Acquisition of endogenous ecotropic MuLV can occur before the late one-cell stage in the genital tract of SWR/J-RF/J hybrid females

Endogenous ecotropic MuLV proviral loci are acquired by the progeny of some SWR/J x (SWR/J x RJ/J)F1] N2 hybrid females obtained by two successive backcrosses of RF/J mice onto the SWR/J background. This results most likely from an infection of early embryos or oocytes by MuLV particles originating from maternal tissues. However, the time and site of infection are not yet known. Using oviductal transfers of embryos at the one-cell stage, we show here that three of 88 N3 embryos from SWR/J x (SWR/J x RF/J)F1] N2 hybrid females transferred to virus-free foster mothers harbored new proviral integrations, whereas none of 61 SWR/J embryos transferred to SWR/J x (SWR/J x RF/J)F1] N2 hybrid females had acquired any proviruses. These data support the infection of oocyte and/or early one-cell embryo as the initial event leading to new proviral insertions.