Timing of mouse Xist expression during preimplantation development

The X-linked Xist gene is imprinted and is expressed from the paternal allele only in preimplantation development. Mouse Xist was analysed in preimplantation embryos. We found that the onset of Xist expression occurs at the G2-phase of the 2-cell embryos. At this same stage of development a number of newly synthesised embryonic transcripts appear, marking the passage from maternal to embryonic control of development. Some of these genes have been proved to be regulated by a zygotic clock which is set upon egg-activation at fertilization. Following treatment of 1-cell embryos with aphidicolin and cytochalasin D inhibitors of DNA replication and cytokinesis, respectively, here we show that Xist expression is dependent on first and second round of DNA replications and is regulated by a zygotic clock.