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Bernadskaya YY, Kuan A, Tjärnberg A, Brandenburg J, Zheng P, Wiechecki K, Kaplan N, Failla M, Bikou M, Madilian O, Wang W, Christiaen L. Cell cycle-driven transcriptome maturation confers multilineage competence to cardiopharyngeal progenitors. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.07.23.604718. [PMID: 39091743 PMCID: PMC11291048 DOI: 10.1101/2024.07.23.604718] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Subscribe] [Scholar Register] [Indexed: 08/04/2024]
Abstract
During development, stem and progenitor cells divide and transition through germ layer- and lineage-specific multipotent states to generate the diverse cell types that compose an animal. Defined changes in biomolecular composition underlie the progressive loss of potency and acquisition of lineage-specific characteristics. For example, multipotent cardiopharyngeal progenitors display multilineage transcriptional priming, whereby both the cardiac and pharyngeal muscle programs are partially active and coexist in the same progenitor cells, while their daughter cells engage in a cardiac or pharyngeal muscle differentiation path only after cell division. Here, using the tunicate Ciona, we studied the acquisition of multilineage competence and the coupling between fate decisions and cell cycle progression. We showed that multipotent cardiopharyngeal progenitors acquire the competence to produce distinct Tbx1/10 (+) and (-) daughter cells shortly before mitosis, which is necessary for Tbx1/10 activation. By combining transgene-based sample barcoding with single cell RNA-seq (scRNA-seq), we uncovered transcriptome-wide dynamics in migrating cardiopharyngeal progenitors as cells progress through G1, S and G2 phases. We termed this process "transcriptome maturation", and identified candidate "mature genes", including the Rho GAP-coding gene Depdc1 , which peak in late G2. Functional assays indicated that transcriptome maturation fosters cardiopharyngeal competence, in part through multilineage priming and proper oriented and asymmetric division that influences subsequent fate decisions, illustrating the concept of "behavioral competence". Both classic feedforward circuits and coupling with cell cycle progression drive transcriptome maturation, uncovering distinct levels of coupling between cell cycle progression and fateful molecular transitions. We propose that coupling competence and fate decision with the G2 and G1 phases, respectively, ensures the timely deployment of lineage-specific programs.
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2
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Schindler-Johnson M, Petridou NI. Collective effects of cell cleavage dynamics. Front Cell Dev Biol 2024; 12:1358971. [PMID: 38559810 PMCID: PMC10978805 DOI: 10.3389/fcell.2024.1358971] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2023] [Accepted: 03/05/2024] [Indexed: 04/04/2024] Open
Abstract
A conserved process of early embryonic development in metazoans is the reductive cell divisions following oocyte fertilization, termed cell cleavages. Cell cleavage cycles usually start synchronously, lengthen differentially between the embryonic cells becoming asynchronous, and cease before major morphogenetic events, such as germ layer formation and gastrulation. Despite exhibiting species-specific characteristics, the regulation of cell cleavage dynamics comes down to common controllers acting mostly at the single cell/nucleus level, such as nucleus-to-cytoplasmic ratio and zygotic genome activation. Remarkably, recent work has linked cell cleavage dynamics to the emergence of collective behavior during embryogenesis, including pattern formation and changes in embryo-scale mechanics, raising the question how single-cell controllers coordinate embryo-scale processes. In this review, we summarize studies across species where an association between cell cleavages and collective behavior was made, discuss the underlying mechanisms, and propose that cell-to-cell variability in cell cleavage dynamics can serve as a mechanism of long-range coordination in developing embryos.
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Affiliation(s)
- Magdalena Schindler-Johnson
- Developmental Biology Unit, European Molecular Biology Laboratory, Heidelberg, Germany
- Faculty of Biosciences, Heidelberg University, Heidelberg, Germany
| | - Nicoletta I. Petridou
- Developmental Biology Unit, European Molecular Biology Laboratory, Heidelberg, Germany
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3
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Ichbiah S, Delbary F, McDougall A, Dumollard R, Turlier H. Embryo mechanics cartography: inference of 3D force atlases from fluorescence microscopy. Nat Methods 2023; 20:1989-1999. [PMID: 38057527 PMCID: PMC10703677 DOI: 10.1038/s41592-023-02084-7] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2023] [Accepted: 10/12/2023] [Indexed: 12/08/2023]
Abstract
Tissue morphogenesis results from a tight interplay between gene expression, biochemical signaling and mechanics. Although sequencing methods allow the generation of cell-resolved spatiotemporal maps of gene expression, creating similar maps of cell mechanics in three-dimensional (3D) developing tissues has remained a real challenge. Exploiting the foam-like arrangement of cells, we propose a robust end-to-end computational method called 'foambryo' to infer spatiotemporal atlases of cellular forces from fluorescence microscopy images of cell membranes. Our method generates precise 3D meshes of cells' geometry and successively predicts relative cell surface tensions and pressures. We validate it with 3D foam simulations, study its noise sensitivity and prove its biological relevance in mouse, ascidian and worm embryos. 3D force inference allows us to recover mechanical features identified previously, but also predicts new ones, unveiling potential new insights on the spatiotemporal regulation of cell mechanics in developing embryos. Our code is freely available and paves the way for unraveling the unknown mechanochemical feedbacks that control embryo and tissue morphogenesis.
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Affiliation(s)
- Sacha Ichbiah
- Center for Interdisciplinary Research in Biology, College of France, CNRS, INSERM, University of PSL, Paris, France
| | - Fabrice Delbary
- Center for Interdisciplinary Research in Biology, College of France, CNRS, INSERM, University of PSL, Paris, France
| | - Alex McDougall
- Laboratory of Developmental Biology of the Villefranche-sur-Mer, Institute of Villefranche-sur-Mer, Sorbonne University, CNRS, Villefranche-sur-Mer, France
| | - Rémi Dumollard
- Laboratory of Developmental Biology of the Villefranche-sur-Mer, Institute of Villefranche-sur-Mer, Sorbonne University, CNRS, Villefranche-sur-Mer, France
| | - Hervé Turlier
- Center for Interdisciplinary Research in Biology, College of France, CNRS, INSERM, University of PSL, Paris, France.
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4
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Treen N, Konishi S, Nishida H, Onuma TA, Sasakura Y. Zic-r.b controls cell numbers in Ciona embryos by activating CDKN1B. Dev Biol 2023; 498:26-34. [PMID: 36965841 DOI: 10.1016/j.ydbio.2023.03.005] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2022] [Revised: 01/25/2023] [Accepted: 03/20/2023] [Indexed: 03/27/2023]
Abstract
The control of cell numbers and the establishment of cell types are two processes that are essential in early embryonic development. We have a reasonable understanding of how these processes occur individually, but we have considerably less sophisticated understanding of how these processes are linked. Tunicates have fixed cell lineages with predictable cell cycles, making them well suited to investigate these processes. In the ascidian Ciona, we show that the transcription factor Zic-r.b, known to be involved in establishing several cell types in early development also activates the expression of the cell cycle inhibitor CDKN1B. Zic-r.b is a major missing component of the cell division clock establishing specific cell numbers. We also show that a larvacean homolog of Zic-r.b is expressed one cell cycle earlier than its Ciona counterpart. The early expression in larvaceans may explain why they have half as many notochord cells as ascidians and may illustrate a general mechanism to evolve changes in morphology.
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Affiliation(s)
- Nicholas Treen
- Shimoda Marine Research Center, University of Tsukuba, 5-10-1, Shimoda, Shizuoka, 415-0025, Japan.
| | - Shohei Konishi
- Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka, 560-0043, Japan
| | - Hiroki Nishida
- Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka, 560-0043, Japan
| | - Takeshi A Onuma
- Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka, 560-0043, Japan
| | - Yasunori Sasakura
- Shimoda Marine Research Center, University of Tsukuba, 5-10-1, Shimoda, Shizuoka, 415-0025, Japan
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5
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Kobayashi K, Tokuoka M, Sato H, Ariyoshi M, Kawahara S, Fujiwara S, Kishimoto T, Satou Y. Regulators specifying cell fate activate cell cycle regulator genes to determine cell numbers in ascidian larval tissues. Development 2022; 149:282402. [PMID: 36278804 DOI: 10.1242/dev.201218] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2022] [Accepted: 10/13/2022] [Indexed: 11/17/2022]
Abstract
In animal development, most cell types stop dividing before terminal differentiation; thus, cell cycle control is tightly linked to cell differentiation programmes. In ascidian embryos, cell lineages do not vary among individuals, and rounds of the cell cycle are determined according to cell lineages. Notochord and muscle cells stop dividing after eight or nine rounds of cell division depending on their lineages. In the present study, we showed that a Cdk inhibitor, Cdkn1.b, is responsible for stopping cell cycle progression in these lineages. Cdkn1.b is also necessary for epidermal cells to stop dividing. In contrast, mesenchymal and endodermal cells continue to divide even after hatching, and Myc is responsible for maintaining cell cycle progression in these tissues. Expression of Cdkn1.b in notochord and muscle is controlled by transcription factors that specify the developmental fate of notochord and muscle. Likewise, expression of Myc in mesenchyme and endoderm is under control of transcription factors that specify the developmental fate of mesenchyme and endoderm. Thus, cell fate specification and cell cycle control are linked by these transcription factors.
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Affiliation(s)
- Kenji Kobayashi
- Laboratory of Cell and Developmental Biology, Graduate School of Bioscience, Tokyo Institute of Technology, Yokohama 226-8501, Japan
| | - Miki Tokuoka
- Laboratory of Cell and Developmental Biology, Graduate School of Bioscience, Tokyo Institute of Technology, Yokohama 226-8501, Japan.,Department of Zoology, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
| | - Hiroaki Sato
- Laboratory of Cell and Developmental Biology, Graduate School of Bioscience, Tokyo Institute of Technology, Yokohama 226-8501, Japan
| | - Manami Ariyoshi
- Department of Chemistry and Biotechnology, Faculty of Science and Technology, Kochi University, Kochi 780-8520, Japan
| | - Shiori Kawahara
- Department of Chemistry and Biotechnology, Faculty of Science and Technology, Kochi University, Kochi 780-8520, Japan
| | - Shigeki Fujiwara
- Department of Chemistry and Biotechnology, Faculty of Science and Technology, Kochi University, Kochi 780-8520, Japan
| | - Takeo Kishimoto
- Laboratory of Cell and Developmental Biology, Graduate School of Bioscience, Tokyo Institute of Technology, Yokohama 226-8501, Japan
| | - Yutaka Satou
- Department of Zoology, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
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6
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Costache V, Prigent Garcia S, Plancke CN, Li J, Begnaud S, Suman SK, Reymann AC, Kim T, Robin FB. Rapid assembly of a polar network architecture drives efficient actomyosin contractility. Cell Rep 2022; 39:110868. [PMID: 35649363 PMCID: PMC9210446 DOI: 10.1016/j.celrep.2022.110868] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2021] [Revised: 03/13/2022] [Accepted: 05/05/2022] [Indexed: 11/30/2022] Open
Abstract
Actin network architecture and dynamics play a central role in cell contractility and tissue morphogenesis. RhoA-driven pulsed contractions are a generic mode of actomyosin contractility, but the mechanisms underlying how their specific architecture emerges and how this architecture supports the contractile function of the network remain unclear. Here we show that, during pulsed contractions, the actin network is assembled by two subpopulations of formins: a functionally inactive population (recruited) and formins actively participating in actin filament elongation (elongating). We then show that elongating formins assemble a polar actin network, with barbed ends pointing out of the pulse. Numerical simulations demonstrate that this geometry favors rapid network contraction. Our results show that formins convert a local RhoA activity gradient into a polar network architecture, causing efficient network contractility, underlying the key function of kinetic controls in the assembly and mechanics of cortical network architectures. RhoA-driven actomyosin contractility plays a key role in driving cell and tissue contractility during morphogenesis. Tracking individual formins, Costache et al. show that the network assembled downstream of RhoA displays a polar architecture, barbed ends pointing outward, a feature that supports efficient contractility and force transmission during pulsed contractions.
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Affiliation(s)
- Vlad Costache
- Sorbonne Université, CNRS, INSERM, Institut de Biologie Paris-Seine IBPS, Laboratoire de Biologie du Développement, Paris, France
| | - Serena Prigent Garcia
- Sorbonne Université, CNRS, INSERM, Institut de Biologie Paris-Seine IBPS, Laboratoire de Biologie du Développement, Paris, France
| | - Camille N Plancke
- Sorbonne Université, CNRS, INSERM, Institut de Biologie Paris-Seine IBPS, Laboratoire de Biologie du Développement, Paris, France
| | - Jing Li
- Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN, USA
| | - Simon Begnaud
- Sorbonne Université, CNRS, INSERM, Institut de Biologie Paris-Seine IBPS, Laboratoire de Biologie du Développement, Paris, France
| | - Shashi Kumar Suman
- Sorbonne Université, CNRS, INSERM, Institut de Biologie Paris-Seine IBPS, Laboratoire de Biologie du Développement, Paris, France
| | - Anne-Cécile Reymann
- IGBMC, CNRS UMR7104, INSERM U1258, and Université de Strasbourg, Illkirch, France
| | - Taeyoon Kim
- Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN, USA.
| | - François B Robin
- Sorbonne Université, CNRS, INSERM, Institut de Biologie Paris-Seine IBPS, Laboratoire de Biologie du Développement, Paris, France.
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7
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Blastocoel morphogenesis: A biophysics perspective. Semin Cell Dev Biol 2021; 130:12-23. [PMID: 34756494 DOI: 10.1016/j.semcdb.2021.10.005] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2021] [Revised: 09/21/2021] [Accepted: 10/08/2021] [Indexed: 11/22/2022]
Abstract
The blastocoel is a fluid-filled cavity characteristic of animal embryos at the blastula stage. Its emergence is commonly described as the result of cleavage patterning, but this historical view conceals a large diversity of mechanisms and overlooks many unsolved questions from a biophysics perspective. In this review, we describe generic mechanisms for blastocoel morphogenesis, rooted in biological literature and simple physical principles. We propose novel directions of study and emphasize the importance to study blastocoel morphogenesis as an evolutionary and physical continuum.
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8
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Godard BG, Dumollard R, Munro E, Chenevert J, Hebras C, McDougall A, Heisenberg CP. Apical Relaxation during Mitotic Rounding Promotes Tension-Oriented Cell Division. Dev Cell 2020; 55:695-706.e4. [PMID: 33207225 DOI: 10.1016/j.devcel.2020.10.016] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/21/2020] [Revised: 09/09/2020] [Accepted: 10/23/2020] [Indexed: 12/16/2022]
Abstract
Global tissue tension anisotropy has been shown to trigger stereotypical cell division orientation by elongating mitotic cells along the main tension axis. Yet, how tissue tension elongates mitotic cells despite those cells undergoing mitotic rounding (MR) by globally upregulating cortical actomyosin tension remains unclear. We addressed this question by taking advantage of ascidian embryos, consisting of a small number of interphasic and mitotic blastomeres and displaying an invariant division pattern. We found that blastomeres undergo MR by locally relaxing cortical tension at their apex, thereby allowing extrinsic pulling forces from neighboring interphasic blastomeres to polarize their shape and thus division orientation. Consistently, interfering with extrinsic forces by reducing the contractility of interphasic blastomeres or disrupting the establishment of asynchronous mitotic domains leads to aberrant mitotic cell division orientations. Thus, apical relaxation during MR constitutes a key mechanism by which tissue tension anisotropy controls stereotypical cell division orientation.
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Affiliation(s)
- Benoit G Godard
- Laboratoire de Biologie du Développement de Villefranche-sur-mer, Institut de la Mer de Villefranche-sur-mer, Sorbonne Université, CNRS, 06230 Villefranche-sur-mer, France; Institute of Science and Technology Austria, Klosterneuburg, Austria
| | - Rémi Dumollard
- Laboratoire de Biologie du Développement de Villefranche-sur-mer, Institut de la Mer de Villefranche-sur-mer, Sorbonne Université, CNRS, 06230 Villefranche-sur-mer, France
| | - Edwin Munro
- Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, IL 60637, USA
| | - Janet Chenevert
- Laboratoire de Biologie du Développement de Villefranche-sur-mer, Institut de la Mer de Villefranche-sur-mer, Sorbonne Université, CNRS, 06230 Villefranche-sur-mer, France
| | - Céline Hebras
- Laboratoire de Biologie du Développement de Villefranche-sur-mer, Institut de la Mer de Villefranche-sur-mer, Sorbonne Université, CNRS, 06230 Villefranche-sur-mer, France
| | - Alex McDougall
- Laboratoire de Biologie du Développement de Villefranche-sur-mer, Institut de la Mer de Villefranche-sur-mer, Sorbonne Université, CNRS, 06230 Villefranche-sur-mer, France
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9
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Fiuza UM, Negishi T, Rouan A, Yasuo H, Lemaire P. A Nodal/Eph signalling relay drives the transition from apical constriction to apico-basal shortening in ascidian endoderm invagination. Development 2020; 147:dev.186965. [DOI: 10.1242/dev.186965] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2019] [Accepted: 07/02/2020] [Indexed: 01/13/2023]
Abstract
Gastrulation is the first major morphogenetic event during animal embryogenesis. Ascidian gastrulation starts with the invagination of 10 endodermal precursor cells between the 64- and late 112-cell stages. This process occurs in the absence of endodermal cell division and in two steps, driven by myosin-dependent contractions of the acto-myosin network. First, endoderm precursors constrict their apex. Second, they shorten apico-basally, while retaining small apical surfaces, thereby causing invagination. The mechanisms that prevent endoderm cell division, trigger the transition between step 1 and step 2, and drive apico-basal shortening have remained elusive. Here, we demonstrate a conserved role for Nodal and Eph signalling during invagination in two distantly related ascidian species, Phallusia mammillata and Ciona intestinalis. Specifically, we show that the transition to step 2 is triggered by Nodal relayed by Eph signalling. Additionally, our results indicate that Eph signalling lengthens the endodermal cell cycle, independently of Nodal. Finally, we find that both Nodal and Eph signals are dispensable for endoderm fate specification. These results illustrate commonalities as well as differences in the action of Nodal during ascidian and vertebrate gastrulation.
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Affiliation(s)
- Ulla-Maj Fiuza
- CRBM, University of Montpellier, CNRS, Montpellier, France
| | - Takefumi Negishi
- Laboratoire de Biologie du Développement de Villefranche-sur-Mer, CNRS, Sorbonne Universités, 06230 Villefranche-sur-Mer, France
| | - Alice Rouan
- Laboratoire de Biologie du Développement de Villefranche-sur-Mer, CNRS, Sorbonne Universités, 06230 Villefranche-sur-Mer, France
| | - Hitoyoshi Yasuo
- Laboratoire de Biologie du Développement de Villefranche-sur-Mer, CNRS, Sorbonne Universités, 06230 Villefranche-sur-Mer, France
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10
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Satou Y. A gene regulatory network for cell fate specification in Ciona embryos. Curr Top Dev Biol 2020; 139:1-33. [DOI: 10.1016/bs.ctdb.2020.01.001] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
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11
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Abstract
Tunicates are a diverse group of invertebrate marine chordates that includes the larvaceans, thaliaceans, and ascidians. Because of their unique evolutionary position as the sister group of the vertebrates, tunicates are invaluable as a comparative model and hold the promise of revealing both conserved and derived features of chordate gastrulation. Descriptive studies in a broad range of tunicates have revealed several important unifying traits that make them unique among the chordates, including invariant cell lineages through gastrula stages and an overall morphological simplicity. Gastrulation has only been studied in detail in ascidians such as Ciona and Phallusia, where it involves a simple cup-shaped gastrula driven primarily by endoderm invagination. This appears to differ significantly from vertebrate models, such as Xenopus, in which mesoderm convergent extension and epidermal epiboly are major contributors to involution. These differences may reflect the cellular simplicity of the ascidian embryo.
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Affiliation(s)
- Konner M Winkley
- Division of Biology, Kansas State University, Manhattan, KS, United States
| | - Matthew J Kourakis
- Department of Molecular, Cellular and Developmental Biology, University of California, Santa Barbara, CA, United States
| | - Anthony W DeTomaso
- Department of Molecular, Cellular and Developmental Biology, University of California, Santa Barbara, CA, United States
| | - Michael T Veeman
- Division of Biology, Kansas State University, Manhattan, KS, United States
| | - William C Smith
- Department of Molecular, Cellular and Developmental Biology, University of California, Santa Barbara, CA, United States.
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12
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Abstract
Cells are arranged into species-specific patterns during early embryogenesis. Such cell division patterns are important since they often reflect the distribution of localized cortical factors from eggs/fertilized eggs to specific cells as well as the emergence of organismal form. However, it has proven difficult to reveal the mechanisms that underlie the emergence of cell positioning patterns that underlie embryonic shape, likely because a systems-level approach is required that integrates cell biological, genetic, developmental, and mechanical parameters. The choice of organism to address such questions is also important. Because ascidians display the most extreme form of invariant cleavage pattern among the metazoans, we have been analyzing the cell biological mechanisms that underpin three aspects of cell division (unequal cell division (UCD), oriented cell division (OCD), and asynchronous cell cycles) which affect the overall shape of the blastula-stage ascidian embryo composed of 64 cells. In ascidians, UCD creates two small cells at the 16-cell stage that in turn undergo two further successive rounds of UCD. Starting at the 16-cell stage, the cell cycle becomes asynchronous, whereby the vegetal half divides before the animal half, thus creating 24-, 32-, 44-, and then 64-cell stages. Perturbing either UCD or the alternate cell division rhythm perturbs cell position. We propose that dynamic cell shape changes propagate throughout the embryo via cell-cell contacts to create the ascidian-specific invariant cleavage pattern.
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13
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Razy-Krajka F, Stolfi A. Regulation and evolution of muscle development in tunicates. EvoDevo 2019; 10:13. [PMID: 31249657 PMCID: PMC6589888 DOI: 10.1186/s13227-019-0125-6] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2018] [Accepted: 06/08/2019] [Indexed: 12/16/2022] Open
Abstract
For more than a century, studies on tunicate muscle formation have revealed many principles of cell fate specification, gene regulation, morphogenesis, and evolution. Here, we review the key studies that have probed the development of all the various muscle cell types in a wide variety of tunicate species. We seize this occasion to explore the implications and questions raised by these findings in the broader context of muscle evolution in chordates.
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Affiliation(s)
- Florian Razy-Krajka
- School of Biological Sciences, Georgia Institute of Technology, Atlanta, USA
| | - Alberto Stolfi
- School of Biological Sciences, Georgia Institute of Technology, Atlanta, USA
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14
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Initiation of the zygotic genetic program in the ascidian embryo. Semin Cell Dev Biol 2018; 84:111-117. [DOI: 10.1016/j.semcdb.2018.02.012] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2017] [Revised: 02/08/2018] [Accepted: 02/08/2018] [Indexed: 12/26/2022]
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15
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Harder M, Reeves W, Byers C, Santiago M, Veeman M. Multiple inputs into a posterior-specific regulatory network in the Ciona notochord. Dev Biol 2018; 448:136-146. [PMID: 30287118 DOI: 10.1016/j.ydbio.2018.09.021] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2018] [Revised: 09/04/2018] [Accepted: 09/28/2018] [Indexed: 11/28/2022]
Abstract
The gene regulatory networks underlying Ciona notochord fate specification and differentiation have been extensively investigated, but the regulatory basis for regionalized expression within the notochord is not understood. Here we identify three notochord-expressed genes, C11.331, C12.115 and C8.891, with strongly enriched expression in the secondary notochord cells at the posterior tip of the tail. C11.331 and C12.115 share a distinctive expression pattern that is highly enriched in the secondary notochord lineage but also graded within that lineage with the strongest expression at the posterior tip. Both genes show similar responses to pharmacological perturbations of Wnt and FGF signaling, consistent with an important role for Wnt and FGF ligands expressed at the tail tip. Reporter analysis indicates that the C11.331 cis-regulatory regions are extensively distributed, with multiple non-overlapping regions conferring posterior notochord-enriched expression. Fine-scale analysis of a minimal cis-regulatory module identifies discrete positive and negative elements including a strong silencer. Truncation of the silencer region leads to increased expression in the primary notochord, indicating that C11.331 expression is influenced by putative regulators of primary versus secondary notochord fate. The minimal CRM contains predicted ETS, GATA, LMX and Myb sites, all of which lead to reduced expression in secondary notochord when mutated. These results show that the posterior-enriched notochord expression of C11.331 depends on multiple inputs, including Wnt and FGF signals from the tip of the tail, multiple notochord-specific regulators, and yet-to-be identified regulators of regional identity within the notochord.
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Affiliation(s)
- Matthew Harder
- Division of Biology, Kansas State University, Manhattan, KS 66506, USA
| | - Wendy Reeves
- Division of Biology, Kansas State University, Manhattan, KS 66506, USA
| | - Chase Byers
- Division of Biology, Kansas State University, Manhattan, KS 66506, USA
| | - Mercedes Santiago
- Division of Biology, Kansas State University, Manhattan, KS 66506, USA
| | - Michael Veeman
- Division of Biology, Kansas State University, Manhattan, KS 66506, USA.
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16
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Hashimoto H, Munro E. Dynamic interplay of cell fate, polarity and force generation in ascidian embryos. Curr Opin Genet Dev 2018; 51:67-77. [PMID: 30007244 DOI: 10.1016/j.gde.2018.06.013] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2018] [Revised: 06/11/2018] [Accepted: 06/22/2018] [Indexed: 10/28/2022]
Abstract
A fundamental challenge in developmental biology is to understand how forces produced by individual cells are patterned in space and time and then integrated to produce stereotyped changes in tissue-level or embryo-level morphology. Ascidians offer a unique opportunity to address this challenge by studying how small groups of cells collectively execute complex, but highly stereotyped morphogenetic movements. Here we highlight recent progress and open questions in the study of ascidian morphogenesis, emphasizing the dynamic interplay of cell fate determination, cellular force generation and tissue-level mechanics.
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Affiliation(s)
- Hidehiko Hashimoto
- Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, IL 60637, United States.
| | - Edwin Munro
- Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, IL 60637, United States; Committee on Development, Regeneration and Stem Cell Biology, University of Chicago, Chicago, IL 60637, United States.
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17
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Silva WTAF. Methylation dynamics during the maternal-to-zygotic genome transition in dioecious species. PLoS One 2018; 13:e0200028. [PMID: 29990374 PMCID: PMC6039002 DOI: 10.1371/journal.pone.0200028] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2018] [Accepted: 06/17/2018] [Indexed: 11/18/2022] Open
Abstract
The starting point of a new generation in sexually reproducing species is fertilization. In many species, fertilization is followed by cell divisions controlled primarily by maternal transcripts, with little to no zygotic transcription. The activation of the zygotic genome (ZGA) is part of a process called maternal-to-zygotic transition (MZT), during which transcripts from the zygotic genome take control of development, setting the conditions for cellular specialization. While we know that epigenetic processes (e.g. methylation) are involved in the MZT, their roles and interplay in the transition are largely unknown. I developed a model and used simulations to elucidate the interaction between possible epigenetic processes, namely methylation processes, involved in the MZT. The model focuses on the dynamics of global methylation levels and how these interact with factors such as a parental repressor and the nucleocytoplasmic ratio to trigger the ZGA, followed by development from fertilization to adulthood. In addition, I included transgenerational effects transmitted to the zygote from both parents through their gametes to show that these may set the stage for plastic developmental processes. I demonstrate that the rates of maintenance methylation and demethylation, which are important for the achievement of the final methylation levels of an individual, exhibit a certain level of flexibility in terms of parameter values. I find that high final methylation levels require more restricted combinations of parameter values. The model is discussed in the context of the current empirical knowledge and provide suggestions for directions of future empirical and theoretical studies.
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Affiliation(s)
- Willian T. A. F. Silva
- Department of Evolutionary Biology, Uppsala University, Norbyvägen 18D, 753 10 Uppsala, Sweden
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18
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Oda-Ishii I, Abe T, Satou Y. Dynamics of two key maternal factors that initiate zygotic regulatory programs in ascidian embryos. Dev Biol 2018; 437:50-59. [PMID: 29550363 DOI: 10.1016/j.ydbio.2018.03.009] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2018] [Revised: 03/12/2018] [Accepted: 03/12/2018] [Indexed: 12/21/2022]
Abstract
In animal embryos, transcription is repressed for a definite period of time after fertilization. In the embryo of the ascidian, Ciona intestinalis (type A; or Ciona robusta), transcription of regulatory genes is repressed before the 8- or 16-cell stages. This initial transcriptional quiescence is important to enable the establishment of initial differential gene expression patterns along the animal-vegetal axis by maternal factors, because the third cell division separates the animal and vegetal hemispheres into distinct blastomeres. Indeed, maternal transcription factors directly activate zygotic gene expression by the 16-cell stage; Tcf7/β-catenin activates genes in the vegetal hemisphere, and Gata.a activates genes in the animal hemisphere. In the present study, we revealed the dynamics of Gata.a and β-catenin, and expression profiles of their target genes precisely. β-catenin began to translocate into the nuclei at the 16-cell stage, and thus expression of β-catenin targets began at the 16-cell stage. Although Gata.a is abundantly present before the 8-cell stage, transcription of Gata.a targets was repressed at and before the 4-cell stage, and their expression began at the 8-cell stage. Transcription of the β-catenin targets may be repressed by the same mechanism in early embryos, because β-catenin targets were not expressed in 4-cell embryos treated with a GSK inhibitor, in which β-catenin translocated to the nuclei. Thus, these two maternal factors have different dynamics, which establish the pre-pattern for zygotic genetic programs in 16-cell embryos.
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Affiliation(s)
- Izumi Oda-Ishii
- Department of Zoology, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
| | - Tetsuya Abe
- Department of Zoology, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
| | - Yutaka Satou
- Department of Zoology, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
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19
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Transgenic Techniques for Investigating Cell Biology During Development. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2018. [PMID: 29542088 DOI: 10.1007/978-981-10-7545-2_14] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Ascidians are increasingly being used as a system for investigating cell biology during development. The extreme genetic and cellular simplicity of ascidian embryos in combination with superior experimental tractability make this an ideal system for in vivo analysis of dynamic cellular processes. Transgenic approaches to cellular and sub-cellular analysis of ascidian development have begun to yield new insights into the mechanisms regulating developmental signaling and morphogenesis. This chapter focuses on the targeted expression of fusion proteins in ascidian embryos and how this technique is being deployed to garner new insights into the cell biology of development.
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20
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Ogura Y, Sasakura Y. Emerging mechanisms regulating mitotic synchrony during animal embryogenesis. Dev Growth Differ 2017; 59:565-579. [DOI: 10.1111/dgd.12391] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2017] [Revised: 07/20/2017] [Accepted: 07/23/2017] [Indexed: 01/21/2023]
Affiliation(s)
- Yosuke Ogura
- Laboratory for Morphogenetic Signaling; RIKEN Center for Developmental Biology; Kobe Japan
| | - Yasunori Sasakura
- Shimoda Marine Research Center; University of Tsukuba; Shizuoka Japan
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21
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Dumollard R, Minc N, Salez G, Aicha SB, Bekkouche F, Hebras C, Besnardeau L, McDougall A. The invariant cleavage pattern displayed by ascidian embryos depends on spindle positioning along the cell's longest axis in the apical plane and relies on asynchronous cell divisions. eLife 2017; 6. [PMID: 28121291 PMCID: PMC5319837 DOI: 10.7554/elife.19290] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2016] [Accepted: 01/24/2017] [Indexed: 12/24/2022] Open
Abstract
The ascidian embryo is an ideal system to investigate how cell position is determined during embryogenesis. Using 3D timelapse imaging and computational methods we analyzed the planar cell divisions in ascidian early embryos and found that spindles in every cell tend to align at metaphase in the long length of the apical surface except in cells undergoing unequal cleavage. Furthermore, the invariant and conserved cleavage pattern of ascidian embryos was found to consist in alternate planar cell divisions between ectoderm and endomesoderm. In order to test the importance of alternate cell divisions we manipulated zygotic transcription induced by β-catenin or downregulated wee1 activity, both of which abolish this cell cycle asynchrony. Crucially, abolishing cell cycle asynchrony consistently disrupted the spindle orienting mechanism underpinning the invariant cleavage pattern. Our results demonstrate how an evolutionary conserved cell cycle asynchrony maintains the invariant cleavage pattern driving morphogenesis of the ascidian blastula. DOI:http://dx.doi.org/10.7554/eLife.19290.001 The position of cells within an embryo early in development determines what type of cells they will become in the fully formed embryo. The embryos of ascidians, commonly known as sea squirts, are ideal for studying what influences cell positioning. These embryos consist of a small number of cells that divide according to an “invariant cleavage pattern”, which means that the positioning and timing of the cell divisions is identical between different individuals of the same species. The pattern of cell division is also largely the same across different ascidian species, which raises questions about how it is controlled. When a cell divides, a structure called the spindle forms inside it to distribute copies of the cell’s genetic material between the new cells. The orientation of the spindle determines the direction in which the cell will divide. Now, by combining 3D imaging of living ascidian embryos with computational modeling, Dumollard et al. show that the spindles in every equally dividing cell tend to all align in the long length of the cell’s “apical” surface. Such alignment allows the cells to be on the outside of the embryo and implements the ascidian invariant cleavage pattern. The cells in the embryo do not all divide at the same time. Indeed, the shape of the cells (and especially their apical surface) depends on two layers of cells in the embryo not dividing at the same time; instead, periods of cell division alternate between the layers. A network of genes in the embryo regulates the timing of these cell divisions and makes it possible for the cells to divide according to an invariant cleavage pattern. However, this network of genes is not the only control mechanism that shapes the early embryo. A structure found in egg cells (and hence produced by the embryo’s mother) causes cells at the rear of the embryo to divide unequally, and this influences the shape of all the cells in the embryo. Thus it appears that maternal mechanisms work alongside the embryo’s gene network to shape the early embryo. The next step will be to determine how physical forces – for example, from the cells pressing against each other – influence the position of the embryo’s cells. How do gene networks relay the biomechanical properties of the embryo to help it take shape? DOI:http://dx.doi.org/10.7554/eLife.19290.002
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Affiliation(s)
- Rémi Dumollard
- Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV) UMR7009, Sorbonne Universités, Université Pierre-et-Marie-Curie, CNRS, Villefranche sur mer, France
| | - Nicolas Minc
- Institut Jacques Monod, UMR7592 CNRS, Paris, France
| | - Gregory Salez
- Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV) UMR7009, Sorbonne Universités, Université Pierre-et-Marie-Curie, CNRS, Villefranche sur mer, France
| | - Sameh Ben Aicha
- Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV) UMR7009, Sorbonne Universités, Université Pierre-et-Marie-Curie, CNRS, Villefranche sur mer, France
| | - Faisal Bekkouche
- Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV) UMR7009, Sorbonne Universités, Université Pierre-et-Marie-Curie, CNRS, Villefranche sur mer, France
| | - Céline Hebras
- Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV) UMR7009, Sorbonne Universités, Université Pierre-et-Marie-Curie, CNRS, Villefranche sur mer, France
| | - Lydia Besnardeau
- Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV) UMR7009, Sorbonne Universités, Université Pierre-et-Marie-Curie, CNRS, Villefranche sur mer, France
| | - Alex McDougall
- Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV) UMR7009, Sorbonne Universités, Université Pierre-et-Marie-Curie, CNRS, Villefranche sur mer, France
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22
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Covert Prepatterning of a Cell Division Wave. Dev Cell 2016; 37:107-8. [PMID: 27093077 DOI: 10.1016/j.devcel.2016.04.005] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
Abstract
A directional wave of mitosis, progressing posterior to anterior across the epidermis, is important for neural tube closure in the invertebrate chordate Ciona intestinalis. In this issue of Developmental Cell, Ogura and Sasakura (2016) show that the patterning of this wave unexpectedly has complex origins in the previous cell cycle.
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23
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Affiliation(s)
- Yosuke Ogura
- a Shimoda Marine Research Center, University of Tsukuba , Shimoda , Shizuoka , Japan
| | - Yasunori Sasakura
- a Shimoda Marine Research Center, University of Tsukuba , Shimoda , Shizuoka , Japan
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24
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Ricci L, Cabrera F, Lotito S, Tiozzo S. Redeployment of germ layers related TFs shows regionalized expression during two non-embryonic developments. Dev Biol 2016; 416:235-248. [PMID: 27208394 DOI: 10.1016/j.ydbio.2016.05.016] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2015] [Revised: 05/11/2016] [Accepted: 05/12/2016] [Indexed: 11/26/2022]
Abstract
In all non-vertebrate metazoan phyla, species that evolved non-embryonic developmental pathways as means of propagation or regeneration can be found. In this context, new bodies arise through asexual reproduction processes (such as budding) or whole body regeneration, that lack the familiar temporal and spatial cues classically associated with embryogenesis, like maternal determinants, or gastrulation. The molecular mechanisms underlying those non-embryonic developments (i.e., regeneration and asexual reproduction), and their relationship to those deployed during embryogenesis are poorly understood. We have addressed this question in the colonial ascidian Botryllus schlosseri, which undergoes an asexual reproductive process via palleal budding (PB), as well as a whole body regeneration by vascular budding (VB). We identified early regenerative structures during VB and then followed the fate of differentiating tissues during both non-embryonic developments (PB and VB) by monitoring the expression of genes known to play key functions in germ layer specification with well conserved expression patterns in solitary ascidian embryogenesis. The expression patterns of FoxA1, GATAa, GATAb, Otx, Bra, Gsc and Tbx2/3 were analysed during both PB and VB. We found that the majority of these transcription factors were expressed during both non-embryonic developmental processes, revealing a regionalization of the palleal and vascular buds. Knockdown of GATAa by siRNA in palleal buds confirmed that preventing the correct development of one of these regions blocks further tissue specification. Our results indicate that during both normal and injury-induced budding, a similar alternative developmental program operates via early commitment of epithelial regions.
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Affiliation(s)
- Lorenzo Ricci
- Sorbonne Universités, UPMC Univ. Paris 06, CNRS, Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV), 06230 Villefranche sur-mer, France
| | - Fabien Cabrera
- Sorbonne Universités, UPMC Univ. Paris 06, CNRS, Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV), 06230 Villefranche sur-mer, France
| | - Sonia Lotito
- Sorbonne Universités, UPMC Univ. Paris 06, CNRS, Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV), 06230 Villefranche sur-mer, France
| | - Stefano Tiozzo
- Sorbonne Universités, UPMC Univ. Paris 06, CNRS, Laboratoire de Biologie du Développement de Villefranche-sur-mer (LBDV), 06230 Villefranche sur-mer, France.
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25
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Ogura Y, Sasakura Y. Developmental Control of Cell-Cycle Compensation Provides a Switch for Patterned Mitosis at the Onset of Chordate Neurulation. Dev Cell 2016; 37:148-61. [DOI: 10.1016/j.devcel.2016.03.013] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2015] [Revised: 01/28/2016] [Accepted: 03/16/2016] [Indexed: 10/21/2022]
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26
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27
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Veeman M, Reeves W. Quantitative and in toto imaging in ascidians: working toward an image-centric systems biology of chordate morphogenesis. Genesis 2015; 53:143-59. [PMID: 25262824 PMCID: PMC4378666 DOI: 10.1002/dvg.22828] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2014] [Revised: 08/20/2014] [Accepted: 09/25/2014] [Indexed: 12/16/2022]
Abstract
Developmental biology relies heavily on microscopy to image the finely controlled cell behaviors that drive embryonic development. Most embryos are large enough that a field of view with the resolution and magnification needed to resolve single cells will not span more than a small region of the embryo. Ascidian embryos, however, are sufficiently small that they can be imaged in toto with fine subcellular detail using conventional microscopes and objectives. Unlike other model organisms with particularly small embryos, ascidians have a chordate embryonic body plan that includes a notochord, hollow dorsal neural tube, heart primordium and numerous other anatomical details conserved with the vertebrates. Here we compare the size and anatomy of ascidian embryos with those of more traditional model organisms, and relate these features to the capabilities of both conventional and exotic imaging methods. We review the emergence of Ciona and related ascidian species as model organisms for a new era of image-based developmental systems biology. We conclude by discussing some important challenges in ascidian imaging and image analysis that remain to be solved.
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Affiliation(s)
- Michael Veeman
- Division of Biology, Kansas State University, Manhattan KS, USA
| | - Wendy Reeves
- Division of Biology, Kansas State University, Manhattan KS, USA
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28
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Farrell JA, O'Farrell PH. From egg to gastrula: how the cell cycle is remodeled during the Drosophila mid-blastula transition. Annu Rev Genet 2014; 48:269-94. [PMID: 25195504 DOI: 10.1146/annurev-genet-111212-133531] [Citation(s) in RCA: 126] [Impact Index Per Article: 12.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Abstract
Many, if not most, embryos begin development with extremely short cell cycles that exhibit unusually rapid DNA replication and no gap phases. The commitment to the cell cycle in the early embryo appears to preclude many other cellular processes that only emerge as the cell cycle slows just prior to gastrulation at a major embryonic transition known as the mid-blastula transition (MBT). As reviewed here, genetic and molecular studies in Drosophila have identified changes that extend S phase and introduce a post-replicative gap phase, G2, to slow the cell cycle. Although many mysteries remain about the upstream regulators of these changes, we review the core mechanisms of the change in cell cycle regulation and discuss advances in our understanding of how these might be timed and triggered. Finally, we consider how the elements of this program may be conserved or changed in other organisms.
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Affiliation(s)
- Jeffrey A Farrell
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138;
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29
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Microinjection and 4D fluorescence imaging in the eggs and embryos of the ascidian Phallusia mammillata. Methods Mol Biol 2014; 1128:175-85. [PMID: 24567214 DOI: 10.1007/978-1-62703-974-1_11] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Time-lapse 4D imaging of fluorescently tagged proteins to follow the dynamics of cellular structures (chromosomes, microtubules, actin, centrosomes, cortical structures like the CAB in ascidians, etc.) combined with targeted gene knockdown during embryonic development is a powerful technique to understand the mechanisms of embryonic development. The eggs and embryos of the primitive marine chordate Phallusia mammillata are an excellent model system for combining live cell imaging with gene knockdown experiments. Here we describe simple methods for microinjecting Phallusia eggs with mRNA encoding fluorescent fusion proteins combined with 4D time-lapse imaging techniques we use to follow all of embryonic development from the egg to late tailbud stage.
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