The complexity of virus systems: the case of endosymbionts

► Viruses are abundant in many endosymbionts despite their intracellular niche. ► WO, a phage of Wolbachia, is one of the most widespread endosymbiont viruses. ► The Modular Theory of phage genome evolution does not apply to WO. ► WO forms a tripartite symbiotic relationship between arthropods, bacteria, and viruses. ► WO has potential as a vector for Wolbachia transformation and in therapeutics.