Viruses viewed as obligatory symbionts of parasitoid wasps

Very few obligatory relationships involve viruses to the remarkable exception of polydnaviruses (PDVs) associated with tens of thousands species of parasitic wasps that develop within the body of lepidopteran larvae. PDV particles, injected along with parasite eggs into the host body, act by manipulating host immune defences, development and physiology, thereby enabling wasp larvae to survive in a potentially harmful environment. Particle production does not occur in infected tissues of parasitized caterpillars, but is restricted to specialized cells of the wasp ovaries. The genome enclosed in the particles encodes almost no viral structural protein, but mostly factors used to manipulate the physiology of the parasitized host. However we recently confirmed the viral nature of PDVs associated with braconid wasps by characterizing a large set of nudivirus genes residing permanently in the wasp chromosome(s). Many of these genes encode structural components of the bracovirus particles and their expression pattern correlates with particle production. They constitute a viral machinery comprising a large number of core genes shared by nudiviruses and baculoviruses. Thus bracovirus particles are made by a complex nudiviral device carrying DNA for the delivery of virulence genes into lepidopteran hosts. Together with gene transfer agents of bacteria, PDVs highlights the fact that viruses should no longer be exclusively considered obligatory parasites, and that in certain cases they are obligatory symbionts.