The diamondback moth, Plutella xylostella parasitized by its endoparasitoid Cotesia plutellae undergoes various physiological alterations which involves immunosuppression and developmental arrest. Its symbiotic virus, C. plutellae bracovirus (CpBV) is highly essential for their successful parasitization which possesses more than 136 genes encoded. CpBV15βunique in CpBV genome is expressed at low levels in early and at higher levels during late parasitization period. This gene product alters the hemocyte-spreading behavior through inhibition of protein synthesis under in vitro conditions. In the current study, we investigated its specific involvement in physiological functions in the host by transient expression and RNA interference techniques. The open reading frame of CpBV15βwas cloned into a eukaryotic expression vector and this recombinant CpBV15β was transfected into healthy non-parasitized 3rd instar P. xylostella by microinjection. CpBV15βwas expressed as early as 24 h and was consistent up to 72 h. Due to the expression of this gene, the hemolymph storage protein levels were significantly reduced and the ability of the hemocytes to adhere and spread on extracellular matrix was altered or reduced, wherein CpBV15βwas detectable in the cytoplasm of hemocytes based on indirect immunofluorescence assay. To confirm the role of CpBV15β, its double stranded RNA could efficiently recover the functional efficacy of hemocytes towards non-self and synthesis of storage proteins. Thus these results clearly demonstrate the role of CpBV15βin altering the host physiology by involving in cellular immune response and host protein synthesis.

Hemolin is a member of the immunoglobulin (Ig) superfamily and contains four Ig domainsthat are similar to neural cell adhesion molecules. It has been regarded as a recognition molecule at immune challenge in insects. This study showed that hemolin of Plutella xylostella was expressed during pupal and adult stages but absent in all larval instars without any immune challenge. It is, however, strongly induced by the injection of Escherichia coli or its lipopolysaccharide in hemocytes, fat body and gut. A double-stranded RNA (dsRNA) interference experiment revealedits role in activation of prophenoloxidase (PPO) in the hemolymph during bacterial infection. Also its involvement in cellular defense was investigated in its mediation of the adherence of hemocytes to rat blood erythrocytes which was knocked down by its dsRNA. Finally, its physiological significance in pupal stage was confirmed by using dsRNA, which significantly prevented adult development. Therefore, it is concluded that hemolin plays roles in both immune and adult development in P. xylostella.