Endogenous primate and feline type C viruses.

1. Endogenous type C viruses have been detected in a wide variety of mammalian species. Multiple copies of related, but not identical, virogene sequences can be found in the DNA of these species. 2. The endogenous type C virogenes are subject to the pressures of natural selection, and closely related species possess related virogene sequences. These genes evolve as cellular entities diverging from one another in a manner which correlates well with taxonomic relatedness of the species. 3. The endogenous type C viruses of baboons and domestic cats are related, but they can be distinguished by biologic and immunologic criteria and by partial nucleic acid sequence homology. Virogene sequences in the DNA of Old World monkeys and domestic cats also show a degree of relatedness not shared by the unique sequence DNA of these species. The data suggest that progenitors of domestic cats were exogenously infected by a type C virus that also gave rise to present-day endogenous type C viruses of Old World monkeys. 4. The genomes of exogenously infectious viruses replicating in permissive host cells appear to evolve much more rapidly than endogenous virogenes which replicate as cellular genes. Laboratory strains of efficiently oncogenic type C viruses are presumed to be derived from activated endogenous viruses which have been selected for virulence and which, in certain cases, have acquired the capacity to replicate in the host's own cells. 5. The ubiquitous presence of endogenous type C viruses among vertegrates and their preservation throughout millions of years of evolution suggests that these genes express normal physiologic functions which provide a selective advantage to the species.