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Microtubule asters are essential in localizing the action of microtubules in processes including mitosis and organelle positioning. In large cells, such as the one-cell sea urchin embryo, aster dynamics are dominated by hydrodynamic pulling forces. However, in systems with more densely positioned nuclei such as the early Drosophila embryo, which packs around 6000 nuclei within the syncytium in a crystalline-like order, it is unclear what processes dominate aster dynamics. Here, we take advantage of a cell cycle regulation Drosophila mutant to generate embryos with multiple asters, independent from nuclei. We use an ex vivo assay to further simplify this biological system to explore the forces generated by and between asters. Through live imaging, drug and optical perturbations, and theoretical modelling, we demonstrate that these asters likely generate an effective pushing force over short distances.

Microtubule asters are essential in localizing the action of microtubules in processes including mitosis and cell positioning. In large cells, such as the one-cell sea urchin embryo, aster dynamics are dominated by hydrodynamic pulling forces. However, in systems with more densely positioned nuclei such as the earlyDrosophilaembryo, which packs around 6000 nuclei within the syncytium in a crystalline-like order, it is unclear what processes dominate aster dynamics. Here, we take advantage of a cell cycle regulationDrosophilamutant to generate embryos with multiple asters, independent from nuclei. We use anex vivoassay to further simplify the system to explore the forces generated by and between asters. Through live imaging, drug and mechanical perturbations, and theoretical modelling, we demonstrate that these asters likely generate an effective pushing force over short distances. Such a pushing force can position asters in a crystalline like order, consistent with experiment. This work demonstrates that the mechanism of aster interactions is likely system size dependent.Significance StatementUsing cytosolic explants fromDrosophilasyncytial embryos combined with quantitative microscopy and perturbations, de-Carvalho et al. reveal the mechanical forces separating microtubule asters. Aster separation drives precise nuclear positioning in multinucleated embryo cells, a vital process for tissue formation and gene expression during subsequent embryo development.

International audience; On a curved surface, epithelial cells can adapt to geometric constraints by tilting and by exchanging their neighbors from apical to basal sides, known as an apicobasal T1 (AB-T1) transition. The relationship between cell tilt, AB-T1 transitions, and tissue curvature still lacks a unified understanding. Here, we propose a general framework for cell packing in curved environments and explain the formation of AB-T1 transitions under different conditions. We find that steep curvature gradients can lead to cell tilting and induce AB-T1 transitions. Conversely, large curvature anisotropy can drive AB-T1 transitions by hydrostatic pressure. The two mechanisms compete to determine the impact of tissue geometry and mechanics on optimized cell rearrangements in 3D.

Morphogenesis of the Central Nervous System (CNS) is a complex process that obeys precise architectural rules. Yet, the mechanisms dictating these rules remain unknown. Analyzing morphogenesis of the Drosophila embryo Ventral Nerve Cord (VNC), we observe that a tight control of JNK signaling is essential for attaining the final VNC architecture. JNK signaling in a specific subset of pioneer neurons autonomously regulates the expression of Fasciclin 2 (Fas 2) and Neurexin IV (Nrx IV) adhesion molecules, probably via the transcription factor zfh1. Interfering at any step in this cascade affects fasciculation along pioneer axons, leading to secondary cumulative scaffolding defects during the structural organization of the axonal network. The global disorder of architectural landmarks ultimately influences nervous system condensation. In summary, our data point to JNK signaling in a subset of pioneer neurons as a key element underpinning VNC architecture, revealing critical milestones on the mechanism of control of its structural organization.

During the development of complicated multicellular organisms, the robust formation of specific cell-cell connections (cell matching) is required for the generation of precise tissue structures. Mismatches or misconnections can lead to various diseases. Diverse mechanical cues, including differential adhesion and temporally varying cell contractility, are involved in regulating the process of cell-cell recognition and contact formation. Cells often start the process of cell matching through contact via filopodia protrusions, mediated by specific adhesion interactions at the cell surface. These adhesion interactions give rise to differential mechanical signals that can be further perceived by the cells. In conjunction with contractions generated by the actomyosin networks within the cells, this differentially coded adhesion information can be translated to reposition and sort cells. Here, we review the role of these different cell matching components and suggest how these mechanical factors cooperate with each other to facilitate specificity in cell-cell contact formation.

Adult epidermal development in Drosophila showcases a striking balance between en masse spreading of the developing adult precursor tissues and retraction of the degenerating larval epidermis. The adult precursor tissues, driven by both intrinsic plasticity and extrinsic mechanical cues, shape the segments of the adult epidermis and appendages. Here, we review the tissue architectural changes that occur during epidermal morphogenesis in the Drosophila pupa, with a particular emphasis on the underlying mechanical principles. We highlight recent developments in our understanding of adult epidermal morphogenesis. We further discuss the forces that drive these morphogenetic events and finally outline open questions and challenges.

Many species show a diverse range of sizes; for example, domestic dogs have large variation in body mass. Yet, the internal structure of the organism remains similar, i.e., the system scales to organism size. Drosophila melanogaster has been a powerful model system for exploring scaling mechanisms. In the early embryo, gene expression boundaries scale very precisely to embryo length. Later in development, the adult wings grow with remarkable symmetry and scale well with animal size. Yet, our knowledge of whether internal organs initially scale to embryo size remains largely unknown. Here, we utilize artificially small Drosophila embryos to explore how three critical internal organs—the heart, hindgut, and ventral nerve cord (VNC)—adapt to changes in embryo morphology. We find that the heart scales precisely with embryo length. Intriguingly, reduction in cardiac cell length, rather than number, appears to be important in controlling heart length. The hindgut, which is the first chiral organ to form, displays scaling with embryo size under large-scale changes in the artificially smaller embryos but shows few hallmarks of scaling within wild-type size variation. Finally, the VNC only displays weak scaling behavior; even large changes in embryo geometry result in only small shifts in VNC length. This suggests that the VNC may have an intrinsic minimal length that is largely independent of embryo length. Overall, our work shows that internal organs can adapt to embryo size changes in Drosophila, but the extent to which they scale varies significantly between organs.

A bewildering array of colour patterns occurs across animals. New work studying five lizard species reveals that reaction-diffusion models can be remarkably predictive of future adult skin patterning, even though molecular details are unknown. This has implications for understanding how complex patterns evolve.

YAP, an effector of the Hippo signalling pathway, promotes organ growth and regeneration. Prolonged YAP activation results in uncontrolled proliferation and cancer. Therefore, exogenous regulation of YAP activity has potential translational applications. We present a versatile optogenetic construct (optoYAP) for manipulating YAP localisation, and consequently its activity and function. We attach a LOV2 domain that photocages a nuclear localisation signal (NLS) to the N-terminus of YAP. In 488 nm light, the LOV2 domain unfolds, exposing the NLS, which shuttles optoYAP into the nucleus. Nuclear import of optoYAP is reversible and tuneable by light intensity. In cell culture, activated optoYAP promotes YAP target gene expression and cell proliferation. Similarly, optofYap can be used in zebrafish embryos to modulate target genes. We demonstrate that optoYAP can override a cell's response to substrate stiffness to generate anchorage-independent growth. OptoYAP is functional in both cell culture and in vivo, providing a powerful tool to address basic research questions and therapeutic applications in regeneration and disease.

All organisms are exposed to changes in their environment throughout their life cycle. When confronted with these changes, they adjust their development and physiology to ensure that they can produce the functional structures necessary for survival and reproduction. While some traits are remarkably invariant, or robust, across environmental conditions, others show high degrees of variation, known as plasticity. Generally, developmental processes that establish cell identity are thought to be robust to environmental perturbation, while those relating to body and organ growth show greater degrees of plasticity. However, examples of plastic patterning and robust organ growth demonstrate that this is not a hard-and-fast rule.In this review, we explore how the developmental context and the gene regulatory mechanisms underlying trait formation determine the impacts of the environment on development in insects. Furthermore, we outline future issues that need to be resolved to understand how the structure of signaling networks defines whether a trait displays plasticity or robustness.

In a previous study, it was reported that Yap1 and Wwtr1 in zebrafish regulates the morphogenesis of the posterior body and epidermal fin fold (Kimelman et al., 2017). We report here that DNA damage induces apoptosis of epidermal basal cells (EBCs) in zebrafish yap1-/-;wwtr1-/- embryos. Specifically, these mutant EBCs exhibit active Caspase-3, Caspase-8, and γH2AX, consistent with DNA damage serving as a stimulus of the extrinsic apoptotic pathway in epidermal cells. Live imaging of zebrafish epidermal cells reveals a steady growth of basal cell size in the developing embryo, but this growth is inhibited in mutant basal cells followed by apoptosis, leading to the hypothesis that factors underscoring cell size play a role in this DNA damage-induced apoptosis phenotype. We tested two of these factors using cell stretching and substrate stiffness assays, and found that HaCaT cells cultured on stiff substrates exhibit more numerous γH2AX foci compared to ones cultured on soft substrates. Thus, our experiments suggest that substrate rigidity may modulate genomic stress in epidermal cells, and that Yap1 and Wwtr1 promotes their survival.

Immigration of mesenchymal cells into the growing fin and limb buds drives distal outgrowth, with subsequent tensile forces between these cells essential for fin and limb morphogenesis. Morphogens derived from the apical domain of the fin, orientate limb mesenchyme cell polarity, migration, division and adhesion. The zebrafish mutant stomp displays defects in fin morphogenesis including blister formation and associated loss of orientation and adhesion of immigrating fin mesenchyme cells. Positional cloning of stomp identifies a mutation in the gene encoding the axon guidance ligand, Slit3. We provide evidence that Slit ligands derived from immigrating mesenchyme act via Robo receptors at the apical ectodermal ridge (AER) to promote release of sphingosine-1-phosphate (S1P). S1P subsequently diffuses back to the mesenchyme to promote their polarisation, orientation, positioning and adhesion to the interstitial matrix of the fin fold. We thus demonstrate the coordination of the Slit-Robo and S1P signalling pathways in fin fold morphogenesis. Our work introduces a mechanism regulating the orientation, positioning and adhesion of its constituent cells. Ministry of Education (MOE) Work in the TJC and TES labs was funded by a Ministry of Education of Singapore AcRF Tier 3 grant (MOE2016-T3-1-005). Work in the FL lab was supported by funding from the National Science Foundation, IOS-1354457. SMP is supported by Senior Research Fellowships (1042589and1156693) from the National Health and Medical Research Council of Australia.

Mirtrons are non-canonical miRNAs arising by splicing and debranching from short introns. A plethora of introns have been inferred by computational analyses as potential mirtrons. Yet, few have been experimentally validated and their functions, particularly in relation to their host genes, remain poorly understood. Here, we found that Drosophila larvae lacking either the mirtron miR-1010 or its binding site in the nicotinic acetylcholine receptor β2 (nAcRβ2) 3′UTR fail to grow properly and pupariate. Increase of cortical nAcRβ2 mediated by neural activity elevates the level of intracellular Ca2+, which in turn activates CaMKII and, further downstream, the transcription factor Adf-1. We show that miR-1010 downregulates nAcRβ2. We reveal that Adf-1 initiates the expression of SKIP, the host gene of miR-1010. Preventing synaptic potentials from overshooting their optimal range requires both SKIP to temper synaptic potentials (incoherent feedforward loop) and miR-1010 to reduce nAcRβ2 mRNA levels (negative feedback loop). Our results demonstrate how a mirtron, in coordination with its host gene, contributes to maintaining appropriate receptor levels, which in turn may play a role in maintaining homeostasis.

Biological systems are highly complex, yet notably ordered structures can emerge. During syncytial stage development of the Drosophila melanogaster embryo, nuclei synchronously divide for nine cycles within a single cell, after which most of the nuclei reach the cell cortex. The arrival of nuclei at the cortex occurs with remarkable positional order, which is important for subsequent cellularisation and morphological transformations. Yet, the mechanical principles underlying this lattice-like positional order of nuclei remain untested. Here, using quantification of nuclei position and division orientation together with embryo explants, we show that short-ranged repulsive interactions between microtubule asters ensure the regular distribution and maintenance of nuclear positions in the embryo. Such ordered nuclear positioning still occurs with the loss of actin caps and even the loss of the nuclei themselves; the asters can self-organise with similar distribution to nuclei in the wild-type embryo. The explant assay enabled us to deduce the nature of the mechanical interaction between pairs of nuclei. We used this to predict how the nuclear division axis orientation changes upon nucleus removal from the embryo cortex, which we confirmed in vivo with laser ablation. Overall, we show that short-ranged microtubule-mediated repulsive interactions between asters are important for ordering in the early Drosophila embryo and minimising positional irregularity.

Morphogen gradients provide essential positional information to gene networks through their spatially heterogeneous distribution. Yet, how morphogen gradients form is still hotly contested, with multiple models proposed for different systems. Here, we focus on the transcription factor Bicoid (Bcd), a morphogen that forms an exponential gradient across the anterior-posterior (AP) axis of the earlyDrosophilaembryo. We utilise fluorescence correlation spectroscopy (FCS) and perturbations to Bcd, to dissect Bcd dynamics at multiple spatial and temporal locations. In both the cytoplasm and nucleus, we find two dynamic modes for Bicoid diffusion dynamics, consisting of fast and slow populations of Bcd. Surprisingly, there are spatial differences in Bcd diffusivity along the AP-axis, with Bcd diffusing more rapidly in the posterior. We establish that such spatially varying differences in the Bcd dynamics are sufficient to explain how Bcd can have a steep exponential gradient in the anterior half of the embryo and yet still have an observable fraction of Bcd near the posterior pole. We subsequently investigated which binding elements of Bcd are playing a role in its dynamics. In the nucleus, we demonstrate that the slower mode of Bcd transport is due to Bcd DNA binding. Addition of the Bcd homeodomain to eGFP::NLS can qualitatively replicate the observed Bcd concentration profile, suggesting this domain is the primary region regulating Bcd dynamics. This study provides a detailed analysis of morphogen dynamics at different spatial and temporal locations, revealing multiple modes of transport. These results explain how a long-ranged gradient can form while retaining a steep profile through much of its range.

SUMMARYImmigration of mesenchymal cells into the growing fin and limb buds drives distal outgrowth, with subsequent tensile forces between these cells essential for fin and limb morphogenesis. Morphogens derived from the apical domain of the fin, orientate limb mesenchyme cell polarity, migration, division and adhesion. The zebrafish mutant stomp displays defects in fin morphogenesis including blister formation and associated loss of orientation and adhesion of immigrating fin mesenchyme cells. Positional cloning of stomp identified a mutation in the gene encoding the axon guidance ligand, Slit3. We provide evidence that Slit ligands derived from immigrating mesenchyme act via Robo receptors at the Apical Ectodermal Ridge (AER) to promote release of sphingosine-1-phosphate (S1P). S1P subsequently diffuses back to the mesenchyme to promote their polarisation, orientation, positioning and adhesion to the interstitial matrix of the fin fold. We thus demonstrate coordination of the Slit-Robo and S1P signalling pathways in fin fold morphogenesis. Our work introduces a mechanism regulating the orientation, positioning and adhesion of its constituent cells.

In a previous study, it was reported that Yap1 and Wwtr1 in zebrafish regulates the morphogenesis of the posterior body and epidermal fin fold (Kimelman, D., et al. 2017). We report here that DNA damage induces apoptosis of epidermal basal cells (EBCs) in zebrafish yap1−/−;wwtr1−/− embryos. Specifically, these mutant EBCs exhibit active Caspase-3, Caspase-8 and γH2AX, consistent with DNA damage serving as a stimulus of the extrinsic apoptotic pathway in epidermal cells. Live imaging of zebrafish epidermal cells reveals a steady growth of basal cell size in the developing embryo, but this growth is inhibited in mutant basal cells followed by apoptosis, leading to the hypothesis that factors underscoring cell size play a role in this DNA damage-induced apoptosis phenotype. We tested two of these factors using cell stretching and substrate stiffness assays, and found that HaCaT cells cultured on stiff substrates exhibit more numerous γH2AX foci compared to ones cultured on soft substrates. Thus, we propose that substrate rigidity modulates genomic stress in the developing epidermal cell, and that Yap1 and Wwtr1 are required for its survival.

In a previous study, it was reported that Yap1 and Wwtr1 in zebrafish regulates the morphogenesis of the posterior body and epidermal fin fold (Kimelman et al., 2017). We report here that DNA damage induces apoptosis of epidermal basal cells (EBCs) in zebrafish

The way in which interactions between mechanics and biochemistry lead to the emergence of complex cell and tissue organization is an old question that has recently attracted renewed interest from biologists, physicists, mathematicians and computer scientists. Rapid advances in optical physics, microscopy and computational image analysis have greatly enhanced our ability to observe and quantify spatiotemporal patterns of signalling, force generation, deformation, and flow in living cells and tissues. Powerful new tools for genetic, biophysical and optogenetic manipulation are allowing us to perturb the underlying machinery that generates these patterns in increasingly sophisticated ways. Rapid advances in theory and computing have made it possible to construct predictive models that describe how cell and tissue organization and dynamics emerge from the local coupling of biochemistry and mechanics. Together, these advances have opened up a wealth of new opportunities to explore how mechanochemical patterning shapes organismal development. In this roadmap, we present a series of forward-looking case studies on mechanochemical patterning in development, written by scientists working at the interface between the physical and biological sciences, and covering a wide range of spatial and temporal scales, organisms, and modes of development. Together, these contributions highlight the many ways in which the dynamic coupling of mechanics and biochemistry shapes biological dynamics: from mechanoenzymes that sense force to tune their activity and motor output, to collectives of cells in tissues that flow and redistribute biochemical signals during development.

Tissue remodelling and organ shaping during morphogenesis are products of mechanical forces generated at the cellular level. These cell-scale forces can be coordinated across the tissue via information provided by biochemical and mechanical cues. Such coordination leads to the generation of complex tissue shape during morphogenesis. In this short review, we elaborate the role of cellular active stresses in vertebrate axis morphogenesis, primarily using examples from postgastrulation development of the zebrafish embryo.

YAP, an effector of the Hippo signalling pathway, promotes organ growth and regeneration. Prolonged YAP activation results in uncontrolled proliferation and cancer. Therefore, exogenous regulation of YAP activity has potential translational applications. We present a versatile optogenetic construct (optoYAP) for manipulating YAP localisation, and consequently its activity and function. We attached a LOV2 domain that photocages a nuclear localisation signal (NLS) to the N-terminus of YAP. In 488 nm light, the LOV2 domain unfolds, exposing the NLS, which shuttles optoYAP into the nucleus. Nuclear import of optoYAP is reversible and tuneable by light intensity. In cell culture, activated optoYAP promotes YAP target gene expression, cell proliferation, and anchorage-independent growth. Similarly, we can utilise optoYAP in zebrafish embryos to modulate target genes. OptoYAP is functional in both cell culture and in vivo, providing a powerful tool to address basic research questions and therapeutic applications in regeneration and disease.

With the rise of new tools, from controlled genetic manipulations and optogenetics to improved microscopy, it is now possible to make clear, quantitative and reproducible measurements of biological processes. The humble fruit fly Drosophila melanogaster, with its ease of genetic manipulation combined with excellent imaging accessibility, has become a major model system for performing quantitative in vivo measurements. Such measurements are driving a new wave of interest from physicists and engineers, who are developing a range of testable dynamic models of active systems to understand fundamental biological processes. The reproducibility of the early Drosophila embryo has been crucial for understanding how biological systems are robust to unavoidable noise during development. Insights from quantitative in vivo experiments in the Drosophila embryo are having an impact on our understanding of critical biological processes, such as how cells make decisions and how complex tissue shape emerges. Here, to highlight the power of using Drosophila embryogenesis for quantitative biology, I focus on three main areas: (1) formation and robustness of morphogen gradients; (2) how gene regulatory networks ensure precise boundary formation; and (3) how mechanical interactions drive packing and tissue folding. I further discuss how such data has driven advances in modelling.

During development, organs must form with precise shapes and sizes. Organ morphology is not always obtained through growth; a classic counterexample is condensation of the nervous system duringDrosophilaembryogenesis. The mechanics underlying such condensation remain poorly understood. Here, we combinein totolive-imaging, biophysical and genetic perturbations, and atomic force microscopy to characterize the condensation of theDrosophilaventral nerve cord (VNC) during embryonic development at both subcellular and tissue scales. This analysis reveals that condensation is not a unidirectional continuous process, but instead occurs through oscillatory contractions alternating from anterior and posterior ends. The VNC mechanical properties spatially and temporally vary during its condensation, and forces along its longitudinal axis are spatially heterogeneous, with larger ones exerted between neuromeres. We demonstrate that the process of VNC condensation is dependent on the coordinated mechanical activities of neurons and glia. Finally, we show that these outcomes are consistent with a viscoelastic model of condensation, which incorporates time delays due to the different time scales on which the mechanical processes act, and effective frictional interactions. In summary, we have defined the complex and progressive mechanics driving VNC condensation, providing insights into how a highly viscous tissue can autonomously change shape and size.

During development, organs must form with precise shapes and sizes. Organ morphology is not always obtained through growth; a classic counterexample is condensation of the nervous system during Drosophila embryogenesis. The mechanics underlying such condensation remain poorly understood. Here, we combine in toto live-imaging, biophysical and genetic perturbations, and atomic force microscopy to characterize the condensation of the Drosophila ventral nerve cord (VNC) during embryonic development at both subcellular and tissue scales. This analysis reveals that condensation is not a unidirectional continuous process, but instead occurs through oscillatory contractions alternating from anterior and posterior ends. The VNC mechanical properties spatially and temporally vary during its condensation, and forces along its longitudinal axis are spatially heterogeneous, with larger ones exerted between neuromeres. We demonstrate that the process of VNC condensation is dependent on the coordinated mechanical activities of neurons and glia. Finally, we show that these outcomes are consistent with a viscoelastic model of condensation, which incorporates time delays due to the different time scales on which the mechanical processes act, and effective frictional interactions. In summary, we have defined the complex and progressive mechanics driving VNC condensation, providing insights into how a highly viscous tissue can autonomously change shape and size.

Many species show a diverse range of sizes; for example, domestic dogs have large variation in body mass. Yet, the internal structure of the organism remains similar,i.e. the system scales to organism size.Drosophila melanogasterhas been a powerful model system for exploring scaling mechanisms. In the early embryo, gene expression boundaries scale very precisely to embryo length. Later in development, the adult wings grow with remarkable symmetry and scale well with animal size. Yet, our knowledge of whether internal organs initially scale to embryo size remains largely unknown. Here, we utilise artificially smallDrosophilaembryos to explore how three critical internal organs – the heart, hindgut and ventral nerve cord (VNC) – adapt to changes in embryo morphology. We find that the heart scales precisely with embryo length. Intriguingly, reduction in cardiac cell length, rather than number, appears to be important in controlling heart length. The hindgut – which is the first chiral organ to form – displays scaling with embryo size under large-scale changes in the artificially smaller embryos but shows few hallmarks of scaling within wild-type size variation. Finally, the VNC only displays weak scaling behaviour; even large changes in embryo geometry result in only small shifts in VNC length. This suggests that the VNC may have an intrinsic minimal length, which is largely independent of embryo length. Overall, our work shows that internal organs can adapt to embryo size changes inDrosophila. but the extent to which they scale varies significantly between organs.

Biological systems are a form of active matter, which often undergo rapid changes in their material state,e.g. liquid to solid transitions. Yet, such systems often also display remarkably ordered structures. It remains an open question as to how local ordering occurs within active systems. Here, we utilise the rapid early development ofDrosophila melanogasterembryos to uncover the mechanisms driving short-ranged order. During syncytial stage, nuclei synchronously divide (within a single cell defined by the ellipsoidal eggshell) for nine cycles after which most of the nuclei reach the cell cortex. Despite the rapid nuclear division and repositioning, the spatial pattern of nuclei at the cortex is highly regular. Such precision is important for subsequent cellularisation and morphological transformations. We utiliseex vivoexplants and mutant embryos to reveal that microtubule asters ensure the regular distribution and maintenance of nuclear positions in the embryo. For large networks of nuclei, such as in the embryo, we predict – and experimentally verify – the formation of force chains. Theex vivoextracts enabled us to deduce the force potential between single asters. We use this to predict how the nuclear division axis orientation in smallex vivosystems depend on aster number. Finally, we demonstrate that, upon nucleus removal from the cortex, microtubule force potentials can reorient subsequent nuclear divisions to minimise the size of pattern defects. Overall, we show that short-ranged microtubule-mediated repulsive interactions between asters can drive ordering within an active system.

Lateral inhibition patterns differentiated cell types among equivalent cells during development in bacteria, metazoans, and plants. Tip-growing rhizoid cells develop among flat epidermal cells in the epidermis of the early-diverging land plant Marchantia polymorpha. We show that the majority of rhizoid cells develop individually, but some develop in linear, one-dimensional groups (chains) of between 2 and 7 rhizoid cells in wild-type plants. The distribution of rhizoid cells can be accounted for within a simple cellular automata model of lateral inhibition. The model predicted that in the absence of lateral inhibition, two-dimensional rhizoid cell groups (clusters) form. These can be larger than those formed with lateral inhibition. M. polymorpha rhizoid differentiation is positively regulated by the ROOT HAIR DEFECTIVE SIX-LIKE1 (MpRSL1) basic-helix-loop-helix (bHLH) transcription factor, which is directly repressed by the FEW RHIZOIDS1 (MpFRH1) microRNA (miRNA). To test if MpFRH1 miRNA acts during lateral inhibition, we generated loss-of-function (lof) mutants without the MpFRH1 miRNA. Two-dimensional clusters of rhizoids develop in Mpfrh1lof mutants as predicted by the model for plants that lack lateral inhibition. Furthermore, two-dimensional clusters of up to 9 rhizoid cells developed in the Mpfrh1lof mutants compared to a maximum number of 7 observed in wild-type groups. The higher steady-state levels of MpRSL1 mRNA in Mpfrh1lof mutants indicate that MpFRH1-mediated lateral inhibition involves the repression of MpRSL1 activity. Together, the modeling and genetic data indicate that MpFRH1 miRNA mediates lateral inhibition by repressing MpRSL1 during pattern formation in the M. polymorpha epidermis.

Spatially distributed signaling molecules, known as morphogens, provide spatial information during development. A host of different morphogens have now been identified, from subcellular gradients through to morphogens that act across a whole embryo. These gradients form over a wide-range of timescales, from seconds to hours, and their time windows for interpretation are also highly variable; the processes of morphogen gradient formation and interpretation are highly dynamic. The morphogen Bicoid (Bcd), present in the early Drosophila embryo, is essential for setting up the future Drosophila body segments. Due to its accessibility for both genetic perturbations and imaging, this system has provided key insights into how precise patterning can occur within a highly dynamic system. Here, we review the temporal scales of Bcd gradient formation and interpretation. In particular, we discuss the quantitative evidence for different models of Bcd gradient formation, outline the time windows for Bcd interpretation, and describe how Bcd temporally adapts its own ability to be interpreted. The utilization of temporal information in morphogen readout may provide crucial inputs to ensure precise spatial patterning, particularly in rapidly developing systems.

International audience; During development, many mutations cause increased variation in phenotypic outcomes, a phenomenon termed decanalization. Phenotypic discordance is often observed in the absence of genetic and environmental variations, but the mechanisms underlying such interindividual phenotypic discordance remain elusive. Here, using the anterior-posterior (AP) patterning of the Drosophila embryo, we identified embryonic geometry as a key factor predetermining patterning outcomes under decanalizing mutations. With the wild-type AP patterning network, we found that AP patterning is robust to variations in embryonic geometry; segmentation gene expression remains reproducible even when the embryo aspect ratio is artificially reduced by more than twofold. In contrast, embryonic geometry is highly predictive of individual patterning defects under decanalized conditions of either increased bicoid (bcd) dosage or bcd knockout. We showed that the phenotypic discordance can be traced back to variations in the gap gene expression, which is rendered sensitive to the geometry of the embryo under mutations.

Summary Actomyosin networks provide the major contractile machinery for regulating cell and tissue morphogenesis during development. These networks undergo dynamic rearrangements, enabling cells to have a broad range of mechanical actions. How cells integrate different mechanical stimuli to accomplish complicated tasks in vivo remains unclear. Here, we explore this problem in the context of cell matching, where individual cells form precise inter-cellular connections between partner cells. To study the dynamic roles of actomyosin networks in regulating precise cell matching, we focused on the process of heart formation during Drosophila embryogenesis, where selective filopodia-binding adhesions ensure precise cell alignment. We find that non-muscle Myosin II clusters periodically oscillate within cardioblasts with ~4-min intervals. We observe that filopodia dynamics—including protrusions, retraction, binding stabilization, and binding separation—are correlated with the periodic localization of Myosin II clusters at the cell leading edge. Perturbing the Myosin II activity and oscillatory pattern alters the filopodia properties and binding dynamics and results in mismatched cardioblasts. By simultaneously changing the activity of Myosin II and filopodia adhesion levels, we further demonstrate that levels of Myosin II and adhesion are balanced to ensure precise connectivity between cardioblasts. Combined, we propose a mechanical proofreading machinery of robust cell matching, whereby oscillations of Myosin II within cardioblasts periodically probe filopodia adhesion strength and ensure correct cell-cell connection formation.

The notochord defines the axial structure of all vertebrates during development. Notogenesis is a result of major cell reorganization in the mesoderm, the convergence and the extension of the axial cells. However, it is currently not fully understood how these processes act together in a coordinated way during notochord formation. The prechordal plate is an actively migrating cell population in the central mesoderm anterior to the trailing notochordal plate cells. We show that prechordal plate cells express Protocadherin 18a (Pcdh18a), a member of the cadherin superfamily. We find that Pcdh18a-mediated recycling of E-cadherin adhesion complexes transforms prechordal plate cells into a cohesive and fast migrating cell group. In turn, the prechordal plate cells subsequently instruct the trailing mesoderm. We simulated cell migration during early mesoderm formation using a lattice-based mathematical framework and predicted that the requirement for an anterior, local motile cell cluster could guide the intercalation and extension of the posterior, axial cells. Indeed, a grafting experiment validated the prediction and local Pcdh18a expression induced an ectopic prechordal plate-like cell group migrating towards the animal pole. Our findings indicate that the Pcdh18a is important for prechordal plate formation, which influences the trailing mesodermal cell sheet by orchestrating the morphogenesis of the notochord. Electronic supplementary material The online version of this article (10.1007/s00418-020-01887-5) contains supplementary material, which is available to authorized users.

Significance In plants, microtubules largely determine the direction of cell expansion and the orientation of cell division planes. However, what processes orient the microtubules has remained debated. Here, we used microfabricated wells to confine and deform wallless plant cells in a controlled way to analyze the response of microtubules to cell geometry and surface tension. We demonstrate that microtubules align with cell geometry by default, whereas when surface tension increases (e.g. when turgor pressure increases), they align with the direction of maximal tension. Not only does this explain many observations in plant tissues, but it also provides a simple mechanism at the core of plant morphogenesis, in which microtubules can spontaneously align with tension, in a typical self-organized system., In plant cells, cortical microtubules (CMTs) generally control morphogenesis by guiding cellulose synthesis. CMT alignment has been proposed to depend on geometrical cues, with microtubules aligning with the cell long axis in silico and in vitro. Yet, CMTs are usually transverse in vivo, i.e., along predicted maximal tension, which is transverse for cylindrical pressurized vessels. Here, we adapted a microwell setup to test these predictions in a single-cell system. We confined protoplasts laterally to impose a curvature ratio and modulated pressurization through osmotic changes. We find that CMTs can be longitudinal or transverse in wallless protoplasts and that the switch in CMT orientation depends on pressurization. In particular, longitudinal CMTs become transverse when cortical tension increases. This explains the dual behavior of CMTs in planta: CMTs become longitudinal when stress levels become low, while stable transverse CMT alignments in tissues result from their autonomous response to tensile stress fluctuations.

Spatially distributed signaling molecules, known as morphogens, provide spatial information during development. A host of different morphogens have now been identified, from subcellular gradients through to morphogens that act across a whole embryo. These gradients form over a wide-range of timescales, from seconds to hours, and their time windows for interpretation are also highly variable; the processes of morphogen gradient formation and interpretation are highly dynamic. The morphogen Bicoid (Bcd), present in the early Drosophila embryo, is essential for setting up the future Drosophila body segments. Due to its accessibility for both genetic perturbations and imaging, this system has provided key insights into how precise patterning can occur within a highly dynamic system. Here, we review the temporal scales of Bcd gradient formation and interpretation. In particular, we discuss the quantitative evidence for different models of Bcd gradient formation, outline the time windows for Bcd interpretation, and describe how Bcd temporally adapts its own ability to be interpreted. The utilization of temporal information in morphogen readout may provide crucial inputs to ensure precise spatial patterning, particularly in rapidly developing systems.

Summary How individual cells form precise connections with partners in a complicated environment has been a longstanding question. However, most cell matching studies have used qualitative approaches, which may miss subtle but significant morphological changes. Here, we describe the use of embryonic Drosophila heart formation as a simplified system to quantitatively study cell matching. We provide a step-by-step protocol for large-scale embryo preparation and immunostaining and imaging details. We also describe steps for quantifying cellular mismatch from the batch images. For complete details on the use and execution of this protocol, please refer to Zhang et al. (2018 and 2020 ).

Throughout development, tissues undergo complex morphological changes, resulting from cellular mechanics that evolve over time and in three-dimensional space. During Drosophila germ-band extension (GBE), cell intercalation is the key mechanism for tissue extension, and the associated apical junction remodelling is driven by polarized myosin-II-dependent contraction. However, the contribution of the basolateral cellular mechanics to GBE remains poorly understood. Here, we characterize how cells coordinate their shape from the apical to the basal side during rosette formation, a hallmark of cell intercalation. Basolateral rosette formation is driven by cells mostly located at the dorsal/ventral part of the rosette (D/V cells). These cells exhibit actin-rich wedge-shaped basolateral protrusions and migrate towards each other. Surprisingly, the formation of basolateral rosettes precedes that of the apical rosettes. Basolateral rosette formation is independent of apical contractility, but requires Rac1-dependent protrusive motility. Furthermore, we identified Src42A as a regulator of basolateral rosette formation. Our data show that in addition to apical contraction, active cell migration driven by basolateral protrusions plays a pivotal role in rosette formation and contributes to GBE.

Significance How do tissues self-organize to generate the complex organ shapes observed in vertebrates? Organ formation requires the integration of chemical and mechanical information, yet how this is achieved is poorly understood for most organs. Muscle compartments in zebrafish display a V shape, which is believed to be required for efficient swimming. We investigate how this structure emerges during zebrafish development, combining live imaging and quantitative analysis of cellular movements. We use theoretical modeling to understand how cell differentiation and mechanical interactions between tissues guide the emergence of a specific tissue morphology. Our work reveals how spatially modulating the mechanical environment around and within tissues can lead to complex organ shape formation., Organ formation is an inherently biophysical process, requiring large-scale tissue deformations. Yet, understanding how complex organ shape emerges during development remains a major challenge. During zebrafish embryogenesis, large muscle segments, called myotomes, acquire a characteristic chevron morphology, which is believed to aid swimming. Myotome shape can be altered by perturbing muscle cell differentiation or the interaction between myotomes and surrounding tissues during morphogenesis. To disentangle the mechanisms contributing to shape formation of the myotome, we combine single-cell resolution live imaging with quantitative image analysis and theoretical modeling. We find that, soon after segmentation from the presomitic mesoderm, the future myotome spreads across the underlying tissues. The mechanical coupling between the future myotome and the surrounding tissues appears to spatially vary, effectively resulting in spatially heterogeneous friction. Using a vertex model combined with experimental validation, we show that the interplay of tissue spreading and friction is sufficient to drive the initial phase of chevron shape formation. However, local anisotropic stresses, generated during muscle cell differentiation, are necessary to reach the acute angle of the chevron in wild-type embryos. Finally, tissue plasticity is required for formation and maintenance of the chevron shape, which is mediated by orientated cellular rearrangements. Our work sheds light on how a spatiotemporal sequence of local cellular events can have a nonlocal and irreversible mechanical impact at the tissue scale, leading to robust organ shaping.

Lateral inhibition patterns differentiated cells during development in bacteria, metazoans and land plants. Tip-growing rhizoid cells develop among flat epidermal cells in the epidermis of the early diverging land plant Marchantia polymorpha. We show that the majority of rhizoid cells develop individually but some develop in linear, one-dimensional clusters of between two and seven rhizoid cells in wild type plants. The distribution of rhizoid cells can be accounted for within a simple model of lateral inhibition. The model also predicted that, in the absence of lateral inhibition, rhizoid cell clusters would be two-dimensional with larger clusters than those formed with lateral inhibition. Rhizoid differentiation in Marchantia polymorpha is positively regulated by the ROOT HAIR DEFECTIVE SIX-LIKE1 (MpRSL1) basic Helix Loop Helix (bHLH) transcription factor which is directly repressed by the FEW RHIZOIDS1 (MpFRH1) miRNA. To test if MpFRH1 miRNA acts during lateral inhibition we generated loss-of-function mutants that did not produce the MpFRH1 miRNA. Two-dimensional clusters of rhizoids develop in Mpfrh1loss-of-function (lof) mutants as predicted by the model for plants that lack lateral inhibition. Furthermore, clusters of up to nine rhizoid cells developed in the Mpfrh1lof mutants compared to a maximum number of seven observed in wild type. The higher steady state levels of MpRSL1 mRNA in Mpfrh1lof mutants indicate that MpFRH1-mediated lateral inhibition involves the repression of MpRSL1 activity. Together the modelling and genetic data indicate that the pattern of cell differentiation in the M. polymorpha epidermis is consistent with a lateral inhibition process in which MpFRH1 miRNA represses MpRSL1. This discovery suggests that novel mechanisms of lateral inhibition may operate in different lineages of land plants, unlike metazoans where the conserved Delta-Notch signaling system controls lateral inhibition in diverse metazoan lineages.

The eukaryotic endomembrane system is controlled by small GTPases of the Rab family, which are activated at defined times and locations in a switch-like manner. While this switch is well understood for an individual protein, how regulatory networks produce intracellular activity patterns is currently not known. Here, we combine in vitro reconstitution experiments with computational modeling to study a minimal Rab5 activation network. We find that the molecular interactions in this system give rise to a positive feedback and bistable collective switching of Rab5. Furthermore, we find that switching near the critical point is intrinsically stochastic and provide evidence that controlling the inactive population of Rab5 on the membrane can shape the network response. Notably, we demonstrate that collective switching can spread on the membrane surface as a traveling wave of Rab5 activation. Together, our findings reveal how biochemical signaling networks control vesicle trafficking pathways and how their nonequilibrium properties define the spatiotemporal organization of the cell.