Fine mapping of genome activation in bovine embryos by RNA sequencing

During maternal-to-embryonic transition control of embryonic development gradually switches from maternal RNAs and proteins stored in the oocyte to gene products generated after embryonic genome activation (EGA). Detailed insight into the onset of embryonic transcription is obscured by the presence of maternal transcripts. Using the bovine model system, we established by RNA sequencing a comprehensive catalogue of transcripts in germinal vesicle and metaphase II oocytes, and in embryos at the 4-cell, 8-cell, 16-cell and blastocyst stages. These were produced by in vitro fertilization of Bos t. taurus oocytes with sperm from a Bos t. indicus bull to facilitate parent-specific transcriptome analysis. Transcripts from 12.4 to 13.7 × 10^3 different genes were detected in the various developmental stages. EGA was analyzed by i) detection of embryonic transcripts which are not present in oocytes; ii) detection of transcripts from the paternal allele; and iii) detection of primary transcripts with intronic sequences. These strategies revealed (i) 220, (ii) 937, and (iii) 6,848 genes to be activated from the 4-cell to the blastocyst stage. The largest proportion of gene activation, i.e. (i) 59%, (ii) 42%, and (iii) 58%, was found in 8-cell embryos, indicating major EGA at this stage. Gene ontology analysis of genes activated at the 4-cell stage identified categories related to translation, RNA processing and transport, consistent with preparation for major EGA. Our study provides the largest transcriptome data set of bovine oocyte maturation and early embryonic development and new insight into the timing of embryonic activation of specific genes.