Coupling of zygotic transcription to mitotic control at the Drosophila mid-blastula transition.

One of the most prominent features at the mid-blastula transition (MBT) observed in most embryos is a pause in cell cycle regulated by the nucleocytoplasmic (N/C) ratio. By using chromosome rearrangements to manipulate the DNA content of embryos, we determined that the threshold for this cell cycle pause in Drosophila is about 70% of the DNA content normally present at cycle 14. Embryos with DNA contents around this value show intermediate cell cycle behaviors. Some pause at cycle 14, some at cycle 15, and some form patches arrested in different mitotic cycles. A second feature at MBT is a massive increase in zygotic transcription and a parallel degradation of maternally supplied RNAs. To determine whether these changes in gene expression are governed by the same N/C ratio that controls cell cycle pause, we compared gene expression in haploid and diploid Drosophila embryos. We find that most maternal RNA degradation and most new transcription correlate with absolute time or developmental stage, and are timed independently of the N/C ratio. We identify a class of zygotically active genes whose expression depends on the N/C ratio and which are only expressed at cycle 15 in haploids. In embryos with patchy cell cycle behavior due to threshold DNA contents, the expression of these genes correlates tightly with the boundaries of the mitotic patches, suggesting either that the mechanism that pauses the mitotic cycle is the same as the one that measures the N/C ratio, or that it is tightly coupled to the mechanism controlling zygotic transcription of N/C ratio genes at the MBT.