X inactive-specific transcript (Xist) expression and X chromosome inactivation in the preattachment bovine embryo

Expression of the X inactive-specific transcript (Xist) is thought to be es sential for the initiation of X chromosome inactivation and dosage compensa tion during female embryo development. In the present study, we analyzed th e patterns of Xist transcription and the onset of X chromosome inactivation in bovine preattachment embryos. Reverse transcription-polymerase chain re action (RT-PCR) revealed the presence of Xist transcripts in all adult fema le somatic tissues evaluated. In contrast, among the male tissues examined, Xist expression was detected only in testis. No evidence for Xist transcri ption was observed after a single round of RT-PCR from pools of in vitro-de rived embryos at the 2- to 4-cell stage. Xist transcripts were detected as a faint amplicon at the 8-cell stage initially, and consistently thereafter in all stages examined up to and including the expanded blastocyst stage. Xist transcripts, however, were subsequently detected from the 2-cell stage onward after nested RT-PCR. Preferential [H-3]thymidine labeling indicativ e of late replication of one of the X chromosomes was noted in female embry os of different developmental ages as follows: 2 of 7 (28.5%) early blastoc ysts, 6 of 13 (46.1%) blastocysts, 8 of 11 (72.1%) expanded blastocysts, an d 14 of 17 (77.7%) hatched blastocysts. These results suggest that Xist exp ression precedes the onset of late replication in the bovine embryo, in a p attern compatible with a possible role of bovine Xist in the initiation of X chromosome inactivation.