Effects of X chromosome number and parental origin on X-linked gene expression in preimplantation mouse embryos

Diploid androgenetic mouse embryos, possessing two sets of paternally inher ited chromosomes, and control fertilized embryos were used to examine the r elative effects of X chromosome number and parental chromosome origin on an drogenone viability and X-linked gene expression. A significant difference in efficiency of blastocyst formation was observed between XX and XY androg enones in some experiments, but this difference was not uniformly observed. Significant effects of both X chromosome number and parental origin on X-l inked gene expression were observed. Male and female control embryos expres sed the Xist RNA initially. This expression was followed by a preferential reduction in Xist RNA abundance in male embryos, indicating that dosage com pensation for the X chromosome may normally require the downregulation of X ist RNA expression in male embryos, in conjunction with the production of s table Xist transcripts in female embryos. By the late blastocyst stage, XX control embryos expressed significantly more Xist RNA than did XY embryos. Unlike their normal counterparts, XX androgenones did not express significa ntly more Xist RNA than did XY androgenones at the late blastocyst stage. A ndrogenones exhibited severe repression of the Pgk1 gene, but during develo pment to the late blastocyst stage Pgk1 mRNA expression increased in XX and rogenones and decreased in XY androgenones. Thus, the initial repression of the Pgkl gene in XX androgenones was lost as the Xist RNA declined in abun dance, and this loss was correlated with a failure of XX androgenones to ex press significantly more Xist RNA than did XY androgenones. These results i ndicate that androgenones may lack a factor that is expressed from the mate rnal genome and required for dosage compensation in preimplantation embryos . The results also indicate that early dosage compensation in preimplantati on embryos may normally be reversible, thus providing flexibility to meet d ifferent developmental requirements of the embryonic and extraembryonic lin eages.