Transient expression of a polydnaviral gene, CpBV15beta, induces immune and developmental alterations of the diamondback moth, Plutella xylostella.

The diamondback moth, Plutella xylostella, parasitized by its endoparasitoid wasp, Cotesia plutellae, undergoes various physiological alterations which include immunosuppression and an extended larval development. Its symbiotic virus, C. plutellae bracovirus (CpBV), is essential for their successful parasitization with more than 136 putative genes encoded in the viral genome. CpBV15beta, a CpBV gene, has been known to play significant role in altering host physiological processes including hemocyte-spreading behavior through inhibition of protein synthesis under in vitro conditions. In the current study, we investigated its specific involvement in physiological processes of the host by transient expression and RNA interference techniques. The open reading frame of CpBV15beta was cloned into a eukaryotic expression vector and this recombinant CpBV15beta was transfected into nonparasitized 3rd instar P. xylostella by microinjection. CpBV15beta was expressed as early as 24h and was consistent up to 72h. Due to the expression of this gene, plasma protein levels were significantly reduced and the ability of the hemocytes to adhere and spread on extracellular matrix was inhibited, wherein CpBV15beta was detectable in the cytoplasm of hemocytes based on an indirect immunofluorescence assay. To confirm the role of CpBV15beta, its double stranded RNA could efficiently recover the hemocyte-spreading behavior and synthesis of plasma proteins suppressed by the transient expression of CpBV15beta. In addition, the larvae transfected with CpBV15beta significantly suffered poor adult development probably due to lack of storage proteins. Thus these results demonstrate the role of CpBV15beta in altering the host physiological processes involving cellular immune response and metamorphic development, which are usually induced by wasp parasitization.