Phylogenetic position of the Diadromus pulchellus ascovirus DNA polymerase among viruses with large double-stranded DNA genomes

Sequences Encoding a Novel Toursvirus Identified from Southern and Northern Corn Rootworms (Coleoptera: Chrysomelidae)

Sequences derived from a novel toursvirus were identified from pooled genomic short read data from U.S. populations of southern corn rootworm (SCR, Diabrotica undecimpunctata howardi Barber) and northern corn rootworm (NCR, Diabrotica barberi Smith & Lawrence). Most viral sequences were identified from the SCR genomic dataset. As proteins encoded by toursvirus sequences from SCR and NCR were almost identical, the contig sets from SCR and NCR were combined to generate 26 contigs. A total of 108,176 bp were assembled from these contigs, with 120 putative toursviral ORFs identified indicating that most of the viral genome had been recovered. These ORFs included all 40 genes that are common to members of the Ascoviridae. Two genes typically present in Ascoviridae (ATP binding cassette transport system permeases and Baculovirus repeated open reading frame), were not detected. There was evidence for transposon insertion in viral sequences at different sites in the two host species. Phylogenetic analyses based on a concatenated set of 45 translated protein sequences clustered toursviruses into a distinct clade. Based on the combined evidence, we propose taxonomic separation of toursviruses from Ascoviridae.

3H-117, a structural protein of Heliothis virescens ascovirus 3h (HvAV-3h)

The open reading frame 117 (3h-117) of Heliothis virescens ascovirus 3h (HvAV-3h), which is a conserved coding region present in all completely sequenced ascovirus members, was characterized in this study. By RT-PCR detection, 3h-117 transcription began at 6-h post-infection (hpi) and remained stable until 168 hpi in HvAV-3h-infected Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae. In addition, 3h-117 putatively encodes a 21.5-kDa protein (3H-117) predicted to be a CTD-like phosphatase. Western blot analysis using a prepared rabbit polyclonal antibody specific to 3H-117 showed that the product could be detected at 24 hpi, which remained stably detectable until 168 hpi. The same analysis also demonstrated that the 3H-117 protein localized in the virions of HvAV-3h. Immunofluorescence analysis showed that at 24 hpi, 3H-117 was mainly located in the nuclei of H. armigera larval fat body cells and later spread into the cytoplasm. In summary, our results indicate that 3H-117 is a structural protein of HvAV-3h.

Complete genome sequence of Ostreid herpesvirus type 1 µVar isolated during mortality events in the Pacific oyster Crassostrea gigas in France and Ireland

Infections with Ostreid herpesvirus 1 (OsHV-1) microvariants in young Pacific oysters are associated with massive mortality events and significant economic losses. Previous studies, focusing on few regions of the genome, have revealed the genomic diversity of these genotypes with respect to the reference type. We used a NGS process to sequence the whole genome of the OsHV-1 µVar in infected individuals, collected during mortality events in France and Ireland. The final genome length of OsHV-1 µVar was approximately 205kbp, shorter than the reference genotype and the overall genome organisation resembled herpes simplex viruses. 94.4% similarity was observed with the OsHV-1 reference genotype. Large indels, including five deletions and three insertions were found to induce the loss and the addition of several ORFs, summed with codon substitutions in 64% of genes shared with the reference type. This diversity raises the question of the exact origin and evolution of OsHV-1 µVar.

Suppression of RNAi by dsRNA-degrading RNaseIII enzymes of viruses in animals and plants

Certain RNA and DNA viruses that infect plants, insects, fish or poikilothermic animals encode Class 1 RNaseIII endoribonuclease-like proteins. dsRNA-specific endoribonuclease activity of the RNaseIII of rock bream iridovirus infecting fish and Sweet potato chlorotic stunt crinivirus (SPCSV) infecting plants has been shown. Suppression of the host antiviral RNA interference (RNAi) pathway has been documented with the RNaseIII of SPCSV and Heliothis virescens ascovirus infecting insects. Suppression of RNAi by the viral RNaseIIIs in non-host organisms of different kingdoms is not known. Here we expressed PPR3, the RNaseIII of Pike-perch iridovirus, in the non-hosts Nicotiana benthamiana (plant) and Caenorhabditis elegans (nematode) and found that it cleaves double-stranded small interfering RNA (ds-siRNA) molecules that are pivotal in the host RNA interference (RNAi) pathway and thereby suppresses RNAi in non-host tissues. In N. benthamiana, PPR3 enhanced accumulation of Tobacco rattle tobravirus RNA1 replicon lacking the 16K RNAi suppressor. Furthermore, PPR3 suppressed single-stranded RNA (ssRNA)--mediated RNAi and rescued replication of Flock House virus RNA1 replicon lacking the B2 RNAi suppressor in C. elegans. Suppression of RNAi was debilitated with the catalytically compromised mutant PPR3-Ala. However, the RNaseIII (CSR3) produced by SPCSV, which cleaves ds-siRNA and counteracts antiviral RNAi in plants, failed to suppress ssRNA-mediated RNAi in C. elegans. In leaves of N. benthamiana, PPR3 suppressed RNAi induced by ssRNA and dsRNA and reversed silencing; CSR3, however, suppressed only RNAi induced by ssRNA and was unable to reverse silencing. Neither PPR3 nor CSR3 suppressed antisense-mediated RNAi in Drosophila melanogaster. These results show that the RNaseIII enzymes of RNA and DNA viruses suppress RNAi, which requires catalytic activities of RNaseIII. In contrast to other viral silencing suppression proteins, the RNaseIII enzymes are homologous in unrelated RNA and DNA viruses and can be detected in viral genomes using gene modeling and protein structure prediction programs.

Binding and processing of small dsRNA molecules by the class 1 RNase III protein encoded by sweet potato chlorotic stunt virus

Sweet potato chlorotic stunt virus (SPCSV; genus Crinivirus, family Closteroviridae) causes heavy yield losses in sweet potato plants co-infected with other viruses. The dsRNA-specific class 1 RNase III-like endoribonuclease (RNase3) encoded by SPCSV suppresses post-transcriptional gene silencing and eliminates antiviral defence in sweet potato plants in an endoribonuclease activity-dependent manner. RNase3 can cleave long dsRNA molecules, synthetic small interfering RNAs (siRNAs), and plant- and virus-derived siRNAs extracted from sweet potato plants. In this study, conditions for efficient expression and purification of enzymically active recombinant RNase3 were established. Similar to bacterial class 1 RNase III enzymes, RNase3-Ala (a dsRNA cleavage-deficient mutant) bound to and processed double-stranded siRNA (ds-siRNA) as a dimer. The results support the classification of SPCSV RNase3 as a class 1 RNase III enzyme. There is little information about the specificity of RNase III enzymes on small dsRNAs. In vitro assays indicated that ds-siRNAs and microRNAs (miRNAs) with a regular A-form conformation were cleaved by RNase3, but asymmetrical bulges, extensive mismatches and 2'-O-methylation of ds-siRNA and miRNA interfered with processing. Whereas Mg(2+) was the cation that best supported the catalytic activity of RNase3, binding of 21 nt small dsRNA molecules was most efficient in the presence of Mn(2+). Processing of long dsRNA by RNase3 was efficient at pH 7.5 and 8.5, whereas ds-siRNA was processed more efficiently at pH 8.5. The results revealed factors that influence binding and processing of small dsRNA substrates by class 1 RNase III in vitro or make them unsuitable for processing by the enzyme.

Phylogenetic position and replication kinetics of Heliothis virescens ascovirus 3h (HvAV-3h) isolated from Spodoptera exigua

Insect-specific ascoviruses with a circular genome are distributed in the USA, France, Australia and Indonesia. Here, we report the first ascovirus isolation from Spodoptera exigua in Hunan, China. DNA-DNA hybridization to published ascoviruses demonstrated that the new China ascovirus isolate is a variant of Heliothis virescens ascovirus 3a (HvAV-3a), thus named HvAV-3h. We investigated the phylogenetic position, cell infection, vesicle production and viral DNA replication kinetics of HvAV-3h, as well as its host-ranges. The major capsid protein (MCP) gene and the delta DNA polymerase (DNA po1) gene of HvAV-3h were sequenced and compared with the available ascovirus isolates for phylogenetic analysis. This shows a close relationship with HvAV-3g, originally isolated from Indonesia, HvAV-3e from Australia and HvAV-3c from United States. HvAV-3h infection induced vesicle production in the SeE1 cells derived from S. exigua and Sf9 cells derived from S. frugiperda, resulting in more vesicles generated in Sf9 than SeE1. Viral DNA replication kinetics of HvAV-3h also demonstrated a difference between the two cell lines tested. HvAV-3h could readily infect three important insect pests Helicoverpa armigera (Hübner), Spodoptera exigua (Hübner) and Spodoptera litura (Fabricius) from two genera in different subfamilies with high mortalities.

Proteomic analysis of Chilo iridescent virus

In this first proteomic analysis of an invertebrate iridovirus, 46 viral proteins were detected in the virions of Chilo iridescent virus (CIV) based on the detection of 2 or more distinct peptides; an additional 8 proteins were found based on a single peptide. Thirty-six of the 54 identified proteins have homologs in another invertebrate and/or in one or more vertebrate iridoviruses. The genes for 5 of the identified proteins, 22L (putative helicase), 118L, 142R (putative RNaseIII), 274L (major capsid protein) and 295L, are shared by all iridoviruses for which the complete nucleotide sequence is known and may therefore be considered as iridovirus core genes. Three identified proteins have homologs only in ascoviruses. The remaining 15 identified proteins are so far unique to CIV. In addition to broadening our insight in the structure and assembly of CIV virions, this knowledge is pivotal to unravel the initial steps in the infection process.

Family Iridoviridae: poor viral relations no longer

Members of the family Iridoviridae infect a diverse array of invertebrate and cold-blooded vertebrate hosts and are currently viewed as emerging pathogens of fish and amphibians. Iridovirid replication is unique and involves both nuclear and cytoplasmic compartments, a circularly permuted, terminally redundant genome that, in the case of vertebrate iridoviruses, is also highly methylated, and the efficient shutoff of host macromolecular synthesis. Although initially neglected largely due to the perceived lack of health, environmental, and economic concerns, members of the genus Ranavirus, and the newly recognized genus Megalocytivirus, are rapidly attracting growing interest due to their involvement in amphibian population declines and their adverse impacts on aquaculture. Herein we describe the molecular and genetic basis of viral replication, pathogenesis, and immunity, and discuss viral ecology with reference to members from each of the invertebrate and vertebrate genera.

Ascoviruses: superb manipulators of apoptosis for viral replication and transmission

Ascoviruses are members of a recently described new family (Ascoviridae) of large double-stranded DNA viruses that attack immature stages of insects belonging to the order Lepidoptera, in which they cause a chronic, fatal disease. Ascoviruses have several unusual characteristics not found among other viruses, the most novel of which are their transmission by endoparasitic wasps and a unique cytopathology that resembles apoptosis. Cell infection induces apoptosis and in some species is associated with synthesis of a virus-encoded executioner caspase and several lipid-metabolizing enzymes. Rather than leading directly to cell death, synthesis of viral proteins results in the rescue of developing apoptotic bodies that are converted into large vesicles in which virions accumulate and continue to assemble. In infected larvae, millions of these virion-containing vesicles begin to disperse from infected tissues 48-72 h after infection into the blood, making it milky white, a major characteristic of the disease. Circulation of virions and vesicles in the blood facilitates mechanical transmission by parasitic wasps. Although ascoviruses appear to be very common, only five species are currently recognized, with the type species being the Spodoptera frugiperda ascovirus 1a. Ascovirus virions are large, enveloped, typically bacilliform or reniform in shape, and, depending on the species, have genomes that range from 119 to 186 kbp. Molecular phylogenetic evidence indicates that ascoviruses evolved from iridoviruses (family Iridoviridae) that attack lepidopteran larvae and are likely the evolutionary source of ichnoviruses (family Polydnaviridae), which assist endoparasitic hymenopterans in overcoming the defense responses of their insect hosts. Thus, as other molecular evidence suggests that iridoviruses evolved from phycodnaviruses (family Phycodnaviridae), an evolutionary pathway is apparent from phycodnaviruses via iridoviruses and ascoviruses to ichnoviruses.

Virus evolution: how far does the double beta-barrel viral lineage extend?

During the past few years one of the most astonishing findings in the field of virology has been the realization that viruses that infect hosts from all three domains of life are often structurally similar. The recent burst of structural information points to a need to create a new way to organize the virosphere that, in addition to the current classification, would reflect relationships between virus families. Using the vertical beta-barrel major capsid proteins and ATPases related to known viral genome-packaging ATPases as examples, we can now re-evaluate the classification of viruses and virus-like genetic elements from a structural standpoint.

Transcriptional analysis of a major capsid protein gene from Spodoptera exigua ascovirus 5a

The major capsid protein (mcp) gene of Spodoptera exigua ascovirus 5a (SeAV-5a) was confirmed by aphidicolin viral DNA replication inhibition analysis to be a late gene. The 5' and 3' ends of mcp gene transcripts have been mapped. Primer extension analyses indicated that transcription of the mcp gene initiates from a cytosine 25 nucleotides (nt) upstream of the translation start codon. Two independent approaches by 3' rapid amplification of cDNA ends (3' RACE) and oligo (dT) cellulose binding assay suggested that SeAV-5a mcp mRNA is polyadenylated. Analyses by 3' RACE also revealed that mcp transcripts terminate at a U, either at 26 or 38 nt downstream of the translation stop codon. The putative 5' transcription control region of the SeAV-5a mcp gene shares similarities with other ascoviruses and Chilo iridescent virus (CIV), containing a conserved TATA-box-like motif (TAATTAAA) and an ATTTGATCTT motif upstream of it. The 3' downstream regions of the mcp gene of all the ascoviruses examined and CIV can form a stem-loop structure, and the ends of the mcp gene transcripts of SeAV-5a are within the predicted stem-loop region. This suggests that the stem-loop structure of the mcp gene might be involved in transcription termination.

Genome of invertebrate iridescent virus type 3 (mosquito iridescent virus)

Iridoviruses (IVs) are classified into five genera: Iridovirus and Chloriridovirus, whose members infect invertebrates, and Ranavirus, Lymphocystivirus, and Megalocytivirus, whose members infect vertebrates. Until now, Chloriridovirus was the only IV genus for which a representative and complete genomic sequence was not available. Here, we report the genome sequence and comparative analysis of a field isolate of Invertebrate iridescent virus type 3 (IIV-3), also known as mosquito iridescent virus, currently the sole member of the genus Chloriridovirus. Approximately 20% of the 190-kbp IIV-3 genome was repetitive DNA, with DNA repeats localized in 15 apparently noncoding regions. Of the 126 predicted IIV-3 genes, 27 had homologues in all currently sequenced IVs, suggesting a genetic core for the family Iridoviridae. Fifty-two IIV-3 genes, including those encoding DNA topoisomerase II, NAD-dependent DNA ligase, SF1 helicase, IAP, and BRO protein, are present in IIV-6 (Chilo iridescent virus, prototype species of the genus Iridovirus) but not in vertebrate IVs, likely reflecting distinct evolutionary histories for vertebrate and invertebrate IVs and potentially indicative of genes that function in aspects of virus-invertebrate host interactions. Thirty-three IIV-3 genes lack homologues in other IVs. Most of these encode proteins of unknown function but also encode IIV3-053L, a protein with similarity to DNA-dependent RNA polymerase subunit 7; IIV3-044L, a putative serine/threonine protein kinase; and IIV3-080R, a protein with similarity to poxvirus MutT-like proteins. The absence of genes present in other IVs, including IIV-6; the lack of obvious colinearity with any sequenced IV; the low levels of amino acid identity of predicted proteins to IV homologues; and phylogenetic analyses of conserved proteins indicate that IIV-3 is distantly related to other IV genera.

Genomic sequence of Spodoptera frugiperda Ascovirus 1a, an enveloped, double-stranded DNA insect virus that manipulates apoptosis for viral reproduction

Ascoviruses (family Ascoviridae) are double-stranded DNA viruses with circular genomes that attack lepidopterans, where they produce large, enveloped virions, 150 by 400 nm, and cause a chronic, fatal disease with a cytopathology resembling that of apoptosis. After infection, host cell DNA is degraded, the nucleus fragments, and the cell then cleaves into large virion-containing vesicles. These vesicles and virions circulate in the hemolymph, where they are acquired by parasitic wasps during oviposition and subsequently transmitted to new hosts. To develop a better understanding of ascovirus biology, we sequenced the genome of the type species Spodoptera frugiperda ascovirus 1a (SfAV-1a). The genome consisted of 156,922 bp, with a G+C ratio of 49.2%, and contained 123 putative open reading frames coding for a variety of enzymes and virion structural proteins, of which tentative functions were assigned to 44. Among the most interesting enzymes, due to their potential role in apoptosis and viral vesicle formation, were a caspase, a cathepsin B, several kinases, E3 ubiquitin ligases, and especially several enzymes involved in lipid metabolism, including a fatty acid elongase, a sphingomyelinase, a phosphate acyltransferase, and a patatin-like phospholipase. Comparison of SfAV-1a proteins with those of other viruses showed that 10% were orthologs of Chilo iridescent virus proteins, the highest correspondence with any virus, providing further evidence that ascoviruses evolved from a lepidopteran iridovirus. The SfAV-1a genome sequence will facilitate the determination of how ascoviruses manipulate apoptosis to generate the novel virion-containing vesicles characteristic of these viruses and enable study of their origin and evolution.

Sequence and organization of the Trichoplusia ni ascovirus 2c (Ascoviridae) genome

The complete Trichoplusia ni ascovirus 2c (TnAV-2c) genome sequence was determined. The circular genome contains 174,059 bp with 165 open reading frames (ORFs) of greater than 180 bp and two major homologous regions (hrs). The genome is quite A+T rich at 64.6%. Fifty-four ORFs had homologues in other insect viruses, such as ascoviruses, iridoviruses, baculoviruses and entomopoxviruses; 30 ORFs showed low identities with those from different parasitic protozoa and 12 ORFs were unique to TnAV-2c. TnAV-2c has 15 ORFs that could be grouped into six gene families. Three major conserved repeating sequences were identified and were interspersed in two regions. BLAST analyses revealed that there were 16 enzymes involved in gene transcription, DNA replication, and nucleotide metabolism. TnAV-2c has 12 and 25 ORFs sharing high identities with ascovirus and iridovirus homologues, respectively. The codon usage bias appears to be more similar to Spodoptera frugiperda ascovirus 1a than to iridoviruses.

The virus infecting the parasitoid Leptopilina boulardi exerts a specific action on superparasitism behaviour

Parasites often induce behavioural changes in their host. However, it is not necessarily easy to determine whether these changes are representative of an adaptation of the parasite (parasite manipulation), an adaptive response of the host or a side-effect of infection. In a solitary parasitoid of Drosophila larvae (Leptopilina boulardi), viral particles (LbFV) modify the host acceptance behaviour of infected females by increasing their tendency to superparasitize. This behavioural alteration allows for the horizontal transmission of the virus within superparasitized Drosophila larvae. To add support for or against the 'manipulation hypothesis', we investigated whether other behavioural components of the parasitoid are affected by viral infection, and whether other forms of horizontal transmission exist. Neither the ability of females to locate host kairomones nor their daily rhythm of locomotor activity was affected by viral infection. However, infected females showed a lower rate of locomotor activity, suggesting a physiological cost of infection. The searching paths of females were also unaffected. Males from infected and uninfected lines showed the same ability to locate females'sexual pheromones. Moreover, alternative modes of horizontal transmission (through food consumption and/or contact with the same Drosophila larvae) did not lead to viral contamination of the parasitoid. The overall specificity of behavioural alteration and of viral horizontal transmission is consistent with the hypothesis that the virus manipulates the behaviour of the parasitoid.

The few virus-like genes of Cotesia congregata bracovirus

The origin of the symbiotic association between parasitoid wasps and bracoviruses is still unknown. From phylogenetic analyses, bracovirus-associated wasp species constitute a monophyletic group, the microgastroid complex. Thus all wasp-bracovirus associations could have originated from the integration of an ancestral virus in the genome of the ancestor of the microgastroids. In an effort to identify a set of virus genes that would give clues on the nature of the ancestral virus, we have recently performed the complete sequencing of the genome of CcBV, the bracovirus of the wasp Cotesia congregata. We describe here the putative proteins encoded by CcBV genome having significant similarities with sequences from known viruses and mobile elements. The analysis of CcBV gene content does not lend support to the hypothesis that bracoviruses originated from a baculovirus. Moreover, no consistent homology was found between CcBV genes and any set of genes constituting the core genome of a known free-living virus. We discuss the significance of the scarce homology found between proteins from CcBV and other viruses or mobile elements.

Characterization of three ascovirus isolates from cotton insects

Three new ascovirus isolates were discovered from lepidopteran larvae in cotton fields in Blackville, South Carolina, USA, and were named TnAV-2c, TnAV-2d, and HvAV-3f. TnAV-2c and TnAV-2d were compared by restriction endonuclease (REN) profiles and found to be similar. HvAV-3f was isolated from Helicoverpa zea, and bears remarkable dissimilarity in REN profiles to the reported SeAV-5a from Spodoptera exigua but DNA hybridization shows they are closely related. Major capsid protein (MCP) and delta DNA polymerase from the three isolates were sequenced, which suggests the three isolates are novel. Phylogenetic analyses showed that TnAV-2c is distantly related to other lepidopteran ascoviruses. HvAV-3f and SeAV-5a may also be variants of the same species based on Southern, Western, and MCP/DNA polymerase gene sequence analyses. High levels of TnAV-2 infection in an H. zea population (as high as 74%) were recorded in a cotton field in Blackville, SC. Observations in this field showed that infection by ascovirus altered the feeding behavior of H. zea larvae.

Viral class 1 RNase III involved in suppression of RNA silencing

Double-stranded RNA (dsRNA)-specific endonucleases belonging to RNase III classes 3 and 2 process dsRNA precursors to small interfering RNA (siRNA) or microRNA, respectively, thereby initiating and amplifying RNA silencing-based antiviral defense and gene regulation in eukaryotic cells. However, we now provide evidence that a class 1 RNase III is involved in suppression of RNA silencing. The single-stranded RNA genome of sweet potato chlorotic stunt virus (SPCSV) encodes an RNase III (RNase3) homologous to putative class 1 RNase IIIs of unknown function in rice and Arabidopsis. We show that RNase3 has dsRNA-specific endonuclease activity that enhances the RNA-silencing suppression activity of another protein (p22) encoded by SPCSV. RNase3 and p22 coexpression reduced siRNA accumulation more efficiently than p22 alone in Nicotiana benthamiana leaves expressing a strong silencing inducer (i.e., dsRNA). RNase3 did not cause intracellular silencing suppression or reduce accumulation of siRNA in the absence of p22 or enhance silencing suppression activity of a protein encoded by a heterologous virus. No other known RNA virus encodes an RNase III or uses two independent proteins cooperatively for RNA silencing suppression.

A novel class of herpesvirus with bivalve hosts

Ostreid herpesvirus 1 (OsHV-1) is the only member of the Herpesviridae that has an invertebrate host and is associated with sporadic mortality in the Pacific oyster (Crassostrea gigas) and other bivalve species. Cryo-electron microscopy of purified capsids revealed the distinctive T=16 icosahedral structure characteristic of herpesviruses, although the preparations examined lacked pentons. The gross genome organization of OsHV-1 was similar to that of certain mammalian herpesviruses (including herpes simplex virus and human cytomegalovirus), consisting of two invertible unique regions (U(L), 167.8 kbp; U(S), 3.4 kbp) each flanked by inverted repeats (TR(L)/IR(L), 7.6 kbp; TR(S)/IR(S), 9.8 kbp), with an additional unique sequence (X, 1.5 kbp) between IR(L) and IR(S). Of the 124 unique genes predicted from the 207 439 bp genome sequence, 38 were members of 12 families of related genes and encoded products related to helicases, inhibitors of apoptosis, deoxyuridine triphosphatase and RING-finger proteins, in addition to membrane-associated proteins. Eight genes in three of the families appeared to be fragmented. Other genes that did not belong to the families were predicted to encode DNA polymerase, the two subunits of ribonucleotide reductase, a helicase, a primase, the ATPase subunit of terminase, a RecB-like protein, additional RING-like proteins, an ion channel and several other membrane-associated proteins. Sequence comparisons showed that OsHV-1 is at best tenuously related to the two classes of vertebrate herpesviruses (those associated with mammals, birds and reptiles, and those associated with bony fish and amphibians). OsHV-1 thus represents a third major class of the herpesviruses.

Characteristics of pathogenic and mutualistic relationships of ascoviruses in field populations of parasitoid wasps

Ascoviruses are disseminated among larvae in lepidopteran populations by parasitic wasps during oviposition. Ascovirus relationships with these wasps vary from pathogenic to mutualistic, and experimentally can be shown possibly to be commensal non-pathogenic virus having little or no effect. Most ascoviruses are pathogens that female wasps vector mechanically. Other ascoviruses have a more intimate relationship with their wasp vectors in that their genome is stably maintained in all wasp nuclei through several generations by vertical transmission. In this relationship, these viruses are mutualistic, enhancing the successful development of the wasp larvae by suppressing lepidopteran defence mechanisms. The DpAV4 ascovirus is a mutualist in certain Diadromus wasps but is pathogenic or not when vectored by other species of this genus. These various biologies suggest that ascovirus/wasp relationships depend on wasp regulatory factors that control virus replication. Thus, certain ascoviruses can potentially have either a pathogenic, mutualistic, or non-pathogenic relationship with a specific wasp vector, the type of relationship being dependent upon the species system in which the relationship evolved. Finally, because ascoviruses appear to be related to ichnoviruses (Polydnaviridae), the DpAV4/Diadromus system constitutes a possible interesting intermediate between the pathogenic ascoviruses and symbiotic viruses that evolved to be ichnoviruses.

Genome sequence of a polydnavirus: insights into symbiotic virus evolution

Little is known of the fate of viruses involved in long-term obligatory associations with eukaryotes. For example, many species of parasitoid wasps have symbiotic viruses to manipulate host defenses and to allow development of parasitoid larvae. The complete nucleotide sequence of the DNA enclosed in the virus particles injected by a parasitoid wasp revealed a complex organization, resembling a eukaryote genomic region more than a viral genome. Although endocellular symbiont genomes have undergone a dramatic loss of genes, the evolution of symbiotic viruses appears to be characterized by extensive duplication of virulence genes coding for truncated versions of cellular proteins.

Genomic sequence of a ranavirus (family Iridoviridae) associated with salamander mortalities in North America

Disease is among the suspected causes of amphibian population declines, and an iridovirus and a chytrid fungus are the primary pathogens associated with amphibian mortalities. Ambystoma tigrinum virus (ATV) and a closely related strain, Regina ranavirus (RRV), are implicated in salamander die-offs in Arizona and Canada, respectively. We report the complete sequence of the ATV genome and partial sequence of the RRV genome. Sequence analysis of the ATV/RRV genomes showed marked similarity to other ranaviruses, including tiger frog virus (TFV) and frog virus 3 (FV3), the type virus of the genus Ranavirus (family Iridoviridae), as well as more distant relationships to lymphocystis disease virus, Chilo iridescent virus, and infectious spleen and kidney necrosis virus. Putative open reading frames (ORFs) in the ATV sequence identified 24 genes that appear to control virus replication and block antiviral responses. In addition, >50 other putative genes, homologous to ORFs in other iridoviral genomes but of unknown function, were also identified. Sequence comparison performed by dot plot analysis between ATV and itself revealed a conserved 14-bp palindromic repeat within most intragenic regions. Dot plot analysis of ATV vs RRV sequences identified several polymorphisms between the two isolates. Finally, a comparison of ATV and TFV genomic sequences identified genomic rearrangements consistent with the high recombination frequency of iridoviruses. Given the adverse effects that ranavirus infections have on amphibian and fish populations, ATV/RRV sequence information will allow the design of better diagnostic probes for identifying ranavirus infections and extend our understanding of molecular events in ranavirus-infected cells.

Evidence for the evolution of ascoviruses from iridoviruses

Ascoviruses (family Ascoviridae) are large, enveloped, double-stranded (ds)DNA viruses that attack lepidopteran larvae and pupae, and are unusual in that they are transmitted by parasitic wasps during oviposition. Previous comparisons of DNA polymerase sequences from vertebrate and invertebrate viruses suggested that ascoviruses are closely related to iridoviruses. This relationship was unexpected because these viruses differ markedly in virion symmetry, genome configuration and cellular pathology. Here we present evidence based on sequence comparisons and phylogenetic analyses of a greater range of ascovirus proteins and their homologues in other large dsDNA viruses that ascoviruses evolved from iridoviruses. Consensus trees for the major capsid protein, DNA polymerase, thymidine kinase and ATPase III from representative ascoviruses, algal viruses (family Phycodnaviridae), vertebrate and invertebrate iridoviruses (family Iridoviridae) and African swine fever virus (ASFV; family Asfarviridae) showed that ascovirus proteins clustered most closely with those of the lepidopteran iridovirus Chilo iridescent virus (CIV) (Invertebrate iridescent virus 6). Moreover, analysis of the presence or absence of homologues of an additional 50 proteins encoded in the genome of Spodoptera frugiperda ascovirus (SfAV-1a) showed that about 40 % occurred in CIV, with lower percentages encoded by the genomes of, respectively, vertebrate iridoviruses, phycodnaviruses and ASFV. The occurrence of three of these genes in SfAV-1a but not CIV was indicative of the evolutionary differentiation of ascoviruses from invertebrate iridoviruses.

Phylogenetic analysis and possible function of bro-like genes, a multigene family widespread among large double-stranded DNA viruses of invertebrates and bacteria

Baculovirus repeated open reading frame (bro) genes and their relatives constitute a multigene family, typically with multiple copies per genome, known to occur among certain insect dsDNA viruses and bacteriophages. Little is known about the evolutionary history and function of the proteins encoded by these genes. Here we have shown that bro and bro-like (bro-l) genes occur among viruses of two additional invertebrate viral families, Ascoviridae and Iridoviridae, and in prokaryotic class II transposons. Analysis of over 100 sequences showed that the N-terminal region, consisting of two subdomains, is the most conserved region and contains a DNA-binding motif that has been characterized previously. Phylogenetic analysis indicated that these proteins are distributed among eight groups, Groups 1-7 consisting of invertebrate virus proteins and Group 8 of proteins in bacteriophages and bacterial transposons. No bro genes were identified in databases of invertebrate or vertebrate genomes, vertebrate viruses and transposons, nor in prokaryotic genomes, except in prophages or transposons of the latter. The phylogenetic relationship between bro genes suggests that they have resulted from recombination of viral genomes that allowed the duplication and loss of genes, but also the acquisition of genes by horizontal transfer over evolutionary time. In addition, the maintenance and diversity of bro-l genes in different types of invertebrate dsDNA viruses, but not in vertebrate viruses, suggests that these proteins play an important role in invertebrate virus biology. Experiments with the unique orf2 bro gene of Autographa californica multicapsid nucleopolyhedrovirus showed that it is not required for replication, but may enhance replication during the occlusion phase of reproduction.

Molecular characterization of the major virion protein gene from the Trichoplusia ni ascovirus

Ascoviruses (AVs) belong to a family of double-stranded DNA viruses that infect Lepidoptera insects and cause the unique pathology of forming virion-containing vesicles in the hemolymph of infected hosts. Virions of AVs are large and contain more than 12 polypeptides. A gene, TnAV-CP, encoding the major structural protein of the Trichoplusia ni ascovirus 2a (TnAV-2a) was cloned by immunoscreening an expression library with antibodies against total TnAV virion proteins. TnAV-CP is an intronless gene with an open reading frame encoding a protein of 455 amino acids. Southern blot showed that it is a single copy gene. A 3.8 kb BamHI fragment containing the complete TnAV-CP gene was cloned and sequenced. Northern analysis detected the transcription of the 1.4 kb TnAV-CP mRNA from 24 h after infection. The predicted TnAV-CP protein was expressed in bacterial expression system and purified to homogeneity. The recombinant protein was used to affinity-purify specific antibodies from the antiserum. The purified antibodies reacted strongly with a single protein of approximately 52 kDa from the total TnAV virion proteins in a Western blot. This protein corresponds to the most abundant structural protein present in the virions of several AVs. Sequence comparison showed that TnAV-CP is most homologous to the putative capsid proteins from AVs infecting noctuid insects, less homologous to that of Diadromus pulchellus ascovirus 4a (DpAV-4a), further supporting the distinction of two subgroups within the family Ascoviridae. Phylogenetic analysis using the putative capsid protein suggested that AVs were closely related to members of Iridoviridae, which corroborated the result based on DNA polymerase delta sequences. The apparent differences between Ascoviridae and Iridoviridae in host range, virion morphology, and genome configuration, and the similarities in genes and methylation of genomic DNA were discussed.

Polydnavirus genome: integrated vs. free virus

Polydnaviruses are unique because of their obligatory association with thousands of parasitoid wasp species from the braconid and ichneumonid families of hymenopterans. PDVs are injected into the parasitized hosts and are essential for parasitism success. However, polydnaviruses are also unique because of their genome composed of multiple dsDNA segments. Cytological evidence has recently confirmed the results of genetic and molecular analyses indicating that PDV segments were integrated in the wasp genome. Moreover a phylogenetic study performed using the age of available fossils to calibrate the molecular clock indicated that the polydnaviruses harboured by braconid wasps have resided within the wasp genome for approximately 70 million years. In the absence of horizontal transmission, the evolution of the PDV genomes has been driven exclusively by the reproductive success they have offered the wasps. The consequences of this particular selection pressure can be observed in the gene content of certain PDV genomes from which increasing sequence data are available. Molecular mechanisms already identified could be involved in the acquisition and loss of genes by the PDV genomes and lead us to speculate on the definition of the virus genome.

Origin and evolution of polydnaviruses by symbiogenesis of insect DNA viruses in endoparasitic wasps

During oviposition, many endoparasitic wasps inject virus-like particles into their insect hosts that enable these parasitoids to evade or directly suppress their hosts' immune system, especially encapsulation by hemocytes. These particles are defined as virions that belong to viruses of the two genera that comprise the family Polydnaviridae, bracoviruses (genus Bracovirus) transmitted by braconid wasps, and ichnoviruses (genus Ichnovirus) transmitted by ichneumonid wasps. Structurally, bracovirus virions resemble nudivirus and baculovirus virions (family Baculoviridae), and ichnovirus virions resemble those of ascoviruses (family Ascoviridae). Whereas nudiviruses, baculoviruses and ascoviruses replicate their DNA and produce progeny virions, polydnavirus DNA is integrated into and replicated from the wasp genome, which also directs virion synthesis. The structural similarity of polydnavirus virions to those of viruses that attack the wasps' lepidopteran hosts, along with polydnavirus transmission and replication biology, suggest that these viruses evolved from insect DNA viruses by symbiogenesis, the same process by which mitochondia and chloroplasts evolved from bacteria. Molecular evidence supporting this hypothesis comes from similarities among structural proteins of ascoviruses and the Campoletis sonorensis ichnovirus. Implications of this hypothesis are that polydnaviruses evolved from viruses, but are no longer viruses, and that DNA packaged into polydnavirus virions is not viral genomic DNA per se, but rather wasp genomic DNA consisting primarily of wasp genes and non-coding DNA. Thus, we suggest that a better understanding of polydnaviruses would result by viewing these not as viruses, but rather as a wasp organelle system that evolved to shuttle wasp genes and proteins into hosts to evade and suppress their immune response.

Parasitoid-mediated transmission of an iridescent virus

We examined the interaction between an invertebrate iridescent virus (IIV) isolated from Spodoptera frugiperda (J.E. Smith) and the solitary ichneumonid endoparasitoid Eiphosoma vitticolle Cresson. In choice tests, parasitoids examined and stung significantly more virus infected than healthy larvae, apparently due to a lack of defense reaction in virus infected hosts. Parasitoid-mediated virus transmission was observed in 100% of the female parasitoids that stung a virus infected host in the laboratory. Each female parasitoid transmitted the virus to an average (+/-SE) of 3.7+/-0.3 larvae immediately after stinging an infected larva. Caged field experiments supported this result; virus transmission to healthy larvae only occurred in cages containing infected hosts (as inoculum) and parasitoids (as vectors). The virus was highly detrimental to parasitoid development because of premature host death and lethal infection of the developing endoparasitoid. Female parasitoids that emerged from virus infected hosts did not transmit the virus to healthy hosts. We suggest that the polyphagous habits of many noctuid parasitoids combined with the catholic host range of most IIVs may represent a mechanism for the transmission of IIVs between different host species in the field.

Effects of the Diadromus pulchellus ascovirus, DpAV-4, on the hemocytic encapsulation response and capsule melanization of the leek-moth pupa, Acrolepiopsis assectella

DpAV-4 is a symbiotic ascovirus found in natural populations of the solitary endoparasitoid wasp Diadromus pulchellus. The female wasp injects this virus into the pupae of the leek-moth Acrolepiopsis assectella during oviposition. The ascovirus replicates in the pupal tissues and the consequent lysis of the cells occurs synchronously with egg hatching and the development of the wasp larva. We show here that encapsulation and capsule melanization were activated when minute nylon monofilaments were implanted into the hemocoel of non parasitized leek-moth pupae and that encapsulation and melanization were inhibited in pupae parasitized by D. pulchellus. When the pupae were infected by DpAV-4, melanization of the nylon monofilaments was abolished, but a capsule was still always formed. These results indicate that DpAV-4 is a free virus able to alter the defence system of the parasitized host so as to improve the development of the parasitoid wasp, D. pulchellus.

Characterization of repetitive DNA regions and methylated DNA in ascovirus genomes

The accompanying phylogenetic study of large double-stranded DNA viruses based on their delta DNA polymerase genes suggests that ascoviruses (family ASCOVIRIDAE:) and iridoviruses (family IRIDOVIRIDAE:) are closely related and may share a common ancestor. This relationship was unexpected because of marked differences between these viruses. Iridoviruses produce icosahedral virions and occur broadly among vertebrates and invertebrates, whereas ascoviruses typically produce reniform or bacilliform virions and are restricted to insect hosts, primarily lepidopterans. Detailed comparisons of these two virus types are not possible because fundamental information on the properties of the virions and their genomes is lacking, especially for ascoviruses. To facilitate further investigation of the putative evolutionary relationship between ascoviruses and iridoviruses, the genomes of representative viruses from each family were compared with respect to physical configuration, presence of DNA repeats and degree of DNA methylation. Genomes from Spodoptera frugiperda (SfAV1), Heliothis virescens (HvAV3) and Diadromus pulchellus (DpAV4) ascoviruses were all found to be circular and partially superhelical and to contain large interspersed repeats of 1-3 kbp. Mosquito (IV type 3), lepidopteran (IV type 6) and isopod (IV type 31) iridovirus genomes were all linear and lacked large regions of repetitive DNA. Ascovirus and iridovirus genomes were methylated and one, DpAV4, had the highest degree of methylation of any reported animal DNA virus. The major differences in the physical and biochemical characteristics of ascoviruses and iridoviruses reported here provide a foundation for further studies of their relatedness while making their possible close relationship and divergence during evolution of even greater interest.