Know more

A first step was to characterize the genomic architecture of the endogenous viral sequences in H. didymator. The H. didymator genome was sequenced, showing that the viral regions were scattered throughout the wasp genome (Legeai et al, 2020). We were able to identify 60 proviral loci in the H. didymator genome, distributed among 32 scaffolds and separated by large portions of wasp's sequences. They correspond either to regions carrying genes from the viral ancestor involved in particle production, or 'replication genes', or to sequences serving as templates for circular DNA molecules, or 'segments', which are then incorporated into the particles and transferred during parasitism (Figure 2).

Figure 2. Organization of polydnavirus sequences integrated into the parasitoid wasp genome. These sequences correspond either to segments (right) or to clusters of replication genes (left). The replication genes originate from the ancestor virus and are involved in the production of the virus particles. They are specifically expressed in the calyx cells of the pupa and adult. These genes are not encapsidated. In contrast, the segments are excised, circularized and then incorporated into the virus particles. They carry a set of genes that will be expressed in the tissues of the parasitized caterpillar. Their products are responsible for the alterations in the caterpillar's physiology necessary for the development of the parasitoid. Figure taken from the team's publication DOI: 10.1684/vir.2020.0835.

We now seek to decipher the function of the viral genes maintained in the wasp genome during evolution. A total of 54 genes potentially belonging to the ichnoviral machinery have been identified in the H. didymator genome (divided into 5 clusters and 1 isolated gene). With few exceptions, none of these genes shows similarity with known genes, and their actual function in viral particle formation remains to be demonstrated. To understand their role, we use RNA interference technology to inhibit their expression in vivo. The consequences of the knockdown on HdIV replication are then studied by combining electron microscopy and different "omics" and functional approaches.

A first study was carried out for six candidate genes, chosen because they are highly expressed in pupal calyxes and because they encode proteins identified during the proteomic analysis of purified virus particles (Lorenzi et al, 2019). This first-ever functional analysis of virus-derived particle production in an ichneumonid wasp showed that the genes studied were required for viral particle morphogenesis and cell trafficking, and that their functions are those expected of classical viral genes (Figure 3).

Figure 3. Schematic representation of the morphogenesis of ichnovirus virions. Ichnoviruses have an atypical morphogenesis, allowing calyx cells to produce particles throughout the life of the female parasitoid. Enveloped virions are assembled in the nucleus of calyx cells, bud through the nuclear envelope, and migrate towards the plasma membrane where they are secreted into the lumen of the oviduct, acquiring a second envelope. By inhibiting the expression of candidate genes by RNA interference, we were able to identify a gene (IVp12-1) involved in the formation of the virogenic stroma, two genes (U23 and IVSP4-1) involved in the assembly of nucleocapsids, two genes (U22 and IVSP3-1) involved in the exit of virions from the nucleus and a gene (IVp53-2) involved in the exit of virions from the cell. Figure taken from the team's publication DOI: 10.1371/journal.ppat.1008210.