Evidence for a direct role of nascent basal bodies during spindle pole initiation in the green alga Spermatozopsis similis.

Basal body replication in the naked biflagellate green alga Spermatozopsis similis was analyzed using standard electron microscopy and immunogold localization of centrin, an ubiquitous centrosomal protein, and p210, a recently characterized basal apparatus component of S. similis. Fibrous disks representing probasal bodies appear at the proximal end of parental basal bodies at the end of interphase and development proceeds via a ring of nine singlet microtubules. Nascent basal bodies dock early to the plasma membrane but p210, usually present in basal body-membrane-linkers of S. similis, was already present on the cytosolic basal body precursors. In addition to the distal connecting fiber and the nuclear basal body connectors (NBBC) of the parental basal bodies, centrin was present on the fibrous probasal bodies, in a linker between probasal bodies and the basal apparatus, in the connecting fiber between nascent basal bodies and their corresponding parent, and, finally, a fiber linking the nascent basal bodies to the nucleus. This NBBC probably is present only in mitotic cells. During elongation a cartwheel of up to seven layers is formed, protruding from the proximal end of nascent basal bodies. Microtubules develop on the cartwheel indicating that it temporarily functions as a microtubule organizing center (MTOC). These microtubules and probably the cartwheels, touch the nuclear envelope at both sides of a nuclear projection. We propose that spindle assembly is initiated at these attachment sites. During metaphase, the spindle poles were close to thylakoid-free lobes of the chloroplast, and the basal bodies were not in the spindle axis. The role of nascent basal bodies during the initial steps of spindle assembly is discussed.     

Ultrastructural and immunocytological studies on the rhizoplast in the chrysophycean alga Ochromonas danica

The rhizoplast, a striated band elongating from the flagellar basal body to the nucleus, is conspicuous in cells of Ochromonas danica Prings. In interphase cells, it runs from the basal body of the anterior flagellum to the space between the nucleus and the Golgi body. In O. danica, the rhizoplast duplicates during mitosis and the two rhizoplasts serve as mitotic poles. In the present study, we reinvestigated mitosis of O. danica using transmission electron microscopy and immunofluorescence microscopy, especially focusing on the rhizoplast. The nuclear envelope became dispersed during metaphase, and the rhizoplasts from two sets of the flagellar basal bodies functioned as the mitotic poles. Immunofluorescence microscopy using anti‐α‐tubulin, anti‐centrin and anti‐γ‐tubulin antibodies showed that centrin molecules were localized at the flagellar basal bodies, whereas γ‐tubulin molecules were detected at the rhizoplast during the whole cell cycle.     

Cytoskeletal dynamics ofApedinella radians (Pedinellophyceae) II. Cell division and maintenance of cell polarity and symmetry

Summary The dynamics of the cytoskeletal proteins centrin, actin, and tubulin were followed during cell division in the unicellular phytoflagellateApedinella radians (Pedinellophyceae). Three centrin, or centrin-like, components appear to coordinate independent developmental events during cell division. The first component, basal body centrin, maintains a physical link between basal bodies and the anterior nuclear membrane. Basal body centrin divides in two at metaphase, and each portion segregates with two basal bodies at anaphase. As the positioning of basal bodies defines the anterior region of the cell, basal body centrin appears to play a role in maintaining cell polarity throughout the cell cycle. The second centrin component consists of an array of filamentous bundles arranged as a six-pointed star. During cell division, the star undergoes a conformational change resulting in two distinct centrin triangles, one distributed to each daughter cell, suggesting that centrin filamentous bundles are involved in maintaining cell (radial) symmetry. The third centrin component is transient and associates with the spindle poles, emerging prior to mitosis and remaining until late anaphase/early telophase. Spindle pole centrin establishes temporary horizontal bipolarity, thereby establishing the spindle axis. Unlike centrin filamentous bundles, actin filamentous bundles depolymerize prior to mitosis, indicating they do not influence cell symmetry during cell division. Mitosis is described for the first time in a pedinellid and features a closed spindle, the absence of rhizoplasts and a persistent spindle.     

The rhizoplast of chrysomonads, a basal body–nucleus connector that polarises the dividing spindle

Summary. An ultrastructure study of the rhizoplast in Synura petersenii, Mallomonas fastigiata, and M. insignis shows that it consists of 15–20 striated rootlets that form a claw or an incomplete cone over the nucleus. These rootlets course along one face of the nucleus between the nuclear membrane and the cis-face of the Golgi stack of cisternae. They converge and merge above the nucleus, forming a stub attached to the proximal section of the two basal bodies. These cross-striated rootlets are composed of closely packed longitudinal microfibrils. By immunofluorescence, the basal bodies and the rootlets forming the claw were decorated by the anti-centrin monoclonal antibody ICL19 raised against the Paramecium tetraurelia acidic centrin protein and by two antibodies raised against the striated parabasal and costal striated fibres of trichomonads. Only the anti-centrin monoclonal antibody 20H5 raised against Chlamydomonas reinhardtii centrin strongly labelled the 20–22 kDa protein bands from the extracted cytoskeleton of S. petersenii by immunoblotting. Electron micrographs of mitosis in S. petersenii cells revealed that the segregated pairs of basal bodies are linked by the striated rootlets of the rhizoplast to the poles of the mitotic spindle. The spindle microtubules arise perpendicularly from the striated rootlets of the basal body–nucleus connector forming the centrosome. In conclusion, in these cells there is a basal body–nucleus connector similar to that of C. reinhardtii and other chlorophytes. It contains centrin proteins, it is involved in the linkage of the basal bodies to the nucleus and is a component of the spindle pole body or centrosome in the dividing cell.Correspondence and reprints: Biologie des Protistes, Université Blaise Pascal de Clermont-Ferrand, Campus des Cézeaux, 63177 Aubière Cedex, France.Present address: Romagnat, France.Received February 7, 2003; accepted May 21, 2003; published online September 23, 2003     

CELL DIVISION IN CHLAMYDOMONAS MOEWUSII1

Cell division in Chlamydomonas moewusii is described. The cells become immobile with flagellar abscission prior to mitosis. The basal bodies migrate toward the nucleus and become intimately associated with the nuclear membrane which is devoid, of ribosomes where adjacent to the basal bodies. The basal bodies replicate at preprophase. The nucleolus fragments at this stage. By prophase the basal body pairs have migrated, to the nuclear poles. Spindle fibers become prominent in the nucleus. The nuclear membrane does not fragment. The nucleus assumes a crescent-form by metaphase. Polar fenestrae are absent. Kinetochores appear at anaphase. An interzonal spindle elongates as the chromosomes move to the nuclear poles. Daughter nuclei become abscised by an ingrowth of nuclear membrane, leaving behind a separated, degenerating interzonal spindle. Ribosomes reappear on the outer nuclear membrane at late telophase. Nucleoli reform early in cytokinesis. The cleavage furrow, associated microtubules, and endoplasmic reticulum comprise the phycoplast. Cytokinesis proceeds rapidly after the completion of telophase. The basal body-nucleus relationship becomes reorganized into the typical interphase condition late in cytokinesis. Specific and predictable organelle rearrangements during mitosis have been described. Cell division in C. moewusii is compared with other algae, especially C. reinhardi.     

Roots1

ABSTRACT Many unicellular eukaryotic organisms possess complex fiber systems that organize and anchor the flagellar basal apparatus in the cell [20, 24]. In 1978 we first published the observation that one of these fiber systems, the striated flagellar root of the quadriflagellate green alga Tetraselmis subcordiformis (=Platymonas subcordiformis), is a contractile organelle [31]. We subsequently found that striated flagellar roots are composed, in part, of the Ca²⁺-binding protein centrin [30]. Since that time, centrin has been found to be a ubiquitous component of the flagellar basal apparatus, basal bodies and centrioles, and centrosomes and mitotic spindle poles of eukaryotic cells (for general reviews see [28, 34]). While we have learned a great deal about centrin from other organisms, our earliest success in understanding the biology of centrin was in large part due to the extraordinary extent to which Tetraselmis cells have elaborated their centrin-based organelles. In this paper, I will return attention to several unanswered questions concerning Tetraselmis striated flagellar root behavior and I will suggest several new directions that students may wish to pursue in order to tease fresh insights from this fascinating organism.     

ULTRASTRUCTURAL FEATURES OF MITOSIS IN PLOEOTIA COSTATA (HETERONEMATALES,EUGLENOPHYTA)1

The ultrastructural features of mitosis in the colorless phagotrophic euglenoid, Ploeotia costata (Farmer and Triemer 1988bn; syn: Serpenomonas costata, Triemer 1986) are described. During interphase the nucleus is rounded and lies adjacent to the reservoir and the four basal bodies, two of which bear flagella. At the onset of mitosis, two additional flagella are generated from the accessory basal bodies such that four basal bodies with flagella now lie at one pole of the prophase nucleus. Microtubules develop in the nucleus prior to migration of one of the basal body pairs to the opposite pole of the nucleus. By metaphase, chromosomes with layered kinetochores are aligned on the equator of the spindle, and a dumbbellshaped nucleolus stretches from pole to pole. Continued elongation of the nucleus results in the separation of the chromosomal masses at anaphase. The distance between the nuclear poles from metaphase to anaphase changes little although the overall length of the nucleus nearly doubles. By telophase a large interzonal spindle develops between the forming daughter nuclei. The extended interzonal spindle breaks near the center prior to cell cleavage.     

TRANSIENT LOCALIZATION OF γ-TUBULIN AROUND THE CENTRIOLES IN THE NUCLEAR DIVISION OF BOERGESENIA FORBESII (SIPHONOCLADALES, CHLOROPHYTA)

Mitosis in Boergesenia forbesii (Harvey) Feldman was studied by immunofluorescence microscopy using anti-β–tubulin, anti-γ–tubulin, and anti-centrin antibodies. In the interphase nucleus, one, two, or rarely three anti-centrin staining spots were located around the nucleus, indicating the existence of centrioles. Microtubules (MTs) elongated randomly from the circumference of the nuclear envelope, but distinct microtubule organizing centers could not be observed. In prophase, MTs located around the interphase nuclei became fragmented and eventually disappeared. Instead, numerous MTs elongated along the nuclear envelope from the discrete anti-centrin staining spots. Anti-centrin staining spots duplicated and migrated to the two mitotic poles. γ–Tubulin was not detected at the centrioles during interphase but began to localize there from prophase onward. The mitotic spindle in B. forbesii was a typical closed type, the nuclear envelope remaining intact during nuclear division. From late prophase, accompanying the chromosome condensation, spindle MTs could be observed within the nuclear envelope. A bipolar mitotic spindle was formed at metaphase, when the most intense staining of γ-tubulin around the centrioles could also be seen. Both spindle MT poles were formed inside the nuclear envelope, independent of the position of the centrioles outside. In early anaphase, MTs between separating daughter chromosomes were not detected. Afterward, characteristic interzonal spindle MTs developed and separated both sets of the daughter chromosomes. From late anaphase to telophase, γ-tubulin could not be detected around the centrioles and MT radiation from the centrioles became diminished at both poles. γ-Tubulin was not detected at the ends of the interzonal spindle fibers. When MTs were depolymerized with amiprophos methyl during mitosis, γ-tubulin localization around the centrioles was clearly confirmed. Moreover, an influx of tubulin molecules into the nucleus for the mitotic spindle occurred at chromosome condensation in mitosis.     

Phosphorylation of centrin during the cell cycle and its role in centriole separation preceding centrosome duplication

Once during each cell cycle, mitotic spindle poles arise by separation of newly duplicated centrosomes. We report here the involvement of phosphorylation of the centrosomal protein centrin in this process. We show that centrin is phosphorylated at serine residue 170 during the G(2)/M phase of the cell cycle. Indirect immunofluorescence staining of HeLa cells using a phosphocentrin-specific antibody reveals intense labeling of mitotic spindle poles during prophase and metaphase of the cell division cycle, with diminished staining of anaphase and no staining of telophase and interphase centrosomes. Cultured cells undergo a dramatic increase in centrin phosphorylation following the experimental elevation of PKA activity, suggesting that this kinase can phosphorylate centrin in vivo. Surprisingly, elevated PKA activity also resulted intense phosphocentrin antibody labeling of interphase centrosomes and in the concurrent movement of individual centrioles apart from one another. Taken together, these results suggest that centrin phosphorylation signals the separation of centrosomes at prophase and implicates centrin phosphorylation in centriole separation that normally precedes centrosome duplication.     

Centrin homologues in higher plants are prominently associated with the developing cell plate

Summary Centrin and calmodulin are members of the EF-hand calcium-binding superfamily of proteins. In this study we compared localisation and immunoblotting of centrin with calmodulin in several monocot (onion and wheat) and dicot (mung bean andArabidopsis) plants. We confirmed that an anti-calmodulin antibody recognised a 17 kDa protein in all species tested and localises to the cytoplasm, mitotic matrix and with microtubules of the preprophase band and phragmoplast. In contrast, immunoblotting using anti-centrin antibodies shows that plant centrins vary from 17 to 20 kDa. Immunofluorescence microscopy with anti-centrin antibodies revealed only weak centrin immunoreactivity in the cytoplasm, nucleus, nuclear envelope, phragmoplast and mitotic matrix in meristematic cells. There was a slightly more intense perinuclear labelling in large differentiating onion cells and between separating anaphase chromosomes. While centrin is known to localise to the mitotic spindle poles in animal and algal cells, there was no appreciable immunoreactivity at the spindle poles in higher plants. In contrast, there was an intense immunofluorescence signal with anti-centrin antibodies in the developing cell plate. Further characterisation of the cell plate labelling by immunogold electron microscopy shows centrin immunoreactivity was closely associated with vesicles in the cell plate. Our observations suggest that centrin may play a role in cell plate formation.Abbreviations BSA bovine serum albumin - MTs microtubules - MTOCs microtubule organising centres - PBS phosphate buffered saline - PBST phosphate buffered saline with Tween-20     

Mitosis in the free-living flagellate Bodo saltans strain PS+ (Kinetoplastidea, Bodonida)

The mitosis in the free-living flagellate Bodo saltans Ps+ with prokaryotic cytobionts in perinuclear space has been studied. The nuclear division in B. saltans Ps+ occurs by closed mitosis type without condensation of chromosomes. Two spatially separated mitotic spindles begin to form consistently at the initial stages of nuclear division. The spindle including about 20 microtubules appears first and later the second spindle with half the number of microtubules comes at the angle of 30-40 degrees. Both spindles rest their ends against the inner nuclear membrane and form 4 distinct poles. The microtubules of the first spindle are associated with 4 pairs of kinetochores, the microtubules of the second one are associated with 2 pairs of kinetochores. The divergence of the kinetochores towards the poles occurs independently in each spindle. The equatorial phase is not revealed in B. saltans Ps+. The poles of both spindles unite in pairs at the elongation phase of mitosis and form the integrated bipolar structure. At this stage of the nuclear division, the kinetochores reach the poles of subspindles and become indistinguishable. Then the nucleus takes the shape of a dumbbell. The inner nuclear membranes of just formed nuclei have layers of condensed chromatin characteristic of the interphase nuclei of kinetoplastidea. The daughter nuclei separate at the phase of reorganization. There are 1-2 prokaryotic endocytobionts in the perinuclear space of the interphase nuclei in B. saltans Ps+. The symbionts multiply during mitosis and their number reaches more than 20 specimens par nucleus.     

IMMUNOLOCALIZATION OF CENTRIN DURING FERTILIZATION AND THE FIRST CELL CYCLE IN FUCUS DISTICHUS AND PELVETIA COMPRESSA (FUCALES,PHAEOPHYCEAE)1

Antibodies that recognize the centrosome-associated protein centrin were used to characterize centrosomal origin and positioning during fertilization and the first cell cycle in Fucus distichus subsp. evanescens (C. Agardh) Powell and Pelvetia compressa (J. Agardh) De Toni. Centrin was identified in sperm, eggs, and zygotes on protein blots, indicating the protein is present in both gametes. Using immunofluorescence microscopy, centrin was found in discrete foci in sperm. In contrast, eggs lack centrosomes and centrin was not detectable by immunofluorescence, indicating that centrin was probably dispersed in the cytoplasm. Two foci of centrin were present on the nuclear envelope of zygotes, but microtubules remained dispersed over the zygotic nucleus. Centrin foci separated over the nuclear envelope as the first cell cycle progressed. Microtubules became concentrated at the centrin foci to form centrosomes that gave rise to the spindle poles at mitosis.     

Transformation of the flagella and associated flagellar components during cell division in the coccolithophoridPleurochrysis carterae

Summary Immunofluorescence microscopy, conventional and high voltage transmission electron microscopy were used to describe changes in the flagellar apparatus during cell division in the motile, coccolithbearing cells ofPleurochrysis carterae (Braarud and Fagerlund) Christensen. New basal bodies appear alongside the parental basal bodies before mitosis and at prophase the large microtubular (crystalline) roots disassemble as their component microtubules migrate to the future spindle poles. By prometaphase the crystalline roots have disappeared; the flagellar axonemes shorten and the two pairs of basal bodies (each consisting of one parental and one daughter basal body) separate so that each pair is distal to a spindle pole. By late prometaphase the pairs of basal bodies bear diminutive flagellar roots for the future daughter cells. The long flagellum of each daughter cell is derived from the parental basal bodies; thus, the basal body that produces a short flagellum in the parent produces a long flagellum in the daughter cell. We conclude that each basal body in these cells is inherently identical but that a first generation basal body generates a short flagellum and in succeeding generations it produces a long flagellum. At metaphase a fibrous band connecting the basal bodies appears and the roots and basal bodies reorient to their interphase configuration. By telophase the crystalline roots have begun to reform and the rootlet microtubules have assumed their interphase appearance by early cytokinesis.Abbreviations CR1, CR2 crystalline roots 1 and 2 - CT cytoplasmic tongue microtubules - DIC differential interference contrast light microscopy - H haptonema - HVEM high voltage transmission electron microscopy - IMF immunofluorescence microscopy - L left flagellum/basal body - M metaphase plate - MT microtubule - N nucleus - R right flagellum/basal body - R1, R2, R3 roots 1, 2, and 3 - TEM transmission electron microscopy     

MITOSIS AND CYTOKINESIS IN ASTEROMONAS GRACILIS, A WALL-LESS GREEN MONAD1

Asteromonas gracilis Artari remains motile throughout cell division. Basal bodies separate and replicate at prophase. They are located lateral to the poles of the closed metaphase spindle. Kinetochores appear at late metaphase. Chromosomes move to the poles and extensions of the nuclear envelope develop into the pyrenoid at anaphase. The interzonal spindle disintegrates at telophase and a diffuse phycoplast is present. Cytokinesis proceeds rapidly from the anterior region of the cell. Newly formed daughter cells have four narrow-banded rootlets and both distal and proximal fibers connect the basal bodies. Features of cell division in Asteromonas are compared to those in other algae, particularly Dunaliella and Chlamydomonas.     

Localization of Tubulin and a Centrin-Homologue in Vegetative Cells and Developing Gametangia of Ectocarpus siliculosus (Dillw.) Lyngb. (Phaeophyceae,Ectocarpales)

The distribution of tubulin and centrin in vegetative cells and during gametogenesis of Ectocarpus siliculosus was studied by immunofluorescence. In interphase cells bundles of microtubules are focused on the centriolar region near the nuclear surface. Some of the bundles ensheath the nucleus while others traverse the cytoplasm in various directions, sometimes reaching the cell cortex. Evaluation of serial optical sections by confocal laser scanning microscopy (CLSM) revealed that the perinuclear and “cytoplasmic” microtubule bundles presumably constitute a single complex. In interphase cells centrin is localized as a single bright spot in the centriolar region. In dividing cells duplication and separation of the microtubular complex and the centrin spot takes place. In post-mitotic cells with two nuclei, the centrioles are located at opposite cell poles, short microtubule bundles emanate from them and partially encompass the nucleus. During gametogenesis a gradual transformation of the vegetative cytoskeleton to the gametic flagellar apparatus occurs.     

Centrin scaffold in Chlamydomonas reinhardtii revealed by immunoelectron microscopy

In the flagellate green alga Chlamydomonas reinhardtii the Ca(2+)-binding EF-hand protein centrin is encoded by a single-copy gene. Previous studies have localized the protein to four distinct structures in the flagellar apparatus: the nucleus-basal body connector, the distal connecting fiber, the flagellar transitional region, and the axoneme. To explain the disjunctive distribution of centrin, the interaction of centrin with as yet unknown specific centrin-binding proteins has been implied. Here, we demonstrate using serial section postembedding immunoelectron microscopy of isolated cytoskeletons that centrin is located in additional structures (transitional fibers and basal body lumen) and that the centrin-containing structures of the basal apparatus are likely part of a continuous filamentous scaffold that extends from the nucleus to the flagellar bases. In addition, we show that centrin is located in the distal lumen of the basal body in a rotationally asymmetric structure, the V-shaped filament system. This novel centrin-containing structure has also been detected near the distal end of the probasal bodies. Taken together, these results suggest a role for a rotationally asymmetric centrin "seed" in the growth and development of the centrin scaffold following replication of the basal apparatus.     

Function of donor cell centrosome in intraspecies and interspecies nuclear transfer embryos

Centrosomes, the main microtubule-organizing centers (MTOCs) in most animal cells, are important for many cellular activities such as assembly of the mitotic spindle, establishment of cell polarity, and cell movement. In nuclear transfer (NT), MTOCs that are located at the poles of the meiotic spindle are removed from the recipient oocyte, while the centrosome of the donor cell is introduced. We used mouse MII oocytes as recipients, mouse fibroblasts, rat fibroblasts, or pig granulosa cells as donor cells to construct intraspecies and interspecies nuclear transfer embryos in order to observe centrosome dynamics and functions. Three antibodies against centrin, gamma-tubulin, and NuMA, respectively, were used to stain the centrosome. Centrin was not detected either at the poles of transient spindles or at the poles of first mitotic spindles. gamma-tubulin translocated into the two poles of the transient spindles, while no accumulated gamma-tubulin aggregates were detected in the area adjacent to the two pseudo-pronuclei. At first mitotic metaphase, gamma-tubulin was translocated to the spindle poles. The distribution of gamma-tubulin was similar in mouse intraspecies and rat-mouse interspecies embryos. The NuMA antibody that we used can recognize porcine but not murine NuMA protein, so it was used to trace the NuMA protein of donor cell in reconstructed embryos. In the pig-mouse interspecies reconstructed embryos, NuMA concentrated between the disarrayed chromosomes soon after activation and translocated to the transient spindle poles. NuMA then immigrated into pseudo-pronuclei. After pseudo-pronuclear envelope breakdown, NuMA was located between the chromosomes and then translocated to the spindle poles of first mitotic metaphase. gamma-tubulin antibody microinjection resulted in spindle disorganization and retardation of the first cell division. NuMA antibody microinjection also resulted in spindle disorganization. Our findings indicate that (1) the donor cell centrosome, defined as pericentriolar material surrounding a pair of centrioles, is degraded in the 1-cell reconstituted embryos after activation; (2) components of donor cell centrosomes contribute to the formation of the transient spindle and normal functional mitotic spindle, although the contribution of centrosomal material stored in the recipient ooplasm is not excluded; and (3) components of donor cell centrosomes involved in spindle assembly may not be species-specific.     

Ultrastructure of mitosis in the aphid-pathogenic fungusErynia neoaphidis

Summary An account of mitosis in the aphid-pathogenic, entomophthoraceous fungusErynia neoaphidis is presented. The mitotic apparatus is characterized by a closed, intranuclear, polarized spindle. Chromosomes are permanently attached by kinetochore microtubules (kcMTs) to the poles during mitosis. The spindle develops as the spindle pole bodies migrate and separate. At metaphase the eccentric spindle contains only kcMTs and is located in a relatively chromatinfree zone. Paired sister kinetochores are arranged in a broad metaphase plate. During anaphase kcMTs shorten, astral and nonchromosomal microtubules develop and elongate and the interpolar distance increases.     

Centrin1 is required for organelle segregation and cytokinesis in Trypanosoma brucei

Centrin is a calcium-binding centrosome/basal body-associated protein involved in duplication and segregation of these organelles in eukaryotes. We had shown that disruption of one of the centrin genes (centrin1) in Leishmania amastigotes resulted in failure of both basal body duplication and cytokinesis. Here, we undertook to define the role of centrin1 (TbCen1) in the duplication and segregation of basal body and its associated organelles kinetoplast and Golgi, as well as its role in cytokinesis of the procyclic form of Trypanosoma brucei by depleting its protein using RNA inhibition methodology. TbCen1-depleted cells showed significant reduction in growth compared with control cells. Morphological analysis of these cells showed they were large and pleomorphic with multiple detached flagella. Both immunofluorescence assays using organelle-specific antibodies and electron microscopic analysis showed that TbCen1-deficient cells contained multiple basal bodies, kinetoplasts, Golgi, and nuclei. These multiple organelles were, however, closely clustered together, indicating duplication without segregation in the absence of centrin. This failure in organelle segregation may be the likely cause of inhibition of cytokinesis, suggesting for the first time a new and unique role for centrin in the segregation of organelles without affecting their multiplication in the procyclic form of T. brucei.     

Mitosis in the Hemoflagellate Trypanosoma cyclops*

SYNOPSIS. The ultrastructure of interphase and mitotic nuclei of the epimastigote form of Trypanosoma cyclops Weinman is described. In the interphase nucleus the nucleolus is located centrally while at the periphery of the nucleus condensed chromatin is in contact with the nuclear envelope. The nucleolus fragments at the onset of mitosis, but granular material of presumptive nucleolar origin is often recognizable in the mitotic nucleus. Peripheral chromatin is in contact with the nuclear envelope throughout mitosis, and it seems reasonable to assume that the nuclear envelope is involved in its segregation to the daughter nuclei. Spindle microtubules extend between the poles of the dividing nucleus and terminate close to the nuclear envelope. The basal body and kinetoplast divide before the onset of mitosis and do not appear to have any morphologic involvement in that process. Spindle pole bodies, kinetochores, and chromosomal microtubules have not been observed.