Identification of bracovirus particle proteins and analysis of their transcript levels at the stage of virion formation.

Polydnaviruses (PDVs) are unique symbiotic viruses associated with parasitic wasps; they replicate only in the calyx cells of a wasp's ovaries and are transferred at oviposition along with the parasitoid egg into the lepidopteran host. The DNA packaged in the viral particles encodes factors that manipulate the host's immune defences and development to benefit the parasitoid. PDVs are found in two subfamilies of ichneumonids (ichnoviruses) and in braconids of the microgastroid complex (bracoviruses). We recently showed that the latter derive from an ancestral nudivirus, as 24 nudivirus-related genes were identified in ovaries of two distantly related braconids at the stage of virion formation. Here, we present a comprehensive analysis of the viral particle proteins of the Chelonus inanitus bracovirus (CiBV). Proteins of purified CiBV particles were analysed by mass spectrometry and amino acid sequences matched to the existing ovarian-cDNA database. In addition, transcript quantities of identified genes were measured by quantitative real-time PCR in female pupae at the onset and peak of virion formation and at corresponding stages in male pupae. This combined approach allowed the identification of 44 CiBV particle proteins: 16 were nudivirus-related, three had similarity to ovarian proteins of another braconid, 11 had similarity to cellular proteins and 14 had no similarity to known proteins. The transcripts of all of them increased in female, but not male, pupae. These data confirm the important contribution of nudivirus genes but also indicate the presence of many lineage- or species-specific proteins possibly involved in the parasitoid-host interaction.

Polydnaviruses of braconid wasps derive from an ancestral nudivirus.

Many species of parasitoid wasps inject polydnavirus particles in order to manipulate host defenses and development. Because the DNA packaged in these particles encodes almost no viral structural proteins, their relation to viruses has been debated. Characterization of complementary DNAs derived from braconid wasp ovaries identified genes encoding subunits of a viral RNA polymerase and structural components of polydnavirus particles related most closely to those of nudiviruses--a sister group of baculoviruses. The conservation of this viral machinery in different braconid wasp lineages sharing polydnaviruses suggests that parasitoid wasps incorporated a nudivirus-related genome into their own genetic material. We found that the nudiviral genes themselves are no longer packaged but are actively transcribed and produce particles used to deliver genes essential for successful parasitism in lepidopteran hosts.