Comparison between the complete nucleotide and derived amino acid sequences from the SI, S2, S3 and S4 dsRNA segments of a large number of reovirus strains has allowed the construction of phylogenetic trees indicating potential evolutionary relationships between various reovirus strains (Fig. 2). Of these four gene segments, the SI dsRNA segment shows the most extreme diversity between viruses belonging to different serotypes. Within strains belonging to a particular serotype the degree of diversity in the SI dsRNA segment is considerably less than the diversity between serotypes. It has been suggested that there were originally three 'progenitor' SI genes representative of the current three viral serotypes, which have subsequently diverged to produce the differences seen between strains of virus belonging to the same serotype.

Comparison of the evolutionary relationships between reovirus strains differ strikingly depending on which dsRNA segment is used for analysis (Fig. 2). This provides strong evidence that reoviruses can and have reassorted their dsRNA segments in nature. Reovirus reassortants can be easily generated in vitro by co-infection of susceptible cells with two distinct strains of virus. Reassortant viruses can also be isolated from mice simultaneously co-infected with two different reovirus strains. These studies provide experimental support for the feasibility of reassort-ment.

In distinction to the evidence supporting reassort-ment of dsRNA segments between reovirus strains is the absence of evidence indicating that recombination occurs between either homologous or heterologous dsRNA segments.

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