Endogenous viral element

An endogenous viral element (EVE) is a DNA sequence derived from a virus, and present within the germline of a non-viral organism . The crucial step in formation of an EVE is the integration of a viral DNA sequence into the genome of a germ cell that goes on to produce a viable organism. The newly established EVE is inherited from one generation to the next as an allele in the host species, and may even reach fixation.

EVEs have been identified in animals, plants and fungi. They may be genetic fragments, or entire viral genomes (proviruses). The most common EVEs in vertebrate genomes are derived from retroviruses. These viruses integrate into the nuclear genome of the host cell as part of their replication cycle, and are thus predisposed to invade the host germline. Furthermore, since integration is a part of the retrovirus replication cycle, endogenous retroviruses (ERVs) may remain capable of producing infectious virus.

In the case of non-retroviral viruses, invasion of the host germ line appears to be a rare, anomalous event, and the resulting EVEs are often only fragments of the parent virus genome. Such fragments are not capable of producing infectious virus, but may express protein or RNA.

EVEs and paleovirology
EVEs are a rare source of retrospective information about ancient viruses. Many are derived from virus integration events that occurred many millions of years ago, and can be viewed as the 'fossilized' sequences of ancient viruses. Ancient EVE insertions are an important component of paleovirological studies that address the long-term evolution of viruses. For example, identification of orthologous EVE insertions has provided minimum ages for several virus families (Parvoviridae, Filoviridae, Bornaviridae ), and genera (Lentivirus ), based on the estimated divergence times of host species.

Co-option and exaptation of EVEs by host species
Occasionally, EVEs may provide a selective advantage to the individuals in which they are inserted. For example, some EVEs protect against infection with related viruses . 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 (ERVWE1) on chromosome seven.