A Technique of Segment Expression and RNA Interference (SERI) Reveals a Specific Physiological Function of a Cysteine-Rich Protein Gene Encoded in Cotesia plutellae Bracovirus

As a provirus, polydnavirus has a segmented DNA genome on chromosome(s) of host wasp. It contains several genes in each segment that presumably play critical roles in regulating physiological processes of target insect parasitized by the wasp. A cysteine-rich protein I (CRP1) is present in the polydnavirus Cotesia plutellue bracovirus (CpBV) genome, but its expression and physiological function in Plutella xylostella parasitized by the viral host C. plutellae is not known. This CpBV-CRP1 encoding 189 amino acids with a putative signal peptide (20 residues) was persistently expressed in parasitized P xylostella with gradual decrease at the late parasitization period. Expression of CpBV-CRP1 was tissue-specific in the fat body/epidermis and hemocyte, but not in the gut. Its physiological function was analyzed by inducing transient expression of a CpBV segment containing CpBV-CRP1 and its promoter, which caused significant reduction in hemocyte -spreading and delayed larval development. When the treated larvae were co-injected with double-stranded RNA of CpBV-CRP1, the expression of CpBV-CRP1 disappeared, whereas other genes encoded in the CpBV segment was expressed. These co-injected larvae significantly recovered the hemocyte-spreading capacity and larval development rate. This study reports that CpBV-CRP1 is expressed in P xylostella parasitized by C plutellae and its physiological function is to alter the host immune and developmental processes.