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. EVEs may be entire viral genomes (proviruses), or fragments of viral genomes. They arise when a viral DNA sequence becomes integrated into the genome of a germ cell that goes on to produce a viable organism. The newly established EVE can be inherited from one generation to the next as an allele in the host species, and may even reach fixation. Endogenous retroviruses and other EVEs that occur as proviruses can potentially remain capable of producing infectious virus in their endogenous state. Replication of such 'active' endogenous viruses can lead to the proliferation of viral insertions in the germline. For most non-retroviral viruses, germline integration appears to be a rare, anomalous event, and the resulting EVEs are often only fragments of the parent virus genome. Such fragments are usually not capable of producing infectious virus, but may express protein or RNA and even cell surface receptors. EVEs have been identified in animals, plants and fungi. In vertebrates EVEs derived from retroviruses (endogenous retroviruses) are relatively common. Because retroviruses integrate into the nuclear genome of the host cell as an inherent part of their replication cycle, they are predisposed to enter the host germline. In addition, EVEs related to parvoviruses, filoviruses, bornaviruses and circoviruses have been identified in vertebrate genomes. In plant genomes, EVEs derived from pararetroviruses are relatively common. EVEs derived from other, non-retrotranscribing virus families, such as Geminiviridae, have also been identified in plants. Moreover, EVEs related to giant viruses (aka GEVEs) of phylum Nucleocytoviricota (NCLDV) similar to Aureococcus anophagefferens virus (AaV) have been found in 2019/2020. EVEs are traditionally identified by similarity to known viruses. In 2021, it has been demonstrated that the k-mer composition of endogenous RNA virus resemble that of their exogenous counterparts.