Zygotic Genome Activation During the Maternal-to-Zygotic Transition

Abstract

Embryogenesis depends on a highly coordinated cascade of genetically encoded events. In animals, maternal factors contributed by the egg cytoplasm initially control development, while the zygotic nuclear genome is quiescent. Subsequently, the genome is activated, embryonic gene products are mobilized and maternal factors are cleared. This transfer of developmental control is called the maternal-to-zygotic transition (MZT). In this review, we discuss recent advances toward understanding the scope, timing and mechanisms that underlie zygotic genome activation at the MZT in animals. We describe high-throughput techniques to measure the embryonic transcriptome and explore how regulation of the cell cycle, chromatin and transcription factors together elicits specific patterns of embryonic gene expression. Finally, we discuss the interplay between zygotic transcription and maternal clearance, and show how these two activities combine to reprogram two terminally differentiated gametes into a totipotent embryo.

DNA replication timing during development anticipates transcriptional programs and parallels enhancer activation

“…The replication timing of genes pre-MBT was of interest because there is little transcription throughout most of the genome until after the MBT (Lee et al 2014). Despite the lack of bulk transcription in pre-MBT embryos, genes were significantly enriched in early replicating areas of the genome (Fig.…”

“…Pre-MBT embryos lack a G1-phase, which is thought to be necessary for the establishment of the replication timing program (Dimitrova and Gilbert 1999;Lu et al 2010). Prior to the MBT, zebrafish embryos rely on maternally deposited mRNA, and there is little or no transcription throughout most of the genome (Lee et al 2014). After the MBT, widespread transcription begins, and certain histone modifications associated with transcriptional activation and repression are added (Vastenhouw et al 2010).…”

N6-Methyladenosine Sequencing Highlights the Involvement of mRNA Methylation in Oocyte Meiotic Maturation and Embryo Development by Regulating Translation in Xenopus laevis

“…During oogenesis in animals like Xenopus laevis, mouse, and human, germinal vesicle (GV)-stage 3 oocytes gradually achieve maximum size, and genomic transcription activity is silenced (1,2). After that, the fully grown GV oocytes resume meiosis and develop to metaphase of the second meiosis (MII) stage.…”

“…The zygote starts mitosis, and embryo development is initiated. As embryo genomic transcription is not reactivated until embryo development to the mid-blastula stage for X. laevis (3) or the 2-cell to 16-cell stages for mammals (4 -7), oocytes or embryos need the maternal RNAs accumulated during oocyte growth to support new protein synthesis.…”

Reprogrammation fonctionnelle de gamètes mâles par des embryons haploïdes parthénotes

“…En effet, lors d'une fécondation « naturelle » ou après ICSI dans un ovocyte en méiose, la chromatine des noyaux de l'ovocyte et des gamètes mâles se remodèle simultanément [8,9]. Mais lorsque le sperme ou les spermatides sont injectés plus tard en phase mitotique des ovocytes activés, il y a un asynchronisme temporel important dans la mise en place des modifications des structures chromatiniennes gamé-tiques mâle et femelle.…”