The Genus Orthoreovirus includes four species and two unassigned viruses. Conclusive species classification requires the direct demonstration of exchange of genetic material via reassortment of genome segments. To date, there is no evidence of reassortment between the four species identified, which reflects the extensive sequence divergence between species.

The four species in the Genus Orthoreovirus can be divided into three distinct subgroups when comparing amino acid sequences of the α2 protein (major inner capsid).

Ndelle virus (NDEV) was previously classified as an unassigned Orbivirus but recent sequence analyses have shown it to have high levels of sequence homology to MRV in at least four genomes segments.

In common with the other genera within the Family Reoviridae, the prime determinant for inclusion of virus isolates within an Orbivirus species is compatibility for reassortment of genome segments during co-infection, thereby exchanging genetic information and generating viable progeny virus strains. However, data providing direct evidence of segment reassortment between isolates is limited and serological comparison [primarily involving the immunodominant serogroup/species-specific antigen VP7 (T13)] from the usual basis of diagnostic assays for each of the virus species (serogroups).

There are 6 antigenic groups of rotaviruses (A-F).

Six species (ARV-A to ARV-F) and some unassigned viruses are recognised on the basis of RNA-RNA hybridisation.

Cypoviruses are currently classified within 14 species that were initially characterised by their distinctive dsRNA electropherotype patterns. Cross-hybridisation analyses of dsRNA, limited comparative RNA sequence data and serological comparisons of cypovirus proteins have confirm the validity of this classification. However, only a few cypoviruses have been analysed in this way.

The system of nomenclature currently used to identify different cypovirus isolates takes account of both the dsRNA electropherotype and the host species from which the virus was originally isolated (e.g., Bombyx mori cypovirus 1, BmCPV-1). The relationships, between different cypoviruses within a single electropherotype, or with other Cypovirus types, are not fully understood at the molecular level. However, sequence analyses of genome segment 10 from three geographically distinct isolates of CPV-5 and comparison of segment 10 from isolates of CPV-1 have shown very high levels of homology (>98%) within a single species. In contrast, comparisons of unrelated types (BmCPV-1 and CfCPV) showed only low levels of sequence homology in genome segment 10.

The current classification system takes account of both the dsRNA electropherotype and the host species from which viruses were originally isolated. The relationships at the molecular level of different cypoviruses within an electropherotype, or to other cypoviruses, is not known. Only electropherotypes 1 and 12 show any significant similarity in their overall genome profiles and levels of RNA cross-hybridisation and serological cross-reaction.

Fijivirus species can be clustered into "groups" of more closely related species. Thus, some species are distinct but others (RBDSV, MRDV, PaSV, and MRCV) appear to be more closely related to each other than any is to other Fijivirus species. Nucleotide and amino acid sequence relatedness between RBDSV, MRDV, PaSV, and MRCV is much greater than it is among other species of Fijivirus. Further information about these viruses may necessitate a revision of the species status of these viruses, but they are retained as distinct species in accordance with previous practice.

Epitopes representing the inner surface of the outer capsid of RDV and RGDV are shared, however the outer surface epitopes of the 3 viruses are distinct.

Plant reoviruses have been observed infecting monocotyledons other than in the family Gramineae (Japan: lily; France: garlic). One report describes a reo-like virus in the lily. Unpublished data (H. Lot, B. Delecolle, G. Boccardo, R. Milne) identified a reo-like virus in garlic with distinctive genomic dsRNA sizes, antigenically distinct from other plant-infecting reoviruses but morphologically similar to Fijiviruses. No vectors have been identified. Reo-like viruses infecting Liliaceae.

Aqua: from Latin aqua, "water"
Colti: sigla from Colorado tick fever
Cypo: sigla from cytoplasmic polyhedrosis
Fiji: from name of country where virus was first isolated
Orbi: from Latin orbis, "ring" or "circle" in recognition of the ring-like structures observed in micrographs of the surface of BTV cores
Oryza: from Latin oryza, "rice"
Phyto: from Greek phyton, "plant"
Reo: sigla from respiratory enteric orphan, due to the early recognition that the viruses caused respiratory and enteric infections, and (incorrect) belief that they were not associated with disease, hence they were considered "orphan" viruses
Rota: from Latin rota, "wheel"

Mertens PPC, Arella M, Attoui H, Belloncik S, Bergoin M, Boccardo G, Booth TF, Chiu W, Diprose JM, Duncan R, Estes MK, Ferber ML, Gorziglia M, Gouet P, Gould AR, Grimes JM, Hewat E, Holmes IH, Hoshino Y, Hill C, Joklik WK, Knowles N, Malby R,Marzachì C, McCrae M, Milne RG, Nibert N, Nunn M, Omura T, Prasad BVV, Prichard I, Roy P, Samal KSK, Schoehn G, Shikata E, Stoltz DB, Stuart DI, Suzuki N, Upadhyaya N, Uyeda I, Waterhouse P, Williams CF, Winton JR, and Zhou HZ.