Introduction
Taxonomic Structure of the Family
Virion Properties
Morphology
Ichnovirus virions consist of nucleocapsids of uniform size (approximately 85 nm 
330 nm), having the form of a prolate ellipsoid, surrounded by 2 unit-membrane envelopes. The inner envelope appears to be assembled de novo within the nucleus of infected calyx cells, while the outer envelope is acquired by budding through the plasma membrane into the oviduct lumen. Bracovirus virions consist of enveloped cylindrical electron-dense nucleocapsids of uniform diameter but of variable length (34-40 nm diameter by 8-150 nm length) and may contain one or more nucleocapsids within a single envelope; the latter appears to be assembled de novo within the nucleus. Bracovirus nucleocapsids in some cases possess long unipolar tail-like appendages.
Physicochemical and Physical Properties
None reported.
Nucleic Acid
Genomes consist of multiple dsDNAs of variable size ranging from approximately 2.0 to more than 31 kbp. No aggregate size for any polydnavirus genome has as yet been determined. Estimates of genome size and complexity range from 150 kbp to in excess of 250 kbp but are complicated by the presence of related DNA sequences shared among two or more DNA genome segments.
Proteins
Virions are structurally complex and contain at least 20-30 polypeptides, with Mr ranging from 10-200 103.
Lipids
Lipids are present, but uncharacterized.
Carbohydrates
Carbohydrates are present, but uncharacterized.
Genome Organization and Replication
Unique among the dsDNA viruses, polydnaviruses have segmented genomes (see above). Chromosomally integrated sequences homologous to viral DNAs are located at multiple sites within the genome of parasitic wasps (parasitoid); this proviral DNA form is responsible for the transmission of viral genomes within parasitoid populations. The polydnavirus genome appears to be unusual in other respects as well: some viral genes contain introns; several viral gene families exist, members of which are distributed on one or more genome segments; transcriptional activity is host-specific, in the sense that some genes are expressed in the wasp ovary while others are expressed only in the parasitized insect host; families of viral genome segments exist in some cases; polydnavirus genomes, at least potentially, are genetically redundant (e.g., they would appear to be diploid). Polydnavirus replication is nuclear, begins during wasp pupal-adult development, and is very likely induced by a change in ecdysone titre. Viral DNA replication appears to involve excision of viral DNA segments via site-specific recombination events. Virus morphogenesis occurs in the calyx epithelium of the ovaries of all female wasps belonging to all affected species. Ichnovirus particles bud directly from the calyx epithelial cells into the lumen of the oviduct. The mode of release of bracovirus particles is presently unclear, but probably involves lysis of affected calyx epithelial cells. Extrachromosomal, circular DNAs are present both in male wasps and in non-ovarian female tissues (but viral morphogenesis has not been demonstrated). Viral replication does not occur in parasitized host insects.
Antigenic Properties
Cross-reacting antigenic determinants are shared by a number of different ichnovirus isolates; in some cases, viral nucleocapsids share at least one major conserved epitope. It has recently been shown that CsIV and C. sonorensis venom protein display common epitopes. Antigenic relationships among the bracoviruses have not as yet been investigated.
Biological Properties
Polydnaviruses have been isolated only from endoparasitic hymenopteran insects (wasps) belonging to the families Ichneumonidae and Braconidae. In nature, polydnavirus genomes are apparently transmitted as proviruses. Polydnavirus particles are injected into host animals during oviposition; virus-specific expression leads to significant changes in host physiology, some of which are responsible for successful parasitism.
List of Species Demarcation Criteria in the Family
Polydnaviruses are unique among viruses in having obligate associations with parasitic wasps in which the viral genome is integrated within the wasp genome, and replicates only from the provirus. These viruses are in fact transmitted within wasp populations only as proviruses (infections of lepidopteran larvae, on the other hand, are non-replicative, and are not heritable). Since polydnavirus genomes are transmitted only as proviruses, virus species can be unambiguously identified by their specific association with the reproductively isolated wasp species which carry the virus. In this situation viral species are simply and unambiguously defined based on their wasp host which carries the proviral DNA. Each polydnavirus-carrying wasp species is genetically distinct from other wasp species and carries a genetically distinct (and replicatively isolated) polydnavirus species.
