Polydnaviruses: Evolution and Applications

Polydnaviruses (“poly” referring to the poly-dispersed DNA segments) (PDVs) are the unique viruses, which are obligatory symbionts with parasitoid wasps (the primary host). Polydnaviridae was formally recognized as a family of viruses in 1991. Corresponding to the PDV-carrying wasp families, Braconidae and Ichneumonidae, PDVs are subdivided into two genera, Bracovirus (BV) and Ichnovirus (IV). The PDV genome is integrated into the wasp’s chromosome as a provirus and vertically transmitted through wasp germ lines. PDV virions only replicate in the calyx cells of female wasps, which are injected into caterpillar hosts (the secondary host). PDV genes are expressed in the secondary host, which suppress the host’s immune system, prevent encapsulation, and regulate the host’s physiology to facilitate parasitism; this results in the death of the secondary host. A breakthrough study on the wasp transcriptome showed that BVs evolved from a nudivirus, while IVs originate from a different virus ancestor that belongs to a new virus family. Due to PDV gene function, PDV-associated gene products are also used for pest control in crops. In this chapter, the evolution, life cycle, functional genes and applications of PDVs will be reviewed.