Polydnaviruses: General Features

Abstract

Polydnaviruses (PDVs) are unique among DNA viruses in having segmented double-stranded DNA (dsDNA) genomes that are largely noncoding. PDVs occur only in obligate mutualistic relationships with parasitic wasps in the families Braconidae and Ichneumonidae. Two genera are recognized, the Bracovirus found in braconid wasps and the Ichnovirus found in ichneumonid wasps. PDV replication has been detected only in the reproductive tract of some female parasitic wasps where the virus replicates from integrated, proviral DNA and subsequently accumulates in the oviduct lumen. Virus particles are injected with the wasp egg, venom, and ovarian proteins into the lepidopteran hosts of the parasitic wasp. PDV gene expression in host cells causes many of the physiological alterations observed in parasitized larvae and enables the survival of the parasitoid and its development within the lepidopteran host. Genome sequence analyses further demonstrate the novel features of this virus. Most viral genes are members of gene families thought to represent diverse virulence genes. Some PDV gene families have introns and the genomes generally lack genes thought to be involved in virus replication. These genome features are shared between BVs and IVs even though the two genera are not thought to be evolutionary related. The common selection pressures on virus replication and function appear to have driven convergent evolution in the two PDV genera.