Picornalike Viruses of Insects

Over the last 30 years, a large number of small (less than 40 nm in diameter) RNA-containing viruses (SRVs) have been isolated from insects and insect cell cultures, the vast majority of which have undetermined relationships with other animal or plant viruses. Nevertheless, two groups of insect SRVs have been sufficiently well characterized to merit their own families, namely, the nodaviruses and the tetraviruses (Murphy et al.,1995; see also Chapters 8 and 9, this volume). In addition to these two well-characterized families of insect SRVs, there are a number of known birnaviruses from insects (Murphy et al., 1995) and a single calicilike virus (Kellen and Hoffman, 1981; Hillman et al., 1982). The remaining 65 or so viruses are to a large extent relatively uncharacterized (Appendix). However, among these a reasonably large 〞group〞 of viruses exist that have characteristics that superficially resemble those of the mammalian picornaviruses. The mammalian picornaviruses are classically defined by having isometric, unenveloped virions of around 30 nm in diameter, with buoyant densities between 1.30 and 1.46 g/ml, depending on the genus (Table I). The virions contain three major capsid proteins of around 30,000 Da (30 kDa) and another small protein of around 7 kDa. The RNA genome is positive sense, polyadenylated at the 3′ end and has a small protein (VPg) covalently linked to its 5′ end.