Citation: Desjardins, C., Gundersen, D.E., Hostetler, J.B., Tallon, L.J., Utterback, T.R., Fuester, R.W., Schatz, M.C., Pedroni, M.J., Fadrosh, D.W., Haas, B.J., Toms, B.S., Chen, D., Nene, V. 2007. Structure and Evolution of a Proviral Locus of Glyptapanteles indiensis Bracovirus. BMC Microbiology 2007, 7:61 Interpretive Summary: Tiny wasps known as parasitoids have potential to control moth pests, such as caterpillars, of agricultural crops and forests. The survival of many of these wasps is enhanced by a virus, called a polydnavirus, that is injected along with the wasp egg into the host caterpillar pest. In this study we use modern molecular techniques to sequence and characterize the DNA of the provirus, the form of the polydnavirus that is a part of the wasp before it is removed and injected into the caterpillar by the wasp. This is the first time researchers have ever examined a large region of the provirus, and many new discoveries were made. The information obtained will help explain how the polydnavirus may be formed and replicate, how the virus may help the wasp to survive, and may lead to new biocontrol strategies. This information will be of interest to university and industry scientists who are interested in virus evolution and/or in developing new virus-based strategies to control pests.
Technical Abstract: Polydnaviruses, a family of double-stranded DNA viruses with segmented genomes, have evolved as obligate endosymbionts of endoparasitoid wasps, and are some of the few viruses known to share mutualistic relationships with eukaryotic hosts. Virus particles are replication deficient and are produced only by female wasps from proviral sequences integrated in the wasp genome. Virus particles are subsequently co-injected with eggs into lepidopteran hosts, where viral gene expression facilitates parasitoid survival and therefore survival of proviral DNA. We report on analyses of the first available large sequence of part of a proviral genome, a 223 kbp locus from Glyptapanteles indiensis that codes for 8 viral genome segments of its associated bracovirus (GiBV). In contrast to current concepts of bracovirus proviral genome organization, our data show that GiBV proviral genome segment sequences are not tightly linked in a single tandem array, but are integrated into multiple loci, potentially allowing control of viral genome segment molarity. Conserved DNA sequence motifs found at the junctions of proviral genome segment sequences in GiBV and viral segment sequences in other polydnavirus genomes suggest that the mechanisms governing segment excision are highly conserved across bracoviruses. Analysis of sequence polymorphism data demonstrates selection acting on non-coding DNA, suggesting additional functional motifs or non-coding RNAs are present in the GiBV genome. The proviral locus bears many striking similarities to prokaryotic pathogenicity islands, including flanking repeats, distinct G+C and dinucleotide compositions, and differences in gene structure and function relative to the host genome. Given that the integration of bracoviruses into wasp genomes happened an estimated 74 million years ago, polydnavirus proviral DNA is ideal for studying the long-term evolution of pathogenicity and other genomic islands. Additionally, as heritable variation is generated in, and selection acts on, the proviral genome rather than the viral genome, these data provide a basis for studying the evolution of mutualistic virus-eukaryote interactions.