Embryonic genome activation (EGA) is the first major transition that occurs after fertilization, and entails a dramatic reprogramming of gene expression that is essential for continued development. Although it has been suggested that EGA in porcine embryos starts at the four-cell stage, recent evidence indicates that EGA may commence even earlier; however, the molecular details of EGA remain incompletely understood. The RNA polymerase II of eukaryotes transcribes mRNAs and most small nuclear RNAs. The largest subunit of RNA polymerase II can become phosphorylated in the C-terminal domain. The unphosphorylated form of the RNA polymerase II largest subunit C-terminal domain (IIa) plays a role in initiation of transcription, and the phosphorylated form (IIo) is required for transcriptional elongation and mRNA splicing. In the present study, we explored the nuclear translocation, nuclear localization, and phosphorylation dynamics of the RNA polymerase II C-terminal domain in immature pig oocytes, mature oocytes, two-, four-, and eight-cell embryos, and the morula and blastocyst. To this end, we used antibodies specific for the IIa and IIo forms of RNA polymerase II to stain the proteins. Unphosphorylated RNA polymerase II stained strongly in the nuclei of germinal vesicle oocytes, whereas the phosphorylated form of the enzyme was confined to the chromatin of prophase I oocytes. After fertilization, both unphosphorylated and phosphorylated RNA polymerase II began to accumulate in the nuclei of early stage one-cell embryos, and this pattern was maintained through to the blastocyst stage. The results suggest that both porcine oocytes and early embryos are transcriptionally competent, and that transcription of embryonic genes during the first three cell cycles parallels expression of phosphorylated RNA polymerase II.

X‐box binding protein‐1 (XBP‐1) is an important regulator of a subset of genes active during endoplasmic reticulum (ER) stress. In the present study, we analyzed XBP‐1 level and location to explore the effect of ER stress on oocyte maturation and developmental competency of porcine embryos in an in vitro culture system. First, we examined the localization of XBP‐1 at different meiotic stages of porcine oocytes and at early stages of parthenogenetic embryo development. Fluorescence staining showed that expression of functional XBP‐1 was weak in mature oocytes and at the one‐cell, two‐cell, and eight‐cell stages of embryos, but abundant at the GV oocyte, four‐cell, morula, and blastocyst stages. In addition, RT‐PCR revealed that both spliced XBP‐1 (XBP‐1s ) and unspliced XBP‐1 (XBP‐1u) were expressed at the GV oocyte, four‐cell, morula, and blastocyst stages. Tunicamycin (TM), an ER stress inducer, blocked porcine embryonic development at the four‐cell stage, exhibiting the effect on embryonic genome activation. Next, porcine embryos cultured in the presence of tauroursodeoxycholate (TUDCA), an ER stress inhibitor, were studied. Total cell numbers and the extent of the ICM increased (p<0.05), whereas the rate of nuclear apoptosis decreased (p<0.05). Moreover, expression of the anti‐apoptotic gene Bcl‐2 increased whereas expression of the pro‐apoptotic genes Bcl‐xl and p53 decreased. The results indicated that inhibition of ER stress enhanced porcine oocyte maturation and embryonic development by preventing ER stress‐mediated apoptosis in vitro.