Bracovirus

Abstract: Polydnaviruses are essential components mediating host–parasitoid relationships between some braconid wasps and their caterpillar hosts largely by suppressing or misdirecting the host immune systems. The polydnavirus–wasp relationship is an unusual apparent mutualism between viruses and eukaryotes and remarkably has evolved to the stage where the two entities no longer can be considered separate. Estimations of the age of the polydnavirus-bearing clade of braconid wasps based on separate calculations from the mitochondrial 16S rRNA and COI genes and the nuclear 28S rRNA gene, calibrated using fossil data, converge to indicate a date of origin of ≈73.7 ± 10 million years ago. This range provides an upper bound on the time during which these wasps and viruses have been functionally associated.

Abstract: Many thousands of endoparasitic wasp species are known to inject polydnavirus (PDV) particles into their caterpillar host during oviposition, causing immune and developmental dysfunctions that benefit the wasp larva. PDVs associated with braconid and ichneumonid wasps, bracoviruses and ichnoviruses respectively, both deliver multiple circular dsDNA molecules to the caterpillar. These molecules contain virulence genes but lack core genes typically involved in particle production. This is not completely unexpected given that no PDV replication takes place in the caterpillar. Particle production is confined to the wasp ovary where viral DNAs are generated from proviral copies maintained within the wasp genome. We recently showed that the genes involved in bracovirus particle production reside within the wasp genome and are related to nudiviruses. In the present work we characterized genes involved in ichnovirus particle production by analyzing the components of purified Hyposoter didymator Ichnovirus particles by LC-MS/MS and studying their organization in the wasp genome. Their products are conserved among ichnovirus-associated wasps and constitute a specific set of proteins in the virosphere. Strikingly, these genes are clustered in specialized regions of the wasp genome which are amplified along with proviral DNA during virus particle replication, but are not packaged in the particles. Clearly our results show that ichnoviruses and bracoviruses particles originated from different viral entities, thus providing an example of convergent evolution where two groups of wasps have independently domesticated viruses to deliver genes into their hosts.

Abstract: Publisher Summary This chapter discusses the origin and evolution of Polydnavirus (PDV) that exists in obligate mutualisms with some parasitic Hymenoptera. A given PDV species replicates only in a single wasp species, and all members of that wasp species carry the associated virus. Two genera of PDVs are recognized––the bracoviruses (BVs) and the ichnoviruses (IVs)–– associated with braconid and ichneumonid wasps, respectively. The PDV genera have similar genome organization and replication/transmission pathways but have no known genetic similarity. Viral gene expression in insects parasitized by PDV-carrying wasps causes physiological alterations that are essential for parasitoid survival and development. PDV life cycles have been described as having “two arms.” PDVs replicate from proviral DNA in specialized cells of the wasp oviduct. Virus replication is first detected in the late pupal stage with virus released from calyx cells by budding or cell lysis and accumulating to high concentrations in the oviduct lumen. PDVs provide an interesting system for viral evolutionary studies. The unique life cycle involves coevolution with two hosts––one mutualistic and one pathogenic––thus, imposing both reductive and diversifying selection pressures on viral genes.