Molecular Characterization of a Novel Endogenous Gammaretrovirus in Killer Whales (Orcinus orca)
Retroviruses have been implicated in various diseases of cetaceans, but none have been identified to date.1,3 Tissues from an adult killer whale (Orcinus orca) were examined for the presence of a retroviral infection. A gammaretrovirus was detected by degenerate PCR in all tissues,2 and rtPCR showed retroviral mRNA expression in tissues and serum. The full-length sequence of the provirus was obtained, and a Taqman-based copy number assay demonstrated no evidence of replication in vivo. PCR on blood samples from 11 clinically healthy captive killer whales and tissues from two wild animals detected proviral DNA in all tissues and all animals, demonstrating the retrovirus is endogenous. PCR on tissues from multiple cetacean species showed homologues of this virus in the DNA of all eight species of delphinids examined, Kogia spp., and harbor porpoises, but not in beluga whales or fin whales. Analysis of the recently completed bottlenose dolphin genome revealed a full-length proviral sequence with 98% nucleotide identity to the killer whale gammaretrovirus. These results suggest that this gammaretrovirus either entered the delphinid ancestors' genome before the divergence of modern dolphins, or that an exogenous variant existed following divergence that was ultimately endogenized into all delphinid species. However, the transcriptional activity and the near intact viral genome suggests a more recent integration into the killer whale genome, favoring the latter hypothesis. While there is no evidence that the virus is clinically significant to killer whales, an exogenous variant affecting other cetacean species cannot be ruled out.
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