Use in paleovirology[edit]
EVEs are a rare source of retrospective information about ancient viruses. Many are derived from germline integration events that occurred millions of years ago, and can be viewed as viral fossils. Such ancient EVEs are an important component of paleovirological studies that address the long-term evolution of viruses. Identification of orthologous EVE insertions enables the calibration of long-term evolutionary timelines for viruses, based on the estimated time since divergence of the ortholog-containing host species groups. This approach has provided minimum ages ranging from 30 to 93 million years for the Parvoviridae, Filoviridae, Bornaviridae and Circoviridae families of viruses,[3] >100 million years in the Flaviviridae,[7] and 12 million years for the Lentivirus genus of the Retroviridae family. EVEs also facilitate the use of molecular clock-based approaches to obtain calibrations of viral evolution in deep time.[8][9]
Co-option and exaptation by host species[edit]
EVEs can sometimes provide a selective advantage to the individuals in which they are inserted. For example, some protect against infection with related viruses.[10][11] In some mammal groups, including higher primates, retroviral envelope proteins have been exapted to produce a protein that is expressed in the placental syncytiotrophoblast, and is involved in fusion of the cytotrophoblast cells to form the syncytial layer of the placenta. In humans this protein is called syncytin, and is encoded by an endogenous retrovirus called (ERVWE1) on chromosome seven. Remarkably, the capture of syncytin or syncytin-like genes has occurred independently, from different groups of endogenous retroviruses, in diverse mammalian lineages. Distinct, syncytin-like genes have been identified in primates, rodents, lagomorphs, carnivores, and ungulates, with integration dates ranging from 10 to 85 million years ago.[12]