Overview
Polydnaviruses (PDVs) are an unusual assemblage of viruses associated with certain parasitoid wasps. Unlike typical pathogens, PDVs function as persistent, mutualistic partners: they are produced by the wasp and delivered into the wasp's insect host to alter host physiology and immune response. For a general introduction to the group see polydnavirus overview.
Classification and hosts
PDVs are represented by two distinct lineages commonly treated as genera: bracoviruses and ichnoviruses. Bracoviruses occur in braconid wasps and ichnoviruses in ichneumonid wasps; each lineage is tied closely to its wasp hosts and differs substantially in sequence and origin. For examples associated with ichneumonid and braconid wasps, see ichnovirus examples and bracovirus examples.
Structure and genome
Polydnaviruses have multipartite genomes: the packaged viral material consists of multiple double-stranded DNA segments. These segments are enclosed within proteinaceous capsids and often an envelope. Structural proteins and packaged segments can be distinguished from the proviral sequences that are integrated in the wasp's chromosomes. For technical descriptions of the genome architecture consult genome resources and studies on viral particle composition such as capsid protein analyses.
Life cycle and function
PDVs do not replicate in the insect host that is parasitized. Instead, replication and particle assembly occur in specialized cells of the female wasp's reproductive tract. When the wasp oviposits, it injects eggs together with viral particles. Genes delivered by the virus are expressed in host tissues and modulate immunity, development, and metabolism so the wasp larva can develop successfully.
Evolutionary and ecological significance
PDVs illustrate a rare evolutionary transition from virus to permanent symbiont: viral sequences have become domesticated within wasp genomes and now perform a reproductive role for their host. Because bracoviruses and ichnoviruses differ markedly in composition, they are thought to have independent origins. Studying PDVs provides insight into host–symbiont coevolution and biological control strategies in agriculture where parasitoid wasps are used to manage pest insects.
Notable distinctions
- Mutualistic relationship: PDVs benefit wasps rather than acting as free pathogens.
- Segmented, double-stranded DNA packaged in particles produced only by wasps.
- Two major types (braco- and ichnoviruses) that are genetically distinct and host-restricted.