Endogenous retroviruses (ERVs) are remnants of ancestral viruses in the host genome. One feline ERV group V (EnvV) member, EnvV-Fca, was detected as a placenta-specific protein secreted from cells. EnvVs are classified into two groups, and genetic analyses show that EnvV2 genes are widespread in vertebrates, with birds, bats, and rodents potentially acting in virus transmission. These findings present a model of retroviral transmission and may elucidate ERV evolution.

Endogenous retroviruses (ERVs) are remnants of ancestral viruses in the host genome. The present study identified the expression of a defective retroviral env gene belonging to the ERV group V member Env (EnvV) in Felis catus (EnvV-Fca). EnV-Fca was specifically detected in the placental trophoblast syncytiotrophobic layer and expressed as a secreted protein in cultured cells. Genetic analyses indicated that EnvV2 genes are widely present in vertebrates and are under purifying selection among carnivores, suggesting a potential benefit for the host. This study suggests that birds, bats, and rodents carrying EnvV2 may play significant roles as intermediate vectors in spreading or cross-transmitting viruses among species. Our findings provide valuable insights into the evolution of ERV in vertebrate hosts.