LAUSR

DNA viruses with efficient host genome integration capability were unknown in eukaryotes until recently. The discovery of virophages, satellite‐like DNA viruses that depend on lytic giant viruses that infect protists, revealed a genetically diverse group of viruses with high genome mobility. Virophages can act as strong inhibitors of their associated giant viruses, and the resulting beneficial effects on their unicellular hosts resemble a population‐based antiviral defense mechanism. By comparing various aspects of genome‐integrating virophages, in particular the virophage mavirus, with other mobile genetic elements and parasite‐derived defense mechanisms in eukaryotes and prokaryotes, we show that virophages share many features with other host–parasite systems. Yet, the dual lifestyle exhibited by mavirus remains unprecedented among eukaryotic DNA viruses, with potentially far‐reaching ecological and evolutionary consequences for the host.