Specificities of ichnoviruses associated with campoplegine wasps: genome, genes and role in host–parasitoid interaction

•Ichnoviruses (IVs) derive from integration of a paleo-ichnovirus into a wasp genome.•IV genomes consist of proviral segments plus genes with roles in particle formation.•A subset of IV genes is expressed in parasitized hosts.•Expression of IV genes is required for successful parasitism of hosts.

Ichnoviruses (IVs), unique symbiotic viruses carried by ichneumonid campoplegine wasps, derive from integration of a paleo-ichnovirus into an ancestral wasp genome. The modern ‘genome’ is composed of both regions that are amplified, circularized and encapsidated into viral particles and non-encapsidated viral genomic regions involved in particle morphogenesis. Packaged genomes include multiple circular dsDNAs encoding many genes mostly organized in gene families. Virus particles are assembled in specialized ovarian cells from which they exit into the oviduct lumen; mature virions are injected during oviposition into the insect host. Expression of viral proteins in infected cells correlates with physiological alterations of the host enabling success of parasitism.