Ancient invasion of an extinct gammaretrovirus in cetaceans

• We find a new endogenous gammaretrovirus incorporated in nine cetacean genomes.
• The newly discovered cetacean gammaretroviruses are highly divergent comparing to extant mammalian counterparts.
• Molecular dating sets the invasion time to 19 million years ago.
• A new species in the genus Gammaretrovirus is proposed.

Endogenous gammaretroviruses (EGVs) have been widely studied in terrestrial mammals but seldom so in marine species. A genomic mining of the bottlenose dolphin (Tursiops truncatus) genome revealed a new EGV, termed Tursiops truncatus endogenous retrovirus (TTEV), which is divergent from extant mammalian EGVs. Molecular clock dating estimated the invasion time of TTEV into the host genome to be approximately 10–19 million years ago (MYA), while a previously identified killer whale endogenous gammaretrovirus (KWERV) was estimated to have invaded the host genome approximately 3–5 MYA. Using a PCR-based technique, we then verified that similar endogenous viruses exist in nine cetacean genomes. Phylogenetic analysis revealed that these cetacean EGVs are highly divergent from their counterparts in other mammals, including KWERV from the killer whale. In sum, we conclude that there have been at least two invasion episodes of EGVs into cetaceans during their evolutionary history.