Chapter 3 - Bluetongue virus, other orbiviruses and other reoviruses: Their relationships and taxonomy

Abstract

Individual reoviruses can infect a wide variety of terrestrial and non-terrestrial vertebrates, terrestrial and non-terrestrial invertebrates, plants, and fungi. Bluetongue virus (BTV), which has been studied in the greatest depth and is the “type” species of the genus Orbivirus, includes 24 distinct serotypes, as well as many different topotypes (groups reflecting the geographical origins of each virus isolate) and nucleotypes (lineages), although these have no “formal” taxonomic status within the virus species itself. Clear overall similarities exist in the structure and replication strategies of the different reoviruses, which are reflected in the phylogenetic relationships of their genome segments. These relationships suggest that all of the reoviruses have evolved from a single common ancestor, in parallel with their different hosts (possibly originally from a marine species), a process known as co-speciation. Subsequent gene deletions or duplications, followed by gene rearrangements, have generated changes in both the number of genome segments and the proteins they encode. Reovirus particles are composed of up to three concentric shells of virus structural proteins (depending on the genus to which they belong) surrounding the segments of the virus genome. Although there are similarities in the innermost sub-core shell structure and protein (the T2 protein), the structure of the outer capsid and outer core layers varies significantly between members of different genera within the family.