RNA interference identifies domesticated viral genes involved in assembly and trafficking of virus-derived particles in ichneumonid wasps

There are many documented examples of viral genes retained in the genomes of multicellular organisms that may in some cases bring new beneficial functions to the receivers. The ability of certain ichneumonid parasitic wasps to produce virus-derived particles, the so-called ichnoviruses (IVs), not only results from the capture and domestication of single viral genes but of almost entire ancestral virus genome(s). Indeed, following integration into wasp chromosomal DNA, the putative and still undetermined IV ancestor(s) evolved into encoding a ‘virulence gene delivery vehicle’ that is now required for successful infestation of wasp hosts. Several putative viral genes, which are clustered in distinct regions of wasp genomes referred to as IVSPERs (Ichnovirus Structural Protein Encoding Regions), have been assumed to be involved in virus-derived particles morphogenesis, but this question has not been previously functionally addressed. In the present study, we have successfully combined RNA interference and transmission electron microscopy to specifically identify IVSPER genes that are responsible for the morphogenesis and trafficking of the virus-derived particles in ovarian cells of the ichneumonid wasp Hyposoter didymator. We suggest that ancestral viral genes retained within the genomes of certain ichneumonid parasitoids possess conserved functions which were domesticated for the purpose of assembling viral vectors for the delivery of virulence genes to parasitized host animals.