Non-poly-DNA viruses, their parasitic wasps, and hosts

Dating the origin of a viral domestication event in parasitoid wasps attacking Diptera

Over the course of evolution, hymenopteran parasitoids have developed a close relationship with heritable viruses, sometimes integrating viral genes into their chromosomes. For example, in Drosophila parasitoids belonging to the Leptopilina genus, 13 viral genes from the Filamentoviridae family have been domesticated to deliver immunosuppressive factors to host immune cells, thereby protecting parasitoid offspring from the host immune response. The present study aims to comprehensively characterize this domestication event in terms of the viral genes involved, the wasp diversity affected by this event and its chronology. Our genomic analysis of 41 Cynipoidea wasps from six subfamilies revealed 18 viral genes that were endogenized during the early radiation of the Eucoilini/Trichoplastini clade around 75 million years ago. Wasps from this highly diverse clade develop not only from Drosophila but also from a variety of Schizophora. This event coincides with the radiation of Schizophora, a highly speciose Diptera clade, suggesting that viral domestication facilitated wasp diversification in response to host diversification. Additionally, in one of the species, at least one viral gene was replaced by another gene derived from a related filamentovirus. This study highlights the impact of viral domestication on the diversification of parasitoid wasps.

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

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

A viral mutualist employs posthatch transmission for vertical and horizontal spread among parasitoid wasps

Mutualistic viruses remain a rarity among known animal–microbe symbioses. Yet, several beneficial viruses have been identified within insects called parasitoid wasps. Most of these viral entities are permanent components of wasp genomes. However, a mutualistic poxvirus found within Diachasmimorpha longicaudata wasps maintains an independent genome and may therefore behave in ways more similar to cellular microbial symbionts. In this study, we discovered unique properties of viral symbiont transmission, including an evolved dependence on parasitoid wasps for virus spread among fruit fly hosts and a distinct mode of faithful virus transmission among parasitoid wasps. These findings demonstrate that certain symbiont transmission pathways have arisen independently across disparate life forms to play pivotal roles in insect biology and evolution.

Heritable symbionts display a wide variety of transmission strategies to travel from one insect generation to the next. Parasitoid wasps, one of the most diverse insect groups, maintain several heritable associations with viruses that are beneficial for wasp survival during their development as parasites of other insects. Most of these beneficial viral entities are strictly transmitted through the wasp germline as endogenous viral elements within wasp genomes. However, a beneficial poxvirus inherited by Diachasmimorpha longicaudata wasps, known as Diachasmimorpha longicaudata entomopoxvirus (DlEPV), is not integrated into the wasp genome and therefore may employ different tactics to infect future wasp generations. Here, we demonstrated that transmission of DlEPV is primarily dependent on parasitoid wasps, since viral transmission within fruit fly hosts of the wasps was limited to injection of the virus directly into the larval fly body cavity. Additionally, we uncovered a previously undocumented form of posthatch transmission for a mutualistic virus that entails external acquisition and localization of the virus within the adult wasp venom gland. We showed that this route is extremely effective for vertical and horizontal transmission of the virus within D. longicaudata wasps. Furthermore, the beneficial phenotype provided by DlEPV during parasitism was also transmitted with perfect efficiency, indicating an effective mode of symbiont spread to the advantage of infected wasps. These results provide insight into the transmission of beneficial viruses among insects and indicate that viruses can share features with cellular microbes during their evolutionary transitions into symbionts.

The Presence of Ancient Core Genes Reveals Endogenization from Diverse Viral Ancestors in Parasitoid Wasps

The Ichneumonoidea (Ichneumonidae and Braconidae) is an incredibly diverse superfamily of parasitoid wasps that includes species that produce virus-like entities in their reproductive tracts to promote successful parasitism of host insects. Research on these entities has traditionally focused upon two viral genera Bracovirus (in Braconidae) and Ichnovirus (in Ichneumonidae). These viruses are produced using genes known collectively as endogenous viral elements (EVEs) that represent historical, now heritable viral integration events in wasp genomes. Here, new genome sequence assemblies for 11 species and 6 publicly available genomes from the Ichneumonoidea were screened with the goal of identifying novel EVEs and characterizing the breadth of species in lineages with known EVEs. Exhaustive similarity searches combined with the identification of ancient core genes revealed sequences from both known and novel EVEs. One species harbored a novel, independently derived EVE related to a divergent large double-stranded DNA (dsDNA) virus that manipulates behavior in other hymenopteran species. Although bracovirus or ichnovirus EVEs were identified as expected in three species, the absence of ichnoviruses in several species suggests that they are independently derived and present in two younger, less widespread lineages than previously thought. Overall, this study presents a novel bioinformatic approach for EVE discovery in genomes and shows that three divergent virus families (nudiviruses, the ancestors of ichnoviruses, and Leptopilina boulardi Filamentous Virus-like viruses) are recurrently acquired as EVEs in parasitoid wasps. Virus acquisition in the parasitoid wasps is a common process that has occurred in many more than two lineages from a diverse range of arthropod-infecting dsDNA viruses.

A Mutualistic Poxvirus Exhibits Convergent Evolution with Other Heritable Viruses in Parasitoid Wasps

For insects known as parasitoid wasps, successful development as a parasite results in the death of the host insect. As a result of this lethal interaction, wasps and their hosts have coevolved strategies to gain an advantage in this evolutionary arms race. Although normally considered to be strict pathogens, some viruses have established persistent infections within parasitoid wasp lineages and are beneficial to wasps during parasitism. Heritable associations between viruses and parasitoid wasps have evolved independently multiple times, but most of these systems remain largely understudied with respect to viral origin, transmission and replication strategies of the virus, and interactions between the virus and host insects. Here, we report a detailed characterization of Diachasmimorpha longicaudata entomopoxvirus (DlEPV), a poxvirus found within the venom gland of Diachasmimorpha longicaudata wasps. Our results show that DlEPV exhibits similar but distinct transmission and replication dynamics compared to those of other parasitoid viral elements, including vertical transmission of the virus within wasps, as well as virus replication in both female wasps and fruit fly hosts. Functional assays demonstrate that DlEPV is highly virulent within fly hosts, and wasps without DlEPV have severely reduced parasitism success compared to those with a typical viral load. Taken together, the data presented in this study illustrate a novel case of beneficial virus evolution, in which a virus of unique origin has undergone convergent evolution with other viral elements associated with parasitoid wasps to provide an analogous function throughout parasitism.IMPORTANCE Viruses are generally considered to be disease-causing agents, but several instances of beneficial viral elements have been identified in insects called parasitoid wasps. These virus-derived entities are passed on through wasp generations and enhance the success of the wasps' parasitic life cycle. Many parasitoid-virus partnerships studied to date exhibit common features among independent cases of this phenomenon, including a mother-to-offspring route of virus transmission, a restricted time and location for virus replication, and a positive effect of virus activity on wasp survival. Our characterization of Diachasmimorpha longicaudata entomopoxvirus (DlEPV), a poxvirus found in Diachasmimorpha longicaudata parasitoid wasps, represents a novel example of beneficial virus evolution. Here, we show that DlEPV exhibits functional similarities to known parasitoid viral elements that support its comparable role during parasitism. Our results also demonstrate unique differences that suggest DlEPV is more autonomous than other long-term viral associations described in parasitoid wasps.

Common themes in three independently derived endogenous nudivirus elements in parasitoid wasps

Endogenous Viral Elements (EVEs) are remnants of viral genomes that are permanently integrated into the genome of another organism. Parasitoid wasps have independently acquired nudivirus-derived EVEs in three lineages. Each parasitoid produces virions or virus-like particles (VLPs) that are injected into hosts during parasitism to function in subversion of host defenses. Comparing the inventory of nudivirus-like genes in different lineages of parasitoids can provide insights into the importance of each encoded function in virus or VLP production and parasitism success. Comparisons revealed the following conserved features: first, retention of genes encoding a viral RNA polymerase and infectivity factors; second, loss of the ancestral DNA polymerase gene; and third, signatures of viral ancestry in patterns of gene retention.

Characterization of a venom gland-associated rhabdovirus in the parasitoid wasp Diachasmimorpha longicaudata

Parasitoid wasps reproduce by laying their eggs on or inside of a host insect, which triggers a defense response in the host insect that kills the developing wasp. To counteract the host's lethal response, some parasitoid wasps are associated with symbiotic viruses that alter host metabolism and development to promote successful development of the wasp embryo. These symbiotic viruses display a number of characteristics that differ from those of pathogenic viruses, but are poorly understood with the exception of one group, the polydnaviruses. Here, we characterize the genome of a non-polydnavirus associated with parasitoid wasps, Diachasmimorpha longicaudata rhabdovirus (DlRhV), and assess its role as a potential mutualistic virus. Our results show that the DlRhV genome contains six open reading frames (ORFs). Three ORFs show sequence homology to known viral genes and one ORF encodes a previously identified protein, called parasitism-specific protein 24 (PSP24), that has been hypothesized to play a role in promoting successful parasitism by D. longicaudata. We constructed a phylogeny that shows that DlRhV is most closely related to other insect-infecting rhabdoviruses. Finally, we report that DlRhV infection does not occur in all populations of D. longicaudata, and is not required for successful parasitism.

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.

Coexistence of intraguild predators and prey in resource-rich environments

The prevalence of intraguild predation (IGP) in productive environments has long puzzled ecologists. Theory predicts the exclusion of intraguild prey from such environments, but data consistently defy this expectation. This suggests that coexistence mechanisms at high resource productivity may differ from those at lower productivity. Here I present a mathematical model that investigates multiple coexistence mechanisms. I incorporate two biological features widely observed in IGP communities: intraspecific interference via cannibalism or superparasitism, and temporal refuges arising from differential sensitivities to abiotic variation. I develop predictions based on three aspects of the IG prey-IG predator interaction: mutual invasibility, transient dynamics, and long-term abundances. These predictions specify the conditions under which coexistence mechanisms reinforce vs. deter one another: when a competition-IGP trade-off allows coexistence at intermediate productivity a temporal refuge for the intraguild prey always allows coexistence at high productivity, but intraspecific interference does so only at a net fitness cost to the intraguild predator. Intraspecific interference that benefits the intraguild predator not only reduces tradeoff-mediated coexistence at intermediate productivity, but also undermines the refuge's coexistence-enhancing effect at high productivity. Different mechanism combinations yield characteristic signatures in time series data during transient dynamics. By judicious measurement of parameters and examining time series for critical signatures, one can elucidate the mechanisms that allow IGP to prevail in resource-rich environments.

Molecular evidence for the evolution of ichnoviruses from ascoviruses by symbiogenesis

BACKGROUND

Female endoparasitic ichneumonid wasps inject virus-like particles into their caterpillar hosts to suppress immunity. These particles are classified as ichnovirus virions and resemble ascovirus virions, which are also transmitted by parasitic wasps and attack caterpillars. Ascoviruses replicate DNA and produce virions. Polydnavirus DNA consists of wasp DNA replicated by the wasp from its genome, which also directs particle synthesis. Structural similarities between ascovirus and ichnovirus particles and the biology of their transmission suggest that ichnoviruses evolved from ascoviruses, although molecular evidence for this hypothesis is lacking.

RESULTS

Here we show that a family of unique pox-D5 NTPase proteins in the Glypta fumiferanae ichnovirus are related to three Diadromus pulchellus ascovirus proteins encoded by ORFs 90, 91 and 93. A new alignment technique also shows that two proteins from a related ichnovirus are orthologs of other ascovirus virion proteins.

CONCLUSION

Our results provide molecular evidence supporting the origin of ichnoviruses from ascoviruses by lateral transfer of ascoviral genes into ichneumonid wasp genomes, perhaps the first example of symbiogenesis between large DNA viruses and eukaryotic organisms. We also discuss the limits of this evidence through complementary studies, which revealed that passive lateral transfer of viral genes among polydnaviral, bacterial, and wasp genomes may have occurred repeatedly through an intimate coupling of both recombination and replication of viral genomes during evolution. The impact of passive lateral transfers on evolutionary relationships between polydnaviruses and viruses with large double-stranded genomes is considered in the context of the theory of symbiogenesis.

Lack of lipogenesis in parasitoids: a review of physiological mechanisms and evolutionary implications

The ability of organisms to adapt to fluctuating food conditions is essential for their survival and reproduction. Accumulating energy reserves, such as lipids, in anticipation of harsh conditions, will reduce negative effects of a low food supply. For Hymenoptera and Diptera, several parasitoid species lack adult lipogenesis, and are unable to store excess energy in the form of lipid reserves. The aim of this review is to provide a synthesis of current knowledge regarding the inability to accumulate lipids in parasitoids, leading to new insights and prospects for further research. We will emphasize physiological mechanisms underlying lack of lipogenesis, the evolution of this adaptation in parasitoids and its biological implications with regard to life history traits. We suggest the occurrence of lack of lipogenesis in parasitoids to be dependent on the extent of host exploitation through metabolic manipulation. Currently available data shows lack of lipogenesis to have evolved independently at least twice, in parasitic Hymenoptera and Diptera. The underlying genetic mechanism, however, remains to be solved. Furthermore, due to the inability to replenish adult fat reserves, parasitoids are severely constrained in resource allocation strategies, in particular the trade-off between survival and reproduction.

The constituents of Microctonus sp. parasitoid venoms

Purified RNA transcripts from venom glands dissected from the parasitoid wasp Microctonus hyperodae were copied, cloned and sequenced using traditional dideoxy sequencing methods. Using mass spectrometry analysis of the trypsinised PAGE gel protein bands we identified the RNA transcripts for the 3 most abundant proteins found in the venom and hence obtained their full protein sequence. Other abundant transcripts were also further sequenced. To reduce the effort required to obtain transcript information we dissected venom glands from a second parasitoid, Microctonus aethiopoides (Morocco biotype). The RNA transcripts were purified and reverse transcribed but instead of cloning the cDNA it was directly sequenced using Roche GS20 pyrosequencing. Results from a single GS20 sequencing run provided data similar to that obtained by the traditional methods used in analysing transcripts from M. hyperodae in a fraction of the time and cost. Comparing the transcripts between the two species showed that a similar range of genes are expressed with the putative orthologs of seven of the eight full length genes characterised from M. hyperodae being found in M. aethiopoides. Pyrosequencing should provide a valuable new method for rapidly sampling transcripts from a wide range of specialised insect tissues.

Superparasitism Evolution: Adaptation or Manipulation?

Superparasitism refers to the oviposition behavior of parasitoid females who lay their eggs in an already parasitized host. This often yields intense competition among larvae that are sharing the same host. Why would a female oviposit in such hostile habitat instead of looking for a better quality, unparasitized host? Here we present a continuous-time model of host-parasitoid interaction and discuss alternative scenarios. This model is first used to analyze the evolution of the superparasitism behavior of a solitary proovigenic parasitoid under both time and egg limitation. Then, following the recent discovery by Varaldi et al., we allow the parasitoid to be infected by a virus that alters the superparasitism behavior of its host to enhance its own horizontal transmission. The analysis of the coevolution of this manipulative behavior with the oviposition behavior of uninfected females clarifies and quantifies the conflict that emerges between the parasitoid and its virus. The model also yields new testable predictions. For example, we expect that uninfected parasitoids should superparasite less after coevolving with the manipulative virus. More generally, this model provides a theoretical framework for analyzing the evolution of the manipulation of parasitoid life-history traits by microparasites.