ICTV Virus Taxonomy Profile: Sedoreoviridae 2022

1Sedoreovirid particles are icosahedral (Table 1). The protein capsid is organized as 1–3 concentric layers of capsid proteins, with an overall diameter of 60–100 nm. Members of the family Sedoreoviridae lack large surface projections on the subviral particle, giving them an almost spherical or ‘smooth’ appearance (Fig. 1), in contrast to members of the family Spinareoviridae, which have spikes or turrets at the 12 icosahedral vertices.

2Sedoreovirids contain 10–12 segments of linear dsRNA comprising 18–26 kbp in total, with individual segments ranging from 0.6 to 5.8 kbp. The positive-sense strands of each duplex are modified with a 5′-terminal type 1 cap structure but no 3′-poly(A) tail. The viral RNAs are mostly monogenic with relatively short 5′- and 3′-non-coding regions, although some segments have a second or third functional ORF.

3Virus cell entry varies between genera but usually results in loss of outer-capsid components. The resulting transcriptionally active particles are released into the cytoplasm. The 5′-capped mRNAs are synthesized by structural enzymatic components of the particle and are released through pores at the icosahedral apices of the virion into the cytoplasm. Viroplasms, also known as virus inclusion bodies, are distributed throughout the cytoplasm. These neo-organelles are sites of viral mRNA synthesis, genome replication and particle assembly. Sets of a single copy of each capped mRNA are incorporated into progeny virus particles. These mRNAs serve as templates for negative-sense strand synthesis, thereby reconstituting genomic encapsidated dsRNAs. Progeny virions are released without compromising cell viability (e.g. budding) or following cell lysis, depending on the cell type.

4Current taxonomy: ictv.global/taxonomy. The family Sedoreoviridae includes several genera (Fig. 2) and >35 species of viruses infecting mammals, birds, crustaceans, arthropods, algae and plants. The number of genome segments (10–12) is characteristic of viruses within a single genus. Other factors distinguishing different genera are host (and vector) range, disease signs and capsid structure. The amino acid sequence of the relatively conserved RNA-directed RNA polymerase can be used for comparison across taxonomic boundaries. Among the members of a species, protein and RNA sequences are relatively conserved, being serologically cross-reactive and including specific RNA packaging signals. This high degree of functional and structural compatibility allows viable progeny virus strains to be generated by reassortment between viruses of the same species.

5Full ICTV Report on the family Sedoreoviridae: ictv.global/report/sedoreoviridae