Universidade Federal Rural do Rio de Janeiro, Brazil1
Virginia-Maryland Regional College of Veterinary Medicine, University of Maryland, College Park, MD 20742, USA2
Maryland Institute for Agriculture and Natural Resources, College Park, MD 20742, USA3
Author for correspondence: Siba Samal.Fax +1 301 935 6079. e-mail ss5{at}umail.umd.edu
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Abstract |
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Aquareoviruses are placed in the genus Aquareovirus in the family Reoviridae (Francki et al., 1991 ). The diameter of aquareovirus in negative stain is approximately 75 nm and the particle exhibits multilayered protein capsid morphology (Shaw et al., 1996
). Aquareovirus is stable at pH 3 and ether-resistant. The genome of aquareovirus is composed of 11 segments of double-stranded (ds)RNA, a similar composition to the members of the genus Rotavirus within the family Reoviridae. However, aquareoviruses do not show any genetic or antigenic relatedness to the members of the genus Rotavirus (Samal et al., 1990
).
Using reciprocal RNARNA blot hybridization, six different genetic groups (genogroups) have been established (designated A to F) among the 42 aquareovirus isolates (Lupiani et al., 1993 , 1994
; Subramanian et al., 1997
). Genogroup A was represented by 18 isolates, genogroup B by 19 isolates, genogroups C, D and E by one isolate each and genogroup F by two isolates. Genogroup A appeared to be the most heterogeneous, containing members from both cold and warm water fish and from fish on several continents. RNARNA hybridization between genogroups A and B showed that segment 10, the genome segment that encodes the major outer capsid protein, was the most variable gene (Lupiani et al., 1993
). The genome segment migration pattern (electrophenotype), as analysed by electrophoresis in 1% agarose gel, is consistent within a single genogroup but shows significant variation between viruses from different genogroups (Samal et al., 1991
). Viruses within a single genogroup do show variations in electrophenotype when their dsRNA genome segments are analysed by electrophoresis in high percentage (>6%) polyacrylamide gels (Lupiani et al., 1993
; Subramanian et al., 1997
).
In this study, we have analysed the electrophoretic pattern of the genomic RNA segments of three additional aquareovirus isolates obtained from different geographical areas of the world and have studied their genetic relatedness by reciprocal RNARNA blot hybridization. This study includes for the first time grass carp haemorrhage virus (GCV), the most pathogenic aquareovirus. Our results identified GCV as a new, seventh genetic group of aquareovirus (genogroup G). These data will be useful in studying the epidemiology, taxonomy and classification of aquareoviruses in the family Reoviridae.
The GCV strain was isolated from a grass carp (Ctenopharyngodon idellus) in the People's Republic of China (Chen & Jiang, 1984 ). This virus causes severe haemorrhagic disease in grass carp, affecting about 85% of fingerling and yearling populations (Jiang & Ahne, 1989
). GCV was previously characterized as a member of the family Reoviridae (Ke et al., 1990
), but its genetic relatedness with other members of the genus Aquareovirus was never determined. The other two aquareoviruses used in this study were isolated from apparently healthy finfish and shellfish during routine examinations. The geoduck clams aquareovirus (CLV) strain was isolated from a geoduck clam (Panope abrupta) from Vallenar Bay, Alaska in 1977. The herring aquareovirus (HRV) strain was isolated from an Atlantic herring (Clupea harengus) in Massachusetts in 1997.
The Chinook salmon embryo cell line (CHSE-214) was used to propagate the GCV, CLV and HRV strains. The CHSE-214 cells were also used to propagate striped bass reovirus (SBR, genogroup A), Chinook salmon reovirus (LBS, genogroup B), golden shiner virus (GSV, genogroup C), channel catfish reovirus (CRV, genogroup D), turbot reovirus (TRV, genogroup E) and salmon reovirus (SCR, genogroup F) strains. The simian embryonic kidney cell line MA104 was used to propagate simian rotavirus SA11. CHSE-214 and MA104 cells were grown at 16 °C and 37 °C, respectively. Aquareovirus, as well as SA11 virus, was purified as described elsewhere (Subramanian et al., 1994 ). Small drops of purified GCV, CLV and HRV were negatively stained with 2% phosphotungstic acid and viewed with a JEOL 1200EX electron microscope. Electron micrographs of negatively stained viruses (Fig. 1
) showed spherical particles 7080 nm in diameter with a double-layered capsid. The ultrastructural features of GCV, CLV and HRV strains were similar to those of aquareovirus, suggesting that these viruses belong to the genus Aquareovirus.
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The results of this study confirm and extend the results of our previous studies (Lupiani et al., 1993 ; Subramanian et al., 1997
), which demonstrated that there is a wide genetic diversity in the genus Aquareovirus and provide an experimental method for grouping aquareoviruses on the basis of genetic relatedness. Reassortment of aquareovirus among genogroups does not seem to occur (unpublished data). However, the possibility of reassortment among strains of a genogroup in nature cannot be ruled out at present. The genogroups determined in this study could be used for comparison with new aquareoviruses as they are isolated. Further biological and serological studies on the isolates in each genogroup are required before the divisions made here can be fully substantiated.
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Acknowledgments |
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References |
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Received 15 February 1999;
accepted 1 June 1999.
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