Endogenous Retroviruses in Host-Virus Coevolution: From Genomic Domestication to Functional Innovation.

Endogenous retroviruses (ERVs) are remnants of retroviral infections that have become stably integrated into host germline genomes. Far beyond passive genomic elements, ERVs actively shape host evolution through complex mechanisms involving genetic innovation, immune modulation, and species adaptation. This review provides a comprehensive synthesis of ERV biology, highlighting recent advances in their classification, amplification mechanisms, and epigenetic silencing. Particular emphasis is placed on the cross-talk between ERVs and exogenous retroviruses (XRVs), demonstrating how receptor competition, recombination, and immune evasion contribute to virus-host co-evolution. We explore ERVs as molecular markers for phylogenetic reconstruction, with case studies such as Koala retrovirus (KoRV) and HERV-K illustrating regional transmission dynamics and co-opted immune functions. Additionally, we discuss the functional domestication of ERVs into regulatory elements, non-coding RNAs, and envelope-derived fusion proteins that influence gene expression, antiviral defense, and placental development.