Taxonomy of Reoviridae
The reovirus family is divided into nine genera. However, only four of those nine genera contain viruses which are known to infect humans. Those genera are:
Rotavirus-- named for the wheel-like appearance of the EM (rota= Latin for wheel)
Orbivirus-- named for the ring-shaped capsomers comprising the inner capsid (orbi= Latin for ring)
Classification of Reoviridae
The reoviral genome is one of the defining characteristics of the family. It has a DOUBLE-STRANDED RNA genome, which makes it unlike any other RNA virus. This genome is linear and segmented. The number of segments seen in a virus depends on the genera the particular virus is found in.
Orthoreovirus and Orbivirus: 10 segments, total of 24 kilobase pairs
Rotavirus: 11 segments, total of 18 kilobase pairs
Coltivirus: 12 segments, total of 27 kilobase pairs
Replication occurs in the cytoplasm, which is not unusal for an RNA virus. However, what is unusual in terms of the reovirus family is that replication occurs within a mostly intact virion particle. In most other viral families, the virion disassembles and uncoats completely before it replicates. The mRNA transcripts produced by reoviruses are full-length, capped at the 5' end, and non-polyadenylated.
The reoviral capsid is also unique among the human viral families. The virion particle is naked (non-enveloped), and spherical in appearance. The capsid shape is icosahedral. However, what distinguishes the reovirus virion is its three capsid layers: two concentric icosahedral capsids at the center of which is an icosahedral inner core. The triangulation number of the icosahdral capsid is T=13, with a left-slanted skew on both the outer and inner capsids. The orthoreoviruses and coltiviruses are approx. 80 nm in diameter. The rotaviruses are approx. 70 nm in diameter.
The reoviruses cause inclusion body formation (a cytopathic effect) that has a characteristic crystallized, beehive pattern. Also, reovirus are equipped with every enzyme that they need for transcribing the double-stranded RNA. The release from the cell is by cell lysis. Due to its segmented genome, reoviruses are capable of genetic reassortment and genetic shift during multiple infections. However, though seen in culture, there has not been evidence of this outside the laboratory.