Analysis of virion structural components reveals vestiges of the ancestral ichnovirus genome

The polydnaviruses (PDVs) are a unique virus type used by an organism (a parasitic wasp) to manipulate the physiology of another organism (a lepidopteran host) in order to ensure successful parasitism. The evolutionary origin of these unusual viruses, found in a??17,500 braconid wasps (Bracoviruses) and a??15,000 ichneumonid wasps (Ichnoviruses), has been a major question for the last decade. We thus undertook an exclusive work aiming at investigating this origin via the characterization of genes encoding structural components for both types of PDVs. The present paper constitutes the first report on the identity and genome organisation of the viral machinery producing Ichnovirus virions. Our results strongly suggest that Ichnoviruses originated from a virus belonging to a group as yet uncharacterized that integrated its genome into that of an ichneumonid wasp ancestor. More importantly, our results demonstrate that the ancestor of Ichnoviruses differs from that of Bracoviruses, which originated from a nudivirus. We have now identified, for the two types of PDVs, the non packaged viral genes and their products involved in producing particles injected into the host during oviposition. Together, these data provide an example of convergent evolution where different groups of wasps have independently domesticated viruses to deliver genes into their hosts.