The earliest transcribed zygotic genes are short, newly evolved, and different across species.

The transition from maternal to zygotic control is fundamental to the life cycle of all multicellular organisms. It is widely believed that genomes are transcriptionally inactive from fertilization until zygotic genome activation (ZGA). Thus, the earliest genes expressed probably support the rapid cell divisions that precede morphogenesis and, if so, might be evolutionarily conserved. Here, we identify the earliest zygotic transcripts in the zebrafish, Danio rerio, through metabolic labeling and purification of RNA from staged embryos. Surprisingly, the mitochondrial genome was highly active from the one-cell stage onwards, showing that significant transcriptional activity exists at fertilization. We show that 592 nuclear genes become active when cell cycles are still only 15 min long, confining expression to relatively short genes. Furthermore, these zygotic genes are evolutionarily younger than those expressed at other developmental stages. Comparison of fish, fly, and mouse data revealed different sets of genes expressed at ZGA. This species specificity uncovers an evolutionary plasticity in early embryogenesis that probably confers substantial adaptive potential.