Interzonal Spindle Research Articles

Mitosis and mitotic wave propagation in the coenocytic alga, Vaucheria terrestris sensu Goetz.

Mitosis and cytoplasmic microtubule (MT) dynamics were observed for the first time in Vaucheria terrestris sensu Goetz. Mitosis could occasionally be seen in part of the cylindrical coenocytic cell. The frequency of encountering cells with dividing nuclei was highest (ca 12%) 4 h after the onset of light in 12 h light/12 h dark regimes; it decreased thereafter and approached zero during the dark period. From the anterior end of every interphase nucleus a unique, long MT bundle extended. Differential-interference optics reveals that there is a filamentous structure in front of the moving nucleus. In prophase, the interphase bundle disappeared and shorter MT bundles emanated from both ends of the nucleus. In metaphase, the cytoplasmic MTs completely disappeared, probably being recycled to spindles. Continuous MTs elongated in anaphase and developed into an interzonal spindle in telophase; this elongated up to as much as 10 microm. The daughter nuclei were pushed away from each other by the interzonal spindle. Mitosis started synchronously in a relatively narrow region, and the mitotic stage propagated as a mitotic wave to adjacent regions, most frequently from tip to base. The role of the mitotic wave in tip growth and morphogenesis of a coenocytic cell is discussed.

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

Mitosis inBoergesenia 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 inB. forbesiiwas 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.

Kinetic Analysis of the Mitotic Cycle of Living Vertebrate Cells by Atomic Force Microscopy

The atomic force microscope (AFM) is becoming an important tool for qualitative and quantitative analyses of biological material. However, the difficulties involved in maintaining long-term, steady-state physiologic conditions and the problems associated with analyzing force curves generated from highly viscoelastic biological structures impede the use of the AFM for studies of kinetic processes in living vertebrate cells. In this report, we describe a simple method to track reproducibly kinetic changes in the localized stiffness of vertebrate cells. We tested our method on a study of vertebrate cells in mitosis and found a marked but transient decrease in stiffness occurs in the mitotic spindle region during anaphase. We propose that physical–chemical changes in the mitotic apparatus, most probably, changes in the state of polymerization of interzonal spindle fibers which also have been reported to undergo a marked reduction in birefringence during anaphase, are responsible for the observed decrease in stiffness. Our methodology affords a new approach to studying mitotic events and should be applicable to studies of a variety of viscoelastic properties of living cells.

Unusual pattern of mitosis in the free-living flagellateDimastigella mimosa (Kinetoplastida)

The three-dimensional ultrastructural organization of the mitotic apparatus ofDimastigella mimosa was studied by computer-aided, serial-section reconstruction. The nuclear envelope remains intact during nuclear division. During mitosis, chromosomes do not condense, whereas intranuclear microtubules are found in close association with six pairs of kinetochores. No discrete microtubule-organizing centers, except kinetochore pairs, could be found within the nucleus. The intranuclear microtubules form six separate bundles oriented at different angles to each other. Each bundle contains up to 8 tightly packed microtubules which push the daughter kinetochores apart. At late anaphase only, midzones of these bundles align along an extended interzonal spindle within the narrow isthmus between segregating progeny nuclei. The nuclear division inD. mimosa can be described as closed intranuclear mitosis with acentric and separate microtubular bundles and weakly condensed chromosomes.

Nuclear phenomena during conjugation of the suctorian Heliophrya erhardi: I. EM-Observations of micronuclear meiosis

Summary We investigated micronuclear meiosis in the conjugating suctorian Heliophrya erhardi by time lapse cinemicrography of living cells, by indirect immunofluorescence (IIF) with monoclonal antibodies against modified tubulins, and by transmission electron microscopy, respectively high voltage electron microscopy (HVEM) of ultrathin and semithin sections. The fine structure of mid meiotic prophase reveals moderately condensed chromatin threads forming a “parachute” arrangement. Since only extremely short and also somewhat atypically structured synaptonemal complexes (SCs) are first discernible as early as diplotene, when partial chromosomal disjunction is seen, it is suggested that we are dealing here with achiasmatic meiosis. The spindle axis is established during prometaphase I by parallel alignment of polar MTs (pMTs) and the congression of chromosomes towards the equator is brought about by outgrowing kinetochore MTs (kMTs) becoming increasingly parallel. Metaphase I is indicated by a complete depolymerization of pMTs, revealing a truncated spindle type consisting exclusively of kMTs which never reach the presumptive poles before the metaphase/anaphase transition, and, as also seen by IIF, several of the small chromosomes converge with their kMTs forming numerous “subspindles”. These are focused on electron dense bodies located at half the distance to the presumptive poles. At late metaphase stages spindles become stabilized by their anchorage at the lamin-like sheets of the inner membrane of the nuclear envelope. Anaphase I is clearly subdivided into anaphase A and into anaphase B. While chromosome segregation in anaphase A I is not conspicuous an enormous interzonal spindle elongation that contributes substantially to chromosomal segregation is formed at anaphase B I. The mean velocity of pole separation of 0.5 μm/min corresponds rather well with the anaphase AI poleward migration. The telophase spindle develops from a centrally located torsional zone of the stembody in anaphase B by separation into two twisted areas which push the chromosomal fronts into opposite directions. During the dumbbell-shaped telophase stage, the daughter nuclei are pinched off by rotational movements of the terminalized contorted regions of the spindle. The nuclear events leading to meiosis II are highly similar to meiosis I.

Cytokinesis in Spirogyra: Integration of Cleavage and Cell-Plate Formation

In Spirogyra, cytokinesis is initiated by a cleavage furrow. When the furrow contacts the elongating interzonal fibers of the telophase spindle, a small phragmoplast appears at the zone of contact, inside of which a cell-plate appears to complete cytokinesis. The use of two sequential mechanisms for achieving cytokinesis may be an intermediate, and possibly a transitional, state in the evolution of the phragmoplast found in higher plants. We tested the relative contribution of the two types of cytokinetic mechanisms by observing the effects of certain drugs on living cells, recorded in time-lapse. The antimicrotubule drug oryzalin causes the spindle to break down rapidly. When applied during early to midcytokinesis, the interzonal spindle collapses onto the cleavage furrow, which continues to grow inward. However, cytokinesis is not complete; examination of such cells 8-12 h later, after the nuclei separate, reveals a small aperture in the center of the cross-wall. This result shows that the phragmoplast/cell-plate is essential for complete cytokinesis, which apparently cannot be accomplished by cleavage alone, and that inward growth of the cleavage furrow is not dependent on microtubules. The antiactin drug cytochalasin D stops cleavage quite rapidly, and the eventual consequences of treatment depend on the stage reached in cleavage. If the cytochalasin is applied early in cytokinesis, cell division is unable to proceed further. Between telophase nuclei the cell does generate a large mass of cytoplasm that contains typical proliferating phragmoplast fibers; cell-plate formation is initiated in this mass but is never able to proceed further, and no coalescence of material into a cross-wall has been recorded. If cytochalasin is applied later, after the cleavage furrow has contacted the interzonal spindle fibers, cleavage stops but now cell-plate formation proceeds and cytokinesis is completed, albeit slowly. This result demonstrates that an interaction between the cleavage furrow and the forming phragmoplast is necessary for the latter to operate normally. These results support our suggestion that Spirogyra illustrates an intermediate stage in the evolution of the phragmoplast, since both cleavage and cell-plate formation are required for cytokinesis. (This suggestion does not require Spirogyra to be on the line of evolution leading to higher plants; rather, Spirogyra appears to be undergoing an evolutionary process similar to what might have occurred in their progenitors.) Furthermore, interaction of the cleavage furrow with the forming cell-plate/phragmoplast is required for cross-wall completion. These results may have significance in considering mechanisms by which higher plants locate their cellplates properly, an ability associated with the evolution and possible function(s) of the preprophase band of microtubules.

Mitosis in the coenocytic green algaBoergesenia forbesii (Harvey) Feldmann (Siphonocladales, Ulvophyceae)

Mitosis in vegetative cells of the siphonocladalean algaBoergesenia forbesii (Harvey) Feldmann was investigated mainly by electron microscopy. The mitotic spindle was centric and closed. The interphase nucleus contained a spherical nucleolus. The nucleolus was slightly dispersed at prophase, but nucleolar materials remained during nearly all stages of mitosis. Kinetochores were evident on chromosomes. The polar regions of nuclear envelope had no fenestrae during mitosis. Anaphase separation of the chromosomes was asynchronous. Elongation of interzonal spindle at telophase separated the two daughter nuclei widely. The ultrastructural features of mitosis inB. forbesii revealed by the present investigation are compared with those of other siphonous and siphonocladous algae in the Ulvophyceae.

ULTRASTRUCTURE OF VEGETATIVE ORGANIZATION AND CELL DIVISION IN THE UNICELLULAR RED ALGA DIXONIELLA GRISEA GEN. NOV. (RHODOPHYTA) AND A CONSIDERATION OF THE GENUS RHODELLA1

ABSTRACTThis study suggests that the genus Rhodella be restricted to that set of features currently observed only in Rhodella maculata Evans and Rhodella violacea (Korn‐mann) Wehrmeyer, that a new genus Dixoniella be established to accommodate the unicellular red alga, Rhodella grisea (Geitler) Fresnel, Billard, Hindák et Pekár‐ková, and that Rhodella cyanea Billard et Fresnel be further studied for probable reclassification. These conclusions are based on ultrastructural comparisons of Dixoniella grisea with published information on the genus Rhodella. The presence of thylakoids in the pyrenoid, a peripheral encircling thylakoid in the chloroplast, a dictyosome/nuclear envelope association, and the lack of a specialized pyrenoid/nucleus association in D. grisea separate this alga from the genus Rhodella. Cell division in D. grisea is not demonstrably different from that in Rhodella, although the unusually well‐defined material of the presumptive microtubule organizing center (MTOC) made it possible to follow the development and behavior of the MTOC to a greater degree than in previously studied red algal cells. The surprising amount of conformity in cell division characters between D. grisea and the genus Rhodella prompted a comparison of cell division characteristics in all red algal unicells studied to date.All unicells show a remarkable degree of similarity except for differences in interzonal spindle length, dissimilarities in size of the nucleus‐associated organelle (NAO), and the unusual NAO of Porphyridium purpureum (Bory) Drew et Ross.

Ultrastructure of zoosporogenesis in the alga Botrydiopsis alpina (Tribophyceae), as revealed by cryofixation and freeze substitution

The fine structure of mitosis and cytokinesis leading to formation of zoospores in the unicellular tribophycean alga Botrydiopsis alpina Vischer has been studied by ultra-rapid cryofixation, freeze substitution, and electron microscopy. Preceding zoosporogenesis, the cell usually enlarges considerably and becomes multinucleate. Throughout zoosporogenesis, nuclear divisions are synchronous with centrioles and dictyosomes intimately associated with nuclear poles. During prophase, nuclei migrate towards the periphery of the cell. During metaphase, the spindle becomes aligned parallel to the cell surface. Anaphase separations of the chromosomes are synchronous and telophase is characterized by a persistent, slightly narrowed, elongated interzonal spindle. Formation of flagellar axonemes mainly commences in the central cytoplasm during prophase and, gradually during mitosis, these profiles are shifted towards the periphery of the developing zoosporangium. Cytokinesis occurs by synchronous, centripetal cleavage from the plasma membrane. Throughout mitosis, special flat vesicles are present underneath the plasma membrane and in conspicuous assemblages around extra-plasmatic pockets which contain extruded flagella. The present observations corroborate the view that centrioles may (in)directly determine the origin of cleavage furrows. The possible function of the flat vesicles is evaluated.

Immunolocalization of pericentriolar material in algal mitotic spindles

AbstractDouble‐label immunofluorescence of tubulin and preicentriolar material (PCM) was carried out with mitotic nuclei in the coenocytic green alga Ernodesmis. Spindle poles are heavily labeled with serum 5051 (anti‐PCM) from midprophase through mid‐ to late anaphase, and bright fluorescence is also evident at the tips of the elongated interzonal spindle in telophase nuclei. Very faint labeling with anti‐PCM is also detected throughout the spindle (and/or its matrix) at all mitotic stages. Control treatments demonstrated that nonspecific surface labeling of chloroplasts with anti‐PCM may be due to some naturally occurring component of human sera rather than to specific labeling by the anti‐PCM serum. Ultrastructural work indicates that the centrosome is always associated with spindle poles through anaphase, but not with the tips of the interzonal telophase Immunoper‐oxidase electron microscopy verifies that anti‐PCM labels the centrosomes of mitotic nuclei in these cells. However, labeling is also present inside the presistent nuclear envelope at the spindle poles, during metaphase, anaphase, and at the tips of the interzonal spindles. Regions of heaviest labeling correspond with amorphous material near the centrioles and at the spindle poles, as evident in conventional electron microscope preparations. The origin of intranuclear amorphous material that labels with anti‐PCM is unclear, but the ends of many spindle microtubules are embedded in it, especially at anaphase, and the tips of microtubules near the amorphous material are often labeled with the antiserum. These results indicate for the first time that serum 5051 does indeed label PCM at the poles of centric spindles in plant cells. Although the location of the labeled material suggests it is associated with the nucleation of spindle microtubules, this conclusion requires more information about microtubule dynamics in these cells. Caution is also warranted in interpreting variant anti‐PCM labeling patterns in other plant cells because of spurious labeling of the spindle itself and other cytoplasmic organelles.

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

ABSTRACTThe 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.

Dynamics of the microtubular cytoskeleton in the green algaAphanochaete magna (Chlorophyta). I. Late mitotic stages and the origin and development of the phycoplast

The spatial and temporal organization of the microtubular cytoskeleton at the transitional stage of mitosis and cytokinesis has been studied in the chaetophoralean green algaAphanochaete magna using indirect immunofluorescence light microscopy and transmission electron microscopic analysis of serial sections including computer-aided three-dimensional reconstruction. At late mitosis, elaborate asterlike microtubule systems including bundles interconnecting both centriolar regions are present. These systems disappear a the onset of interzonal spindle disintegration. The incipient phycoplast consists of a star-shaped microtubule assemblage projecting from the intact interzonal spindle. It develops strongly at the time of spindle disintegration, later on it becomes compressed by daughter nuclei movement. Cell plate formation is associated with a two-dimensional phycoplast. Phycoplast microtubules remain for a while associated with the completed cross wall but finally they depolymerize. The general occurrence of astral microtubule systems (includingA. magna) is evaluated. The subsequent developmental stages of the phycoplast, formation, maturation and depolymerization, are discussed.

THE ULTRASTRUCTURE OF MITOSIS AND CYTOKINESIS IN THE SARCINOIDCHLOROKYBUS ATMOPHYTICUS(CHLOROPHYTA, CHAROPHYCEAE) REVEALED BY RAPID FREEZE FIXATION AND FREEZE SUBSTITUTION1

ABSTRACTMitosis and cytokinesis in vegetative cells of the sarcinoid green alga Chlorokybus atmophyticus Geitler were examined with rapid freeze fixation, freeze substitution, and transmission electron microscopy. The taxonomic placement of C. atmophyticus in the class Charophyceae sensu Stewart and Mattox is corroborated by some mitotic and cytokinetic features including development of a microtubular sheath around the prophase nucleus, the almost constant chromosome to pole distance during anaphase, telophase nuclei widely separated by a persistent interzonal spindle, and centripetal plasma membrane invagination. Features, previously unknown in the Charophyceae, include the specific position of the peroxisome lying between the nucleus and adjacent cell wall during interphase and mitosis, the extensive array of microtubules radiating from the centrioles located at the presumptive poles at prophase, involvement of coated vesicles in the furrowing process, and occurrence of transversely aligned cleavage microtubules. Placement of Chlorokybus in the order Klebsormidiales is proposed.

Ultrastructure of Mitosis in Entosiphon sulcatum (Euglenida)1

ABSTRACT The ultrastructural features of cell division in the biflagellate, phagotrophic euglenoid, Entosiphon sulcatum, have been examined. Prophase is marked by the appearance of daughter feeding apparatuses and the emergence of two additional flagella. Pairs of flagella begin to migrate laterally along the surface of the elongating nucleus and remain lateral to the developing spindle poles. As the nucleolus elongates, it becomes dumbbell‐shaped and the chromosomes move to the center of the nucleus, forming a loosely organized metaphase plate. Microtubules from opposing spindle poles attach to one of the pair of kinetochores found on each chromosome. The initial chromosome separation occurs during anaphase as the nucleus elongates. The length of the chromosomal microtubules does not decrease until late anaphase/early telophase. As the nucleus elongates, it forms a dumbbell‐shaped structure. Most of the remaining microtubules are positioned in the interzone between the forming daughter nuclei. The interzonal spindle becomes somewhat constricted but remains intact until it is broken by the impinging cleavage furrow. Replication of the pellicular strips is not completed until late in cytokinesis.

Mitosis and cytokinesis inTribonema regulare (Tribophyceae, Chrysophyta)

The ultrastructure of mitosis and cytokinesis of the uninucleateTribonema regulare has been investigated by employing transmission electron microscopy. Prophase is characterized by settlement of a pair of centrioles at the presumptive poles of the spindle, metaphase by equatorial bulging of the nucleus, anaphase by non-synchronous separation of the chromosomes, and telophase by a persistent, strongly elongated, interzonal spindle. Throughout mitosis, at each pole dictyosomes are associated with the polar gaps of the nuclear envelope that otherwise remains intact. Cytokinesis does not immediately follow mitosis; from the static images it can be concluded that it is necessary for the daughter nuclei to approach each other before cytokinesis is initiated by complete division of the protoplast via plasma membrane cleavage. Afterwards, a ring of cell wall material is deposited close near the lateral wall in the plane of protoplast separation followed by a simultaneous or centripetal development of a single integral partitioning septum. Once the septum is completed, the cylindrical portion of the H-shaped segment is manufactured. The phylogenetic position ofTribonema amongst those algae, which may have evolved from unicells into filaments, is discussed.

On the mechanism of anaphase A: evidence that ATP is needed for microtubule disassembly and not generation of polewards force.

As anaphase began, mitotic PtK1 and newt lung epithelial cells were permeabilized with digitonin in permeabilization medium (PM). Permeabilization stopped cytoplasmic activity, chromosome movement, and cytokinesis within about 3 min, presumably due to the loss of endogenous ATP. ATP, GTP, or ATP-gamma-S added in the PM 4-7 min later restarted anaphase A while kinetochore fibers shortened. AMPPNP could not restart anaphase A; ATP was ineffective if the spindle was stabilized in PM + DMSO. Cells permeabilized in PM + taxol varied in their response to ATP depending on the stage of anaphase reached: one mid-anaphase cell showed initial movement of chromosomes back to the metaphase plate upon permeabilization but later, anaphase A resumed when ATP was added. Anaphase A was also reactivated by cold PM (approximately 16 degrees C) or PM containing calcium (1-10 mM). Staining of fixed cells with antitubulin showed that microtubules (MTs) were relatively stable after permeabilization and MT assembly was usually promoted in asters. Astral and kinetochore MTs were sensitive to MT disassembly conditions, and shortening of kinetochore MTs always accompanied reactivation of anaphase A. Interphase and interzonal spindle MTs were relatively stable to cold and calcium until extraction of cells was promoted by longer periods in the PM, or by higher concentrations of detergent. Since we cannot envisage how both cold treatment or relatively high calcium levels can reactivate spindle motility in quiescent, permeabilized, and presumably energy-depleted cells, we conclude that anaphase A is powered by energy stored in the spindle. The nucleotide triphosphates effective in reactivating anaphase A could be necessary for the kinetochore MT disassembly without which anaphase movement cannot proceed.

Ultrastructure of mitosis and cytokinesis inZygnema sp. (Zygnematales, Chlorophyta)

Mitosis and cytokinesis have been studied in the green algaZygnema C. A. Agardh using interference-contrast light and transmission electron microscopy. At prophase, the nucleolus disintegrates and numerous extranuclear microtubules near the nuclear periphery penetrate into the nucleoplasm. When aligned in the equatorial plane of the open metaphase spindle the chromosomes are coated with persistent nucleolar fragments. At anaphase, vacuoles intrude into the interzonal spindle region and seemingly contribute to the anaphase movement of the chromosomes. At telophase, the spindle is persistent and the reforming nuclei are separated by cytoplasmic strands containing microtubules, interspersed with vacuoles. Extensive bundles of microtubules, dictyosomes and parallel, slightly inflated ER-profiles extend from the poles of the telophase nucleus along the longitudinal side of the chloroplast. Conceivably, these microtubules guide the nucleus during its post-mitotic migration towards its central interphase position between the two halves of the dividing chloroplast. Throughout the mitotic cycle, ubiquitous dictyosomes, positioned near the chloroplast core, seem very active. Arrays of microtubules run towards these dictyosomes and may conduct the dictyosome-vesicles to the cleavage plane. At metaphase, septum growth becomes visible as an annular ingrowth of the plasmalemma. At late telophase or at entering interphase, an extensive clump of vesicles, associated with longitudinal bundles of microtubules, appears between the leading edges of the advanced furrow. Apparent fusion of these vesicles with the head of the centripetally-growing furrow results in its completion. The pattern of mitosis and cytokinesis inZygnema is compared with that of closely related green algae.

Ultrastructural features of mitosis inLeishmania adleri

The interphase nucleus ofLeishmania adleri has clumps of chromatin associated with the nuclear envelope and a large centrally located nucleolus. Prior to mitosis the basal bodies replicate at the cell anterior. Subsequently, dense plaques appear in the equatorial region of the nucleus at the time of spindle development. Microtubules appear in the nucleus adjacent to the nuclear envelope and embedded in the matrix of the plaques. A central spindle composed of a single bundle of microtubules develops and spans the nucleus. Plaques and nucleolar components laterally associate with the spindle and migrate towards the poles. The central spindle elongates to three to four times its original length separating the forming daughter nuclei and producing an interzonal spindle. A remnant of the interzonal spindle remains attached to each of the daughter nuclei until late into cytokinesis. The kinetoplast does not divide until after the completion of mitosis.

PHRAGMOPLASTS IN CYTOKINESIS OFCEPHALEUROS PARASITICUS(CHLOROPHYTA) VEGETATIVE GELLS1

ABSTRACTCytokinesis in apical cells of actively growing cultures ofCephaleuros parasiticusKarsten sporangiate thalli was examined with transmission electron microscopy. A massive, interzonal cytokinetic microtubule spindle is anchored at its poles to the medial surfaces of the daughter nuclei at telophase. Later, the daughter nuclei are widely separated and no longer associated with the interzonal spindle; however, the spindle retains its shape and becomes a distinct phragmoplast with an array of vesicles, presumably derived from dictyosomes, aligned in the division plane. Fusion of the vesicles gives rise to a thin cell plate. Some bundles of microtubules in the spindle appear to mark the sites of plasmodesmata formation, but no endoptasmic reticulum is directly involved in plasmodesmata formation. No infurrowing or phycoplast array of microtubules is involved in the cytokinesis. The relationship, if any. between the metaphase‐anaphase mitotic microtubule system and the interzonal cytokinetic spindle has not been determined. Cephaleuros parasiticus isone of only four green algae now known to contain a higher plant‐like phragmoplast and cytokinetic process. The observations reported can be interpreted as very strong evidence for a phylogenetic affinity between the Trentepohliaceae and the Charophyceae, but consideration of ulvophycean features of the Trentepohliaceae such as motile cell ultrastructure and life histories precludes unequivocal assignment of the family to either the Charophyceae or Ulvophyceae.

Ultrastructural Features of Mitosis in Anisonema sp. (Euglenida)1

ABSTRACTThe fine structure of stages in mitosis in a colorless euglenoid, Anisonema sp., reveals that chromosomes remain condensed throughout the life cycle and are attached to the nuclear envelope at interphase. The onset of mitosis is marked by the anterior migration of the nucleus towards the base of the reservoir and by elongation of the nucleolus. The nuclear envelope persists throughout mitosis. Microtubules are generated in the peripheral nucleoplasm adjacent to the envelope and attach to the chromosomes while they are still associated with the envelope. The region of microtubular contact develops into a distinct layered kinetochore as the developing spindle with attached chromosomes separates from the nuclear envelope and moves into the nucleoplasm. The mature spindle consists of a number of subspindles each containing about 8–10 microtubules and a few associated chromosomes. Both chromosomal and non‐chromosomal microtubules are present in each subspindle and extend towards the envelope terminating at or near the nuclear pores. Chromosomal segregation is concomitant with nuclear elongation. By late division, an interzonal spindle develops in the dumbbell‐shaped nucleus and nucleolar separation occurs. Continued invagination of the nuclear envelope in the region of the interzonal spindle eventually separates the daughter nuclei. A remnant of the interzonal spindle persists in the cytoplasm until cytokinesis.