Functional characterization of five nudivirus core genes in Venturia canescens virus-like particles

Permanent integration of partial or entire viral genomes into germ line cells of eukaryotic hosts is referred to as endogenization. Some endogenous virus elements (EVEs) have been fixed in the host populations, and been co-opted for new functions. Virus-like particles (VLPs) are typical cases of beneficial EVEs found in parasitoid wasps. Venturia canescens (Hymenoptera: Ichneumonidae) is an endoparasitoid wasp that produces VLPs (VcVLPs) and uses VcVLPs to protect its eggs against host defenses. Phylogenetic analyses showed that VcVLPs are derived from nudiviruses, which are closely related to pathogenic baculoviruses. VcVLP genome components consist of 21 genes homologous to nudivirus core genes. Sixteen of them were predicted to have roles in transcription and envelope formation because they also exhibit homology with core genes present in baculoviruses. While the role of all baculovirus core genes were experimentally studied in model species, none of the nudivirus core genes have been functionally studied. The goal of our study was to functionally characterize the remaining 5 nudivirus core genes (OrNVorf18-like, OrNVorf41-like, OrNVorf47-like, OrNVorf61-like, and OrNVorf76-like) without predicted functions. Coupling RNAi knockdown methods with functional assays, our results indicate that OrNVorf18-like, OrNVorf41-like, and OrNVorf61-like are involved in VcVLP formation and knockdown of OrNVorf47-like reduced the ability of VcVLPs to attach to wasp eggs. However, knockdown of OrNVorf76-like didn’t result in any phenotypes. In addition, parasitism assays showed that knockdown of OrNVorf18-like, OrNVorf41-like, OrNVorf61-like and OrNVorf47-like caused wasp eggs encapsulated by host blood cells, suggesting their essential role in protecting wasp eggs against host encapsulation.