Multiple copies of each CpBV segment on host genome reflect their unequal replication in an endoparasitoid wasp, Cotesia plutellae

An endoparasitoid wasp, Cotesia plutellae, parasitizes young larvae of the diamondback moth, Plutella xylostella. The parasitization alters host physiological processes, such as immunosuppression, extension of larval period, and diverting nutrient usage. The wasp possesses its specific symbiotic polydnavirus, C. plutellae bracoviurs (CpBV), which plays a significant role in the host physiological alterations. CpBV is replicated only in female ovary during late pupa and adult stages in response to ecdysteroid signal. A Broad gene and a DNA polymerase were determined to be involved in the viral replication by RNA interference experiments against putative replication-associated genes predicted from EST database generated from ovary transcrips by 454 pyrosequencing. Unequal viral segment levels after the replication was analyzed in copy numbers of each CpBV segment on the wasp chromosome(s). For example, four copies of segment #3 of CpBV (CpBV-S3) were located on the wasp chromosome(s) by segment terminal PCRs and Southern hybridization. Our analysis suggests that the unequal CpBV segment levels are due to their different copy numbers on the wasp genome.