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

Abstract

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 socalled 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 virusderived 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. Author summaryThousands of parasitic wasp from the ichneumonid family rely on virus-derived particles, named Ichnoviruses (Polydnavirus family), to ensure their successful development. The particles are produced in a specialized ovarian tissue of the female wasp named calyx. Virions are assembled in the calyx cell nuclei and stored in the oviduct before being transferred to the parasitoid host upon female wasp oviposition. Genes encoding proteins PLOS Pathogens | https://doi.

Genomic architecture of endogenous ichnoviruses reveals distinct evolutionary pathways leading to virus domestication in parasitic wasps

“…In ichneumonids, increase in gene copies is one, but also probably not the only, mechanism involved in transcriptional control. Indeed, IVSPER genes differ in their expression level in H. didymator calyx [35], which suggests involvement of other gene-specific mechanisms. Moreover, the clustering of replication genes in ichneumonids may facilitate coordinated regulation of their expression.…”

“…For annotation, EST reads from venom [78], as well as reads obtained using GS FLX (Roche/454), Titanium chemistry from total insects [78] and from ovaries [79], were mapped to the genome with GMAP [80], and Illumina reads published previously [35], or from the 1KITE consortium (http://1kite.org/subprojects.html) using STAR [81], and new calyx RNA-seq (SRA accession: PRJNA590863) with TopHat2 v2.1.0 [82]. BRAKER1 v1.10 [41] was used to predict genes in the genome of H. didymator using default settings.…”

“…Half of the nudiviral genes identified within the genome of the braconids Cotesia congregata and Microplitis demolitor are located in a "nudiviral cluster;" the other nudiviral genes are dispersed and isolated in the wasp genome [15,23,31]. Genes involved in ichnovirus particle formation, but lacking similarity with known virus genes, have been identified in two ichneumonids belonging to two subfamilies, the campoplegine Hyposoter didymator [16,35] and the banchine Glypta fumiferanae [20]. Analysis of bacterial artificial chromosomes (BAC) for these two species revealed that the approximately 40 replication genes that have been identified are organized in three large clusters, named "Ichnovirus Structural Protein Encoding Regions" (IVSPER) [16,20].…”

The Unconventional Viruses of Ichneumonid Parasitoid Wasps

“…This very first functional analysis of the production of particles of viral origin in an ichneumonid wasp clearly shows that the genes examined are necessary for morphogenesis and cell trafficking of A first functional study has been undertaken recently for six candidate replication genes, chosen because of their high level of expression in the replicative tissue (calyx) and because they encode proteins identified during the proteomic analysis of the purified viral particles [36]. Their involvement in particle formation was demonstrated through inhibition of their expression by RNA interference (RNAi), an approach that has proven very effective in this biological system: IVp12-1 is involved in the formation of the virogenic stroma, two genes (U23 and IVSP4-1) are involved in the assembly of nucleocapsids, two genes (U22 and IVSP3-1) are involved in the release of virions from the nucleus and IVp53-2 is involved in the release of virions from the cell.…”

“…The functions of the three NCLDV-like genes identified in the IVSPERs of G. fumiferanae [31], two of which were later found to have homologs in campoplegine wasps [30], should be investigated, perhaps using an RNAi approach similar to the one described above [36]. These genes were found to be overexpressed in the replicative calyx tissue of G. fumiferanae, relative to the ovarioles [31], and were hypothesized to be part of the replicative machinery of the progenitor virus.…”