The origin and structure of endogenous retroviral DNA

DNA related to the genomes of avian leukosis viruses (ALV) and mouse mammary tumor viruses (MMTV) is widely distributed among normal chickens and mice. We have used restriction mapping techniques to compare individuals from inbred and outbred populations with respect to genetically-transmitted proviral DNA, in efforts to understand its origins and functions. We have found that outbred animals have variable numbers of ALV and MMTV proviruses located at various sites in their host genomes, with some animals apparently free of such endogenous proviruses. Inbred or partially inbred animals have very similar or identical patterns, suggesting that proviruses behave as stable genetic elements once introduced into the germ line. Moreover, several cellular genes, including the homologous of the src gene of avian sarcoma virus, are invariant, or nearly so, in similar tests of these animals. We conclude that the proviruses under study have been introduced into the germ lines of chickens and mice long after speciation, that they are still segregating among contemporary animals, and that they are unlikely to provide essential functions to their hosts.