Polydnaviruses are symbiotic viruses associated with some parasitic Hymenoptera that are vertically transmitted as proviruses within wasp genomes. To study this symbiotic association a gene encoding an abundant Campoletis sonorensis polydnavirus virion protein was characterized. This gene is not encapsidated but resides in the wasp genome where it is expressed only during virus replication. Immunolocalization studies detected the encoded 44-kDa protein only in oviduct tissue with ultrastructural studies detecting epitopes between or on virion envelopes. Expression and localization of the 44-kDa protein are consistent with its being a viral structural protein but localization of the gene only within the wasp genome is atypical, raising the possibility that this protein is adventitiously packaged during virion assembly. To address this possibility, quantitative dot blot and genomic Southern blot hybridizations were performed to determine whether the copy number of the p44 gene increased disproportionately during replication, as would be expected for a gene encoding a virion protein. The copy number of the p44 gene increases in tissues supporting virus replication but is unchanged in other tissues, suggesting that this gene is amplified in replicative cells. The data indicate that genes encoding polydnavirus virion proteins may be distributed between wasp and encapsidated viral genomes.

Polydnaviruses are the only known group of mutualistic viruses. They are required for successful parasitization in many braconid and ichneumonid parasitoids. The intimacy of this mutualistic association is indicated by the integration and vertical transmission of polydnaviruses in wasp genomes and by their asymptomatic, developmentally regulated replication. The evolution of this mutualism raises several interesting issues that require a better understanding of the viral genome and viral replication. To develop probes for virus replication and morphogenesis, we have begun to characterize several viral structural proteins. A 699 bp cDNA encoding the p12 viral structural protein was cloned and sequenced. The p12 gene localizes to viral segment Y and encodes a predicted protein of 92 amino acids that does not encode a signal peptide and is unrelated to known peptide or nucleic acid sequences. The p12 mRNA is detected at the onset of virus replication. mRNA titers increase with increasing rates of virus replication. Polyclonal antisera raised against histidine‐tagged p12 protein expressed in bacteria reacted specifically with the p12 polypeptide in Western blots of CsPDV virions. The p12 polypeptide was not detected in non‐replicative wasp or lepidopteran tissues by Western blot analyses but was readily detected in protein extracts of wasp ovaries. The data indicate that the p12 gene is a viral gene encoding a virion protein and provides a specific probe for virus replication that will be useful for studying the evolution of this group of mutualistic viruses. Arch. Insect Biochem. Physiol. 40:30–40, 1999. © 1999 Wiley‐Liss, Inc.

Polydnaviruses are the only known group of mutualistic viruses. They are required for successful parasitization in many braconid and ichneumonid parasitoids. The intimacy of this mutualistic association is indicated by the integration and vertical transmission of polydnaviruses in wasp genomes and by their asymptomatic, developmentally regulated replication. The evolution of this mutualism raises several interesting issues that require a better understanding of the viral genome and viral replication. To develop probes for virus replication and morphogenesis, we have begun to characterize several viral structural proteins. A 699 bp cDNA encoding the p12 viral structural protein was cloned and sequenced. The p12 gene localizes to viral segment Y and encodes a predicted protein of 92 amino acids that does not encode a signal peptide and is unrelated to known peptide or nucleic acid sequences. The p12 mRNA is detected at the onset of virus replication. mRNA titers increase with increasing rates of virus replication. Polyclonal antisera raised against histidine-tagged p12 protein expressed in bacteria reacted specifically with the p12 polypeptide in Western blots of CsPDV virions. The p12 polypeptide was not detected in non-replicative wasp or lepidopteran tissues by Western blot analyses but was readily detected in protein extracts of wasp ovaries. The data indicate that the p12 gene is a viral gene encoding a virion protein and provides a specific probe for virus replication that will be useful for studying the evolution of this group of mutualistic viruses.