Large-scale invasion of unicellular eukaryotic genomes by integrating DNA viruses

Significance Protists are a diverse collection of predominantly unicellular eukaryotic organisms that are not animals, plants, or fungi. They make up most of the eukaryotic tree of life, are major components of nearly all ecosystems and are critical for carbon and nutrient cycling. In this study, we found that large parts of protist genomes are viral in origin and that these viral integrations are comparable in scale to prophage integrations in bacterial genomes. Protist EVEs were distantly related to virophages, a group of viruses which parasitize on larger “giant viruses” that infect and kill their eukaryotic hosts. Many EVEs appear to be functional viruses, which suggests that diverse arrays of these elements may be part of a host antivirus system.