Origin and evolution of symbiotic viruses associated with parasitoid wasps

• Parasitoid wasps use polydnaviruses to transfer virulence genes into their host.
• Polydnaviruses evolved at least twice independently from different viruses.
• Most essential viral functions have been maintained including particle infectivity.
• Unlike other viruses PDVs replicate sequences that are not packaged in the particles.
• Bracovirus virulence gene families are produced mainly by chromosomal duplications.

The Polydnaviridae (PDV), including the Bracovirus (BV) and Ichnovirus (IV) genera, originated from the integration of viruses in the genomes of two parasitoid wasp lineages. In a remarkable example of convergent evolution BVs evolved from the domestication of a nudivirus, while IVs originate from a different ancestral virus belonging to a new virus entity. In both cases the ancestor genomes have been maintained in wasp genomes as endogenous viral elements involved in production of particles containing DNA encoding virulence genes that are injected into lepidopteran hosts. However many PDV virulence genes appear to be of eukaryotic origin, and expansion and diversification of these genes have led to the production of novel PDVs in different wasp species that promote survival of offspring in particular hosts.