Date Published: May 9, 2013
Publisher: Public Library of Science
Author(s): Gaelen R. Burke, Sarah A. Thomas, Jai H. Eum, Michael R. Strand, David S. Schneider.
Viruses are usually thought to form parasitic associations with hosts, but all members of the family Polydnaviridae are obligate mutualists of insects called parasitoid wasps. Phylogenetic data founded on sequence comparisons of viral genes indicate that polydnaviruses in the genus Bracovirus (BV) are closely related to pathogenic nudiviruses and baculoviruses. However, pronounced differences in the biology of BVs and baculoviruses together with high divergence of many shared genes make it unclear whether BV homologs still retain baculovirus-like functions. Here we report that virions from Microplitis demolitor bracovirus (MdBV) contain multiple baculovirus-like and nudivirus-like conserved gene products. We further show that RNA interference effectively and specifically knocks down MdBV gene expression. Coupling RNAi knockdown methods with functional assays, we examined the activity of six genes in the MdBV conserved gene set that are known to have essential roles in transcription (lef-4, lef-9), capsid assembly (vp39, vlf-1), and envelope formation (p74, pif-1) during baculovirus replication. Our results indicated that MdBV produces a baculovirus-like RNA polymerase that transcribes virus structural genes. Our results also supported a conserved role for vp39, vlf-1, p74, and pif-1 as structural components of MdBV virions. Additional experiments suggested that vlf-1 together with the nudivirus-like gene int-1 also have novel functions in regulating excision of MdBV proviral DNAs for packaging into virions. Overall, these data provide the first experimental insights into the function of BV genes in virion formation.
Microorganisms form associations with metazoan hosts that range from beneficial symbiosis (mutualists) to parasitic (pathogens). Mutualists serve as important sources of evolutionary innovation for hosts, while pathogens often acquire genes from hosts or other organisms that facilitate their own survival and cause disease. Although most research on obligate mutualists focuses on bacteria, several fungi and protozoans are also known to form beneficial partnerships –. Viruses in contrast are almost always thought to form parasitic associations –. A notable exception to this is the family Polydnaviridae, which consists entirely of large DNA viruses that are obligate mutualists of insects called parasitoid wasps , . Polydnaviruses (PDVs) thus provide an opportunity for understanding the adaptations involved in the evolution of viruses into mutualists from pathogenic ancestors.
Phylogenetic data strongly support that BVs evolved from a nudivirus ca. 100 Mya, and that nudiviruses and baculoviruses shared a more ancient common viral ancestor ca. 200 Mya earlier , , . Detailed studies of AcMNPV and select other species also provide important insights into the function of baculovirus core genes. In contrast, the hypothesized function of baculovirus core gene homologs in BVs (and nudiviruses) is founded on inferences from the baculovirus literature and/or expression patterns in wasp ovaries during replication. Thus, the primary goal of this investigation was to assess whether RNAi methods could be used to disrupt BV gene expression, and then to use these methods with a subset of genes to determine whether their roles in replication were consistent with or differed from baculoviruses.