The maternal-to-zygotic transition revisited

Abstract

The development of animal embryos is initially directed by maternal gene products. Then, during the maternal-to-zygotic transition (MZT), developmental control is handed to the zygotic genome. Extensive research in both vertebrate and invertebrate model organisms has revealed that the MZT can be subdivided into two phases, during which very different modes of gene regulation are implemented: initially, regulation is exclusively post-transcriptional and post-translational, following which gradual activation of the zygotic genome leads to predominance of transcriptional regulation. These changes in the gene expression program of embryos are precisely controlled and highly interconnected. Here, we review current understanding of the mechanisms that underlie handover of developmental control during the MZT.

A single-cell transcriptomics CRISPR-activation screen identifies new epigenetic regulators of zygotic genome activation

“…Zygotic genome activation (ZGA) is the first transcriptional event that takes place in an embryo (reviewed in Vastenhouw et al 2019) and is a critical step in early development. In mouse, following an initial minor wave of ZGA in the late zygote, the major wave of ZGA occurs at the mid-to-late twocell embryo stage and is characterized by the transcriptional activation of thousands of genes (reviewed in Vastenhouw et al 2019;Jukam et al 2017;Svoboda 2018.;Yartseva & Giraldez 2015).…”

“…Zygotic genome activation (ZGA) is the first transcriptional event that takes place in an embryo (reviewed in Vastenhouw et al 2019) and is a critical step in early development. In mouse, following an initial minor wave of ZGA in the late zygote, the major wave of ZGA occurs at the mid-to-late twocell embryo stage and is characterized by the transcriptional activation of thousands of genes (reviewed in Vastenhouw et al 2019;Jukam et al 2017;Svoboda 2018.;Yartseva & Giraldez 2015). Significantly, in addition to the transcriptome, the epigenetic and chromatin landscape is drastically remodelled during this transition, including reprogramming of histone post-translational modifications, global chromatin accessibility and three-dimensional structure and global DNA demethylation (reviewed in Fraser & Lin 2016;Eckersley-Maslin, Alda-Catalinas & Reik 2018;Jansz & Torres-Padilla 2019).…”

Pluripotency factors determine gene expression repertoire at zygotic genome activation

“…The zygotic genome awakens through a process known as maternal-to-zygotic transition (MZT), during which the degradation of maternal transcripts is coordinated with Zygotic Genome Activation (ZGA). The current models of ZGA take into account the gradual increase in the ratio of transcriptional activators to transcriptional repressors, accompanied with local changes of chromatin accessibility, which together create a permissive environment for ZGA to occur (Schulz and Harrison, 2018;Vastenhouw et al, 2019). In zebrafish, Xenopus and Drosophila, where development starts with rapid cell cycles, excessive maternal core histones serve as general transcriptional repressors before ZGA occurs (Almouzni and Wolffe, 1995;Amodeo et al, 2015;Joseph et al, 2017;Shindo and Amodeo, 2019;Wilky et al, 2019).…”

Combinatorial action of transcription factors in open chromatin contributes to early cellular heterogeneity and organizer mesendoderm specification

“…During this process, cells respond to extracellular signals as dictated by cell-autonomous constraints such as chromatin state and the presence of factors mediating the response. The embryo is transcriptionally quiescent until zygotic genome activation (ZGA) (Paranjpe and Veenstra, 2015;Vastenhouw et al, 2019), which gradually occurs during the mid-blastula stage in Xenopus. This is accompanied by slowing down of cell divisions and introduction of cell cycle gap phases.…”