Reproduction Abstracts

Before fertilization, oocyte genome is not transcribed and this transcriptionally silent state continues after fertilization. It is known that in mice, the first gene expression in the life occurs at mid-1-cell stage and the expression pattern dramatically alters during development following the program of gene expression. However, the profile of transcribed genes and the mechanism regulating their expression at the 1-cell stage has not been clarified, since the amount of zygotic transcripts are too small compared to the maternal mRNA which has been accumulated during oocyte growth. Here, we show the unique characteristics of gene expression in 1-cell stage embryos. RNA sequence analysis revealed that the transcripts were deficiently spliced. Based on the expression of introns, we identified 4575 genes transcribed at the 1-cell stage and found that their expression pattern was unique. We also found that intergenic regions are pervasively expressed. Analysis for the regulation of the expression of those genes and reporter gene assay revealed that precocious gene expression, i.e. core promoter elements-independent, cryptically initiated and terminated transcription, occurs in 1-cell stage embryos. The first gene expression during life, which is the initiation of transcriptional cascades, is thus loosely regulated. The significance of this unique gene expression will be discussed.