THE FINE STRUCTURE OF THE CILIA FROM CTENOPHORE SWIMMING-PLATES

The ctenophore swimming-plate has been examined with the electron microscope. It has been recognized as an association of long cilia in tight hexagonal packing. One of the directions of the hexagonal packing is parallel to the long edge of the swimming-plate and is perpendicular to the direction of the ciliary beat. All the cilia in the swimming-plate are identically oriented. The effective beat in the movement of the swimming-plate is directed towards the aboral pole of the animal, and this is also the side of the unpaired peripheral filament in all the cilia. The direction of the ciliary beat is fixed in relation to the position of the filaments of the cilia. The swimming-plate cilium differs from other types of cilia and flagella in having a filament arrangement that can be described as 9 + 3 as opposed to the conventional 9 + 2 pattern. The central filaments appear in a group of two "tubular" filaments and an associated compact filament. The compact filament might have a supporting function. It has been called "midfilament." Two of the peripheral nine filaments (Fig. 1, Nos. 3 and 8) are joined to the ciliary membrane by means of slender lamellae, which divide the cilium into two unequal compartments. These lamellae have been called "compartmenting lamellae." Some observations of the arrangement of the compartmenting lamelae indicate that they function by cementing the cilia together in lateral rows. The cilia of the rows meet at a short distance from each other, leaving a gap of 30 A only. The meeting points are close to the termini of the compartmenting ridges. An electron-dense substance is sometimes seen bridging the gap. Some irregularities are noted with regard to the arrangement of the compartmenting lamellae particularly at the peripheral rows of cilia. In many cilia in these rows there are small vesicles beneath the ciliary membrane.     

Cilia and the life of ctenophores

Ctenophores, or comb jellies, are a distinct phylum of marine zooplankton with eight meridional rows of giant locomotory comb plates. Comb plates are the largest ciliary structures known, and provide unique experimental advantages for investigating the biology of cilia. Here, I review published and unpublished work on how ctenophores exploit both motile and sensory functions of cilia for much of their behavior. The long‐standing problem of ciliary coordination has been elucidated by experiments on a variety of ctenophores. The statocyst of ctenophores is an example of how mechanosensory properties of motile cilia orient animals to the direction of gravity. Excitation or inhibition of comb row beating provides adaptive locomotory responses, and global reversal of beat direction causes escape swimming. The diverse types of prey and feeding mechanisms of ctenophores are related to radiation in body form and morphology. The cydippid Pleurobrachia catches copepods on tentacles and undergoes unilateral ciliary reversal to sweep prey into its mouth. Mnemiopsis uses broad muscular lobes and ciliated auricles to capture and ingest prey. Beroë has giant smooth muscles and toothed macrocilia to rapidly engulf or bite through ctenophore prey, and uses reversible tissue adhesion to keep its mouth closed while swimming. Ciliary motor responses are calcium‐dependent, triggered by voltage‐activated calcium channels located along the length (reversed beating) or at the base (activation of beating) of ciliary membranes. Ciliary and muscular responses to stimuli are regulated by epithelial and mesogleal nerve nets with ultrastructurally identifiable synapses onto effector cells. Post‐embryonic patterns of comb row development in larval and adult stages are described and compared with regeneration of comb plates after surgical removal. Truly, cilia and ctenophores, like love and marriage, go together like a horse and carriage.     

Microtubule-membrane interactions in cilia. II. Photochemical cross-linking of bridge structures and the identification of a membrane-associated dynein-like ATPase

Photochemical cross-linking of both Tetrahymena and Aequipecten ciliary membrane proteins with the lipophilic reagent 4,4'-dithiobisphenylazide links together a high molecular weight dynein-like ATPase, membrane tubulin, and at least two other proteins. Electron microscopy of detergent-extracted cilia reveals that the cross-linked complex remains attached to the outer-doublet microtubules by a microtubule-membrane bridge. Cleavage of the reagent's disulfide bond releases the bridge- membrane complex and the dynein-like membrane-associated ATPase. Electron microscopy was used to ensure that the dynein-like protein did not result from the solubilization of the dynein arms attached to the outer-doublet microtubules. The dynein-like protein has been isolated using sucrose gradients and is similar to axonemal dynein with respect to its sedimentation characteristics nucleotide specificity, and divalent cation requirements. Photochemical cross-linking of ciliary membrane porteins in vivo results initially in the modification of ciliary beat and, eventually, in the cessation of ciliary movement. These results suggest that a dynein-like ATPase comprises the bridge which links the ciliary membrane to the outer-doublet microtubules and that this bridge is involved in the modulation of normal ciliary movement.     

A comparative study of ciliary differentiations in the branchial stigmata of ascidians

In a correlated thin sectioning and freeze-fracturing study, we have examined species belonging to the orders of the ascidian class: Stolidobranchiata (Botryllus schlosseri, Botrylloides leachi, Molgula socialis, Styela plicata), Phlebobranchiata (Ascidiella aspersa, Phallusia ingeria, Ciona intestinalis) and Aplousobranchiata (Clavelina lepadiformis). Though the branchial basket varies in the complexity and filtration efficiency in the three orders, the ciliated epithelia aroand the stigmata contain a common pattern of organization; seven rows of flattened cells, each bearing a single row of long cilia flanked by a single row of microvilli. All the species examined possess ciliary specializations represented by: (a) bridges connecting doublets number 5 and 6 as well as 9.1 and 2; (b) dense material lying between the above mentioned axonemal doublets (5-6 and 1-2) and the ciliary membrane, sometimes in the shape of longitudinal strands or often as lines of dots; (c) a fuzzy coat protruding from the ciliary membrane, consisting of tufts or scattered filaments; (d) intramembrane particles (IMPs) associated with the P-face of the membrane, often arranged in clusters and orderly alignments related to the anderlying axonemal doublets; these IMPs decorate the opposite sides of each cilium facing the adjacent cilia forming the ciliary rows of adjacent cells and are absent on the lateral sides. The stigmatal cilia propel water through the stigmata and their effective strokes follow a line at right angles to the row of cilia in each cell. The usual direction of the effective stroke is toward doublets 5-6. It is possible, therefore, to refer to structure in relation to the ciliary beat cycle. The importance of these specializations is unknown, but the structures appear to vary in the different species. A correlation between the richness of the specializations and the complexity of the branchial basket was not evidenced. It was suggested that the ciliary specializations relate to the peculiar organization of the stigmatal margin and that all are involved in the regulation of the ciliary activity.     

Detection of plasma membrane cholesterol by filipin during microvillogenesis and ciliogenesis in quail oviduct

Using filipin as a probe for the presence of membrane cholesterol, the evolution of cholesterol distribution in the apical plasma membrane was studied during estrogen-induced ciliogenesis in quail oviduct and compared with the distribution of intramembrane particles (IMPs). Ciliary growth is preceded by the first step of microvillus differentiation. Microvilli emerge in membrane domains rich in IMPs and devoid of filipin-cholesterol (f-c) complexes. However growing microvillus membrane shows f-c complexes. During ciliary growth, microvilli lengthen from 0.5 to 2 microns, indicating that the microvillar membrane is not a membrane reservoir for ciliogenesis. During ciliary growth, the characteristic ciliary necklace IMP rows appear progressively at the base of cilia. The first IMP row is organized in a membrane circlet lacking of f-c complexes, whereas the new shaft membrane in the middle of the circlet exhibits numerous complexes. These two different domains of the cilia keep their specificity during ciliary growth. Only the ciliary tip shows fewer complexes than the shaft membrane. The apical membrane of differentiated ciliated cells is thus composed of various domains, the ciliary shaft full of f-c complexes and poor in IMPs, the ciliary necklace is devoid of f-c complexes and rich in IMPs, the microvilli membrane is rich in both IMPs and f-c complexes, and the interciliary membrane is poor in both f-c complexes and IMPs, whereas the undifferentiated cells exhibit an apical membrane in which f-c complexes and IMPs are distributed homogeneously.     

The Permanence of the Infraciliature in Suctoria: An Electronmicroscopic Study of Pattern Formation in Tokophrya infusionum*

SYNOPSIS. The adult Tokophrya infusionum does not possess cilia, but has 20–30 barren basal bodies arranged in 6 short rows adjacent to the contractile vacuole pore. During reproduction, which is by internal budding, the contractile vacuole sinks into the parent along with the invaginating membranes that form the embryo and the wall of the brood pouch. The 6 rows of basal bodies radiate away from the pore and elongate to form 5 long ciliary rows, that encircle the anterior half of the embryo, and 1 short row at the posterior end. The contractile vacuole pore, along with several barren basal bodies, remains in the parent when the embryo is completed. The pore rises to the surface when the embryo is born. New basal bodies are then formed in the parent to replace those which were incorporated into the embryo, and formation of another embryo may begin. The cilia of the embryo are partially resorbed 10 min after the start of metamorphosis, with depolymerization of the ciliary microtubules. Later, the cilia and most of the basal bodies disappear completely, except for a group of barren basal bodies near the embryo's contractile vacuole pore, which form 6 rows and serve as an anlage for the basal bodies and cilia that arise during embryogenesis. There is, therefore, an organized infraciliature in Suctoria throughout their life cycle, and a distinct continuity of basal bodies across the generations.     

The role of cortical orientation in the control of the direction of ciliary beat in Paramecium

The swimming behavior of many ciliate protozoans depends on graded changes in the direction of the ciliary effective stroke in response to depolarizing stimuli (i.e., the avoiding reaction of Paramecium). We investigated the problem of whether the directional response of cilia with a variable plane of beat is related to the polarity of the cell as a whole or to the orientation of the cortical structures themselves. To do this, we used a stock of Paramecium aurelia with part of the cortex reversed 180 degrees. We determined the relation of the orientation of the kineties (ciliary rows) to the direction of beat in these mosaic paramecia by cinemicrography of particle movements near living cells and by scanning electron microscopy of instantaneously fixed material. We found that the cilia of the inverted rows always beat in the direction opposite to that of normally oriented cilia during both forward and backward swimming. In addition, metachronal waves of ciliary coordination were present on the inverted patch, travelling in the direction opposite to those on the normal cortex. The reference point for the directional response of Paramecium cilia to stimuli thus resides within the cilia or their immediate cortical surroundings.     

CILIARY MEMBRANE DIFFERENTIATIONS IN TETRAHYMENA PYRIFORMIS : Tetrahymena Has Four Types of Cilia

We have examined thin sections and replicas of freeze-fractured cilia of Tetrahymena pyriformis. The ciliary necklace located at the base of all freeze-fractured oral and somatic cilia has been studied in thin sections. Since electron-dense linkers have been found to connect both microtubule doublets and triplets to the ciliary membrane at the level of the necklace, the linkers and the associated necklace seem to be related to the transition region between the doublets and triplets of a cilium. Plaque structures, consisting of small rectangular patches of particles located distal to the ciliary necklace, are found in strain GL, but are absent in other strains examined in this study. In freeze-cleaved material, additional structural differentiations are observed in the distal region of the ciliary membranes of somatic and oral cilia. Somatic cilia contain many randomly distributed particles within their membrane. Oral cilia can be divided into three categories on the basis of the morphology of their freeze-fractured membranes: (a) undifferentiated cilia with very few randomly distributed particles: (b) cilia with particles arranged in parallel longitudinal rows spaced at intervals of 810–1080 Å that are located on one side of the cilium; and (c) cilia with patches of particles arranged in short rows oriented obliquely to the main axis of the cilium. The latter particles, found on one side of the cilium, seem to serve as attachment sites for bristles 375–750 Å long and 100 Å wide which extend into the surrounding medium. The particles with bristles are located at the tips of cilia in the outermost membranelle and may be used to detect food particles and/or to modify currents in the oral region so that food particles are propelled more efficiently into the buccal cavity. Examination of thin-sectioned material indicates that the particles in oral cilia which form the longitudinal rows could be linked to microtubule doublets. Linkage between microtubule doublets and adjacent membrane areas on one side of the cilium could modify the form of ciliary beat by restricting the sliding of the microtubules. It is suggested that membrane-microtubule interactions may form the basis for the various forms of ciliary beat observed in different organisms.     

Scanning electron microscopy of Mycoplasma pneumoniae on the membrane of individual ciliated tracheal cells

Cell monolayer cultures were prepared from hamster tracheal explants by a collagenase exposure and subsequent incubation in Waymouth's MAB 87/3 medium. The epithelial outgrowth occurred on glass cover slips. Cilia on the monolayers continued to beat normally after the "parent" explant was removed. Monolayer cultures infected with Mycoplasma pneumoniae had significant amounts of attachment. A morphological analysis of the attachment was conducted with scanning electron microscopy. Clusters, cocci, and filaments of M. pneumoniae all attached to the epithelial cells, but the filaments were especially common. Mycoplasmas were seen in association with both ciliated and nonciliated cell membranes. On ciliated cells, mycoplasmas were on the ciliary strands and on the cell membrane. When located immediately adjacent to or in between cilia, mycoplasmas were oriented vertically with the constricted attachment tip oriented down toward the host cell membrane. When located more than a micron away from the ciliary fibers, mycoplasmas lay horizontally along the epithelial cell membrane. The photographic data suggest that clusters or "sperules" of mycoplasmas may liberate individual mycoplasmas that attach to the cell membrane. It appears that the receptor sites for M. pneumoniae are rather uniformly distributed along the ciliated cell membrane, and are not restricted to the interciliary areas.     

Ultrastructure of gill cilia and ciliary rootlets of Chaetoderma nitidulum Lovén 1844 (Mollusca, Chaetodermomorpha)

Cilia and associated structures on the gill lamellae on the ctenidum of Chaetoderma nitidulum were studied. The gill cilia are very long and have a whip-like narrow portion distally, where only three microtubule doublets continue to the distal tip. In the transition zone between the cilium and the centriolar triplet section of the basal body there is a dense plate, an aggregation of granules and a ciliary necklace with four strands. Further down there is a short cross-striated basal foot and two conical cross-striated ciliary rootlets. The first rootlet is flattened and directed forward. It connects distally with the basal feet of other adjacent cilia. The second rootlet is rounded in cross-section and vertically directed. The epithelial structures of Chaetoderma show similarities with other Mollusca. We found no structural characters that could support the current hypothesis of a close relationship of Xenoturbella to the Mollusca.     

NEUROID TRANSMISSION IN CILIATED EPITHELIUM

Evidence from several sources indicates that there is present in certain types of ciliated tissue a primitive form of conduction regulating the rate of beat of the cilia. (Kraft, 1890; Engelmann, 1898). A similar type of conduction has been observed by Parker (1910) in the sponge Stylotella. In the foregoing experiments a study was made of such conduction in the ciliated tissue of the gills of the clam Unio. Observations were made on the effects of temperature changes on the rate of beat of cilia adjacent to areas not themselves directly influenced. The following results were obtained. 1. The transmission through the gill of the effects of warmth applied locally is apparent through increased rate of ciliary beat on adjacent gill tissue in all directions from the region of application. Effects are not observed laterally at a distance greater than 9 to 11 mm. from the nearest edge of the stimulated area. The narrowness of the gill makes it impossible to determine the vertical limits of the transmission. 2. Effects of low temperatures are not observable beyond the limits of the region of direct application. These results differ from those of Kraft in tissue from the frog''s pharynx, where conduction was shown to take place only down a row of beating cilia. On the other hand they agree with his results in indicating that the effects of warmth only are transmitted. The phenomenon might be explained by the stimulating effect of the action current of the directly excited cilia on the neighboring relatively quiet cilia. A similar explanation has been offered by Lillie for waves of coordinated beating in the rows of swimming plates of ctenophores. Such an explanation, though in accord with the work on Unio, is inconsistent with certain of the observations of Kraft on the tissue from the frog''s pharynx.     

Novel bridge of axon-like processes of epithelial cells in the aboral sense organ of ctenophores

We describe by light and electron microscopy a novel structure in the aboral sense organ (apical organ) of cydippid (Pleurobrachia) and lobate (Mnemiopsis) ctenophores. An elevated bundle of long, thin, microtubule-filled processes arises from the apical ends of two groups of epithelial cells located on opposite sides of the apical organ along the tentacular plane of the body. This bundle of axon-like processes arches over the epithelial floor like a bridge, with branches at both ends running toward opposing pairs of ciliary balancers that are motile pacemakers for the rows of locomotory ciliary comb plates. The bridge in Pleurobrachia is approximately 40 microm long and 3-4 microm wide and consists of approximately 60 closely packed processes, 0.2-0.8 microm thick, containing vesicles and numerous microtubules running parallel to their long axes. There are approximately 30 epithelial cells in each of the two groups giving rise to the bridge and each cell forms a single process, so roughly half of the processes in the bridge must originate from cells on one side and diverge into branches to a pair of balancers on the opposite side of the apical organ. The 150-200 cilia in each balancer arise from morphologically complex cellular projections with asymmetric lateral extensions directed towards a fork of the bridge. Presynaptic triad structures and vesicles are found in this region but clear examples of synaptic contacts between bridge processes and balancer cells have not yet been traced. Cydippid larvae of Mnemiopsis have a conspicuous bridge along the tentacular plane of the apical organ. Beroid ctenophores that lack tentacles at all stages do not have a bridge. We discuss the possibility that the bridge is an electrical conduction pathway to balancers that coordinates tentacle-evoked swimming responses of ctenophores, such as global ciliary excitation.     

Cytoskeleton-membrane interactions in the cnidocil complex of hydrozoan nematocytes

Summary Each cnidocil complex of the hydrozoans Tubularia larynx and Hydra vulgaris consists of 9 or 7–10 large stereovilli (=stereocilia), respectively, and a modified cilium, the cnidocil. The cnidocils comprise the regular 9 microtubule doublets, up to 30 additional microtubules, as well as a central filament body. Adjacent stereovilli are linked together by intermembrane connectors forming the stereovillar cone. The distal tips of the stereovilli surround the cnidocil in a closed tubular arrangement measuring up to 0.7 m in length. Within this contact region the cnidocil is linked to the stereovillar tube by another set of intermembrane connectors, which seem to hold the cnidocil in a central position within the stereovillar cone. Stereovillar membrane and actin core are linked by 16-nm long cross bridges, which display a periodicity of 16 nm and emerge from the actin core. Within the cnidocils periodically arranged membrane-cytoskeleton bridges are uniformly restricted to the contact region. Here, 24-nm long cross bridges, which are spaced by a regular distance of 20 nm, interconnect the A-tubules of the microtubule doublets and the membrane. The cnidociliary membrane is differentiated into distinct domains as revealed by freeze-fracturing. Within the contact region of the nematocytes of Tubularia larynx, intramembrane particles are arranged in 9 rows of 700 nm length and 50 nm width, separated by particlefree areas. Intramembrane particles are irregularly distributed distal to the contact region. Considering recent physiological results we presume that the latter represent chemoreceptor units, while mechanical stimuli are transmitted via the intermembrane connectors and the microtubule-membrane bridges to mechanosensitive channels within the domain of the cnidociliary membrane in the contact region.     

Computation of the internal forces in cilia: application to ciliary motion, the effects of viscosity, and cilia interactions.

This paper presents a simple and reasonable method for generating a phenomenological model of the internal mechanism of cilia. The model uses a relatively small number of parameters whose values can be obtained by fitting to ciliary beat shapes. Here, we use beat patterns observed in Paramecium. The forces that generate these beats are computed and fit to a simple functional form called the "engine." This engine is incorporated into a recently developed hydrodynamic model that accounts for interactions between neighboring cilia and between the cilia and the surface from which they emerge. The model results are compared to data on ciliary beat patterns of Paramecium obtained under conditions where the beats are two-dimensional. Many essential features of the motion, including several properties that are not built in explicitly, are shown to be captured. In particular, the model displays a realistic change in beat pattern and frequency in response to increased viscosity and to the presence of neighboring cilia in configurations such as rows of cilia and two-dimensional arrays of cilia. We found that when two adjacent model cilia start beating at different phases they become synchronized within several beat periods, as observed in experiments where two flagella are brought into close proximity. Furthermore, examination of various multiciliary configurations shows that an approximately antiplectic wave pattern evolves autonomously. This modeling evidence supports earlier conjectures that metachronism may occur, at least partially, as a self-organized phenomenon due to hydrodynamic interactions between neighboring cilia.     

THE CILIARY NECKLACE : A Ciliary Membrane Specialization

Cilia, primarily of the lamellibranch gill (Elliptio and Mytilus), have been examined in freeze-etch replicas. Without etching, cross fractures rarely reveal the 9 + 2 pattern, although suggestions of ninefold symmetry are present. In etched preparations, longitudinal fractures through the matrix show a triplet spoke alignment corresponding to the spoke periodicity seen in thin sections. Dynein rows can be visualized along the peripheral microtubules in some preparations. Fracture faces of the ciliary membrane are smooth with few membrane particles, except in the regions adjacent to the basal plate. In the transition region below the plate, a unique particle arrangement, the ciliary necklace, is found. In the Elliptio gill, on fracture face A the necklace is comprised of three well-defined rows or strands of membrane particles that encircle the ciliary shaft. The rows are scalloped and each scallop corresponds to a peripheral doublet microtubule. In thin sections at the level of these particles, a series of champagne-glass structures link the microtubular doublets to the ciliary membrane. The ciliary necklace and this "membrane-microtubule" complex may be involved in energy transduction or the timing of ciliary beat. Comparative studies show that these features are present in all somatic cilia examined including those of the ameboflagellate Tetramitus, sea urchin embryos, rat trachea, and nonmotile cilia of cultured chick embryo fibroblasts. The number of necklace strands differs with each species. The necklace has not been found in rat or sea urchin sperm.     

Different functions of tight junctions in the ascidian branchial basket

The stigmatal cells in the branchial basket of ascidians from a number of genera have been examined as to the nature and distribution of their intercellular junctions. The branchial wall consists of ciliated and parietal cells; the ciliated cells are arranged in seven rows and are associated by junctions with other cells in the same row as well as with those in adjacent rows. They are also associated by junctions with peripheral parietal cells. Junctions between adjacent ciliated cells in all cases exhibit tight junctions or zonulae occludentes. However, these cell borders also possess fasciae or zonulae adhaerentes if they are in the same row and the ciliary rootlets insert-into these junctions. If the cells are in adjacent rows they exhibit adhaerentes junctions only in species belonging to the orders Phlebobranchiata and Aplousobranchiata. In contrast, if the cells in adjacent rows belong to the order Stolidobranchiata. they never exhibit any adhaerentes junctions and the ciliary rootlets of the basal bodies from the cilia insert instead into the tight junctions and the non-junctional membrane below them. At the homologous junctional borders between adjacent parietal cells and also at heterologous junctional borders between parietal and ciliated cells, tight junctions alone occur, with no co-existing adhaerentes junctions along their lateral borders. Again, fibrils from ciliary rootlets insert into zonulae occludentes. This shows that tight junctions are capable both of forming permeability barriers, in that they can be seen to prevent the entry of exogenous tracers such as lanthanum, and of acting as adhesive devices.     

Fine Structure and Function of Pharynx Cilia in Glossobalanus minutus Kowalewsky (Enteropneusta)

Two kinds of cilia have been observed in the pharynx of Glossobalanus minutus Kowalewsky. From the present study, a ciliary specialization can be found in order to move a determinate substance, i.e. mucus or water. Mucus-moving cilia (type I cilia) have a single basal centriole and poorly developed ciliary rootlets. Their tips are rounded, bearing an inner, asymmetrical cap attached to some tubules. Water-moving cilia (type II cilia) are exclusively located at lateral epithelia of branchial bars, giving rise to the water current through the gills. They have two basal centrioles, proximal and distal, and a complex system of ciliary rootlets made up of a principal rootlet, a secondary or accessory rootlet and a 'fan' rootlet. The tips of type II cilia have a long process with some tubules inside. All basal structures are precisely orientated in order to assure a good coordination of ciliary beat. The possible functional significance of ciliary substructure is also discussed. From these observations a model for mucus and water currents through gill slits is postulated.     

Structural conformation of ciliary dynein arms and the generation of sliding forces in Tetrahymena cilia

The sliding tubule model of ciliary motion requires that active sliding of microtubules occur by cyclic cross-bridging of the dynein arms. When isolated, demembranated Tetrahymena cilia are allowed to spontaneously disintegrate in the presence of ATP, the structural conformation of the dynein arms can be clearly resolved by negative contrast electron microscopy. The arms consist of three structural subunits that occur in two basic conformations with respect to the adjacent B subfiber. The inactive conformation occurs in the absence of ATP and is characterized by a uniform, 32 degrees base-directed polarity of the arms. Inactive arms are not attached to the B subfiber of adjacent doublets. The bridged conformation occurs strictly in the presence of ATP and is characterized by arms having the same polarity as inactive arms, but the terminal subunit of the arms has become attached to the B subfiber. In most instances the bridged conformation is accompanied by substantial tip-directed sliding displacement of the bridged doublets. Because the base-directed polarity of the bridged arms is opposite to the direction required for force generation in these cilia and because the bridges occur in the presence of ATP, it is suggested that the bridged conformation may represent the initial attachment phase of the dynein cross-bridge cycle. The force-generating phase of the cycle would then require a tip-directed deflection of the arm subunit attached to the B subfiber.     

Morphological Description and Molecular Phylogeny of Two Species of Levicoleps (Ciliophora,Prostomatida), L. taehwae nov. spec. and L. biwae jejuensis nov. subspec., collected in Korea

Two colepid ciliates, Levicoleps taehwae nov. spec. and L. biwae jejuensis nov. subspec., were collected from the brackish water of the Taehwa River and a small freshwater pond in Jeju Island, South Korea, respectively. Their living morphology, infraciliature, and small subunit (SSU) rRNA gene sequences were determined using standard methods. Barrel‐shaped L. taehwae nov. spec. is a small ciliate with an average size of 45 × 25 μm in vivo, about 15 ciliary rows each composed of 12 monokinetids and two perioral dikinetids, and two 20 μm‐long caudal cilia. The sequence length and GC content of the SSU rRNA gene are 1,669 bp, 44.5%. This novel species is similar in body size to Coleps hirtus, and has six armor tiers and hirtus‐type tier plates, and the same number of ciliary rows as C. hirtus; however, it can be distinguished from the latter by the absence of armor spines and its sequence similarity of SSU rRNA gene is about 92.8% which indicates that it is a distinct form. Levicoleps biwae jejuensis nov. subspec., is a medium colepid ciliate which has a barrel‐shaped body, about 22 somatic kineties and 16 transverse ciliary rows, three mini adoral organelles, and four 15 μm‐long caudal cilia. The sequence length and GC content of the SSU rRNA gene are 1,666 bp and 44.4%.     

Gradients of proliferation of ciliary basal bodies and the determination of the position of the oral primordium in Tetrahymena.

The pattern of proliferation of new basal bodies in ciliary rows (somatic proliferation) in Tetrahymena was observed. Starved and refed cells were used, because proliferation in these cells is more pronounced than that under other circumstances. The formation of new basal bodies is locally determined by the position of "old" pre-existing basal body (short range determination). However, the probability of proliferation associated with any given "old" basal body differs very much. This probability is determined by the spatial coordinates of the particular region of the cell (long range determination); however some randomness in this process was also observed. Two different gradients of proliferation were found. The first gradient is circumferential with a maximum number of new basal bodies added in ciliary rows n, 1, 2 and 3 and the minimum number added in ciliary rows 7, 8 and 9. The second is an antero-posterior gradient with the highest number of new basal bodies added in the midbody region. Moreover, at least in some cases, new oral primordia first appear, as a random proliferation of new basal bodies adjacent to a few old cilia of ciliary row No. 1, resembling somatic proliferation. Then 2,3 or even more clumps of basal bodies appear, each having one old cilium posteriorly. These clumps, however, are not linear groups within the ciliary row but instead they form small fields of basal bodies. These findings suggest, that the same two-gradient system for new basal body addition operates during somatic proliferation and also determines the position of the new oral primordium as the site of the highest gradient value at the intersection of two gradients.