Nucleotide sequences of four RNA segments of a reovirus isolated from the mud crab Scylla serrata provide evidence that this virus belongs to a new genus in the family Reoviridae

Incidence of mud crab reovirus (MCRV) outbreak in polyculture ponds of Andhra Pradesh, south east coast of India

Reovirus designated as Mud crab reovirus (MCRV) is associated with the mass mortalities of mud crabs resulting in significant economic loss to crab and shrimp-mud crab polyculture farmers in the Nagayalanka, Krishna district, Andhra Pradesh. The 100 % chronic mass mortalities have been attributed to the outbreak of Mud crab reovirus (MCRV) in the polyculture farms. The moribund crabs showed autotomy, discoloration of carapace, loss of appetite, slow movement and loose gills. Histopathological observations of the infected mud crabs showed an atrophied hepatopancreas, complete degeneration of tissues along with viral inclusions in hepatopancreas, gills and muscles. Further analysis using Transmission electron microscopy (TEM), showed that the viral particles had a diameter of 70 nm and exhibited a non-enveloped, icosahedral shape arranged in a crystalline manner. The virus mainly infects the connective tissue of hepatopancreas, gills, muscle and develops in the cytoplasm. RT-PCR reconfirmed the presence of reovirus in the hepatopancreas of spontaneously infected mud crab Scylla serrata. The current study shows the importance of monitoring the MCRV prevalence in polyculture farms to minimize its spread and precautionary measures can be taken by screening the brooders from the crab hatchery and stocking of wild crabs without screening should be avoided in order to prevent MCRV outbreak.

Diversity and classification of reoviruses in crustaceans: A proposal

A variety of reoviruses have been described in crustacean hosts, including shrimp, crayfish, prawn, and especially in crabs. However, only one genus of crustacean reovirus - Cardoreovirus - has been formally recognized by ICTV (International Committee on Taxonomy of Viruses) and most crustacean reoviruses remain unclassified. This arises in part from ambiguous or incomplete information on which to categorize them. In recent years, increased availability of crustacean reovirus genomic sequences is making the discovery and classification of crustacean reoviruses faster and more certain. This minireview describes the properties of the reoviruses infecting crustaceans and suggests an overall classification of brachyuran crustacean reoviruses based on a combination of morphology, host, genome organization pattern and phylogenetic sequence analysis.

Identification and characterization of host cell proteins interacting with Scylla serrata reovirus non-structural protein p35

We have previously shown that non-structural protein p35, encoded by Scylla serrata reovirus (SsRV) S10, may act as a viroporin. To characterize the role of p35 protein in the modulation of cellular function, a yeast two-hybrid system was used to screen a cDNA library derived from S. serrata to find its interacting partner. Protein interactions were confirmed in vitro by GST pull-down. Full cDNAs of p35 interactors were cloned by the rapid amplification of cDNA ends. After two-hybrid library screening, we isolated partial cDNAs encoding hemocyanin, cryptocyanin, and TAX1BP1. Interaction of p35 with each of hemocyanin, cryptocyanin, and TAX1BP1 was confirmed by GST pull-down. The full-length cDNA fragments of hemocyanin, cryptocyanin, and TAX1BP1 were 2287, 2422, and 3437 bp, respectively, and they encoded three putative proteins with molecular masses of ~76.9, ~79.2, and ~107.2 kDa, respectively. This study casts new light on the function and physiological significance of p35 during the SsRV replication cycle.

Identification and RNA segment assignment of six structural proteins of Scylla serrata reovirus

Scylla serrata reovirus (SsRV) is one of the most prevalent viral pathogens of the mud crab (S. serrata). The virus represents an unassigned novel genus in the Reoviridae family, and contains 12 double-stranded RNA genomic segments. Previous analysis of virion proteins concluded that SsRV contains at least eight structural proteins, ranging from 25 to 160 kDa. Here, tandem time-of-flight mass spectrometry and Western blotting were used to re-identify the structural proteins. The results indicate that proteins encoded by SsRV segments S1, S3, S6, S9, S11, and S12 are structural proteins.

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.

Scylla serrata reovirus p35 protein expressed in Escherichia coli cells alters membrane permeability

To promote viral entry, replication, release, and spread to neighboring cells, many cytolytic animal viruses encode proteins responsible for modification of host cell membrane permeability and for formation of ion channels in host cell membranes. Scylla serrata reovirus (SsRV) is a major pathogen that can severely damage mud crab (S. serrata) aquaculture. Protein p35, which is encoded by segment 10 of SsRV, contains two transmembrane domains. In this study, we found that SsRV p35 can induce membrane permeability changes when expressed in Escherichia coli. SsRV p35 expressed in bacterial cells existed as monomers under reducing conditions but formed homodimers and homotrimers under non-reducing conditions. These findings demonstrate that p35 may act as a viroporin; further studies are needed to elucidate the detailed structure-function relationships of this protein.

Near-Full-Length Genome Sequence of a Novel Reovirus from the Chinese Mitten Crab, Eriocheir sinensis

A novel Eriocheir sinensis reovirus (EsRV) was identified using deep-sequencing techniques in crabs afflicted with trembling disease (TD). Near-full-length genome sequences of 12 segments of EsRV were obtained. The genome of EsRV will facilitate further studies on the causative agent of TD.

Cimodo virus belongs to a novel lineage of reoviruses isolated from African mosquitoes

A novel reovirus, designated Cimodo virus (CMDV), was isolated from mosquitoes collected in a rainforest region in Côte d’Ivoire. The entire genome comprised 24 835 bp divided into 12 segments ranging from 585 to 4080 bp. The icosahedral non-enveloped virions were 80 nm in diameter. Eight major viral proteins of about 150, 135, 120, 80, 66, 59, 42 and 30 kDa were identified and seven proteins were mapped to the corresponding genome segments by liquid chromatography mass spectrometry. Predicted protein genes diverged by >77 % encoded amino acids from their closest reovirus relatives. The deep phylogenetic branching suggests that CMDV defines an as-yet-unidentified genus within the subfamily Spinareovirinae.

Identification and genomic characterization of a novel fish reovirus, Hubei grass carp disease reovirus, isolated in 2009 in China

A novel fish reovirus, Hubei grass carp disease reovirus (HGDRV; formerly grass carp reovirus strain 104, GCRV104), was isolated from diseased grass carp in China in 2009 and the full genome sequence was determined. This reovirus was propagated in a grass carp kidney cell line with a typical cytopathic effect. The total size of the genome was 23 706 bp with a 51 mol% G+C content, and the 11 dsRNA segments encoded 12 proteins (two proteins encoded by segment 11). A nucleotide sequence similarity search using blastn found no significant matches except for segment 2, which partially matched that of the RNA-dependent RNA polymerase (RdRp) from several viruses in the genera Aquareovirus and Orthoreovirus of the family Reoviridae. At the amino acid level, seven segments (Seg-1 to Seg-6, and Seg-8) matched with species in the genera Aquareovirus (15-46 % identities) and Orthoreovirus (12-44 % identities), while for four segments (Seg-7, Seg-9, Seg-10 and Seg-11) no similarities in these genera were found. Conserved terminal sequences, 5'-GAAUU----UCAUC-3', were found in each HGDRV segment at the 5' and 3' ends, and the 5'-terminal nucleotides were different from any known species in the genus Aquareovirus. Phylogenetic analysis based on RdRp amino acid sequences from members of the family Reoviridae showed that HGDRV clustered with aquareoviruses prior to joining a branch common with orthoreoviruses. Based on these observations, we propose that HGDRV is a new species in the genus Aquareovirus that is distantly related to any known species within this genus.

Isolation and partial characterization of a new reovirus in the Chinese mitten crab, Eriocheir sinensis H Milne Edwards

A second type of freshwater crab reovirus has been isolated from Chinese mitten crab, Eriocheir sinensis H. Milne Edwards, in China; we named it E. sinensis reovirus (EsRV816). The negatively stained virion is a non-enveloped icosahedral particle, 60 ± 5 nm in diameter. Its genome is composed of 10 dsRNA linear pieces exhibiting an electrophoretic pattern of 5/3/2. The largest segment (RNA-1) was cloned and sequenced. The deduced amino acid sequence, corresponding to the RdRp of the virus, showed 26% identity with the RdRp of Operophtera brumata (L.) cypovirus 19 in the genus Cypovirus and 24% identity with RdRp of Nilaparvata lugens (Stal) reovirus in the genus Fijivirus. On the basis of its ultrastructure and physicochemical properties, this virus is quite different from other crab reoviruses, and particularly with another freshwater crab reovirus EsRV905, recently classified in a new genus Cardoreovirus. This virus (EsRV816) possesses all the characters of the members of the reoviridae family and could represent a new genus.

Evolutionary genomics of mycovirus-related dsRNA viruses reveals cross-family horizontal gene transfer and evolution of diverse viral lineages

Background

Double-stranded (ds) RNA fungal viruses are typically isometric single-shelled particles that are classified into three families, Totiviridae, Partitiviridae and Chrysoviridae, the members of which possess monopartite, bipartite and quadripartite genomes, respectively. Recent findings revealed that mycovirus-related dsRNA viruses are more diverse than previously recognized. Although an increasing number of viral complete genomic sequences have become available, the evolution of these diverse dsRNA viruses remains to be clarified. This is particularly so since there is little evidence for horizontal gene transfer (HGT) among dsRNA viruses.

Results

In this study, we report the molecular properties of two novel dsRNA mycoviruses that were isolated from a field strain of Sclerotinia sclerotiorum, Sunf-M: one is a large monopartite virus representing a distinct evolutionary lineage of dsRNA viruses; the other is a new member of the family Partitiviridae. Comprehensive phylogenetic analysis and genome comparison revealed that there are at least ten monopartite, three bipartite, one tripartite and three quadripartite lineages in the known dsRNA mycoviruses and that the multipartite lineages have possibly evolved from different monopartite dsRNA viruses. Moreover, we found that homologs of the S7 Domain, characteristic of members of the genus phytoreovirus in family Reoviridae are widely distributed in diverse dsRNA viral lineages, including chrysoviruses, endornaviruses and some unclassified dsRNA mycoviruses. We further provided evidence that multiple HGT events may have occurred among these dsRNA viruses from different families.

Conclusions

Our study provides an insight into the phylogeny and evolution of mycovirus-related dsRNA viruses and reveals that the occurrence of HGT between different virus species and the development of multipartite genomes during evolution are important macroevolutionary mechanisms in dsRNA viruses.

Structural insights into the classification of Mud Crab Reovirus

Cryo-electron microscopy was applied to analyze mud crab reovirus (MCRV), which causes 'sleeping disease' in mud crab, Scylla serrata, a marine species cultured in China. We present here the three dimensional structure of MCRV at 13.8Å resolution. The outer capsid shell is composed of 260 trimers with complete T=13 icosahedral symmetry. A major difference between MCRV and previously reported aquareoviruses is that it lacks a pentameric turret structure. These results together with recently published molecular biological evidence (Deng et al., 2012) indicate that, from a structural perspective, MCRV should be classified as a new member of the family Reoviridae.