A comparison of six cypovirus isolates by cross-hybridisation of their dsRNA genome segments

Mechanisms of Cell Entry by dsRNA Viruses: Insights for Efficient Delivery of dsRNA and Tools for Improved RNAi-Based Pest Control

While RNAi is often heralded as a promising new strategy for insect pest control, a major obstacle that still remains is the efficient delivery of dsRNA molecules within the cells of the targeted insects. However, it seems overlooked that dsRNA viruses already have developed efficient strategies for transport of dsRNA molecules across tissue barriers and cellular membranes. Besides protecting their dsRNA genomes in a protective shell, dsRNA viruses also display outer capsid layers that incorporate sophisticated mechanisms to disrupt the plasma membrane layer and to translocate core particles (with linear dsRNA genome fragments) within the cytoplasm. Because of the perceived efficiency of the translocation mechanism, it is well worth analyzing in detail the molecular processes that are used to achieve this feat. In this review, the mechanism of cell entry by dsRNA viruses belonging to the Reoviridae family is discussed in detail. Because of the large amount of progress in mammalian versus insect models, the mechanism of infections of reoviruses in mammals (orthoreoviruses, rotaviruses, orbiviruses) will be treated as a point of reference against which infections of reoviruses in insects (orbiviruses in midges, plant viruses in hemipterans, insect-specific cypoviruses in lepidopterans) will be compared. The goal of this discussion is to uncover the basic principles by which dsRNA viruses cross tissue barriers and translocate their cargo to the cellular cytoplasm; such knowledge subsequently can be incorporated into the design of dsRNA virus-based viral-like particles for optimal delivery of RNAi triggers in targeted insect pests.

Genome Sequence Analysis of CsRV1: A Pathogenic Reovirus that Infects the Blue Crab Callinectes sapidus Across Its Trans-Hemispheric Range

The blue crab, Callinectes sapidus Rathbun, 1896, which is a commercially important trophic link in coastal ecosystems of the western Atlantic, is infected in both North and South America by C. sapidus Reovirus 1 (CsRV1), a double stranded RNA virus. The 12 genome segments of a North American strain of CsRV1 were sequenced using Ion Torrent technology. Putative functions could be assigned for 3 of the 13 proteins encoded in the genome, based on their similarity to proteins encoded in other reovirus genomes. Comparison of the CsRV1 RNA-dependent RNA polymerase (RdRP) sequence to genomes of other crab-infecting reoviruses shows that it is similar to the mud crab reovirus found in Scylla serrata and WX-2012 in Eriocheir sinensis, Chinese mitten crab, and supports the idea that there is a distinct “Crabreo” genus, different from Seadornavirus and Cardoreovirus, the two closest genera in the Reoviridae. A region of 98% nucleotide sequence identity between CsRV1 and the only available sequence of the P virus of Macropipus depurator suggests that these two viruses may be closely related. An 860 nucleotide region of the CsRV1 RdRP gene was amplified and sequenced from 15 infected crabs collected from across the geographic range of C. sapidus. Pairwise analysis of predicted protein sequences shows that CsRV1 strains in Brazil can be distinguished from those in North America based on conserved residues in this gene. The sequencing, annotation, and preliminary population metrics of the genome of CsRV1 should facilitate additional studies in diverse disciplines, including structure-function relationships of reovirus proteins, investigations into the evolution of the Reoviridae, and biogeographic research on the connectivity of C. sapidus populations across the Northern and Southern hemispheres.

Histopathology of cotton bollworm midgut infected with Helicoverpa armigera cytoplasmic polyhedrosis virus

This research was carried out to examine cytopathological effects of Helicoverpa armigera Cytoplasmic polyhedrosis virus (HaCPV) on infected midgut cotton bollworm (Helicoverpa armigera) using transmission and scanning electron microscope. The symptoms on infected host larvae of the host, compared with healthy ones, were getting swollen with milky-white and fragile Histopathological examinations showed infection with HaCPV small polyhedral inclusion bodies (PIB) after 1 or 2 days which were observed in columnar cells of midgut. Virions were partially or completely occupied in a polyhedral matrix to form polyhedral inclusion bodies (PIB) at periphery of virogenic stroma. PIBs were measured 0.5 to 3.5 μm and virions about 46 nm in diameter. Microvilli of infected columnar cells were affected and degenerated immediately prior to rupture of the cell. Some infected columnar cells ruptured to release PIB into the gut lumen 3 days after infection. In addition, PIB were found in goblet cells, 5 or 6 days after infection. Infected goblet cells degenerate to such an extent that only a few of the original microvillus-like cytoplasmic projections and cell organells were left. These cytopathic effects caused in the midgut by HaCPV on cotton bollworm larvae are essentially similar to those have been reported for lepidoperan and dipteran infection by CPV.

Reovirus genomes from plant-feeding insects represent a newly discovered lineage within the family Reoviridae

A complex set of double-stranded RNAs (dsRNAs) was isolated from threecornered alfalfa hopper (Spissistilus festinus), a plant-feeding hemipteran pest. A subset of these dsRNAs constitute the genome of a new reovirus, provisionally designated Spissistilus festinus reovirus (SpFRV). SpFRV was present in threecornered alfalfa hopper populations in the San Joaquin Valley of California, with incidence ranging from 10% to 60% in 24 of 25 sample sets analyzed. The 10 dsRNA segments of SpFRV were completely sequenced and shown to share conserved terminal sequences (5'-AGAGA and CGAUGUUGU-3') of the positive-sense strand that are distinct from known species of the family Reoviridae. Comparisons of the RNA directed RNA polymerase (RdRp) indicated SpFRV is most closely related (39.1% amino acid identity) to another new reovirus infecting the angulate leafhopper (Acinopterus angulatus) and provisionally designated Acinopterus angulatus reovirus (AcARV). The RdRp of both viruses was distantly related to Raspberry latent virus RdRp at 27.0% (SpFRV) and 30.0% (AcARV) or Rice ragged stunt virus RdRp at 26.2% (SpFRV) and 29.0% (AcARV) amino acid identity. RdRp phylogeny confirmed that SpFRV and AcARV are sister taxa sharing a most recent common ancestor. SpFRV segment 6 encodes a protein containing two NTP binding motifs that are conserved in homologs of reoviruses in the subfamily Spinareovirinae. The protein encoded by SpFRV segment 4 was identified as a guanylyltransferase homolog. SpFRV segments 1, 3, and 10 encode homologs of reovirus structural proteins. No homologs were identified for proteins encoded by SpFRV segments 5, 7, 8, and 9. Collectively, the low level of sequence identity with other reoviruses, similar segment terminal sequences, RdRp phylogeny, and host taxa indicate that SpFRV and AcARV may be considered members of a proposed new genus of the family Reoviridae (subfamily Spinareovirinae), with SpFRV assigned as the type species.

Biological and molecular studies of a cypovirus from the black fly Simulium ubiquitum (Diptera: Simuliidae)

A cypovirus from the black fly Simulium ubiquitum (SuCPV) was isolated and examined using biological and molecular techniques. SuCPV produces small (typically 0.25mum), polyhedral shaped inclusion bodies (polyhedra), in which the virus particles become multiply embedded. SuCPV is the third cypovirus isolated from Diptera, but the first from Simuliidae that has been characterized using molecular analyses. SuCPV has a genome composed of 10 segments of dsRNA, with an electrophoretic migration pattern that is different from those of recent UsCPV-17 and CrCPV-17 isolates from the mosquitoes Uranotaenia sapphirina and Culex restuans, respectively. The SuCPV electropherotype appears to show significant differences from those of the previously characterized lepidopteran cypoviruses. Sequence analysis of SuCPV segment 10 shows that it is unrelated to either of the two CPV isolates from Diptera or to the CPV species for which Seg-10 has been previously characterized from Lepidoptera. A comparison of the terminal regions of SuCPV genome segments to those of CPV-1, 2, 4, 5 14, 15, 16, 17, 18, and 19 also revealed only low levels of conservation. We therefore, propose that SuCPV is classified within a new Cypovirus species, which we have tentatively identified as Cypovirus-20. We have therefore referred to this virus isolate as S. ubiquitum CPV-20 (SuCPV-20).

Phylogenetic analysis of Heliothis armigera cytoplasmic polyhedrosis virus type 14 and a series of dwarf segments found in the genome

Full-length nucleotide sequences for the genome segments (S1-S6) of Heliothis armigera cytoplasmic polyhedrosis virus type 14 (HaCPV-14) have been characterized. Each segment consists of a single open reading frame with conserved motifs AGAA and AGCU at the 5' and 3' ends, respectively. Comparison of the proteins of HaCPV-14 with those of other members of the family Reoviridae suggests that S1 encodes an RNA-dependent RNA polymerase (RdRp), whilst S2 encodes a major capsid protein of the virus. Phylogenetic analysis of RdRps from 16 viruses in the family Reoviridae reveals that the genera Cypovirus and Oryzavirus may have originated from a common insect virus ancestor. A series of viable dwarf segments originating from S5 of HaCPV-14 has been identified. Analysis of the predicted secondary structures for these dwarf segments suggests that the signals essential for replication and packaging are located within the terminal sequences of these segments.

Identification and genome characterization of Heliothis armigera cypovirus types 5 and 14 and Heliothis assulta cypovirus type 14

Genomic characterization of Heliothis armigera cypovirus (HaCPV) isolated from China showed that insects were co-infected with several cypoviruses (CPVs). One of the CPVs (HaCPV-5) could be separated from the others by changing the rearing conditions of the Heliothis armigera larvae. This finding was further confirmed by nucleotide sequencing analysis. Genomic sequences of segments S10-S7 from HaCPV-14, S10 and S7 from HaCPV-5, and S10 from Heliothis assulta CPV-14 were compared. Results from database searches showed that the nucleotide sequences and deduced amino acid sequences of the newly identified CPVs had high levels of identity with those of reported CPVs of the same type, but not with CPVs of different types. Putative amino acid sequences of HaCPV-5 S7 were similar to that of the protein from Rice ragged stunt virus (genus Oryzavirus, family Reoviridae), suggesting that CPVs and oryzaviruses are related more closely than other genera of the family Reoviridae. Conserved motifs were also identified at the ends of each RNA segment of the same virus type: type 14, 5'-AGAAUUU...CAGCU-3'; and type 5, 5'-AGUU...UUGC-3'. Our results are consistent with classification of CPV types based on the electrophoretic patterns of CPV double-stranded RNA.

Molecular and biological characterization of a Cypovirus from the mosquito Culex restuans

A cypovirus from the mosquito Culex restuans (named CrCPV) was isolated and its biology, morphology, and molecular characteristics were investigated. CrCPV is characterized by small (0.1-1.0 microm), irregularly shaped inclusion bodies that are multiply embedded. Laboratory studies demonstrated that divalent cations influenced transmission of CrCPV to Culex quinquefasciatus larvae; magnesium enhanced CrCPV transmission by approximately 30% while calcium inhibited transmission. CrCPV is the second cypovirus from a mosquito that has been confirmed by using molecular analysis. CrCPV has a genome composed of 10 dsRNA segments with an electropherotype similar to the recently discovered UsCPV-17 from the mosquito Uranotaenia sapphirina, but distinct from the lepidopteran cypoviruses BmCPV-1 (Bombyx mori) and TnCPV-15 (Trichoplusia ni). Nucleotide and deduced amino acid sequence analysis of CrCPV segment 10 (polyhedrin) suggests that CrCPV is closely related (83% nucleotide sequence identity and 87% amino acid sequence identity) to the newly characterized UsCPV-17 but is unrelated to the 16 remaining CPV species from lepidopteran hosts. A comparison of the terminal segment regions of CrCPV and UsCPV-17, an additional method for differentiating various Cypovirus species, revealed a high level of conservation. Therefore, we propose that CrCPV is a member of the Cypovirus-17 group and designate this species as CrCPV-17.

Morphological and molecular characterization of a Cypovirus (Reoviridae) from the mosquito Uranotaenia sapphirina (Diptera: Culicidae)

A novel cypovirus has been isolated from the mosquito Uranotaenia sapphirina (UsCPV) and shown to cause a chronic infection confined to the cytoplasm of epithelial cells of the gastric ceca and posterior stomach. The production of large numbers of virions and inclusion bodies and their arrangement into paracrystalline arrays gives the gut of infected insects a distinctive blue iridescence. The virions, which were examined by electron microscopy, are icosahedral (55 to 65 nm in diameter) with a central core that is surrounded by a single capsid layer. They are usually packaged individually within cubic inclusion bodies (polyhedra, approximately 100 nm across), although two to eight virus particles were sometimes occluded together. The virus was experimentally transmitted per os to several mosquito species. The transmission rate was enhanced by the presence of magnesium ions but was inhibited by calcium ions. Most of the infected larvae survived to adulthood, and the adults retained the infection. Electrophoretic analysis of the UsCPV genome segments (using 1% agarose gels) generated a migration pattern (electropherotype) that is different from those of the 16 Cypovirus species already recognized. UsCPV genome segment 10 (Seg-10) showed no significant nucleotide sequence similarity to the corresponding segment of the other cypoviruses that have previously been analyzed, and it has different "conserved" termini. A BLAST search of the UsCPV deduced amino acid sequence also showed little similarity to Antheraea mylitta CPV-4 (67 of 290 [23%]) or Choristoneura fumiferana CPV-16 (33 of 111 [29%]). We conclude that UsCPV should be recognized as a member of a new Cypovirus species (Cypovirus 17, strain UsCPV-17).

Commensal and mutualistic relationships of reoviruses with their parasitoid wasp hosts

During evolution, certain endoparasitoid wasps have developed mechanisms to suppress the defence systems of their hosts. For this purpose, these species, all of which belong to the families Ichneumonidae and Braconidae, inject various kinds of virus-like particles. The most studied of these particles are classified as polydnaviruses (family Polydnaviridae) which are symbiotic viruses. Over the past decade, it has also been shown that several wasp species harbour reoviruses (family Reoviridae), and that two of these suppress host defence, allowing the development of the parasitoid eggs. In this paper, we summarize the key features of these viruses and their relationships with their wasp hosts. Five reoviruses are known that appear to be non-pathogenic for the wasps. Three of these, McRVLP, HeRV, OpRVLP, use their wasp hosts as vectors, and do not appear to be involved in host defence suppression. The fourth, DpRV-1, is a commensal reovirus detected in most field populations of the wasp, Diadromus pulchellus. This reovirus is always found associated with an ascovirus, DpAV-4a, which is indispensable for host immune suppression. Although DpRV-1 has not been shown to directly increase D. pulchellus parasitic success, it may contribute to this success by retarding DpAV-4a replication in the wasp. The fifth reovirus, DpRV-2, occurs in a specific population of D. pulchellus in which DpRV-1 and DpAV-4 are absent. This virus has a mutualistic relationship with its wasp host, as its injection by females during oviposition is essential for host immunosuppression. Interestingly, these viruses belong to several different reovirus genera.

Lipofectin increases the specific activity of cypovirus particles for cultured insect cells

Cytoplasmic polyhedrosis viruses (CPV) are classified as 14 distinct species (electropherotypes) within the genus Cypovirus, family Reoviridae. Cypovirus research has been limited by a lack of appropriate cell culture systems (for each of these virus species) in which the majority of cells can become productively infected. Lipofection increased the infection rate of Lymantria dispar 652 cells, by virus particles (derived from polyhedra) of Orgyia pseudosugata type 5 cypovirus (Op-5 CPV), from 3 to 44%. Lipofection also significantly increased the percentage of Trichoplusia ni 368 cells infected with the same virus (from < 1 to approximately 7%). The spread of cypovirus infection between cells was either very slow or insignificant, and infected cells appeared to remain viable for long periods. Virus infection was detected by the observation of polyhedra formation in individual cells and it was therefore possible to develop a simple quantitative assay system to measure virus titre (TCID50). Cryo-electron microscopy showed that cypovirus particles formed a complex with the lipid, involving their envelopment within the liposome membrane. It was concluded that the increased infectivity of the virus by lipofection was due to a more efficient cell entry mechanism, probably involving fusion between liposome and cell membranes.