Spontaneous expression of murine endogenous viruses in hamster X mouse hybrid cells

The spontaneous expression patterns of murine leukemia virus (MuLV) from mouse spleen cultures and hamster × mouse spleen hybrid cells were compared. The hybrid cells preferentially segregated mouse while retaining a complete set of hamster chromosomes. Virus expression patterns showed considerable mouse strain differences in both parental and hybrid cells. None of 13 BALB/c spleen cultures expressed virus while 6 of 20 hybrid clones expressed N‐tropic or N‐tropic and X‐tropic viruses. All AKR spleen cultures and 5 of 7 hybrids expressed N‐tropic but not X‐tropic virus. X‐tropic virus expression was detected in all 6 NZB spleen cultures and in 5 of 8 hybrids. Virus expression did not occur in NIH Swiss or NIH Swiss/Nude (nu/nu) spleen or hybrid cultures. A major structural locus (Akv‐1) for N‐tropic virus expression has been assigned to chromosome 7 in AKR mice by recombi‐national genetics and is closely linked to the gene for glucose phosphate isomerase (Gpi‐1). Analysis of AKR hybrids confirmed the requirement of chromosome 7 for initial expression of mouse N‐tropic virus and, in addition, excluded the role of all other mouse chromosomes in this phenomenon. Eighteen of 20 BALB/c clones retained chromosome 7, including 6 virus‐positive clones, suggesting that chromosome 7 may be necessary but is not sufficient for N‐tropic virus expression in this strain. Hybrid clones with examples of asynteny were found for all other chromosomes. Long‐term passage and sub‐cloning studies of AKR and BALB/c virus‐positive hybrids indicated that virus‐positive clones segregated independently of chromosome 7 markers. Spontaneous expression of X‐tropic virus in NZB hybrid clones was syntenic with the gene for‐enylate kinase (Ak‐1) which is assigned to chromosome 2. A relatively small number of NZB hybrid clones were studied and, a firm gene assignment cannot be made, because only one discordant clone each was found for chromosomes 1, 3, 10, 19. Our studies indicate that spontaneous expression of MuLV strains does not require the following MuLV‐related genes: Fv‐1 (chromosome 4), Fv‐2 (chromosome 9), Rec‐1 (replication of ecotropic virus, chromosome 5), Gv‐1 and H‐2 (chromosome 17).