Roles for polarity and nuclear determinants in specifying daughter cell fates after an asymmetric cell division in the maize leaf

Asymmetric cell divisions occur repeatedly during plant development, but the mechanisms by which daughter cells are directed to adopt different fates are not well understood [1,2]. Previous studies have demonstrated roles for positional information in specification of daughter cell fates following asymmetric divisions in the embryo [3] and root [4]. Unequally inherited cytoplasmic determinants have also been proposed to specify daughter cell fates after some asymmetric cell divisions in plants [1,2,5], but direct evidence is lacking. Here we investigate the requirements for specification of stomatal subsidiary cell fate in the maize leaf by analyzing four mutants disrupting the asymmetric divisions of subsidiary mother cells (SMCs). We show that subsidiary cell fate does not depend on proper localization of the new cell wall during the SMC division, and is not specified by positional information acting on daughter cells after completion of the division. Instead, our data suggest that specification of subsidiary cell fate depends on polarization of SMCs and on inheritance of the appropriate daughter nucleus. We thus provide evidence of a role for unequal inheritance of an intracellular determinant in specification of cell fate after an asymmetric plant cell division.     

Mutations in the Caenorhabditis elegans Gene vab-3 Reveal Distinct Roles in Fate Specification and Unequal Cytokinesis in an Asymmetric Cell Division

Asymmetric cell divisions in which a precursor cell distributes fate potential unequally between the two daughter cells represent one of the major mechanisms for fate specification during development. Such mechanisms suggest at least two distinct cellular activities: factors that act to establish asymmetry in the precursor cell and factors that are distributed or activated unequally and function to make the daughter cells different from each other. In Caenorhabditis elegans , cytokinesis of the first division of the male-specific postembryonic blast cell B is unequal, and the two daughters adopt different fates. Others have observed that the genes lin-17 and lin-44 are required, respectively, to establish and to orient this asymmetric division. Mutations in lin-17 and lin-44 coordinately disrupt cytokinesis and fate specification. We describe the function of the gene vab-3 in the B cell lineage. Mutations in vab-3 disrupt the fate of the anterior daughter of B, B.a. However, unlike lin-17 and lin-44 , mutations in vab-3 can disrupt fate without the corresponding disruption of unequal cytokinesis. Analysis of lin-17; vab-3 double mutants suggests that vab-3 acts after lin-17 for B.a fate specification. Double mutant analysis has also identified additional functions of lin-17 in the B lineage subsequent to this first division.     

Asymmetric cell divisions in flowering plants - one mother, "two-many" daughters

Plant development shows a fascinating range of asymmetric cell divisions. Over the years, however, cellular differentiation has been interpreted mostly in terms of a mother cell dividing mitotically to produce two daughter cells of different fates. This popular view has masked the significance of an entirely different cell fate specification pathway, where the mother cell first becomes a coenocyte and then cellularizes to simultaneously produce more than two specialized daughter cells. The "one mother - two different daughters" pathways rely on spindle-assisted mechanisms, such as translocation of the nucleus/spindle to a specific cellular site and orientation of the spindle, which are coordinated with cell-specific allocation of cell fate determinants and cytokinesis. By contrast, during "coenocyte-cellularization" pathways, the spindle-assisted mechanisms are irrelevant since cell fate specification emerges only after the nuclear divisions are complete, and the number of specialized daughter cells produced depends on the developmental context. The key events, such as the formation of a coenocyte and migration of the nuclei to specific cellular locations, are coordinated with cellularization by unique types of cell wall formation. Both one mother - two different daughters and the coenocyte-cellularization pathways are used by higher plants in precise spatial and time windows during development. In both the pathways, epigenetic regulation of gene expression is crucial not only for cell fate specification but also for its maintenance through cell lineage. In this review, the focus is on the coenocyte-cellularization pathways in the context of our current understanding of the asymmetric cell divisions. Instances where cell differentiation does not involve an asymmetric division are also discussed to provide a comprehensive account of cell differentiation.     

Asymmetric cell division and cell fate in plants

A variety of approaches has recently been employed to investigate how sister cells adopt distinct fates following asymmetric divisions during plant development. Surgical and drug studies have been used to analyze asymmetric divisions during both early embryogenesis in brown algae and pollen development in tobacco. Genetic screens have been used to identify genes in Arabidopsis thaliana that are required for specific asymmetric cell divisions during pollen and root development. These studies indicate that cell polarity and division orientation are closely tied to the process of cell fate specification, and suggest that differential inheritance of determinants and positional information may both be involved in the specification of cell fates following asymmetric cell division.     

Timers in Early Development of Sea Urchin Embryos

To elucidate the timing mechanisms in the early development of sea urchin embryos, we measured the times of initiation of the first four cleavages, of ciliary movement, of primary mesenchyme cell ingression, and of gastrulation at four temperatures ranging from 11 to 20°C. The times of cleavage and of initiation of ciliary movement showed similar temperature dependency, indicating that these events may be controlled by a common timer (the first timer). Although batches of eggs often showed variation in the period between fertilization and the first cleavage, their subsequent cleavages were more regular. This indicates that the first timer may not start at fertilization. The ingression of mesenchyme cells and the onset of gastrulation showed similar temperature dependency that was higher than that of other events, suggesting the existence of a second timer. Temperature shift experiments indicate that the second timer starts at the mid-blastula (the 8–9th cleavage) stage when divisions of blastomeres become asynchronous.     

Systems‐level quantification of division timing reveals a common genetic architecture controlling asynchrony and fate asymmetry

Coordination of cell division timing is crucial for proper cell fate specification and tissue growth. However, the differential regulation of cell division timing across or within cell types during metazoan development remains poorly understood. To elucidate the systems‐level genetic architecture coordinating division timing, we performed a high‐content screening for genes whose depletion produced a significant reduction in the a synchrony of d ivision between s ister cells (ADS) compared to that of wild‐type during Caenorhabditis elegans embryogenesis. We quantified division timing using 3D time‐lapse imaging followed by computer‐aided lineage analysis. A total of 822 genes were selected for perturbation based on their conservation and known roles in development. Surprisingly, we find that cell fate determinants are not only essential for establishing fate asymmetry, but also are imperative for setting the ADS regardless of cellular context, indicating a common genetic architecture used by both cellular processes. The fate determinants demonstrate either coupled or separate regulation between the two processes. The temporal coordination appears to facilitate cell migration during fate specification or tissue growth. Our quantitative dataset with cellular resolution provides a resource for future analyses of the genetic control of spatial and temporal coordination during metazoan development.     

A cell-intrinsic timer that operates during oligodendrocyte development

Multicellular organisms develop on a predictable schedule that depends on both cell-intrinsic timers and sequential cell-cell interactions mediated by extracellular signals. The interplay between intracellular timers and extracellular signals is well illustrated by the development of oligodendrocytes, the cells that make the myelin in the vertebrate central nervous system. An intrinsic timing mechanism operates in each oligodendrocyte precursor cell to limit the length of time the cell divides before terminally differentiating. This mechanism consists of two components, a timing component, which depends on the mitogen platelet-derived growth factor (PDGF) and measures elapsed time, and an effector component, which depends on thyroid hormone and stops cell division and initiates differentiation at the appropriate time. The cell-cycle inhibitor p27/Kip1 accumulates in the precursor cells as they proliferate and is part of both components of the timer. It seems likely that similar timing mechanisms operate in other cell lineages. BioEssays 22:64-71, 2000.     

Ephrin-Eph signalling drives the asymmetric division of notochord/neural precursors in Ciona embryos

Asymmetric cell divisions produce two sibling cells with distinct fates, providing an important means of generating cell diversity in developing embryos. Many examples of such cell divisions have been described, but so far only a limited number of the underlying mechanisms have been elucidated. Here, we have uncovered a novel mechanism controlling an asymmetric cell division in the ascidian embryo. This division produces one notochord and one neural precursor. Differential activation of extracellular-signal-regulated kinase (ERK) between the sibling cells determines their distinct fates, with ERK activation promoting notochord fate. We first demonstrate that the segregation of notochord and neural fates is an autonomous property of the mother cell and that the mother cell acquires this functional polarity via interactions with neighbouring ectoderm precursors. We show that these cellular interactions are mediated by the ephrin-Eph signalling system, previously implicated in controlling cell movement and adhesion. Disruption of contacts with the signalling cells or inhibition of the ephrin-Eph signal results in the symmetric division of the mother cell, generating two notochord precursors. Finally, we demonstrate that the ephrin-Eph signal acts via attenuation of ERK activation in the neural-fated daughter cell. We propose a model whereby directional ephrin-Eph signals functionally polarise the notochord/neural mother cell, leading to asymmetric modulation of the FGF-Ras-ERK pathway between the daughter cells and, thus, to their differential fate specification.     

Patterns of cell division and expression of asymmetric cell fate determinants in postembryonic neuroblast lineages of Drosophila

We have studied the division of postembryonic neuroblasts (Nbs) in the outer proliferation center (OPC) and central brain anlagen of Drosophila. We focused our attention on three aspects of these processes: the pattern of cellular division, the topological orientation of those divisions, and the expression of asymmetric cell fate determinants. Although larval Nbs are of embryonic origin, our results indicate that their properties appear to be modified during development. Several conclusions can be summarized: (i) In early larvae, Nbs divide symmetrically to give rise to two Nbs while in the late larval brain most Nbs divide asymmetrically to bud off an intermediate ganglion mother cell (GMC) that very rapidly divides into two ganglion cells (GC). (ii) Symmetric and asymmetric divisions of OPC Nbs show tangential and radial orientations, respectively. (iii) This change in the pattern of division correlates with the expression of inscuteable, which is apically localized only in asymmetric divisions. (iv) The spindle of asymmetrically dividing Nb is always oriented on an apical-basal axis. (v) Prospero does not colocalize with Miranda in the cortical crescent of mitotic Nbs. (vi) Prospero is transiently expressed in one of the two sibling GCs generated by the division of GMCs. The implications of these results on cell fate specification and differentiation of adult brain neurons are discussed.     

Retinoic acid modulates the pattern of cell division in embryos ofLymnaea stagnalis (Mollusca)

All-trans retinoic acid is well known as a modulator of positional specification in vertebrate development. A similar mechanism may operate in molluscan development. Molluscan development is characterized by an invariant pattern of cell divisions, which allows the study of individual cells in the developing organism. Low concentrations of exogenous retinoic acid applied during gastrulation affect the cell division pattern in the early larval stage of the molluscLymnaea stagnalis. A few cells from the apical plate, a larval organ consisting of seven large cleavage-arrested cells, were induced by retinoic acid to resume cell division. They typically formed an area of proliferating small cells that resembles the adjacent areas of precursor cells of adult ectoderm. The identification of individual cells that are transformed by retinoic acid may provide new insights into the mechanisms underlying positional specification within the embryo.     

Eya1 acts upstream of Tbx1, Neurogenin 1, NeuroD and the neurotrophins BDNF and NT-3 during inner ear development

Cell fate specification during inner ear development is dependent upon regional gene expression within the otic vesicle. One of the earliest cell fate determination steps in this system is the specification of neural precursors, and regulators of this process include the Atonal-related basic helix-loop-helix genes, Ngn1 and NeuroD and the T-box gene, Tbx1. In this study we demonstrate that Eya1 signaling is critical to the normal expression patterns of Tbx1, Ngn1, and NeuroD in the developing mouse otocyst. We discuss a potential mechanism for the absence of neural precursors in the Eya1-/- inner ears and the primary and secondary mechanisms for the loss of cochleovestibular ganglion cells in the Eya1bor/bor hypomorphic mutant.     

Sterols are required for cell‐fate commitment and maintenance of the stomatal lineage in Arabidopsis

Asymmetric cell division is important for regulating cell proliferation and fate determination during stomatal development in plants. Although genes that control asymmetric division and cell differentiation in stomatal development have been reported, regulators controlling the process from asymmetric division to cell differentiation remain poorly understood. Here, we report a weak allele (fk–J3158) of the Arabidopsis sterol C–14 reductase gene FACKEL (FK) that shows clusters of small cells and stomata in leaf epidermis, a common phenomenon that is often seen in mutants defective in stomatal asymmetric division. Interestingly, the physical asymmetry of these divisions appeared to be intact in fk mutants, but the cell‐fate asymmetry was greatly disturbed, suggesting that the FK pathway links these two crucial events in the process of asymmetric division. Sterol profile analysis revealed that the fk–J3158 mutation blocked downstream sterol production. Further investigation indicated that cyclopropylsterol isomerase1 (cpi1), sterol 14α–demethylase (cyp51A2) and hydra1 (hyd1) mutants, corresponding to enzymes in the same branch of the sterol biosynthetic pathway, displayed defective stomatal development phenotypes, similar to those observed for fk. Fenpropimorph, an inhibitor of the FK sterol C–14 reductase in Arabidopsis, also caused these abnormal small‐cell and stomata phenotypes in wild‐type leaves. Genetic experiments demonstrated that sterol biosynthesis is required for correct stomatal patterning, probably through an additional signaling pathway that has yet to be defined. Detailed analyses of time‐lapse cell division patterns, stomatal precursor cell division markers and DNA ploidy suggest that sterols are required to properly restrict cell proliferation, asymmetric fate specification, cell‐fate commitment and maintenance in the stomatal lineage cells. These events occur after physical asymmetric division of stomatal precursor cells.     

Bacterial filaments recover by successive and accelerated asymmetric divisions that allow rapid post-stress cell proliferation

Filamentation is a reversible morphological change triggered in response to various stresses that bacteria might encounter in the environment, during host infection or antibiotic treatments. Here we re-visit the dynamics of filament formation and recovery using a consistent framework based on live-cells microscopy. We compare the fate of filamentous Escherichia coli induced by cephalexin that inhibits cell division or by UV-induced DNA-damage that additionally perturbs chromosome segregation. We show that both filament types recover by successive and accelerated rounds of divisions that preferentially occur at the filaments' tip, thus resulting in the rapid production of multiple daughter cells with tightly regulated size. The DNA content, viability and further division of the daughter cells essentially depends on the coordination between chromosome segregation and division within the mother filament. Septum positioning at the filaments' tip depends on the Min system, while the nucleoid occlusion protein SlmA regulates the timing of division to prevent septum closure on unsegregated chromosomes. Our results not only recapitulate earlier conclusions but provide a higher level of detail regarding filaments division and the fate of the daughter cells. Together with previous reports, this work uncovers how filamentation recovery allows for a rapid cell proliferation after stress treatment.     

Division patterns in the thallus ofPelvetia compressa embryos and the effects of gravity

Summary The pattern of divisions in the thallus ofPelvetia compressa embryos was determined with respect to the embryonic growth axis. To detect all possible division planes, embryos were viewed from two vantages which permitted observations of (1) the thallus pole and (2) the longitudinal embryonic profile. Following formation of rhizoid and thallus cells by any asymmetrical division transverse to the embryonic axis that is established prior to any divisions, the thallus cell divided twice along the embryonic axis (axial divisions) in orthogonal planes, and then divided transverse to the growth axis. This division pattern produced an eight-cell thallus with four cells in each of two layers. The spatial relation between gravity and the first axial division was investigated, and gravity was found to have little effect on the alignment of this division. The reproducible pattern of divisions in the thallus indicates spatial control of spindle positioning.Abbreviations ASW artificial seawater - AF after fertilization     

Identification of genes driving lineage divergence from single-cell gene expression data in C. elegans

The nematode Caenorhabditis elegans (C. elegans) is an ideal model organism to study the cell fate specification mechanisms during embryogenesis. It is generally believed that cell fate specification in C. elegans is mainly mediated by lineage-based mechanisms, where the specification paths are driven forward by a succession of asymmetric cell divisions. However, little is known about how each binary decision is made by gene regulatory programs. In this study, we endeavor to obtain a global understanding of cell lineage/fate divergence processes during the early embryogenesis of C. elegans. We reanalyzed the EPIC data set, which traced the expression level of reporter genes at single-cell resolution on a nearly continuous time scale up to the 350-cell stage in C. elegans embryos. We examined the expression patterns for a total of 131 genes from 287 embryos with high quality image recordings, among which 86 genes have replicate embryos. Our results reveal that during early embryogenesis, divergence between sister lineages could be largely explained by a few genes. We predicted genes driving lineage divergence and explored their expression patterns in sister lineages. Moreover, we found that divisions leading to fate divergence are associated with a large number of genes being differentially expressed between sister lineages. Interestingly, we found that the developmental paths of lineages could be differentiated by a small set of genes. Therefore, our results support the notion that the cell fate patterns in C. elegans are achieved through stepwise binary decisions punctuated by cell divisions. Our predicted genes driving lineage divergence provide good starting points for future detailed characterization of their roles in the embryogenesis in this important model organism.     

The cell cycle of<Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>: the role of the commitment point

Chlamydomonas reinhardtii cells can double their size several times during the light period before they enter the division phase. To explain the role of the commitment point (defined as the moment in the cell cycle after which cells can complete the cell cycle independently of light) and the moment of initiation of cell division we investigated whether the timing of commitment to cell division and cell division itself are dependent upon cell size or if they are under control of a timer mechanism that measures a period of constant duration. The time point at which cells attain commitment to cell division was dependent on the growth rate and coincided with the moment at which cells have approximately doubled in size. The timing of cell division was temperature-dependent and took place after a period of constant duration from the onset of the light period, irrespective of the light intensity and timing of the commitment point. We concluded that at the commitment point all the prerequisites are checked, which is required for progression through the cell cycle; the commitment point is not the moment at which cell division is initiated but it functions as a checkpoint, which ensures that cells have passed the minimum cell size required for the cell division.     

Widespread RNA segregation in a spiralian embryo

Asymmetric cell divisions are a crucial mode of cell fate specification in multicellular organisms, but their relative contribution to early embryonic patterning varies among taxa. In the embryo of the mollusc Ilyanassa, most of the early cell divisions are overtly asymmetric. During Ilyanassa early cleavage, mRNAs for several conserved developmental patterning genes localize to interphase centrosomes, and then during division they move to a portion of the cortex that will be inherited by one daughter cell. Here we report an unbiased survey of RNA localization in the Ilyanassa embryo, and examine the overall patterns of centrosomal localization during early development. We find that 3-4% of RNAs are specifically localized to centrosomes during early development, and the remainder are either ubiquitously distributed throughout the cytoplasm or weakly enriched on centrosomes compared with levels in the cytoplasm. We observe centrosomal localization of RNAs in all cells from zygote through the fifth cleavage cycle, and asymmetric RNA segregation in all divisions after the four-cell stage. Remarkably, each specifically localized message is found on centrosomes in a unique subset of cells during early cleavages, and most are found in unique sets of cells at the 24-cell stage. Several specifically localized RNAs are homologous to developmental regulatory proteins in other embryos. These results demonstrate that the mechanisms of localization and segregation are extraordinarily intricate in this system, and suggest that these events are involved in cell fate specification across all lineages in the early Ilyanassa embryo. We propose that greater reliance on segregation of determinants in early cleavage increases constraint on cleavage patterns in molluscs and other spiralian groups.