Cypovirus


Cypovirus, short for cytoplasmic polyhedrosis virus, is a genus of viruses, in the family Reoviridae, in the subfamily Spinareovirinae. Cypoviruses have only been isolated from insects. There are currently 16 species in this genus including the type species Cypovirus 1. Diseases associated with this genus include: larvae chronic disease. Morphologically, these viruses have much in common with the much more widely studied nucleopolyhedroviruses, a genus of arthropod viruses in the Baculovirus family. However, Cypoviruses have an RNA genome and replicate in the cytoplasm of the infected cells while nucleopolyhedroviruses have a DNA genome and replicate in the nucleus.

Taxonomy

Group: dsRNA

Structure and proteins

Classification of Cypoviruses is based on the electrophoretic migration profiles of their genome segments. Cypovirus has only a single capsid shell, which is similar to the orthoreovirus inner core. They exhibit striking capsid stability and is fully capable of endogenous RNA transcription and processing. The overall folds of Cypovirus proteins are similar to those of other reoviruses, however, they have insertional domains and unique structures that contribute to their extensive intermolecular interactions. The Cypovirus turret protein contains two methylase domains with a highly conserved helix-pair/β-sheet/helix-pair sandwich fold but lacks the β-barrel flap present in orthoreovirus λ2. The stacking of turret protein functional domains and the presence of constrictions and A spikes along the mRNA release pathway indicate a mechanism that uses pores and channels to regulate the highly coordinated steps of RNA transcription, processing, and release.
Viruses in Cypovirus are non-enveloped, with icosahedral geometries, and T=2 symmetry. The diameter is around 65 nm. Genomes are linear and segmented, around 4.2kb in length. The genome codes for 10 to 12 proteins.
GenusStructureSymmetryCapsidGenomic arrangementGenomic segmentation
CypovirusIcosahedralT=13, T=2Non-envelopedLinearSegmented

Life cycle

Viral replication is cytoplasmic. Entry into the host cell is achieved by attachment to host receptors, which mediates endocytosis. Replication follows the double-stranded RNA virus replication model. Double-stranded RNA virus transcription is the method of transcription. Translation takes place by ribosomal skipping. The virus exits the host cell by monopartite non-tubule guided viral movement, and existing in occlusion bodies after cell death and remaining infectious until finding another host.
Insect serve as the natural host. Transmission routes are fecal-oral, parental, and egg transmission.
Infection occurs when a susceptible insect consumes the polyhedra, usually as a contaminant on the insect’s food. The polyhedra dissolve in the digestive tract of the insect, releasing the virus particles that penetrate the gut epithelial cells. Replication of the virus is often confined to these cells and the progeny virus, in the form of new polyhedra are excreted in the insect feces, thus contaminating more foliage resulting in the spread of the disease to additional insects. The progression of the disease can be rather slow, but the virus infection is normally fatal.
GenusHost detailsTissue tropismEntry detailsRelease detailsReplication siteAssembly siteTransmission
CypovirusInsectsMidgut; goblet; fatCell receptor endocytosisCell deathCytoplasmCytoplasmPolyhedra: oral-fecal; vertical: eggs