Transcription-dependent induction of G(1) phase during the zebra fish midblastula transition

The early development of the zebra fish (Danio rerio) embryo is characterized by a series of rapid and synchronous cell cycles with no detectable transcription. This period is followed by the midblastula transition (MET), during which the cell cycle gradually lengthens, cell synchrony is lost, and zygotic transcription is initially detected. In this work, we examined the changes in the pattern of the cell cycle during MBT in zebra fish and whether these changes are dependent on the initiation of zygotic transcription. To characterize the pattern of the early zebra fish cell cycles, the embryonic DNA content was determined by flow cytometric analysis. We found that G(1) phase is below detection levels during the first 10 cleavages and can be initially detected at the onset of MET. Inhibition of zygotic transcription, by microinjection of actinomycin D, abolished the appearance of G(1) phase at MET. Premature activation of zygotic transcription, by microinjection of nonspecific;DNA, induced G(1) phase before the onset of MET, while coinjection of actinomycin D and nonspecific DNA abolished this early appearance of G; phase. We therefore suggest that during the early development of the zebra fish embryo, G(1) phase appears at the onset of MBT and that the activation of transcription at MBT is essential and sufficient for the G(1)-phase induction.