved larvae. Further, some genes were upregulated by envenomation in a stage-specific manner. For example, hsp25 was upregulated after envenomation in the feeding larvae, but hsp90 and an immune-associated gene, hemolin, were upregulated in the wandering larvae. However, both envenomation and starvation resulted in the downregulation of genes associated with development and metabolism. Taken together, P. interpunctella upregulated stress- and immune-responsive genes, but downregulated genes associated with development and metabolism after envenomation. This study provides important information for understanding the molecular mechanisms of host responses to parasitism.

Comparative transcriptome analysis of venom glands from Cotesia vestalis and Diadromus collaris, two endoparasitoids of the host Plutella xylostella

Venoms secreted by the venom gland (VG) of parasitoid wasp help ensure successful parasitism by host immune suppression and developmental regulation. Cotesia vestalis, a larval endoparasitoid, and Diadromus collaris, a pupal endoparasitoid, parasitize the diamondback moth (DBM), Plutella xylostella. To explore and compare the venom components of two endoparasitoids, we sequenced transcriptomes of the VGs and wasp bodies without VGs (BWVGs) of the two endoparasitoids. Statistically enriched GO terms and KEGG pathways of the two VGs compared to respective whole-body background were similar and reflected active protein biosynthesis activities in the two VGs. 1,595 VG specific genes of the D. collaris VG and 1,461 VG specific genes of the C. vestalis VG were identified by comparative transcript profiling. A total of 444 and 513 genes encoding potential secretory proteins were identified and defined as putative venom genes in D. collaris VG and C. vestalis VG, respectively. The putative venom genes of the two wasps showed no significant similarity or convergence. More venom genes were predicted in D. collaris VG than C. vestalis VG, especially hydrolase-coding genes. Differences in the types and quantities of putative venom genes shed light on different venom functions.

Protein Discovery: Combined Transcriptomic and Proteomic Analyses of Venom from the Endoparasitoid Cotesia chilonis (Hymenoptera: Braconidae)

Many species of endoparasitoid wasps provide biological control services in agroecosystems. Although there is a great deal of information on the ecology and physiology of host/parasitoid interactions, relatively little is known about the protein composition of venom and how specific venom proteins influence physiological systems within host insects. This is a crucial gap in our knowledge because venom proteins act in modulating host physiology in ways that favor parasitoid development. Here, we identified 37 possible venom proteins from the polydnavirus-carrying endoparasitoid Cotesia chilonis by combining transcriptomic and proteomic analyses. The most abundant proteins were hydrolases, such as proteases, peptidases, esterases, glycosyl hydrolase, and endonucleases. Some components are classical parasitoid venom proteins with known functions, including extracellular superoxide dismutase 3, serine protease inhibitor and calreticulin. The venom contains novel proteins, not recorded from any other parasitoid species, including tolloid-like proteins, chitooligosaccharidolytic β-N-acetylglucosaminidase, FK506-binding protein 14, corticotropin-releasing factor-binding protein and vascular endothelial growth factor receptor 2. These new data generate hypotheses and provide a platform for functional analysis of venom components.

Transdifferentiation and Proliferation in Two Distinct Hemocyte Lineages in Drosophila melanogaster Larvae after Wasp Infection

Cellular immune responses require the generation and recruitment of diverse blood cell types that recognize and kill pathogens. In Drosophila melanogaster larvae, immune-inducible lamellocytes participate in recognizing and killing parasitoid wasp eggs. However, the sequence of events required for lamellocyte generation remains controversial. To study the cellular immune system, we developed a flow cytometry approach using in vivo reporters for lamellocytes as well as for plasmatocytes, the main hemocyte type in healthy larvae. We found that two different blood cell lineages, the plasmatocyte and lamellocyte lineages, contribute to the generation of lamellocytes in a demand-adapted hematopoietic process. Plasmatocytes transdifferentiate into lamellocyte-like cells in situ directly on the wasp egg. In parallel, a novel population of infection-induced cells, which we named lamelloblasts, appears in the circulation. Lamelloblasts proliferate vigorously and develop into the major class of circulating lamellocytes. Our data indicate that lamellocyte differentiation upon wasp parasitism is a plastic and dynamic process. Flow cytometry with in vivo hemocyte reporters can be used to study this phenomenon in detail.

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.

Effects of the endoparasitoid Cotesia chilonis (Hymenoptera: Braconidae) parasitism, venom, and calyx fluid on cellular and humoral immunity of its host Chilo suppressalis (Lepidoptera: Crambidae) larvae

The larval endoparasitoid Cotesia chilonis injects venom and bracoviruses into its host Chilo suppressalis during oviposition. Here we study the effects of the polydnavirus (PDV)-carrying endoparasitoid C. chilonis (Hymenoptera: Braconidae) parasitism, venom and calyx fluid on host cellular and humoral immunity, specifically hemocyte composition, cellular spreading, encapsulation and melanization. Total hemocyte counts (THCs) were higher in parasitized larvae than in unparasitized larvae in the late stages following parasitization. While both plasmatocyte and granulocyte fractions and hemocyte mortality did not differ between parasitized and unparasitized hosts, in vitro spreading behavior of hemocytes was inhibited significantly by parasitism throughout the course of parasitoid development. C. chilonis parasitism suppressed the encapsulation response and melanization in the early stages. Venom alone did not alter cellular immune responses, including effects on THCs, mortality, hemocyte composition, cell spreading and encapsulation, but venom did inhibit humoral immunity by reducing melanization within 6h after injection. In contrast to venom, calyx fluid had a significant effect on cell spreading, encapsulation and melanization from 6h after injection. Dose-response injection studies indicated the effects of venom and calyx fluid synergized, showing a stronger and more persistent reduction in immune system responses than the effect of either injected alone.

Insights into the venom composition and evolution of an endoparasitoid wasp by combining proteomic and transcriptomic analyses

Parasitoid wasps are abundant and diverse hymenopteran insects that lay their eggs into the internal body (endoparasitoid) or on the external surface (ectoparasitoid) of their hosts. To make a more conducive environment for the wasps’ young, both ecto- and endoparasitoids inject venoms into the host to modulate host immunity, metabolism and development. Endoparasitoids have evolved from ectoparasitoids independently in different hymenopteran lineages. Pteromalus puparum, a pupal endoparasitoid of various butterflies, represents a relatively recent evolution of endoparasitism within pteromalids. Using a combination of transcriptomic and proteomic approaches, we have identified 70 putative venom proteins in P. puparum. Most of them show higher similarity to venom proteins from the related ectoparasitoid Nasonia vitripennis than from other more distantly related endoparasitoids. In addition, 13 venom proteins are similar to venoms of distantly related endoparasitoids but have no detectable venom matches in Nasonia. These venom proteins may have a role in adaptation to endoparasitism. Overall, these results lay the groundwork for more detailed studies of venom function and adaptation to the endoparasitic lifestyle.

Venom Proteins from Parasitoid Wasps and Their Biological Functions

Parasitoid wasps are valuable biological control agents that suppress their host populations. Factors introduced by the female wasp at parasitization play significant roles in facilitating successful development of the parasitoid larva either inside (endoparasitoid) or outside (ectoparasitoid) the host. Wasp venoms consist of a complex cocktail of proteinacious and non-proteinacious components that may offer agrichemicals as well as pharmaceutical components to improve pest management or health related disorders. Undesirably, the constituents of only a small number of wasp venoms are known. In this article, we review the latest research on venom from parasitoid wasps with an emphasis on their biological function, applications and new approaches used in venom studies.

Transcriptome of a specialized extra-embryonic cell, teratocyte, and its host immunosuppressive role revealed by ex vivo RNA interference

The specialized wasp cells teratocytes (TCs) are derived from the embryonic serosal membrane of some parasitic hymenopteran insects. As a parasitic factor, TCs are multifunctional in host regulation, such as host nutritional deprivation, immunosuppression and developmental arrest; however, little is understood about their genetic constituents. The present study provides a comprehensive view of the genes expressed by TCs through a transcriptome analysis based on RNA sequencing technology. The assembled 34 686 contigs (>200 base pairs) were annotated into different functional categories, indicating a distinct distribution in gene transcripts compared with those of haemocytes and fat body. The TC transcriptome contained components of insulin signalling and biosyntheses of juvenile hormone and 20-hydroxyecdysone. TCs also expressed various groups of digestive enzymes, indicating that they have nutritional role for the growing parasitoid larvae in parasitism. Furthermore, through this transcriptome analysis two kinds of immunosuppressive serine protease inhibitors (serpins) and Rho GTPase-activating proteins (RhoGAPs) were annotated. To determine the biological functions of these factors, we devised ex vivo RNA interference (RNAi) by conducting knockdown of gene expression in in vitro-cultured TCs followed by injection of the treated TCs to test insects. Ex vivo RNAi revealed that some serpins and RhoGAPs expressed in TCs inhibited host cellular immunity. This study reports a transcriptome of the unique TC animal cell and its immunosuppressive genetic factors using ex vivo RNAi technology.

Harnessing the natural Drosophila-parasitoid model for integrating insect immunity with functional venomics

Drosophila species lack most hallmarks of adaptive immunity yet are highly successful against an array of natural microbial pathogens and metazoan enemies. When attacked by figitid parasitoid wasps, fruit flies deploy robust, multi-faceted innate immune responses and overcome many attackers. In turn, parasitoids have evolved immunosuppressive strategies to match, and more frequently to overcome, their hosts. We present methods to examine the evolutionary dynamics underlying anti-parasitoid host defense by teasing apart the specialized immune-modulating venoms of figitid parasitoids and, in turn, possibly delineating the roles of individual venom molecules. This combination of genetic, phylogenomic, and "functional venomics" methods in the Drosophila-parasitoid model should allow entomologists and immunologists to tackle important outstanding questions with implications across disciplines and to pioneer translational applications in agriculture and medicine.

Insights into function and evolution of parasitoid wasp venoms

Most species in the order Hymenoptera are parasitoids that lay eggs and develop in or on the body of arthropod hosts. Several factors contribute to successful parasitism including venoms that wasps inject into hosts when ovipositing. Here, we review the composition, function and diversity of parasitoid venoms with emphasis on studies of wasps that parasitize hosts in the genus Drosophila. The comparative literature indicates that some closely related species parasitizing the same host do not share any abundant venom protein while unrelated species sometimes have the same major venom component. Within species, studies also identify intraspecific variation that suggests parasitoid venoms may rapidly evolve. Overall, however, our picture of venom function remains largely unclear and will require additional comparative data on the composition of venoms from a greater diversity of species than exists currently. Further advances will come mainly from experimental data using functional tools, such as RNA interference.

Identification of the main venom protein components of Aphidius ervi, a parasitoid wasp of the aphid model Acyrthosiphon pisum

Background

Endoparasitoid wasps are important natural enemies of the widely distributed aphid pests and are mainly used as biological control agents. However, despite the increased interest on aphid interaction networks, only sparse information is available on the factors used by parasitoids to modulate the aphid physiology. Our aim was here to identify the major protein components of the venom injected at oviposition by Aphidius ervi to ensure successful development in its aphid host, Acyrthosiphon pisum.

Results

A combined large-scale transcriptomic and proteomic approach allowed us to identify 16 putative venom proteins among which three γ-glutamyl transpeptidases (γ-GTs) were by far the most abundant. Two of the γ-GTs most likely correspond to alleles of the same gene, with one of these alleles previously described as involved in host castration. The third γ-GT was only distantly related to the others and may not be functional owing to the presence of mutations in the active site. Among the other abundant proteins in the venom, several were unique to A. ervi such as the molecular chaperone endoplasmin possibly involved in protecting proteins during their secretion and transport in the host. Abundant transcripts encoding three secreted cystein-rich toxin-like peptides whose function remains to be explored were also identified.

Conclusions

Our data further support the role of γ-GTs as key players in A. ervi success on aphid hosts. However, they also evidence that this wasp venom is a complex fluid that contains diverse, more or less specific, protein components. Their characterization will undoubtedly help deciphering parasitoid-aphid and parasitoid-aphid-symbiont interactions. Finally, this study also shed light on the quick evolution of venom components through processes such as duplication and convergent recruitment of virulence factors between unrelated organisms.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-342) contains supplementary material, which is available to authorized users.

Development of RNAi in a Drosophila endoparasitoid wasp and demonstration of its efficiency in impairing venom protein production

Endoparasitoid wasps are essential regulators of insect pests in ecosystems as well as important biological control auxiliaries. Traits important for parasitism success, such as the injection of venom proteins at oviposition, have thus been mainly studied. However, identification of the key genes involved among the large number of genes identified was still prevented by the lack of functional approaches. Here, we report the development of RNA interference (RNAi) in Leptopilina boulardi, a figitid endoparasitoid that performs its entire development inside the Drosophila host. Having set up conditions for in vitro development of parasitoid late larval stages or pupae, we first targeted the cinnabar gene by microinjecting double-stranded RNA (dsRNA), leading to its silencing and production of red-eyed individuals. We then demonstrated that expression of the gene encoding LbGAP, a virulence factor found in a high amount in L. boulardi venom, could be specifically and almost completely silenced. Finally, a time-course analysis revealed that LbGAP silencing lasted during the entire lifetime of L. boulardi. This is the first report of the efficient silencing of venom protein-encoding genes in parasitoid wasps. Overall, RNAi opens the way for a large-scale functional analysis of parasitoid venom factors as well as other traits involved in parasitism success and more largely in the biology of these ecologically important organisms.

The cell-mediated immunity of Drosophila melanogaster: hemocyte lineages, immune compartments, microanatomy and regulation

In the animal kingdom, innate immunity is the first line of defense against invading pathogens. The dangers of microbial and parasitic attacks are countered by similar mechanisms, involving the prototypes of the cell-mediated immune responses, the phagocytosis and encapsulation. Work on Drosophila has played an important role in promoting an understanding of the basic mechanisms of phylogenetically conserved modules of innate immunity. The aim of this review is to survey the developments in the identification and functional definition of immune cell types and the immunological compartments of Drosophila melanogaster. We focus on the molecular and developmental aspects of the blood cell types and compartments, as well as the dynamics of blood cell development and the immune response. Further advances in the characterization of the innate immune mechanisms in Drosophila will provide basic clues to the understanding of the importance of the evolutionary conserved mechanisms of innate immune defenses in the animal kingdom.

Insights from natural host-parasite interactions: the Drosophila model

Immune responses against opportunistic pathogens have been extensively studied in Drosophila, leading to a detailed map of the genetics behind innate immunity networks including the Toll, Imd, Jak-Stat, and JNK pathways. However, immune mechanisms of other organisms, such as plants, have primarily been investigated using natural pathogens. It was the use of natural pathogens in plant research that revealed the plant R-Avr system, a specialized immune response derived from antagonistic coevolution between plant immune proteins and their natural pathogens' virulence proteins. Thus, we recommend that researchers begin to use natural Drosophila pathogens to identify novel immune strategies that may have arisen through antagonistic coevolution with common natural pathogens. In this review, we address the benefits of using natural pathogens in research, describe the known natural pathogens of Drosophila, and discuss the future prospects for research on natural pathogens of Drosophila.

Parasitoid wasp virulence: A window into fly immunity

In nature, larvae of the fruit fly Drosophila melanogaster are commonly infected by parasitoid wasps. Following infection, flies mount an immune response termed cellular encapsulation in which fly immune cells form a multilayered capsule that covers and kills the wasp egg. Parasitoids have thus evolved virulence factors to suppress cellular encapsulation. To uncover the molecular mechanisms underlying the antiwasp response, we and others have begun identifying and functionally characterizing these virulence factors. Our recent work on the Drosophila parasitoid Ganaspis sp.1 has demonstrated that a virulence factor encoding a SERCA-type calcium pump plays an important role in Ganaspis sp.1 virulence. This venom SERCA antagonizes fly immune cell calcium signaling and thereby prevents the activation of the encapsulation response. In this way, the study of wasp virulence factors has revealed a novel aspect of fly immunity, namely a role for calcium signaling in fly immune cell activation, which is conserved with human immunity, again illustrating the marked conservation between fly and mammalian immune responses. Our findings demonstrate that the cellular encapsulation response can serve as a model of immune cell function and can also provide valuable insight into basic cell biological processes.

Inhibition of host cell encapsulation through inhibiting immune gene expression by the parasitic wasp venom calreticulin

Parasitoid wasps inject venom into the host to protect their offspring against host immune responses. In our previous study, we identified a calreticulin (CRT) in Pteromalus puparum venom. In this study, we expressed the wild-type and the coiled-coil domain deletion mutant P. puparum calreticulins (PpCRTs) in Escherichia coli and prepared polyclonal antibody in rabbit against PpCRT. Western blot analysis showed that PpCRT protein was not only present in the venom but also in all the tissues tested. Real time PCR results indicated that PpCRT mRNA was highly expressed in the venom gland. The transcript level of PpCRT in the venom gland was peaked at 2 days post-eclosion, while the PpCRT protein in the venom was maintained at a constant level. Both recombinant wild-type and mutant PpCRT proteins could bind to the surface of P. puparum eggs. Recombinant PpCRT inhibited hemocyte spreading and cellular encapsulation of the host Pieris rapae in vitro, and the coiled-coil domain is important for the inhibitory function of PpCRT. Immunocytochemistry results showed that PpCRT entered P. rapae hemocytes, and the coiled-coil domain played a role in this process. After injection of recombinant PpCRT into P. rapae pupae, real time PCR results showed that PpCRT inhibited transcript levels of host encapsulation-related genes, including calreticulin and scavenger receptor genes. In conclusion, our results suggest that P. puparum venom protects its offspring against host cellular immune responses via its functional component PpCRT to inhibit the expression of host cellular response-related genes.

Extensive inter- and intraspecific venom variation in closely related parasites targeting the same host: the case of Leptopilina parasitoids of Drosophila

The arms race between immune suppressive parasites that produce virulence factors and hosts that evolve resistance to these factors is suggested to be a key driver for the diversification of both partners. However, little is known regarding the diversity of virulence factors in closely related parasites or the mechanisms underlying the variation of virulence. One of the best-described model to address this issue is the interaction between Leptopilina parasitic wasps and their Drosophila hosts, in which variation of virulence is well documented. Thanks to a combined transcriptomic and proteomic approach, we have identified the main secreted proteins in the venom of Leptopilina heterotoma (Gotheron strain, 66 proteins) and of two well-characterized strains of Leptopilina boulardi, ISm and ISy (65 and 49 proteins, respectively). Results revealed significant quantitative differences in venom components between the L. boulardi strains, in agreement with their different virulence properties. Strikingly, the two related Leptopilina species did not share any abundant venom protein. The main identified proteins in L. boulardi were RhoGAPs and serpins while an aspartylglucosaminidase (AGA) was found abundant in L. heterotoma. The extensive quantitative variation observed between these species may be related with their use of different virulence strategies and/or to differences in their host range (specialist versus generalist). Altogether, our data suggests that parasitoid venom can quickly evolve, mainly through rapid changes in regulation of gene expression. It also evidences venom evolutionary processes largely described in other venomous animals i.e. the convergent recruitment of venom proteins between phylogenetically unrelated organisms, and the role of duplications in the emergence of multigenic families of virulence factors.

Rac1 mediates cytokine-stimulated hemocyte spreading via prostaglandin biosynthesis in the beet armyworm, Spodoptera exigua

Cell spreading is an integral component of insect hemocytic immune reactions to infections and invasions. Cell spreading is accomplished by cytoskeleton rearrangement, which is activated by three major immune mediators, biogenic monoamines, plasmatocyte-spreading peptide (PSP), and eicosanoids, particularly prostaglandin E2 (PGE2). However, little is known about how these immune mediators activate hemocyte spreading at the intra-cellular level. A small G protein, Rac1, acts in cytoskeleton arrangements in mammalian cells. Based on this information, we identified a Rac1 transcript (SeRac1) in hemocytes prepared from Spodoptera exigua. SeRac1 was expressed in most developmental stages and in the two main immunity-conferring tissues, hemocytes and fat body, in larvae. In response to bacterial challenge, its expression was up-regulated by >37-fold at 2h post-injection and returned to a basal level about 2h later. Silencing SeRac1 expression inhibited hemocyte spreading in response to three immune mediators, octopamine, 5-hydroxytryptamine, and PSP. Addition of PGE2 to SeRac1-silenced larvae rescued the influence of these three mediators on hemocyte spreading. These compounds also increased phospholipase A2 activity via SeRac1, which leads to prostaglandin biosynthesis. We infer that SeRac1 transduces OA, 5-HT, and PSP signaling via activating biosynthesis of prostaglandins and possibly other eicosanoids.

Integrative approach reveals composition of endoparasitoid wasp venoms

The fruit fly Drosophila melanogaster and its endoparasitoid wasps are a developing model system for interactions between host immune responses and parasite virulence mechanisms. In this system, wasps use diverse venom cocktails to suppress the conserved fly cellular encapsulation response. Although numerous genetic tools allow detailed characterization of fly immune genes, lack of wasp genomic information has hindered characterization of the parasite side of the interaction. Here, we use high-throughput nucleic acid and amino acid sequencing methods to describe the venoms of two related Drosophila endoparasitoids with distinct infection strategies, Leptopilina boulardi and L. heterotoma. Using RNA-seq, we assembled and quantified libraries of transcript sequences from female wasp abdomens. Next, we used mass spectrometry to sequence peptides derived from dissected venom gland lumens. We then mapped the peptide spectral data against the abdomen transcriptomes to identify a set of putative venom genes for each wasp species. Our approach captured the three venom genes previously characterized in L. boulardi by traditional cDNA cloning methods as well as numerous new venom genes that were subsequently validated by a combination of RT-PCR, blast comparisons, and secretion signal sequence search. Overall, 129 proteins were found to comprise L. boulardi venom and 176 proteins were found to comprise L. heterotoma venom. We found significant overlap in L. boulardi and L. heterotoma venom composition but also distinct differences that may underlie their unique infection strategies. Our joint transcriptomic-proteomic approach for endoparasitoid wasp venoms is generally applicable to identification of functional protein subsets from any non-genome sequenced organism.

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

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

Venom gland extract is not required for successful parasitism in the polydnavirus-associated endoparasitoid Hyposoter didymator (Hym. Ichneumonidae) despite the presence of numerous novel and conserved venom proteins

The venom gland is a conserved organ in Hymenoptera that shows adaptations associated with life-style diversification. Few studies have investigated venom components and function in the highly diverse parasitic wasps and all suggest that the venom regulates host physiology. We explored the venom of the endoparasitoid Hyposoter didymator (Campopleginae), a species with an associated polydnavirus produced in the ovarian tissue. We investigated the effects of the H. didymator venom on two physiological traits of the host Spodoptera frugiperda (Noctuidae): encapsulation response and growth rate. We found that H. didymator venom had no significant effect on host cellular immunity or development, suggesting that it does not contribute to parasitism success. The host physiology seemed to be modified essentially by the ovarian fluid containing the symbiotic polydnaviruses. Proteomic analyses indicated that the H. didymator venom gland produces a large variety of proteins, consistent with the classical hymenopteran venom protein signature, including: reprolysin-like, dipeptidyl peptidase IV, hyaluronidase, arginine kinase or allergen proteins. The venom extracts also contained novel proteins, encoded by venom genes conserved in Campopleginae ichneumonids, and proteins with similarities to active molecules identified in other parasitoid species, such as calreticulin, reprolysin, superoxide dismutase and serpin. However, some of these proteins appear to be produced only in small amounts or to not be secreted. Possibly, in Campopleginae carrying polydnaviruses, the host-modifying activities of venom became redundant following the acquisition of polydnaviruses by the lineage.

"It stings a bit but it cleans well": venoms of Hymenoptera and their antimicrobial potential

Venoms from Hymenoptera display a wide range of functions and biological roles. These notably include manipulation of the host, capture of prey and defense against competitors and predators thanks to endocrine and immune systems disruptors, neurotoxic, cytolytic and pain-inducing venom components. Recent works indicate that many hymenopteran species, whatever their life style, have also evolved a venom with properties which enable it to regulate microbial infections, both in stinging and stung animals. In contrast to biting insects and their salivary glands, stinging Hymenoptera seem to constitute an under-exploited ecological niche for agents of vector-borne disease. Few parasitic or mutualistic microorganisms have been reported to be hosted by venom-producing organs or to be transmitted to stung animals. This may result from the presence of potent antimicrobial molecules in venoms, histological features of venom apparatuses and selective effects of venoms on immune defenses of targeted organisms. The present paper reviews for the first time the venom antimicrobial potential of solitary and social Hymenoptera in molecular, ecological, and evolutionary perspectives.

Variability of venom components in immune suppressive parasitoid wasps: from a phylogenetic to a population approach

Endoparasitoid wasps develop at the expense of other insects, leading to their death. Eggs deposited inside the host body induce an immune response, which results in the formation of a melanized cellular capsule around the egg. To evade or counteract this response, endoparasitoids have evolved different strategies, the most often reported being injection into the host of immunosuppressive factors, notably venom proteins, along with the egg. The analysis of venom components has been performed independently in species of different taxa, but the present picture is far from complete. Intriguingly, the question of the level of venom variability inside species has been neglected, although it may partly determine the potential for parasitoid adaptation. Here, we present a short review of our present knowledge of venom components in endoparasitoids, as well as of the only well-known example of intraspecific variability in a venom immune suppressive protein being responsible for variation in parasitoid virulence. We then present data evidencing inter-individual variation of venom protein profiles, using a gel electrophoresis approach, both in laboratory strains and field populations of a figitid and a braconid species. Whether occurrence of such variability may permit a selection of parasitoid venom components driven by the host remains to be tested, notably in the context of the production and use of biological control auxiliaries.