Genus Aquareovirus
|
|
|
|
Type Species |
Aquareovirus A |
(ARV-A) |
Distinguishing Features
Aquareoviruses physically resemble orthoreoviruses but possess 11 dsRNA genome segments. They infect a variety of aquatic animals, including finfish and Crustacea. Aquareoviruses replicate in cell cultures of piscine and mammalian origins at temperatures between 15 and 25°C. Large syncytia are produced as a typical cytopathic effect of infection.
Virion Properties
Morphology
Virus particles are spherical in appearance but have icosahedral symmetry (Fig. 13, Top). The virions are approximately 80 nm in diameter, with two concentric capsid shells that appear to be made up of three layers of proteins. The outer capsid (~ 10 nm thick) surrounds a core ~ 60 nm in diameter. The boundary between the outer capsid and inner core is evident as a prominent white ring in negatively stained electron micrographs (Fig. 13, Bottom). Small projections on the surface of the core particle interconnect inner and outer capsid layers. The core particle also contains an inner layer, which contains the 11 dsRNA genome segments and the internal core proteins (the transcriptase complexes). The aquareovirus particle morphology is strikingly similar to that of the orthoreovirus “intermediate subviral particle” (ISVP). However, a noticeable distinction is that aquareovirus particles lack the hemagglutinin spikes observed in orthoreoviruses.
Physicochemical and Physical Properties
Virion density in CsCl is 1.36 g/cm3. Sedimentation coefficient of the virus particles is ~550S. Virus infectivity is stable between pH 3 and pH 10. Virus infectivity is not affected by treatment with ether or chloroform. None of the viral proteins is removed from the particle by treatment with 3 mM EDTA or cesium salts. Aquareoviruses held at 4, 16 or 23°C in minimal essential medium (MEM) with 5% serum showed no significant reduction in infectivity over a period of 28 days. However, all virus infectivity is lost after incubation at 45°C for 7 days. Virus infectivity is rapidly inactivated by heating to 56°C.
Nucleic Acid
The aquareovirus genome is composed of 11 segments of dsRNA that are packaged in equimolar ratios. The Mr of the dsRNA segments range from 0.4-2.5 
106 and the genome has a total Mr of approximately 16.0 106. The genomic RNAs are named “segment 1” to “segment 11” in order of increasing electrophoretic mobility in 1% agarose gels. The genome segments migrate as three size classes. There are three large (segments 1-3: Mr 2.5 to 2.3 106), three medium (segments 4-6: Mr 1.9 to 1.6 106) and five small segments (segments 7-11: Mr 1.0 to 0.37 106). Six distinct species have been identified by reciprocal RNA-RNA hybridization studies (ARV-A to F). The genome segment migration pattern (electropherotype), as analyzed by electrophoresis in 1% agarose gel, is consistent within a single species but shows significant variation between species. Viruses within a single species do show variations in electropherotype, when their dsRNA genome segments are analyzed by electrophoresis in high percentage polyacrylamide gels (>6%).
Proteins
The virions of ARV-A viruses contain 7 structural proteins: VP1, Mr 130 103, inner capsid (core); VP2, Mr 127 103, inner capsid (core); VP3, Mr 126 103, inner capsid (core); VP4, Mr 73 103, inner capsid (core); VP5, Mr 71 103, minor, outer capsid; VP6, Mr 46 103, inner capsid (core); VP7, Mr 35 103, major outer capsid. They also encode five non-structural proteins: NS1, Mr 97 103; NS2, Mr 39 103; NS3, Mr 29 103; NS4, Mr 28 103; NS5, Mr 15 103. The observed variations in dsRNA electropherotype suggest that viruses from different species may have proteins with significant differences in size.
Lipids
Aquareoviruses have no known lipid components.
Carbohydrates
VP7 of ARV-A isolates may be glycosylated.
Genome Organization and Replication
The coding assignments of SBRV, a strain of Aquareovirus A (ARV-A), have been determined (Table 10). Genome segments 1-10 of the SBRV genome encode one protein each, while segment 7 encodes two proteins.
Antigenic Properties
Aquareovirus outer capsid proteins (CP) lack haemaglutinating activity. Viruses possess type specific and group specific antigenic determinants. Members within a single species may be antigenically related. Members of different species are antigenically distinct. Minor antigenic cross reactivity has only been demonstrated between ARV-A and ARV-B. Distinct serotypes probably exist within each species. The major outer CP of ARV-A (VP7) is not the major neutralizing antigen.
Biological Properties
Aquareoviruses have been isolated from poikilotherm vertebrates as well as invertebrates (hosts include fish, molluscs, etc.) obtained from both fresh and sea water. The viruses replicate efficiently in fish cell lines at temperatures ranging from 15°C to 25°C. They produce a characteristic cytopathic effect consisting of large syncytia. Generally, the viruses are of low pathogenicity in their host species. However, grass carp reovirus is highly pathogenic in grass carp. The infectivity of aquareoviruses is enhanced by treatment with trypsin or chymotrypsin, which correlates with digestion of the outer capsid protein VP7.
List of Species Demarcation Criteria in the Genus
Within the family Reoviridae, the prime determinant for inclusion of virus isolates within a single virus species is their ability to exchange genetic information during co-infection, by genome segment reassortment, thereby generating viable progeny virus strains. However, data providing direct evidence of segment reassortment between isolates of aquareoviruses are not available. RNA cross-hybridization studies and serological comparisons are therefore, the methods most commonly used to examine the level of similarity that exists between isolates. The methods which form the “species parameters” can in effect be used to predict the compatibility of related viruses for genome segment reassortment.
Members of a single Aquareovirus species may be identified by:
1. |
Their ability to exchange genetic material by genome segment reassortment during dual infections, thereby producing viable progeny virus strains. |
2. |
Cross-hybridization assays (Northern or dot blot), with probes made from viral RNA or cDNA. For example, in Northern hybridization assays, conditions (stringency) that do not allow <17% mismatch will not show any hybridization between viruses from two different species, while viruses within a species will show hybridization. |
3. |
RNA sequence analysis (viruses within different species should have low levels of sequence homology among the cognate genome segments). For example, genome segment 10 that encodes the major outer CP, VP7, will show > 45% sequence variation between viruses from two different species. These nucleotide sequence differences should also be reflected in the amino acid sequence variation (> 64%) of the VP7 proteins. |
4. |
Serological comparisons of antigens or antibodies by neutralization (or other) assays using, either polyclonal antisera or monoclonal antibodies against conserved antigens. For example, cross-neutralization assays, using polyclonal rabbit antisera separate different aquareoviruses into the same groupings as the cross-hybridization assays. |
5. |
Analysis of “electropherotype” by agarose gel electrophoresis (AGE). For example, viruses within ARV-A will show a relatively uniform electropherotype, while viruses belonging to ARV-B will show a different, but also relatively uniform, electropherotype. However, similarities in migration rates of some genome segments can exist between species. |
6. |
Identification of the conserved terminal regions of the genome segments (some closely related species can also have identical terminal sequences on at least some segments). |
Six species (ARV-A to F) and some unassigned viruses have been recognized on the basis of RNA-RNA hybridization
List of Species in the Genus
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
Species in the Genus
|
|
|
|
Aquareovirus A |
|
(ARV-A) |
|
American oyster reovirus 13p2 |
|
(13p2V) |
|
Angel fish reovirus |
|
(AFRV) |
|
Atlantic salmon reovirus HBR |
|
(HBRV) |
|
Atlantic salmon reovirus ASV |
|
(ASRV) |
|
Atlantic salmon reovirus TSV |
|
(TSRV) |
|
Chinook salmon reovirus DRC |
|
(DRCRV) |
|
Chum salmon reovirus CSV |
|
(CSRV) |
|
Masou salmon reovirus MSV |
|
(MSRV) |
|
Smelt reovirus |
|
(SRV) |
|
Striped bass reovirus |
Seg 10: [U83396] |
(SBRV) |
|
Aquareovirus B |
|
(ARV-B) |
|
Chinook salmon reovirus B |
|
(GRCV) |
|
Chinook salmon reovirus LBS |
|
(LBSV) |
|
Chinook salmon reovirus YRC |
|
(YRCV) |
|
Chinook salmon reovirus ICR |
|
(ICRV) |
|
Coho salmon reovirus CSR |
|
(CSRV) |
|
Coho salmon reovirus ELC |
|
(ELCV) |
|
Coho salmon reovirus SCS |
Seg 10: [U90430] |
(SCSV) |
|
Aquareovirus C |
|
(ARV-C) |
|
Golden shiner reovirus |
|
(GSRV) |
|
Aquareovirus D |
|
(ARV-D) |
|
Channel catfish reovirus |
|
(CCRV) |
|
Aquareovirus E |
|
(ARV-E) |
|
Turbot reovirus |
|
(TRV) |
|
Aquareovirus F |
|
(ARV-F) |
|
Chum salmon reovirus PSR |
|
(PSRV) |
|
Coho salmon reovirus SSR |
|
(SSRV) |
Tentative Species in the Genus
|
|
|
Chub reovirus |
(CHRV) |
|
Grass carp reovirus |
(GCRV) |
|
Hard clam reovirus |
(HCRV) |
|
Landlocked salmon reovirus |
(LSRV) |
|
Tench reovirus |
(TNRV) |
