Little is known of the fate of viruses involved in long-term obligatory associations with eukaryotes. For example, many species of parasitoid wasps have symbiotic viruses to manipulate host defenses and to allow development of parasitoid larvae. The complete nucleotide sequence of the DNA enclosed in the virus particles injected by a parasitoid wasp revealed a complex organization, resembling a eukaryote genomic region more than a viral genome. Although endocellular symbiont genomes have undergone a dramatic loss of genes, the evolution of symbiotic viruses appears to be characterized by extensive duplication of virulence genes coding for truncated versions of cellular proteins. Evolution; Associations; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Manduca sexta (Sphingidae); Hymenopteran parasites; Cotesia congregata; Nucleotide sequence of parasite symbiotic polydnavirus injected into host Cotesia congregata (Braconidae); Evolution; Coevolution; Mutualism; Bracovirus; Nucleotide sequence of symbiotic polydnavirus injected into lepidopteran host, evolution relations; Lepidopteran hosts; Manduca sexta