Characterization of Campoletis sonorensis ichnovirus genome integration into lepidopteran cells

Polydnaviruses (PDVs) are a group of insect viruses that reside within certain parasitoid wasps. When parasitizing, these wasps inject an egg as well as PDV into caterpillar hosts. The PDV serves to suppress the caterpillar's immune system thus allowing the wasp egg to develop. PDVs share an intimate relationship with their wasp host; they do not replicate outside of the wasp body and their genome is stably integrated into the genome of the wasp.  In contrast, the PDV relationship with their caterpillar host is supposedly transient, and the genome persists only as circular segments. Recently, this view was challenged when certain polydnavirus segments were found to persist in PDV-exposed lepidopteran cells, stably integrating into their cell genome. We show that the Campoletis sonorensis ichnovirus (CsIV) is not only consistent with this new paradigm but that integration is pervasive. We screened for the presence of all 24 CsIV genome segments in CsIV-exposed Sf9 cells using PCR (Polymerase Chain Reaction) and detected the persistence of more than 50% of them. Segment Q persists in CsIV-injected Heliothis virescens larvae from 4th instar through adulthood, but is not transmitted to F₁ offspring. Southern blotting data indicates that the integration of CsIV segment D is not random in both C. sonorensis males and Sf9 cell lines. These data represent the first characterization of the integration phenomenon across all segments of a PDV genome and implies major significance of PDV integration in viral function and parasitoid success.