All animal embryos pass through a stage during which developmental control is handed from maternally provided gene products to those synthesized from the zygotic genome. This maternal-to-zygotic transition (MZT) has been extensively studied in model organisms, including echinoderms, nematodes, insects, fish, amphibians and mammals. In all cases, the MZT can be subdivided into two interrelated processes: first, a subset of maternal mRNAs and proteins is eliminated; second, zygotic transcription is initiated. The timing and scale of these two events differ across species, as do the cellular and morphogenetic processes that sculpt their embryos. In this article, we discuss conserved and distinct features within the two component processes of the MZT.

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The maternal-to-zygotic transition: a play in two acts

http://tglab.princeton.edu/wp-content/uploads/2013/09/Garcia_et_al_CurrentBiology_2013.pdf

Quantitative Imaging of Transcription in Living Drosophila Embryos Links Polymerase Activity to Patterning

Following fertilization, the two specified gametes must unite to create an entirely new organism. The genome is initially transcriptionally quiescent, allowing the zygote to be reprogrammed into a totipotent state. Gradually, the genome is activated through a process known as the maternal-to-zygotic transition, which enables zygotic gene products to replace the maternal supply that initiated development. This essential transition has been broadly characterized through decades of research in several model organisms. However, we still lack a full mechanistic understanding of how genome activation is executed and how this activation relates to the reprogramming of the zygotic chromatin architecture. Recent work highlights the central role of transcriptional activators and suggests that these factors may coordinate transcriptional activation with other developmental changes.

/pdf/mechanisms-regulating-zygotic-genome-activation-1fda6gpyg8.pdf

Mechanisms regulating zygotic genome activation.

TrxG and PcG Proteins but Not Methylated Histones Remain Associated with DNA through Replication

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.

The maternal-to-zygotic transition revisited.

Developmental control of late replication and S phase length.

https://www.cell.com/current-biology/pdf/S0960-9822(08)00088-2.pdf

RNAi of Mitotic Cyclins in Drosophila Uncouples the Nuclear and Centrosome Cycle

We examined the contribution of S phase in timing cell cycle progression during Drosophila embryogenesis using an approach that deletes S phase rather than arresting its progress. Injection of Drosophila Geminin, an inhibitor of replication licensing, prevented subsequent replication so that the following mitosis occurred with uninemic chromosomes, which failed to align. The effect of S phase deletion on interphase length changed with development. During the maternally regulated syncytial blastoderm cycles, deleting S phase shortened interphase, and deletion of the last of blastoderm S phase (cycle 14) induced an extra synchronous division and temporarily deferred mid-blastula transition (MBT) events. In contrast, deleting S phase after the MBT in cycle 15 did not dramatically affect mitotic timing, which appears to retain its dependence on developmentally programmed zygotic transcription. We conclude that normal S phase and replication checkpoint activities are important timers of the undisturbed cell cycle before, but not after, the MBT.

/pdf/dna-replication-times-the-cell-cycle-and-contributes-to-the-5av3y8vliz.pdf

DNA replication times the cell cycle and contributes to the mid-blastula transition in Drosophila embryos

Cyclins are key cell cycle regulators, yet few analyses test their role in timing the events that they regulate. We used RNA interference and real-time visualization in embryos to define the events regulated by each of the three mitotic cyclins of Drosophila melanogaster, CycA, CycB, and CycB3. Each individual and pairwise knockdown results in distinct mitotic phenotypes. For example, mitosis without metaphase occurs upon knockdown of CycA and CycB. To separate the role of cyclin levels from the influences of cyclin type, we knocked down two cyclins and reduced the gene dose of the one remaining cyclin. This reduction did not prolong interphase but instead interrupted mitotic progression. Mitotic prophase chromosomes formed, centrosomes divided, and nuclei exited mitosis without executing later events. This prompt but curtailed mitosis shows that accumulation of cyclin function does not directly time mitotic entry in these early embryonic cycles and that cyclin function can be sufficient for some mitotic events although inadequate for others.

/pdf/influence-of-cyclin-type-and-dose-on-mitotic-entry-and-4pmhniljzy.pdf

Mark L. McCleland

Papers

Developmental control of late replication and S phase length.

RNAi of Mitotic Cyclins in Drosophila Uncouples the Nuclear and Centrosome Cycle

DNA replication times the cell cycle and contributes to the mid-blastula transition in Drosophila embryos

Influence of cyclin type and dose on mitotic entry and progression in the early Drosophila embryo.