Reovirus exists in the form of 13 particle species that differ in their content of protein sigma 1.

When electrophoresed in 0.7% agarose gels, populations of reovirus particles can be resolved into 13 well-defined bands that possess from 0 to 12 projection/spike-associated trimers of protein sigma 1. This state of affairs is not an artifact of purification, of the techniques used to demonstrate it, of aggregation, or of virus particle instability. Complexes of monoclonal antibody against protein sigma 1 with virus particles that possess only 1, or, to a lesser extent, 2 sigma 1 trimers are less stable (that is, more readily dissociated by sonication) than complexes of antibody and virus particles that possess 3 or more sigma 1 trimers. The specific infectivity of virus particles that possess 3 or more sigma 1 trimers is essentially the same; virus particles that possess only 2 sigma 1 trimers are about two-thirds as infectious; and particles that possess only 1 sigma 1 trimer still possess very significant infectivity (about one-third of maximum). Reovirus particles that possess no sigma 1 trimers (about 1 in 30) are essentially noninfectious. The reason reovirus particles do not possess a full complement of sigma 1 trimers is presumably the fact that only very small amounts of protein sigma 1 are synthesized in infected cells; and since possession of 3 such trimers is sufficient for maximal infectivity, and since the average number of sigma 1 trimers per reovirus particle is 7.1, there is presumably no selection for variants that synthesize larger amounts of sigma 1. On the contrary, such variants may well be at a selective disadvantage.