rigin and Characteristics of Polydnaviruses Carried by Parasitoid Wasps

Viruses are usually small infectious agents and form a pathogenic association with their hosts, but a notable exception to this is polydnaviruses (PDV) which are symbiotic viruses, the obligate mutualists of endoparasitoid wasps that form a giant group in Hymenoptera. PDV virions are injected into caterpillar hosts when parasitoid wasps lay eggs, and induce host immune suppression and developmental arrest to ensure survival of wasp larvae in caterpillar hosts. PDV genome is integrated in a wasp chromosome as a provirus, and therefore PDV virion only replicates in wasp calyx cells which are specialized cells of the wasp ovary, and never express virulence genes in wasp cells. Phylogenetic analyses of viral gene sequences indicate that the members of Bracovirus (BV), one of two genera of the Polydnaviridae, are closely related to pathogenic nudiviruses and baculoviruses. Moreover, the genome enclosed in the virions encodes almost no viral structural proteins, but interestingly a large set of homologs of nudivirial genes found in the wasp genome encode structural components of the BV virions. Thus BVs do not appear to be nudivirus remnants, but instead employ the same mechanism as nudivirus to delivery virulence genes into caterpillar hosts. Overall, these indicate that nudivirus is probably the ancestor of PDVs.