Externally deposited eggs begin development with an immense cytoplasm and a single overwhelmed nucleus. Rapid mitotic cycles restore normality as the ratio of nuclei to cytoplasm (N/C) increases. A threshold N/C has been widely proposed to activate zygotic genome transcription and onset of morphogenesis at the mid-blastula transition (MBT). To test whether a threshold N/C is required for these events, we blocked N/C increase by down-regulating cyclin/Cdk1 to arrest early cell cycles in Drosophila. Embryos that were arrested two cell cycles prior to the normal MBT activated widespread transcription of the zygotic genome including genes previously described as N/C dependent. Zygotic transcription of these genes largely retained features of their regulation in space and time. Furthermore, zygotically regulated post-MBT events such as cellularization and gastrulation movements occurred in these cell cycle–arrested embryos. These results are not compatible with models suggesting that these MBT events are directly coupled to N/C. Cyclin/Cdk1 activity normally declines in tight association with increasing N/C and is regulated by N/C. By experimentally promoting the decrease in cyclin/Cdk1, we uncoupled MBT from N/C increase, arguing that N/C-guided down-regulation of cyclin/Cdk1 is sufficient for genome activation and MBT.

体外沉积的卵开始发育时具有巨大的细胞质和单个被压垮的细胞核。随着细胞核与细胞质的比率 (N/C) 增加，快速有丝分裂周期恢复正常。阈值 N/C 已被广泛提出来激活合子基因组转录和中囊胚转变 (MBT) 的形态发生。为了测试这些事件是否需要阈值 N/C，我们通过下调细胞周期蛋白/Cdk1 来阻止果蝇的早期细胞周期，从而阻止 N/C 增加。在正常 MBT 之前停滞两个细胞周期的胚胎激活了合子基因组的广泛转录，包括先前描述为 N/C 依赖性的基因。这些基因的合子转录在很大程度上保留了它们在空间和时间上的调节特征。此外，在这些细胞周期停滞的胚胎中发生了受合子调控的 MBT 后事件，例如细胞化和原肠胚形成运动。这些结果与表明这些 MBT 事件直接与 N/C 相关的模型不兼容。 Cyclin/Cdk1 活性的下降通常与 N/C 的增加密切相关，并受 N/C 的调节。通过实验促进 cyclin/Cdk1 的减少，我们将 MBT 与 N/C 的增加分开，认为 N/C 引导的 cyclin/Cdk1 下调足以激活基因组和 MBT。