Regulation of Zygotic Transcription and Cell Cycle Checkpoints in Early Embryogenesis

For many organisms, the first goal of embryogenesis is to accumulate a large cell population to accommodate gastrulation. To achieve this quickly, embryos employ specialized cell cycles called cleavages that consist of continuous rounds of DNA replication and division. Cell proliferation occurs rapidly because cleavage cycles lack the gap phases and cell cycle checkpoints found in canonical cell cycles. Further, the genetic materials required to sustain cleavage cycles are preloaded during oogenesis, aiding efficient cell cycle progression. After a constant, organism-specific number of cleavages, many metazoan embryos undergo the mid-blastula transition (MBT), which initiates extensive cell cycle remodeling. Cell cycles lengthen, gap phases appear and checkpoint function is acquired. At the same time, the nearly quiescent zygotic genome is activated and transcriptional activity dramatically increases. This dissertation describes how these simultaneous MBT events are regulated. Chapter 2 addresses how zygotic transcription and cell cycle remodeling are coordinated. By artificially slowing cleavage cycles in zebrafish embryos, I demonstrate that increases in transcriptional activity are independent of cell cycle elongation and embryo age. I conclude that zygotic transcription is regulated by the nuclear-to-cytoplasmic (N:C) ratio, which increases after each round of replication in cleavage-stage embryos. Chapter 2 also shows the mechanisms governing DNA damage checkpoint acquisition at the MBT. DNA damage checkpoint acquisition does not require zygotic transcription. Instead, using immunostaining to examine checkpoint signaling, I show that cleavage-stage embryos cannot activate the checkpoint protein Chk1 kinase after damage induction. I conclude that the lack of Chk1 activity prior to the MBT limits DNA damage checkpoint function during cleavage cycles. Chapter 3 investigates how the spindle assembly checkpoint (SAC) is acquired at the MBT. I show that SAC acquisition is independent of the N:C ratio and other MBT events like cell cycle elongation and zygotic transcription. I conclude that SAC acquisition is age-dependent, and relies on a timer mechanism to regulate maternally-supplied SAC components. The studies reported in this dissertation demonstrate the various mechanisms embryos use to orchestrate simultaneous MBT events