Widespread Presence, in Chickens, of DNA Complementary to the RNA Genome of Avian Leukosis Viruses

DNA-RNA hybridization experiments have demonstrated the widespread presence in chickens of DNA complementary to the RNA of avian myeloblastosis virus. All apparently normal chicken embryos, or adult chickens that were tested, contained viral DNA in amounts ranging from 1.7 to 4.6 viral genome equivalents per cell. Embryos that were negative or positive for the group-specific antigen of avian leukosis viruses contained the same amount of viral DNA. Embryos from a strain of chickens free of leukosis viruses of groups A and B that develop 40% fewer spontaneous leukotic tumors than the original strain contained an average of 2.1 viral DNA equivalents per cell, whereas the original strain contained 3.2 viral DNA equivalents. By comparison, leukemic cells and cells infected with avian myeloblastosis virus or Rous sarcoma virus contained between 4 and 13 viral DNA equivalents. This study provides direct biochemical evidence for the presence in normal chicken cells of avian leukosis virus DNA that is inheritable and that represents either complete or incomplete viral genomes.