Truly ubiquitous CRESS DNA viruses scattered across the eukaryotic tree of life

Until recently, most viruses detected and characterized were of economic significance, associated with agricultural and medical diseases. This was certainly true for the eukaryote-infecting circular Rep (replication-associated protein)-encoding single-stranded DNA (CRESS DNA) viruses, which were thought to be a relatively small group of viruses. With the explosion of metagenomic sequencing over the past decade and increasing use of rolling-circle replication for sequence amplification, scientists have identified and annotated copious numbers of novel CRESS DNA viruses – many without known hosts but which have been found in association with eukaryotes. Similar advances in cellular genomics have revealed that many eukaryotes have endogenous sequences homologous to viral Reps, which not only provide “fossil records” to reconstruct the evolutionary history of CRESS DNA viruses but also reveal potential host species for viruses known by their sequences alone. The Rep protein is a conserved p...