Females of the endoparasitoid wasp Microplitis demolitor produce a polydnavirus (MdPDV) that is injected into lepidopteran hosts upon oviposition. In parasitized hosts, MdPDV infects primarily hemocytes that mediate a defense reaction known as encapsulation. Viral gene expression in infected hemocytes leads to pathogenic effects, enabling the wasp's progeny to develop within the host unmolested by defense reactions. To study the cause of these cytopathic effects, namely apoptosis of granular cells and loss of adhesion by plasmatocytes, the cDNAs of five viral genes have been isolated from infected Pseudoplusia includens (Lepidoptera: Noctuidae) hemocytes. Based on structural features, these transcripts can be grouped into two gene families, the epidermal growth factor (EGF)-like and the direct repeat (Rep-) gene family. The Rep-gene family consists of two members, designated as Glc1.8 and 3.1 according to the size of their mRNAs. These genes are structurally identical except for the copy number of a core direct repeat arranged in tandem arrays. Immunocytochemical studies indicate that Glc1.8 and 3.1 are expressed in MdPDV-infected hemocytes as cell-surface proteins. Since infected plasmatocytes do not spread onto foreign targets, gene silencing experiments using double-stranded RNA (dsRNA) directed against Glc1.8 and 3.1 were carried out. Inspection of hemocytes treated with Glc1.8/3.1-specific dsRNA revealed that (1) both Rep-genes can be successfully silenced by RNA interference (RNAi) and (2) plasmatocytes not expressing Glc1.8/3.1 but other MdPDV genes regain their ability to respond to a foreign surface and spread. RNAi studies linking the expression of EGF-like genes to the occurrence of apoptosis are underway.
Species 1: Hymenoptera Braconidae Microplitis demolitor
Species 2: Lepidoptera Noctuidae Pseudoplusia includens
Keywords: Parasitic wasps, polydnavirus
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