The mother centriole plays an instructive role in defining cell geometry

A "poly-matter network" conception of biological inheritance

Here we intend to shift the "DNA- and information-centric" conception of biological inheritance, with the accompanying exclusion of any non-DNA matter, to a "poly-matter network" framework which, in addition to DNA, considers the action of other cellular membranous constituents. These cellular structures, in particular organelles and plasma membranes, express "landscapes" of specific topologies at their surfaces, which may become altered in response to certain environmental factors. These so-called "membranous environmental landscapes" (MELs), which replicate by self-organization / autopoiesis rather than self-assembly, are transferred from donor to acceptor cells by various - vesicular and non-vesicular - mechanisms and exert novel features in the acceptor cells. The "DNA-centric" conception may be certainly explanatorily sufficient for the transfer of heritable phenotype variation to acceptor cells following the copying of DNA in donor cells and thereby for the phenomenon of biological inheritance of traits. However, it is not causally sufficient. With the observation of phenotype variation, as initially manifested during bacterial transformation, the impact of environmental factors, such as nutrition and stress, in the differential regulation of gene expression has been widely accepted and resulted in intense efforts to resolve the underlying epigenetic mechanisms. However, these are explained under a conceptual frame where the DNA (and associated proteins) are the only matter of inheritance. In contrast, it is our argumentation that inheritance can only be adequately understood as the transfer of DNA in concert with non-DNA matter in a "poly-matter network" conception. The adequate inclusion of the transfer of non-DNA matter is still a desideratum of future genetic research, which may pave the way for the experimental elucidation not only of how DNA and membrane matter act in concert to enable the inheritance of innate traits, but also whether they interact for that of acquired biological traits. Moreover, the "poly-matter network" conception may open new perspectives for an understanding of the pathogenesis of "common complex" diseases.

Chlamydomonas as a model system to study cilia and flagella using genetics, biochemistry, and microscopy

The unicellular green alga, Chlamydomonas reinhardtii, has played a central role in discovering much of what is currently known about the composition, assembly, and function of cilia and flagella. Chlamydomonas combines excellent genetics, such as the ability to grow cells as haploids or diploids and to perform tetrad analysis, with an unparalleled ability to detach and isolate flagella in a single step without cell lysis. The combination of genetics and biochemistry that is possible in Chlamydomonas has allowed many of the key components of the cilium to be identified by looking for proteins that are missing in a defined mutant. Few if any other model organisms allow such a seamless combination of genetic and biochemical approaches. Other major advantages of Chlamydomonas compared to other systems include the ability to induce flagella to regenerate in a highly synchronous manner, allowing the kinetics of flagellar growth to be measured, and the ability of Chlamydomonas flagella to adhere to glass coverslips allowing Intraflagellar Transport to be easily imaged inside the flagella of living cells, with quantitative precision and single-molecule resolution. These advantages continue to work in favor of Chlamydomonas as a model system going forward, and are now augmented by extensive genomic resources, a knockout strain collection, and efficient CRISPR gene editing. While Chlamydomonas has obvious limitations for studying ciliary functions related to animal development or organ physiology, when it comes to studying the fundamental biology of cilia and flagella, Chlamydomonas is simply unmatched in terms of speed, efficiency, cost, and the variety of approaches that can be brought to bear on a question.

Two metrics on rooted unordered trees with labels

Background

The early development of a zygote can be mathematically described by a developmental tree. To compare developmental trees of different species, we need to define distances on trees. If children cells after a division are not distinguishable, developmental trees are represented by the space \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {T}}$$\end{document}T of rooted trees with possibly repeated labels, where all vertices are unordered. If children cells after a division are partially distinguishable, developmental trees are represented by the space \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {P}}$$\end{document}P of rooted trees with possibly repeated labels, where vertices can be ordered or unordered.

Results

On \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {T}}$$\end{document}T, the space of rooted unordered trees with possibly repeated labels, we define two metrics: the best-match metric and the left-regular metric, which show some advantages over existing methods. On \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {P}}$$\end{document}P, the space of rooted labeled trees with ordered or unordered vertices, there is no metric, and we define a semimetric, which is a variant of the best-match metric. To compute the best-match distance between two trees, the expected time complexity and worst-case time complexity are both \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {O}}(n^2)$$\end{document}O(n2), where n is the tree size. To compute the left-regular distance between two trees, the expected time complexity is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {O}}(n)$$\end{document}O(n), and the worst-case time complexity is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {O}}(n\log n)$$\end{document}O(nlogn).

Conclusions

For rooted labeled trees with (fully/partially) unordered vertices, we define metrics (semimetric) that have fast algorithms to compute and have advantages over existing methods. Such trees also appear outside of developmental biology, and such metrics can be applied to other types of trees which have more extensive applications, especially in molecular biology.

Evolutionary conservation of centriole rotational asymmetry in the human centrosome

Centrioles are formed by microtubule triplets in a nine-fold symmetric arrangement. In flagellated protists and in animal multiciliated cells, accessory structures tethered to specific triplets render the centrioles rotationally asymmetric, a property that is key to cytoskeletal and cellular organization in these contexts. In contrast, centrioles within the centrosome of animal cells display no conspicuous rotational asymmetry. Here, we uncover rotationally asymmetric molecular features in human centrioles. Using ultrastructure expansion microscopy, we show that LRRCC1, the ortholog of a protein originally characterized in flagellate green algae, associates preferentially to two consecutive triplets in the distal lumen of human centrioles. LRRCC1 partially co-localizes and affects the recruitment of another distal component, C2CD3, which also has an asymmetric localization pattern in the centriole lumen. Together, LRRCC1 and C2CD3 delineate a structure reminiscent of a filamentous density observed by electron microscopy in flagellates, termed the 'acorn'. Functionally, the depletion of LRRCC1 in human cells induced defects in centriole structure, ciliary assembly and ciliary signaling, supporting that LRRCC1 cooperates with C2CD3 to organizing the distal region of centrioles. Since a mutation in the LRRCC1 gene has been identified in Joubert syndrome patients, this finding is relevant in the context of human ciliopathies. Taken together, our results demonstrate that rotational asymmetry is an ancient property of centrioles that is broadly conserved in human cells. Our work also reveals that asymmetrically localized proteins are key for primary ciliogenesis and ciliary signaling in human cells.

NudCL2 is an autophagy receptor that mediates selective autophagic degradation of CP110 at mother centrioles to promote ciliogenesis

Primary cilia extending from mother centrioles are essential for vertebrate development and homeostasis maintenance. Centriolar coiled-coil protein 110 (CP110) has been reported to suppress ciliogenesis initiation by capping the distal ends of mother centrioles. However, the mechanism underlying the specific degradation of mother centriole-capping CP110 to promote cilia initiation remains unknown. Here, we find that autophagy is crucial for CP110 degradation at mother centrioles after serum starvation in MEF cells. We further identify NudC-like protein 2 (NudCL2) as a novel selective autophagy receptor at mother centrioles, which contains an LC3-interacting region (LIR) motif mediating the association of CP110 and the autophagosome marker LC3. Knockout of NudCL2 induces defects in the removal of CP110 from mother centrioles and ciliogenesis, which are rescued by wild-type NudCL2 but not its LIR motif mutant. Knockdown of CP110 significantly attenuates ciliogenesis defects in NudCL2-deficient cells. In addition, NudCL2 morphants exhibit ciliation-related phenotypes in zebrafish, which are reversed by wild-type NudCL2, but not its LIR motif mutant. Importantly, CP110 depletion significantly reverses these ciliary phenotypes in NudCL2 morphants. Taken together, our data suggest that NudCL2 functions as an autophagy receptor mediating the selective degradation of mother centriole-capping CP110 to promote ciliogenesis, which is indispensable for embryo development in vertebrates.

Pattern Formation and Complexity in Single Cells

In the context of animal or plant development, we tend to think of cells as small, simple, building blocks, such that complex patterns or shapes can only be constructed from large numbers of cells, with cells in different parts of the organism taking on different fates. However, cells themselves are far from simple, and often take on complex shapes with a remarkable degree of intracellular patterning. How do these patterns arise? As in embryogenesis, the development of structure inside a cell can be broken down into a number of basic processes. For each part of the cell, morphogenetic processes create internal structures such as organelles, which might correspond to organs at the level of a whole organism. Given that mechanisms exist to generate parts, patterning processes are required to ensure that the parts are distributed in the correct arrangement relative to the rest of the cell. Such patterning processes make reference to global polarity axes, requiring mechanisms for axiation which, in turn, require processes to break symmetry. These fundamental processes of symmetry breaking, axiation, patterning, and morphogenesis have been extensively studied in developmental biology but less so at the subcellular level. This review will focus on developmental processes that give eukaryotic cells their complex structures, with a focus on cytoskeletal organization in free-living cells, ciliates in particular, in which these processes are most readily apparent.

Analysis of biological noise in the flagellar length control system

Any proposed mechanism for organelle size control should be able to account not only for average size but also for the variation in size. We analyzed cell-to-cell variation and within-cell variation of length for the two flagella in Chlamydomonas, finding that cell-to-cell variation is dominated by cell size, whereas within-cell variation results from dynamic fluctuations. Fluctuation analysis suggests tubulin assembly is not directly coupled with intraflagellar transport (IFT) and that the observed length fluctuations reflect tubulin assembly and disassembly events involving large numbers of tubulin dimers. Length variation is increased in long-flagella mutants, an effect consistent with theoretical models for flagellar length regulation. Cells with unequal flagellar lengths show impaired swimming but improved gliding, raising the possibility that cells have evolved mechanisms to tune biological noise in flagellar length. Analysis of noise at the level of organelle size provides a way to probe the mechanisms determining cell geometry.

Archaeal Origins of Eukaryotic Cell and Nucleus

Symbiosis is a major evolutionary force, especially at the cellular level. Here we discuss several older and new discoveries suggesting that besides mitochondria and plastids, eukaryotic nuclei also have symbiotic origins. We propose an archaea-archaea scenario for the evolutionary origin of the eukaryotic cells. We suggest that two ancient archaea-like cells, one based on the actin cytoskeleton and another one based on the tubulin-centrin cytoskeleton, merged together to form the first nucleated eukaryotic cell. This archaeal endosymbiotic origin of eukaryotic cells and their nuclei explains several features of eukaryotic cells which are incompatible with the currently preferred autogenous scenarios of eukaryogenesis.

Centering and Shifting of Centrosomes in Cells

Centrosomes have a nonrandom localization in the cells: either they occupy the centroid of the zone free of the actomyosin cortex or they are shifted to the edge of the cell, where their presence is justified from a functional point of view, for example, to organize additional microtubules or primary cilia. This review discusses centrosome placement options in cultured and in situ cells. It has been proven that the central arrangement of centrosomes is due mainly to the pulling microtubules forces developed by dynein located on the cell cortex and intracellular vesicles. The pushing forces from dynamic microtubules and actomyosin also contribute, although the molecular mechanisms of their action have not yet been elucidated. Centrosomal displacement is caused by external cues, depending on signaling, and is drawn through the redistribution of dynein, the asymmetrization of microtubules through the capture of their plus ends, and the redistribution of actomyosin, which, in turn, is associated with basal-apical cell polarization.

Positioning of the Centrosome and Golgi Complex

For over a century, the centrosome has been an organelle more easily tracked than understood, and the study of its peregrinations within the cell remains a chief underpinning of its functional investigation. Increasing attention and new approaches have been brought to bear on mechanisms that control centrosome localization in the context of cleavage plane determination, ciliogenesis, directional migration, and immunological synapse formation, among other cellular and developmental processes. The Golgi complex, often linked with the centrosome, presents a contrasting case of a pleiomorphic organelle for which functional studies advanced somewhat more rapidly than positional tracking. However, Golgi orientation and distribution has emerged as an area of considerable interest with respect to polarized cellular function. This chapter will review our current understanding of the mechanism and significance of the positioning of these organelles.

The Centrosome as a Geometry Organizer

'Does the geometric design of centrioles imply their function? Several principles of construction of a microscopically small device for locating the directions of signal sources in microscopic dimensions: it appears that the simplest and smallest device that is compatible with the scrambling influence of thermal fluctuations, as are demonstrated by Brownian motion, is a pair of cylinders oriented at right angles to each other. Centrioles locate the direction of hypothetical signals inside cells' (Albrecht-Buehler G, Cell Motil, 1:237-245; 1981).Despite a century of devoted efforts (articles on the centrosome always begin like this) its role remains vague and nebulous: does the centrosome suffer from bad press? Likely it does, it has an unfair image problem. It is dispensable in mitosis, but a fly zygote, artificially deprived of centrosomes, cannot start its development; its sophisticated architecture (200 protein types, highly conserved during evolution) constitutes an enigmatic puzzle; centrosome reduction in gametogenesis is a challenging brainteaser; its duplication cycle (only one centrosome per cell) is more complicated than chromosomes. Its striking geometric design (two ninefold symmetric orthogonal centrioles) shows an interesting correspondence with the requirements of a cellular compass: a reference system organizer based on a pair of orthogonal goniometers; through its two orthogonal centrioles, the centrosome may play the role of a cell geometry organizer: it can establish a finely tuned geometry, inherited and shared by all cells. Indeed, a geometrical and informational primary role for the centrosome has been ascertained in Caenorhabditis elegans zygote: the sperm centrosome locates its polarity factors. The centrosome, through its aster of microtubules, possesses all the characteristics necessary to operate as a biophysical geometric compass: it could recognize cargoes equipped with topogenic sequences and drive them precisely to where they are addressed (as hypothesized by Albrecht-Buehler nearly 40 years ago). Recently, this geometric role of the centrosome has been rediscovered by two important findings; in the Kupffer's vesicle (the laterality organ of zebrafish), chiral cilia orientation and rotational movement have been described: primary cilia, in left and right halves of the Kupffer's vesicle, are symmetrically oriented relative to the midline and rotate in reverse direction. In mice node (laterality organ) left and right perinodal cells can distinguish flow directionality through their primary cilia: primary cilium, ninefold symmetric, is strictly connected to the centrosome that is located immediately under it (basal body). Kupffer's vesicle histology and mirror behaviour of mice perinodal cells suggest primary cilia are enantiomeric geometric organelles. What is the meaning of the geometric design of centrioles and centrosomes? Does it imply their function?

Coordination of eukaryotic cilia and flagella

Propulsion by slender cellular appendages called cilia and flagella is an ancient means of locomotion. Unicellular organisms evolved myriad strategies to propel themselves in fluid environments, often involving significant differences in flagella number, localisation and modes of actuation. Remarkably, these appendages are highly conserved, occurring in many complex organisms such as humans, where they may be found generating physiological flows when attached to surfaces (e.g. airway epithelial cilia), or else conferring motility to male gametes (e.g. undulations of sperm flagella). Where multiple cilia arise, their movements are often observed to be highly coordinated. Here I review the two main mechanisms for motile cilia coordination, namely, intracellular and hydrodynamic, and discuss their relative importance in different ciliary systems.

Integration of intracellular signaling: Biological analogues of wires, processors and memories organized by a centrosome 3D reference system

BACKGROUND

Myriads of signaling pathways in a single cell function to achieve the highest spatio-temporal integration. Data are accumulating on the role of electromechanical soliton-like waves in signal transduction processes. Theoretical studies strongly suggest feasibility of both classical and quantum computing involving microtubules.

AIM

A theoretical study of the role of the complex composed of the plasma membrane and the microtubule-based cytoskeleton as a system that transmits, stores and processes information.

METHODS

Theoretical analysis presented here refers to (i) the Penrose-Hameroff theory of consciousness (Orchestrated Objective Reduction; Orch OR), (ii) the description of the centrosome as a reference system for construction of the 3D map of the cell proposed by Regolini, (iii) the Heimburg-Jackson model of the nerve pulse propagation along axons' lipid bilayer as soliton-like electro-mechanical waves.

RESULTS AND CONCLUSION

The ideas presented in this paper provide a qualitative model for the decision-making processes in a living cell undergoing a differentiation process.

OUTLOOK

This paper paves the way for the real-time live-cell observation of information processing by microtubule-based cytoskeleton and cell fate decision making.

Cell polarity: having and making sense of direction-on the evolutionary significance of the primary cilium/centrosome organ in Metazoa

Cell-autonomous polarity in Metazoans is evolutionarily conserved. I assume that permanent polarity in unicellular eukaryotes is required for cell motion and sensory reception, integration of these two activities being an evolutionarily constrained function. Metazoans are unique in making cohesive multicellular organisms through complete cell divisions. They evolved a primary cilium/centrosome (PC/C) organ, ensuring similar functions to the basal body/flagellum of unicellular eukaryotes, but in different cells, or in the same cell at different moments. The possibility that this innovation contributed to the evolution of individuality, in being instrumental in the early specification of the germ line during development, is further discussed. Then, using the example of highly regenerative organisms like planarians, which have lost PC/C organ in dividing cells, I discuss the possibility that part of the remodelling necessary to reach a new higher-level unit of selection in multi-cellular organisms has been triggered by conflicts among individual cell polarities to reach an organismic polarity. Finally, I briefly consider organisms with a sensorimotor organ like the brain that requires exceedingly elongated polarized cells for its activity. I conclude that beyond critical consequences for embryo development, the conservation of cell-autonomous polarity in Metazoans had far-reaching implications for the evolution of individuality.

Bothrosome Formation in Schizochytrium aggregatum (Labyrinthulomycetes, Stramenopiles) during Zoospore Settlement

Labyrinthulomycetes are characterized by the presence of ectoplasmic nets originating from an organelle known as the bothrosome, whose evolutionary origin is unclear. To address this issue, we investigated the developmental process from a zoospore to a vegetative cell in Schizochytrium aggregatum. After disappearance of the flagellum during zoospore settlement, the bothrosome emerged at the anterior-ventral pole of the cells. A new Golgi body also appeared at this stage, and the bothrosome was positioned close to both the new and the old Golgi bodies. This observation suggested that the Golgi body is related to the formation of the bothrosome. Actin appeared as a spot in the same location as the newly appeared bothrosome, as determined by immunofluorescence labeling. An immunoelectron microscopic analysis revealed that actin was present in the ectoplasmic nets and in the cytoplasm around the bothrosome, indicating that the electron-dense materials of the bothrosome are not the polar center of F-actin. This suggests that actin filaments pull the endoplasmic reticulum to the bothrosome and induce the membrane to become evaginated within ectoplasmic nets.

Centrosome positioning in non-dividing cells

Centrioles and centrosomes are found in almost all eukaryotic cells, where they are important for organising the microtubule cytoskeleton in both dividing and non-dividing cells. The spatial location of centrioles and centrosomes is tightly controlled and, in non-dividing cells, plays an important part in cell migration, ciliogenesis and immune cell functions. Here, we examine some of the ways that centrosomes are connected to other organelles and how this impacts on cilium formation, cell migration and immune cell function in metazoan cells.

Jessica Feldman: Microtubule-organizing function dives off centrosomes

Feldman studies how cell patterning and cytoskeletal organization are controlled.

Quantitative analysis and modeling of katanin function in flagellar length control

A mutation in a microtubule-severing enzyme, katanin, causes flagella to become short due to a reduced cytoplasmic precursor pool. These results suggest that competition between flagella and cytoplasmic microtubules for a limited tubulin pool is facilitated by katanin, which is confirmed by stochastic models.

Flagellar length control in Chlamydomonas reinhardtii provides a simple model system in which to investigate the general question of how cells regulate organelle size. Previous work demonstrated that Chlamydomonas cytoplasm contains a pool of flagellar precursor proteins sufficient to assemble a half-length flagellum and that assembly of full-length flagella requires synthesis of additional precursors to augment the preexisting pool. The regulatory systems that control the synthesis and regeneration of this pool are not known, although transcriptional regulation clearly plays a role. We used quantitative analysis of length distributions to identify candidate genes controlling pool regeneration and found that a mutation in the p80 regulatory subunit of katanin, encoded by the PF15 gene in Chlamydomonas, alters flagellar length by changing the kinetics of precursor pool utilization. This finding suggests a model in which flagella compete with cytoplasmic microtubules for a fixed pool of tubulin, with katanin-mediated severing allowing easier access to this pool during flagellar assembly. We tested this model using a stochastic simulation that confirms that cytoplasmic microtubules can compete with flagella for a limited tubulin pool, showing that alteration of cytoplasmic microtubule severing could be sufficient to explain the effect of the pf15 mutations on flagellar length.

Choosing sides--asymmetric centriole and basal body assembly

Centrioles and basal bodies (CBBs) are microtubule-rich cylindrical structures that nucleate and organize centrosomes and cilia, respectively. Despite their apparent ninefold rotational symmetry, the nine sets of triplet microtubules in CBBs possess asymmetries in their morphology and in the structures that associate with them. These asymmetries define the position of nascent CBB assembly, the orientation of ciliary beating, the orientation of spindle poles and the maintenance of cellular geometry. For some of these functions, the orientation of CBBs is first established during new CBB biogenesis when the daughter structure is positioned adjacent to the mother. The mother CBB organizes the surrounding environment that nascent CBBs are born into, thereby providing a nest for the new CBB to develop. Protists, including ciliates and algae, highlight the importance of this environment with the formation of asymmetrically placed scaffolds onto which new basal bodies assemble and are positioned. Recent studies illuminate the positioning of nascent centrioles relative to a modular pericentriolar material (PCM) environment and suggest that, like ciliates, centrosomes organize an immediate environment surrounding centrioles for their biogenesis and positioning. In this Commentary, I will explore the positioning of nascent CBB assembly as the first event in building cellular asymmetries and describe how the environment surrounding both basal bodies and centrioles may define asymmetric assembly.

A high-resolution morphological and ultrastructural map of anterior sensory cilia and glia in Caenorhabditis elegans

Many primary sensory cilia exhibit unique architectures that are critical for transduction of specific sensory stimuli. Although basic ciliogenic mechanisms are well described, how complex ciliary structures are generated remains unclear. Seminal work performed several decades ago provided an initial but incomplete description of diverse sensory cilia morphologies in C. elegans. To begin to explore the mechanisms that generate these remarkably complex structures, we have taken advantage of advances in electron microscopy and tomography, and reconstructed three-dimensional structures of fifty of sixty sensory cilia in the C. elegans adult hermaphrodite at high resolution. We characterize novel axonemal microtubule organization patterns, clarify structural features at the ciliary base, describe new aspects of cilia-glia interactions, and identify structures suggesting novel mechanisms of ciliary protein trafficking. This complete ultrastructural description of diverse cilia in C. elegans provides the foundation for investigations into underlying ciliogenic pathways, as well as contributions of defined ciliary structures to specific neuronal functions. DOI: http://dx.doi.org/10.7554/eLife.01948.001.

The awesome power of dikaryons for studying flagella and basal bodies in Chlamydomonas reinhardtii

Cilia/flagella and basal bodies/centrioles play key roles in human health and homeostasis. Among the organisms used to study these microtubule-based organelles, the green alga Chlamydomonas reinhardtii has several advantages. One is the existence of a temporary phase of the life cycle, termed the dikaryon. These cells are formed during mating when the cells fuse and the behavior of flagella from two genetically distinguishable parents can be observed. During this stage, the cytoplasms mix allowing for a defect in the flagella of one parent to be rescued by proteins from the other parent. This offers the unique advantage of adding back wild-type gene product or labeled protein at endogenous levels that can used to monitor various flagellar and basal body phenotypes. Mutants that show rescue and ones that fail to show rescue are both informative about the nature of the flagella and basal body defects. When rescue occurs, it can be used to determine the mutant gene product and to follow the temporal and spatial patterns of flagellar assembly. This review describes many examples of insights into basal body and flagellar proteins' function and assembly that have been discovered using dikaryons and discusses the potential for further analyses.

Site-specific basal body duplication in Chlamydomonas

Correct centriole/basal body positioning is required for numerous biological processes, yet how the cell establishes this positioning is poorly understood. Analysis of centriolar/basal body duplication provides a key to understanding basal body positioning and function. Chlamydomonas basal bodies contain structural features that enable specific triplet microtubules to be specified. Electron tomography of cultures enriched in mitotic cells allowed us to follow basal body duplication and identify a specific triplet at which duplication occurs. Probasal bodies elongate in prophase, assemble transitional fibers (TF) and are segregated with a mature basal body near the poles of the mitotic spindle. A ring of nine-singlet microtubules is initiated at metaphase, orthogonal to triplet eight. At telophase/cytokinesis, triplet microtubule blades assemble first at the distal end, rather than at the proximal cartwheel. The cartwheel undergoes significant changes in length during duplication, which provides further support for its scaffolding role. The uni1-1 mutant contains short basal bodies with reduced or absent TF and defective transition zones, suggesting that the UNI1 gene product is important for coordinated probasal body elongation and maturation. We suggest that this site-specific basal body duplication ensures the correct positioning of the basal body to generate landmarks for intracellular patterning in the next generation.

It's never too early to get it Right: A conserved role for the cytoskeleton in left-right asymmetry

For centuries, scientists and physicians have been captivated by the consistent left-right (LR) asymmetry of the heart, viscera, and brain. A recent study implicated tubulin proteins in establishing laterality in several experimental models, including asymmetric chemosensory receptor expression in C. elegans neurons, polarization of HL-60 human neutrophil-like cells in culture, and asymmetric organ placement in Xenopus. The same mutations that randomized asymmetry in these diverse systems also affect chirality in Arabidopsis, revealing a remarkable conservation of symmetry-breaking mechanisms among kingdoms. In Xenopus, tubulin mutants only affected LR patterning very early, suggesting that this axis is established shortly after fertilization. This addendum summarizes and extends the knowledge of the cytoskeleton's role in the patterning of the LR axis. Results from many species suggest a conserved role for the cytoskeleton as the initiator of asymmetry, and indicate that symmetry is first broken during early embryogenesis by an intracellular process.

A unified model for left-right asymmetry? Comparison and synthesis of molecular models of embryonic laterality

Understanding how and when the left-right (LR) axis is first established is a fundamental question in developmental biology. A popular model is that the LR axis is established relatively late in embryogenesis, due to the movement of motile cilia and the resultant directed fluid flow during late gastrulation/early neurulation. Yet, a large body of evidence suggests that biophysical, molecular, and bioelectrical asymmetries exist much earlier in development, some as early as the first cell cleavage after fertilization. Alternative models of LR asymmetry have been proposed that accommodate these data, postulating that asymmetry is established due to a chiral cytoskeleton and/or the asymmetric segregation of chromatids. There are some similarities, and many differences, in how these various models postulate the origin and timing of symmetry breaking and amplification, and these events' linkage to the well-conserved subsequent asymmetric transcriptional cascades. This review examines experimental data that lend strong support to an early origin of LR asymmetry, yet are also consistent with later roles for cilia in the amplification of LR pathways. In this way, we propose that the various models of asymmetry can be unified: early events are needed to initiate LR asymmetry, and later events could be utilized by some species to maintain LR-biases. We also present an alternative hypothesis, which proposes that individual embryos stochastically choose one of several possible pathways with which to establish their LR axis. These two hypotheses are both tractable in appropriate model species; testing them to resolve open questions in the field of LR patterning will reveal interesting new biology of wide relevance to developmental, cell, and evolutionary biology.

Centrosome positioning in vertebrate development

The centrosome, a major organizer of microtubules, has important functions in regulating cell shape, polarity, cilia formation and intracellular transport as well as the position of cellular structures, including the mitotic spindle. By means of these activities, centrosomes have important roles during animal development by regulating polarized cell behaviors, such as cell migration or neurite outgrowth, as well as mitotic spindle orientation. In recent years, the pace of discovery regarding the structure and composition of centrosomes has continuously accelerated. At the same time, functional studies have revealed the importance of centrosomes in controlling both morphogenesis and cell fate decision during tissue and organ development. Here, we review examples of centrosome and centriole positioning with a particular emphasis on vertebrate developmental systems, and discuss the roles of centrosome positioning, the cues that determine positioning and the mechanisms by which centrosomes respond to these cues. The studies reviewed here suggest that centrosome functions extend to the development of tissues and organs in vertebrates.

Statistical method for comparing the level of intracellular organization between cells

Systems level approaches to analyzing complex emergent behavior require quantitative characterization of alterations of behavior on both the microscale and macroscale. Here we consider the problem of cellular organization and describe a statistical methodology for quantitative comparison of the internal organization between different populations of similar physical objects, such as cells. This comparison is achieved with several steps of analysis. Starting with three-dimensional or two-dimensional images of cells, images are segmented to identify individual cells. Locations of internal points of interest, such as organelles or proteins, are recorded. To define the configuration of internal points in each cell, the individual cells are subjected to bounded Voronoi tessellation: subdividing the bounded volume or area of the cell into subvolumes determined by the locations of the internal points of interest. A statistical methodology is applied to yield a metric for similarity in degree of organization between populations. We applied this methodology to test whether centrioles play a role in global cellular organization, using mutants of the green alga Chlamydomonas reinhardtii with known alterations in centriole number, structure, and position as a model system. Comparing mutant populations and wild-type populations revealed a dramatic difference in the degree of organization in the mutant strains. These computational and experimental results provide statistical support for prior observational studies and support the idea that centrioles play a role in generating or maintaining global cellular organization. Our results confirm that this method can be used to sensitively compare the extent and type of organization within cells.

Centriole asymmetry determines algal cell geometry

The mechanisms that determine the shape and organization of cells remain largely unknown. Green algae such as Chlamydomonas provide excellent model systems for studying cell geometry owing to their highly reproducible cell organization. Structural and genetic studies suggest that asymmetry of the centriole (basal body) plays a critical determining role in organizing the internal organization of algal cells, through the attachment of microtubule rootlets and other large fiber systems to specific sets of microtubule triplets on the centriole. Thus to understand cell organization, it will be critical to understand how the different triplets of the centriole come to have distinct molecular identities.

Organelle size equalization by a constitutive process

How cells control organelle size is an elusive problem. Two predominant models for size control can be distinguished: (1) induced control, where organelle genesis, maintenance, and disassembly are three separate programs that are activated in response to size change, and (2) constitutive control, where stable size results from the balance between continuous organelle assembly and disassembly. The problem has been studied in Chlamydomonas reinhardtii because the flagella are easy to measure, their size changes only in the length dimension, and the genetics are comparable to yeast. Length dynamics in Chlamydomonas flagella are quite robust: they maintain a length of about 12 μm and recover from amputation in about 90 min with a growth rate that decreases smoothly to zero as the length approaches 12 μm. Despite a wealth of experimental studies, existing data are consistent with both induced and constitutive control models for flagella. Here we developed novel microfluidic trapping and laser microsurgery techniques in Chlamydomonas to distinguish between length control models by measuring the two flagella on a single cell as they equilibrate after amputation of a single flagellum. The results suggest that cells equalize flagellar length by constitutive control.

Mechanisms of regulating cell topology in proliferating epithelia: impact of division plane, mechanical forces, and cell memory

Regulation of cell growth and cell division has a fundamental role in tissue formation, organ development, and cancer progression. Remarkable similarities in the topological distributions were found in a variety of proliferating epithelia in both animals and plants. At the same time, there are species with significantly varied frequency of hexagonal cells. Moreover, local topology has been shown to be disturbed on the boundary between proliferating and quiescent cells, where cells have fewer sides than natural proliferating epithelia. The mechanisms of regulating these topological changes remain poorly understood. In this study, we use a mechanical model to examine the effects of orientation of division plane, differential proliferation, and mechanical forces on animal epithelial cells. We find that regardless of orientation of division plane, our model can reproduce the commonly observed topological distributions of cells in natural proliferating animal epithelia with the consideration of cell rearrangements. In addition, with different schemes of division plane, we are able to generate different frequency of hexagonal cells, which is consistent with experimental observations. In proliferating cells interfacing quiescent cells, our results show that differential proliferation alone is insufficient to reproduce the local changes in cell topology. Rather, increased tension on the boundary, in conjunction with differential proliferation, can reproduce the observed topological changes. We conclude that both division plane orientation and mechanical forces play important roles in cell topology in animal proliferating epithelia. Moreover, cell memory is also essential for generating specific topological distributions.

Axonemal positioning and orientation in three-dimensional space for primary cilia: what is known, what is assumed, and what needs clarification

Two positional characteristics of the ciliary axoneme--its location on the plasma membrane as it emerges from the cell, and its orientation in three-dimensional (3D) space--are known to be critical for optimal function of actively motile cilia (including nodal cilia), as well as for modified cilia associated with special senses. However, these positional characteristics have not been analyzed to any significant extent for primary cilia. This review briefly summarizes the history of knowledge of these two positional characteristics across a wide spectrum of cilia, emphasizing their importance for proper function. Then the review focuses what is known about these same positional characteristics for primary cilia in all major tissue types where they have been reported. The review emphasizes major areas that would be productive for future research for understanding how positioning and 3D orientation of primary cilia may be related to their hypothesized signaling roles within different cellular populations.

Effect of localization, length and orientation of chondrocytic primary cilium on murine growth plate organization

The research investigates the role of the immotile chondrocytic primary cilium in the growth plate. This study was motivated by (i) the recent evidence of the mechano-sensorial function of the primary cilium in kidney tubule epithelial cells and (ii) the distinct three-dimensional orientation patterns that the chondrocytic primary cilium forms in articular cartilage in the presence or the absence of loading. For our investigation, we used the Smad1/5(CKO) mutant mouse, whose disorganized growth plate is due to the conditional deletion of Smad 1 and 5 proteins that also affect the so-called Indian Hedgehog pathway, whose physical and functional topography has been shown to be partially controlled by the primary cilium. Fluorescence and confocal microscopy on stained sections visualized ciliated chondrocytes. Morphometric data regarding position, orientation and eccentricity of chondrocytes, and ciliary localization on cell membrane, length and orientation, were collected and reconstructed from images. We established that both localization and orientation of the cilium are definite, and differently so, in the Smad1/5(CKO) and control mice. The orientation of the primary cilium, relative to the major axis of the chondrocyte, clusters at 80° with respect to the anterior-posterior direction for the Smad1/5(CKO) mice, showing loss of the additional clustering present in the control mice at 10°. We therefore hypothesized that the clustering at 10° contains information of columnar organization. To test our hypothesis, we prepared a mathematical model of relative positioning of the proliferative chondrocytic population based on ciliary orientation. Our model belongs to the category of "interactive particle system models for self-organization with birth". The model qualitatively reproduced the experimentally observed chondrocytic arrangements in growth plate of each of the Smad1/5(CKO) and control mice. Our mathematically predicted cell division process will need to be observed experimentally to advance the identification of ciliary function in the growth plate.

Origins of cellular geometry

Cells are highly complex and orderly machines, with defined shapes and a startling variety of internal organizations. Complex geometry is a feature of both free-living unicellular organisms and cells inside multicellular animals. Where does the geometry of a cell come from? Many of the same questions that arise in developmental biology can also be asked of cells, but in most cases we do not know the answers. How much of cellular organization is dictated by global cell polarity cues as opposed to local interactions between cellular components? Does cellular structure persist across cell generations? What is the relationship between cell geometry and tissue organization? What ensures that intracellular structures are scaled to the overall size of the cell? Cell biology is only now beginning to come to grips with these questions.

The daughter four-membered microtubule rootlet determines anterior-posterior positioning of the eyespot in Chlamydomonas reinhardtii

The characteristic geometry of the unicellular chlorophyte Chlamydomonas reinhardtii has contributed to its adoption as a model system for cellular asymmetry and organelle positioning. The eyespot, a photosensitive organelle, is localized asymmetrically in the cell at a precisely defined position relative to the flagella and cytoskeletal microtubule rootlets. We have isolated a mutant, named pey1 for posterior eyespot, with variable microtubule rootlet lengths. The length of the acetylated daughter four-membered (D4) microtubule rootlet correlates with the position of the eyespot, which appears in a posterior position in the majority of cells. The correlation of rootlet length with eyespot positioning was also observed in the cmu1 mutant, which has longer acetylated microtubules, and the mlt1 mutant, in which the rootlet microtubules are shorter. Observation of eyespot positioning after depolymerization of rootlet microtubules indicated that eyespot position is fixed early in eyespot development and becomes independent of the rootlet. Our data demonstrate that the length of the D4 rootlet is the major determinant of eyespot positioning on the anterior-posterior axis and are suggestive that the gene product of the PEY1 locus is a novel regulator of acetylated microtubule length.

The conversion of centrioles to centrosomes: essential coupling of duplication with segregation

Centrioles are self-reproducing organelles that form the core structure of centrosomes or microtubule-organizing centers (MTOCs). However, whether duplication and MTOC organization reflect innate activities of centrioles or activities acquired conditionally is unclear. In this paper, we show that newly formed full-length centrioles had no inherent capacity to duplicate or to organize pericentriolar material (PCM) but acquired both after mitosis through a Plk1-dependent modification that occurred in early mitosis. Modified centrioles initiated PCM recruitment in G1 and segregated equally in mitosis through association with spindle poles. Conversely, unmodified centrioles segregated randomly unless passively tethered to modified centrioles. Strikingly, duplication occurred only in centrioles that were both modified and disengaged, whereas unmodified centrioles, engaged or not, were "infertile," indicating that engagement specifically blocks modified centrioles from reduplication. These two requirements, centriole modification and disengagement, fully exclude unlimited duplication in one cell cycle. We thus uncovered a Plk1-dependent mechanism whereby duplication and segregation are coupled to maintain centriole homeostasis.

Asymmetric properties of the Chlamydomonas reinhardtii cytoskeleton direct rhodopsin photoreceptor localization

The eyespot of the unicellular green alga Chlamydomonas reinhardtii is a photoreceptive organelle required for phototaxis. Relative to the anterior flagella, the eyespot is asymmetrically positioned adjacent to the daughter four-membered rootlet (D4), a unique bundle of acetylated microtubules extending from the daughter basal body toward the posterior of the cell. Here, we detail the relationship between the rhodopsin eyespot photoreceptor Channelrhodopsin 1 (ChR1) and acetylated microtubules. In wild-type cells, ChR1 was observed in an equatorial patch adjacent to D4 near the end of the acetylated microtubules and along the D4 rootlet. In cells with cytoskeletal protein mutations, supernumerary ChR1 patches remained adjacent to acetylated microtubules. In mlt1 (multieyed) mutant cells, supernumerary photoreceptor patches were not restricted to the D4 rootlet, and more anterior eyespots correlated with shorter acetylated microtubule rootlets. The data suggest a model in which photoreceptor localization is dependent on microtubule-based trafficking selective for the D4 rootlet, which is perturbed in mlt1 mutant cells.

Apparent diffusive motion of centrin foci in living cells: implications for diffusion-based motion in centriole duplication

The degree to which diffusion contributes to positioning cellular structures is an open question. Here we investigate the question of whether diffusive motion of centrin granules would allow them to interact with the mother centriole. The role of centrin granules in centriole duplication remains unclear, but some proposed functions of these granules, for example, in providing pre-assembled centriole subunits, or by acting as unstable 'pre-centrioles' that need to be captured by the mother centriole (La Terra et al 2005 J. Cell Biol. 168 713-22), require the centrin foci to reach the mother. To test whether diffusive motion could permit such interactions in the necessary time scale, we measured the motion of centrin-containing foci in living human U2OS cells. We found that these centrin foci display apparently diffusive undirected motion. Using the apparent diffusion constant obtained from these measurements, we calculated the time scale required for diffusion to capture by the mother centrioles and found that it would greatly exceed the time available in the cell cycle. We conclude that mechanisms invoking centrin foci capture by the mother, whether as a pre-centriole or as a source of components to support later assembly, would require a form of directed motility of centrin foci that has not yet been observed.

Building the centriole

Centrioles are conserved microtubule-based organelles that lie at the core of the animal centrosome and play a crucial role in nucleating the formation of cilia and flagella in most eukaryotes. Centrioles have a complex ultrastructure with ninefold symmetry and a well-defined length. This structure is assembled from a host of proteins, including a variety of disease gene products. Over a century after the discovery of centrioles, the mechanisms underlying the assembly of these fascinating organelles, in particular the establishment of ninefold symmetry and the control of centriole length, are now starting to be uncovered.

Orientation of primary cilia of articular chondrocytes in three-dimensional space

Primary cilia have functions as sensory organelles integral to signal transduction and establishment of cell polarity. In articular cartilage the primary cilium has been hypothesized to function as an antenna to sense the biomechanical environment, regulate the secretion of extracellular matrix components, and maintain cellular positional information, leading to high tissue anisotropy. We used analysis of electron microscopy serial sections to demonstrate positional attributes of the primary cilium of adult equine articular chondrocytes in situ. Data for ~500 axonemes, comparing superficial to radiate chondrocytes from both load-bearing and non-load-bearing regions, were graphed using spherical co-ordinates θ, φ. The data demonstrate the axoneme has a definable orientation in 3D space differing in superficial and radiate zone chondrocytes, cells that differ by 90° in the orientation of their major axes to the articular surface. Axonemal orientation is more definable in load-bearing than in non-load-bearing areas. The position of emergence of the axoneme from the cell also is variable. In load-bearing regions of the superficial zone, extension of the axoneme is from the cellular side facing the subchondral bone. In radiate zone cells, axonemes extend from either face of the chondrocyte, that is, both toward the articular surface or toward the subchondral bone. In non-load-bearing regions this consistency is lost. These observations relate to current hypotheses concerning establishment of tissue anisotropy in articular cartilage during development, involving both migration of cells from the joint periphery and a restricted zone of division within the tissue resulting in the columnar arrangement of radiate zone cells.

Common themes in centriole and centrosome movements

Centrioles are found in nearly all eukaryotic cells and are required for growth and maintenance of the radial array of microtubules, the mitotic spindle, and cilia and flagella. Different types of microtubule structures are often required at different places in a given cell; centrioles must move around to nucleate these varied structures. Here, we draw together recent data on diverse centriole movements to decipher common themes in how centrioles move. Par proteins establish and maintain the required cellular asymmetry. The actin cytoskeleton facilitates movement of multiple basal bodies. Microtubule forces acting on the cell cortex, and nuclear-cytoskeletal links, are important for positioning individual centrosomes, and during cell division. Knowledge of these common mechanisms can inform the study of centriole movements across biology.

Influence of centriole number on mitotic spindle length and symmetry

The functional role of centrioles or basal bodies in mitotic spindle assembly and function is currently unclear. Although supernumerary centrioles have been associated with multipolar spindles in cancer cells, suggesting centriole number might dictate spindle polarity, bipolar spindles are able to assemble in the complete absence of centrioles, suggesting a level of centriole-independence in the spindle assembly pathway. In this report we perturb centriole number using mutations in Chlamydomonas reinhardtii, and measure the response of the mitotic spindle to these perturbations in centriole number. Although altered centriole number increased the frequency of monopolar and multipolar spindles, the majority of spindles remained bipolar regardless of the centriole number. But even when spindles were bipolar, abnormal centriole numbers led to asymmetries in tubulin distribution, half-spindle length and spindle pole focus. Half spindle length correlated directly with number of centrioles at a pole, such that an imbalance in centriole number between the two poles of a bipolar spindle correlated with increased asymmetry between half spindle lengths. These results are consistent with centrioles playing an active role in regulating mitotic spindle length. Mutants with centriole number alteration also show increased cytokinesis defects, but these do not correlate with centriole number in the dividing cell and may therefore reflect downstream consequences of defects in preceding cell divisions.

Geometric order in proliferating epithelia: impact of rearrangements and cleavage plane orientation

Regulation of proliferation is required for normal development of tissues and prevention of cancer formation. Continuous control of proliferation leads to regular shaped cells forming characteristic tissue patterns. Epithelial tissues serve as a model system for studying tissue morphogenesis. Several groups have studied epithelial morphogenesis using topological or geometric models, with various assumptions. In this study, we have developed a method to simulate the dynamic process of proliferating epithelia using an off-lattice cellular model. Our method realistically models the shape, size, geometry, lineage, cleavage plane orientation as well as topological properties of individual cells. We find that cellular rearrangements and cleavage plane orientation are critical in the formation of the observed cellular pattern of epithelia, including a high percentage of hexagons in proliferating epithelial cells. It is likely that the rearrangements and orientation of the cleavage plane reduces the overall stress on the cell. We show that a high percentage of hexagons in proliferating epithelia can be obtained using uniform growth rates, which was considered unlikely in previous studies. Our off-lattice cellular model provides an improvement over existing topological for studying the dynamics of proliferating epithelium.

Centrioles, centrosomes, and cilia in health and disease

Centrioles are barrel-shaped structures that are essential for the formation of centrosomes, cilia, and flagella. Here we review recent advances in our understanding of the function and biogenesis of these organelles, and we emphasize their connection to human disease. Deregulation of centrosome numbers has long been proposed to contribute to genome instability and tumor formation, whereas mutations in centrosomal proteins have recently been genetically linked to microcephaly and dwarfism. Finally, structural or functional centriole aberrations contribute to ciliopathies, a variety of complex diseases that stem from the absence or dysfunction of cilia.

ASQ2 encodes a TBCC-like protein required for mother-daughter centriole linkage and mitotic spindle orientation

An intriguing feature of centrioles is that these highly complicated microtubule-based structures duplicate once per cell cycle, affording the cell precise control over their number. Each cell contains exactly two centrioles, linked together as a pair, one of which is a mother centriole formed in a previous cell cycle and the other of which is a daughter centriole whose assembly is templated by the mother. Neither the molecular basis nor the functional role of mother-daughter centriole linkage is understood. We have identified a mutant, asq2, with defects in centriole linkage. asq2 mutant cells have variable numbers of centrioles and centriole positioning defects. Here, we show that ASQ2 encodes the conserved protein Tbccd1, a member of a protein family including a tubulin folding cochaperone and the retinitis pigmentosa protein RP2, involved in tubulin quality control during ciliogenesis. We characterize mitosis in asq2 cells and show that the majority of cells establish a bipolar spindle but have defects in spindle orientation. Few asq2 cells have centrioles at both poles, and these cells have properly positioned spindles, indicating that centrioles at the poles might be important for spindle orientation. The defects in centriole number control, centriole positioning, and spindle orientation appear to arise from perturbation of centriole linkage mediated by Tbccd1/Asq2p.

Modeling and inferring cleavage patterns in proliferating epithelia

The regulation of cleavage plane orientation is one of the key mechanisms driving epithelial morphogenesis. Still, many aspects of the relationship between local cleavage patterns and tissue-level properties remain poorly understood. Here we develop a topological model that simulates the dynamics of a 2D proliferating epithelium from generation to generation, enabling the exploration of a wide variety of biologically plausible cleavage patterns. We investigate a spectrum of models that incorporate the spatial impact of neighboring cells and the temporal influence of parent cells on the choice of cleavage plane. Our findings show that cleavage patterns generate "signature" equilibrium distributions of polygonal cell shapes. These signatures enable the inference of local cleavage parameters such as neighbor impact, maternal influence, and division symmetry from global observations of the distribution of cell shape. Applying these insights to the proliferating epithelia of five diverse organisms, we find that strong division symmetry and moderate neighbor/maternal influence are required to reproduce the predominance of hexagonal cells and low variability in cell shape seen empirically. Furthermore, we present two distinct cleavage pattern models, one stochastic and one deterministic, that can reproduce the empirical distribution of cell shapes. Although the proliferating epithelia of the five diverse organisms show a highly conserved cell shape distribution, there are multiple plausible cleavage patterns that can generate this distribution, and experimental evidence suggests that indeed plants and fruitflies use distinct division mechanisms.

Is left-right asymmetry a form of planar cell polarity?

Consistent left-right (LR) patterning is a clinically important embryonic process. However, key questions remain about the origin of asymmetry and its amplification across cell fields. Planar cell polarity (PCP) solves a similar morphogenetic problem, and although core PCP proteins have yet to be implicated in embryonic LR asymmetry, studies of mutations affecting planar polarity, together with exciting new data in cell and developmental biology, provide a new perspective on LR patterning. Here we propose testable models for the hypothesis that LR asymmetry propagates as a type of PCP that imposes coherent orientation onto cell fields, and that the cue that orients this polarization is a chiral intracellular structure.

The green algal eyespot apparatus: a primordial visual system and more?

Most flagellate green algae exhibiting phototaxis posses a singular specialized light sensitive organelle, the eyespot apparatus (EA). Its design principles are similar in all green algae and produce, in conjunction with the movement pattern of the cell, a highly directional optical device. It enables an oriented movement response with respect to the direction and intensity of light. The functional EA involves local specializations of different compartments (plasma membrane, cytosol, and chloroplast) and utilizes specialized microbial-type rhodopsins, which act as directly light-gated ion channels. Due to their elaborate structures and the presence of retinal-based photoreceptors in some lineages, algal EAs are thought to play an important role in the evolution of photoreception and are thus not only of interest to plant biologists. In green algae considerable progress in the molecular dissection of components of this primordial visual system has been made by genetic and proteomic approaches in recent years. This review summarizes general aspects of the green algal EA as well as recent progress in the identification of proteins related to it. Further, novel data supporting a link between eyespot globules and plastoglobules will be presented and potential additional roles of the EA besides those in photoreception will be discussed.

Predation and eukaryote cell origins: a coevolutionary perspective

Cells are of only two kinds: bacteria, with DNA segregated by surface membrane motors, dating back approximately 3.5Gy; and eukaryotes, which evolved from bacteria, possibly as recently as 800-850My ago. The last common ancestor of eukaryotes was a sexual phagotrophic protozoan with mitochondria, one or two centrioles and cilia. Conversion of bacteria (=prokaryotes) into a eukaryote involved approximately 60 major innovations. Numerous contradictory ideas about eukaryogenesis fail to explain fundamental features of eukaryotic cell biology or conflict with phylogeny. Data are best explained by the intracellular coevolutionary theory, with three basic tenets: (1) the eukaryotic cytoskeleton and endomembrane system originated through cooperatively enabling the evolution of phagotrophy; (2) phagocytosis internalised DNA-membrane attachments, unavoidably disrupting bacterial division; recovery entailed the evolution of the nucleus and mitotic cycle; (3) the symbiogenetic origin of mitochondria immediately followed the perfection of phagotrophy and intracellular digestion, contributing greater energy efficiency and group II introns as precursors of spliceosomal introns. Eukaryotes plus their archaebacterial sisters form the clade neomura, which evolved from a radically modified derivative of an actinobacterial posibacterium that had replaced the ancestral eubacterial murein peptidoglycan by N-linked glycoproteins, radically modified its DNA-handling enzymes, and evolved cotranslational protein secretion, but not the isoprenoid-ether lipids of archaebacteria. I focus on this phylogenetic background and on explaining how in response to novel phagotrophic selective pressures and ensuing genome internalisation this prekaryote evolved efficient digestion of prey proteins by retrotranslocation and 26S proteasomes, then internal digestion by phagocytosis, lysosomes, and peroxisomes, and eukaryotic vesicle trafficking and intracellular compartmentation.