In vitro integration of an ichnovirus genome segment into the genomic DNA of lepidopteran cells

Polydnaviruses (PDVs) are dsDNA viruses transmitted by ichneumonid and braconid endoparasitoids to their lepidopteran hosts during oviposition. Wasp carriers are asymptomatic and transmit the virus to their progeny through the germ line; replication is confined to the calyx region of the wasp ovary, where the virus accumulates in the fluid bathing the eggs. In the lepidopteran host, however, no virus replication takes place, but PDV gene expression is essential for successful parasitism. Sustained gene expression in the absence of virus replication thus requires that the circular PDV genome segments persist for days within host cells. Available evidence suggests that most genome segments persist as episomes, but recent studies have indicated that some genome segments may undergo integration within lepidopteran genomic DNA, at least in vitro. In the present study, an integrated form of a Tranosema rostrale ichnovirus (TrIV) genome segment was cloned from genomic DNA extracted from infected Choristoneura fumiferana CF-124T cells and junction regions on either side of the viral DNA sequence were sequenced. This is the first proven example of integration of an ichnovirus genome segment in infected lepidopteran cells. Interestingly, circular forms of this genome segment do not appear to persist in these cells; none the less, a gene (TrFrep1) carried by this genome segment displays long-term transcription in infected cultured cells.