Equal expression of X-linked genes such as G6PD and PGK in females and males and 
the initiation of X-chromosome inactivation are critically dependent on the 
expression of the X-inactive specific transcript (Xist). The objective of the 
present study was to determine the effects of in vitro production (IVP) and 
nuclear transfer (NT) on the relative abundance (RA) of the X-linked transcripts 
G6PD, PGK, and Xist in preimplantation bovine embryos. In experiment 1, 
sex-determined IVP or in vivo-produced embryos were analyzed for mRNA expression 
of the 3 genes. The sex ratio was 36% vs. 64% in IVP blastocysts and thus 
deviated significantly from the expected ratio of 50% in the vivo control group. 
The RA of G6PD transcripts was significantly higher in female IVP embryos than 
in male embryos. In contrast, no significant differences were seen between in 
vivo-derived female embryos and their male counterparts. At the morula stage, 
female IVP embryos transcribed significantly more PGK mRNA than did male 
embryos. However, blastocysts did not exhibit significant differences in PGK 
transcripts. No differences were observed for in vivo-derived embryos with 
regard to the RA of PGK transcripts. The RA of Xist mRNA was significantly 
higher in all female embryos than in their male counterparts. In experiment 2, 
IVP, in vivo-developed, NT-derived, and parthenogenetic embryos carrying two X 
chromosomes of either maternal and paternal origin or of maternal origin only 
(parthenogenotes) were analyzed for the RA of the 3 genes. In NT-derived 
morulae, the RA of G6PD transcripts was significantly increased compared with 
their IVP and in vivo-generated counterparts. G6PD transcript levels were 
significantly increased in IVP blastocysts compared with in vivo-generated and 
parthenogenetic embryos. At the morula stage, PGK transcripts were similar in 
all groups, but the RA of PGK transcripts was significantly higher in IVP 
blastocysts than in their in vivo-generated, parthenogenetic, and NT-derived 
counterparts. The RA of Xist was significantly elevated in NT-derived morulae 
compared with IVP, in vivo-generated, and parthenogenetic embryos. NT-derived 
blastocysts showed an increased Xist expression compared with that of IVP, in 
vivo-generated, and parthenogenetic embryos. Results of the present study show 
for the first time that differences in X-chromosome-linked gene transcript 
levels are related to a perturbed dosage compensation in female and male IVP and 
female NT-derived embryos. This finding warrants further studies to improve IVP 
systems and NT protocols to ensure the production of embryos with normal gene 
expression patterns.