Numerous cases of symbiogenesis between endocellular bacteria and a wide variety of eukaryotic hosts have been characterized. However, recent work has demonstrated that this evolutionary process was not restricted to bacteria. It also occurred between endocellular eukaryotes such as unicellular algae and fungal endophyte in plants [55, 56] . Endosymbiogenesis was also proposed as the evolutionary mechanism that allowed some invertebrate viruses with a large double-stranded DNA genome related to the nudiviruses and the ascoviruses [22] , to have led, respectively, to the origin of bracoviruses and ichnoviruses, which are currently recognized as forming two genera within the family Polydnaviridae. Although presently there is no definitive evidence ruling out the hypothesis that the resemblance between ichnovirus and ascovirus virions is only an evolutionary convergence, the genomic differences between ascovirus and ichnoviruses are in good agreement with the symbiogenetic hypothesis. Indeed, they match an evolutionary scenario of endosymbiogenesis during which, from a single integration event of symbiotic virus genome, viral genes were lost and/or translocated from the provirus to other chromosomal regions (Fig. 5 ). In parallel, host genes of interest for the wasp parasitoid were integrated and diversified by selection and gene duplication in the proviral DNA. In this scenario, the more ancient symbiogenesis, the rarer the traces of genes from viral origin in the ichnovirus genome would be. This constitutes a constraint that dramatically limits the possibility to investigate the evolutionary links between ascovirus and ichnovirus. Results of our analyses demonstrate that the situation is also complicated by the fact that lateral gene transfers unrelated to the origin of ichnoviruses cause important misleading background noise. Moreover, the scenario in Figure 5 is close to a previously proposed version [57] , but is not consistent with results presented here, nor with recently accumulated knowledge on DNA transfer from organelles into the nucleus. Since endocellular environments favour lateral transfers between virus and wasp nucleus, it can be proposed that genes of virus origin that are involved in the ichnovirus biology were passively integrated in one or several loci, step by step over time, alone or through transfers of gene clusters, or even the entire viral genome. Since parasitoid wasps are able to vector different viruses [44, 45] , this second scenario opens the exciting possibility that virus genes involved in the ichnovirus biology might correspond to a gene patchwork resulting from transfers from viruses belonging to different NCLDV and non-NCLVD families. Because of the background noise due to lateral gene transfers found in these systems, elucidating the origins of ichnoviruses will be very time-consuming, requiring new accurate experimental approaches to generate more robust evidence. Sequencing wasp genomes to identify proteins of viral origin that are components of virions and involved in the assembly of these may well contribute to our understanding of how ichnoviruses and bracoviruses evolved from other insect DNA viruses.