Genetic background affects integration frequency of ecotropic proviral sequences into the mouse germ line

Germ line acquisition of ecotropic proviruses occurs at a high frequency in the progeny of SWR/J-RF/J hybrid mice carrying two genetically linked RF/J ecotropic proviral loci, Emv-16 and Emv-17 (N. A. Jenkins and N. G. Copeland, Cell 43:811-819, 1985). To determine if genetic background affects proviral integration frequency, I analyzed a series of crosses in which the two RF/J proviral loci were transferred onto different provirus-negative background strains. Unlike SWR/J-RF/J hybrid progeny, few CBA/CaJ-RF/J hybrid mice were identified that carried new germ line proviral loci. These results indicate that genetic factors other than the linked RF/J proviral loci contribute to the increased frequency of germ line provirus integration seen in the SWR/J-RF/J hybrids. The frequency of proviral acquisition appeared to increase when females carrying Emv-16, Emv-17, and at least one new proviral locus were further backcrossed, suggesting that integration frequency can be increased by genetic manipulation. The breeding data are consistent with the hypothesis that virus from the mother infects the egg or the early embryo. Analysis of the transmission frequency and cosegregation patterns of new proviral loci indicated that viral integration occurs after the first round of DNA replication and before the germ line is set aside during embryogenesis, with a majority of viral integrations occurring at the two-cell stage of development, and independent viral integrations can occur in the same or in different cells of the embryo.