Interphase-arrested Drosophila embryos activate zygotic gene expression and initiate mid-blastula transition events at a low nuclear-cytoplasmic ratio

Abstract

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.

Autonomous clocks that regulate organelle biogenesis, cytoskeletal organization, and intracellular dynamics

“…An important and recently debated ( Strong et al, 2020 ; Syed et al, 2021 ) example of such a potentially autonomous developmental timing mechanism that can run independently of the cell cycle progression is the program that triggers cellularization during mid-blastula transition (MBT) in embryogenesis ( Figure 3A ). Organismal development unfolds on a tight schedule and in specific sequences that are unlikely to be reproduced by stochasticity.…”

“…The major temporal trigger of maternal-to-zygotic transition and its associated events – both in flies and other species – has long been assumed to be the exponential increase in nuclear-to-cytoplasmic (N/C) ratio as a function of cell cycle progression ( Edgar et al, 1986 ; Newport and Kirschner, 1982 ). With elegant experiments that genetically perturb the progression of cell cycle in fly embryos, recent work suggests the contrary: neither the activation of zygotic genome nor the timing of cellularization is governed by an accurate N/C ratio or cell cycle progression, and both events appear to occur when the nuclear divisions are halted 2–3 rounds in prior ( Figure 3A ; McCleland and O’Farrell, 2008 ; Strong et al, 2020 ). These experiments also reveal a subtle but critical difference between the onset times of cellularization and the zygotic genome activation.…”

“…These experiments also reveal a subtle but critical difference between the onset times of cellularization and the zygotic genome activation. Although the slowing of cell cycle progression (in arrested embryos) could explain the premature trigger of zygotic genome activation independently of a set N/C ratio ( Strong et al, 2020 ), the timing of cellularization remains relatively similar to wild-type conditions, eliminating the possibility that a mere slowing in nuclear cycles could elicit the trigger of cellularization. These findings hint at a potential autonomous timing mechanism responsible for the onset of this hallmark morphological event.…”

The nuclear to cytoplasmic ratio directly regulates zygotic transcription in Drosophila through multiple modalities

“…Recent RNA-seq has shown that the vast majority of genes, including the genes used in this study, are capable of initiating detectable transcription if the cell cycle is stopped one or two cycles prematurely (33). This demonstrates that pre-ZGA N/C ratios are not strictly incompatible with transcription.…”

“…In Drosophila and zebrafish, it has been observed that the early interphases simply are not long enough to sustain robust transcription of most genes (20,21,(29)(30)(31)(32). One recent study found that if the cell cycle is artificially paused, the vast majority of ZGA genes can be induced one or two cell cycles prematurely in Drosophila (33). On the other hand, ZGA has been implicated as upstream of cell cycle slowing.…”