the virions, hence, the family name Baculoviridae (23). These rod-shaped virions are found occluded in large polyhedral shaped, proteinaceous capsules (polyhedra, 0.1-15 μm in diameter) or in smaller granular capsules (granula) 0.3 till 0.5 μm in length and 0.1 to 0.3 μm in diameter (43). These capsules are often collectively called occlusion bodies (OBs). Baculoviruses share this occlusion phenotype with the genetically unrelated cypoviruses (CPV) and entomopoxviruses (EPV).
The family Baculoviridae comprises four genera, Alphabaculovirus, Betabaculovirus, Gammabaculovirus and Deltabaculovirus (118). The Alphabaculovirus genus contains the nucleopolyhedroviruses (NPVs) of lepidopteran insects and the Betabaculovirus genus encompasses the granuloviruses (GVs) of lepidopteran hosts. The NPVs from hymenopterans form the genus Gammabaculovirus and the Deltabaculovirus genus encompasses the NPVs from dipteran hosts. The Alphabaculoviruses are further divided in group Ⅰ and group Ⅱ NPVs on the basis of phylogenetic analysis (99) and the type of envelope fusion protein (GP64 or F, respectively). The virions of NPVs as found in the occlusion bodies may contain single (S) or multiple (M) nucleopcapsids, but this is not a taxonomical denominator. Collectively about 700 baculoviruses have been described genetically, but only a minority have been characterised to the extent that they can be called a species. Baculoviruses occur in a very wide range of insect hosts, but each virus by itself in general has a narrow host range. The most notable example of a baculovirus with multiple hosts is Autographa californica MNPV, the type species of the Alphabaculoviruses, belonging to the group Ⅰ NPVs.
Baculoviruses have a complex 'life' cycle. They infect their larval host orally and the virions, upon release from the proteinaceous capsules (hence their name occlusion derived virions = ODVs), replicate in the epithelial cells of the larval midgut. Infectious virions (budded virions = BVs) are produced in these cells and released into the hemolymph and tracheal system to invade and infect other organs and tissues of the insect larva. Replication of the Gammabaculoviruses is restricted to the midgut (144). At the end of the infection virions are occluded into OBs, which are released into the environment from the insect body upon death. In the OB form baculoviruses can persist in the environment for many years. For some baculoviruses, notably AcMNPV, the replication cycle can also be completed in cell culture using BVs as inoculum and this has greatly enhanced our current understanding of the cell biology and genetics of baculovirus infections (for more details (279)).
Baculoviruses contain a double-stranded, circular and superhelical DNA molecule, which replicates in the cell nucleus. After synthesis the DNA is packaged in a number of proteins to form nucleocapsids. One or more nucleocapsids are wrapped in an envelope, which is de novo formed in the nucleus of infected cells. Single or multiple virion packages are occluded in GVs and NPVs, respectively. The size of baculovirus genomes depends on the species and ranges from 80 to 180 kilobasepairs (kbp), hence encompassing a variable numbers of ORFs (266). Gene homology, gene content and gene location can be used to construct reliable phylogenetic trees to show the relatedness among baculoviruses (99).
The transcription of baculoviruses occurs in a cascaded fashion and four classes of transcripts are discriminated. The immediate early (IE) transcripts are made by host RNA polymerases, a process independent of de novo protein synthesis. Delayed early (DE) transcripts require translation of IE viral transcripts for their synthesis. Late (L) transcripts are expressed after the onset of DNA replication and very late (VL) transcripts are those that are still expressed very late after infection, sometimes at very high levels (polyhedrin, p10). Baculovirus late and very late transcription occurs via a virus-encoded RNA polymerase, which is α-amanitin insensitive. Baculovirus genes are hence categorized