CELL DIVISION IN THE FILAMENTOUS PLEURASTRUM AND ITS COMPARISON WITH THE UNICELLULAR PLATYMONAS (CHLOROPHYCEAE)1

At prophase in Pleurastrum, extranuclear spindle microtubules develop from the region of centrioles, which lie lateral to the nucleus midway between the future sites of the metaphase spindle poles. The microtubules then move laterally to overarch the nucleus and finally become incorporated into the spindle. The centrioles do not migrate and therefore lie in the same plane as the chromosomes at metaphase. At telophase, 2, more different systems of microtubules develop from the vicinity of the centrioles—a phycoplast and extensive arrays of microtubules that ensheath the daughter nuclei. Cell division in the filamentous Pleurastrum is compared to that in the green flagellate, Platymonas. The similarities between cell division in the 2 algae are interpreted as evidence: (i) that rhizoplasts (which in Platymonas resemble myofibrils) are somehow homologous to microtubules; and, (ii) that cell division in Pleurastrum differs from cell division in other examined filamentous chlorophycean genera because Pleurastrum has an independent evolutionary origin from a monad with Platymonas-like characteristics.     

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.     

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.     

MITOSIS AND CYTOKINESIS IN PLATYMONAS SUBCORDIFORMIS,A SCALY GREEN MONAD1

Scaly green monads are often placed in a separate class, Prasinophyceae, and have been considered to be among the most, primitive of green algae. Platymonas possesses rhizoplasts which resemble sarcomeric structures. At prophase, extranuclear spindle micro-tubules emanate from a granular region which appears to arise through dissolution or dispersion of the rhizoplasts. It is probable that the rhizoplasts are largely consumed during the formation of the spindle and only small fragments are left at metaphase. The rhizoplasts can be seen again at telophase but are short at this stage. The basal bodies are not at the spindle poles but remain at their interphase position. The interzonal spindle collapses early at telophase, and shortly thereafter cleavage microtubules appear. These microtubules extend from the region of the basal bodies to the posterior of the cell. The events of cell division are compared with these events in other green algae and in Ochromonas. The functional and phylogenetic significance of the observations is discussed.     

MITOSIS IN THE OCTAFLAGELLATE PRASINOPHYTE,PYRAMIMONAS AMYLIFERA (CHLOROPHYTA)1

Cell division is described in the octaflagellate prasinophyte Pyramimonas amylifera Conrad and is compared in related genera. Basal bodies replicate at preprophase and move toward the poles. Cells remain motile throughout division. The nuclear envelope disperses and chromosomes begin to condense at prophase. Pairs of multilayered kinetochores are evident on the chromosomes of the metaphase plate. Spindle microtubules extending from the region of the basal bodies and rhizoplasts attach to the kinetochores or extend from pole to pole. Numerous vesicles and ribosomes have entered the nuclear region and the incipient cleavage furrow invaginates. The chromosomes move toward the poles at anaphase leaving a broad interzonal spindle between the two chromosomal plates. The nuclear envelope reforms first around the chromatin on the side adjacent to the spindle poles and later on the interzonal side. The cleavage furrow progresses into the interzonal spindle at telophase. By late telophase the nucleoli have reformed and the chromosomes have decondensed. The interzonal spindle has not been observed late in telophase. As the cleavage furrow nears completion the cells begin to twist and contort, ultimately separating the two cells.     

A COMPARATIVE STUDY OF CELL DIVISION IN TRICHOSARCINA POLYMORPHA AND PSEUDENDOCLONIUM BASILIENSE (CHLOROPHYCEAE)1

Pseudendoclonium basiliense and Trichosarcina polymorpha are essentially identical with regard to the fine structural details of cell division even though one was previously classified in the Chaetophorales and the other in the Ulvales. Cell division in the 2 genera is also shown to be like that in Ulva, as previously suggested might be the case. The combination of mitotic and cytokinetic characteristics common to the 3 genera is distinctive: (1) precocious development of a thick cleavage furrow, (2) centrioles distinctly lateral to polar fenestrae, (3) collapse of the interzonal spindle at telophase, and. (4) a cleavage furrow not associated with microtubules. It is suggested that features of vegetative cell division presently provide the best, characteristics for defining the Ulvaceae and that the use of growth habit should be abandoned. Despite the fact that a phycoplast is not present, in these algae, it is concluded that their affinities lie with genera that do possess a phycoplast.     

An ultrastructural study of the flagellateTetraselmis cordiformis stein (Chlorophyceae) with emphasis on the flagellar apparatus

Summary The ultrastructure of the freshwater flagellateTetraselmis cordiformis Stein (Chlorophyceae) was investigated. The general morphology could be described as typical prasinophycean (Prasinophyceae sensu Christensen) and the organism shares all generic characteristics ofPlatymonas West. The flagellar apparatus has been examined in detail. The four flagella emerge from an apical trough in the theca and are arranged in a zig-zag row. They are covered by three types of scales. Four cruciate flagellar roots of compound type are present. One part is microtubular (4-2-4-2 system) and the other prominent part is fibrillar with distinctive cross striations. The four roots are short and terminate at the bottom of the apical through, where they attach the flagellar apparatus to the theca. The four-stranded root shows no changes in root tubule configuration. In addition to the cruciate root system there are two massive rhizoplasts. The rhizoplasts exhibit different striation patterns along their length. Taxonomic implications and flagellar root system structure as it relates to current theories of evolution in green algae are discussed.     

PHRAGMOPLASTS IN CYTOKINESIS OF CEPHALEUROS PARASITICUS (CHLOROPHYTA) VEGETATIVE GELLS 1

Cytokinesis in apical cells of actively growing cultures of Cephaleuros parasiticus Karsten 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.     

The ultrastructure of mitosis inIsochrysis galbana parke (Prymnesiophyceae)

Summary Mitosis and cytokinesis have been studied in the flagellate algaIsochrysis galbana Parke (Prymnesiophyceae). Nuclear division is preceded by replication of the flagella and haptonema, the Golgi body and the chloroplast; fission in the chloroplast occurs in the region of the pyrenoid. During prophase, spindle microtubules radiating from two ill-defined poles are formed. The nuclear envelope breaks down and the chromatin condenses. At metaphase the spindle is fully developed, some pole-to-pole microtubules passing through the well-defined chromatin plate, others terminating at it. No kinetochores or individual chromosomes were observed. By late metaphase, many Golgi-derived vesicles may be seen against the two poleward faces of the metaphase plate. During anaphase, the two daughter masses of chromatin move towards the poles. In early telophase, the nuclear envelope of each daughter nucleus is complete only on the side towards the adjacent chloroplast, remaining open on the interzonal side. However, during telophase each nucleus becomes reorientated so that it lies lateral to the long axis of the spindle and with its open side towards the chloroplasts. By late telophase, each new nuclear envelope is complete and confluence with the adjacent chloroplast ER established.Cytokinesis and subsequent segregation of the daughter cells are effected by the dilation of Golgi- and ER-derived vesicles in the interzonal region. No microtubular structures are involved. Comparisons with the results from other studies of mitosis in members of thePrymnesiophyceae show that they all have a number of features in common, but that there are differences in detail between species.     

FURTHER STUDIES OF PRESUMEDLY PRIMITIVE GREEN ALGAE,INCLUDING THE DESCRIPTION OF PEDINOPHYCEAE CLASS. NOV. AND RESULTOR GEN. NOV.1

The tiny jumping flagellate originally described as Pedinomonas mikron Throndsen was isolated into pure culture from Australian waters and its ultrastructure critically examined. Pedinomonas mikron differs in behavior and in features of the flagellar apparatus from P. minor, the type species from freshwater, and is referred to the new genus Resultor. The two genera are closely related and form the new class Pedinophyceae, which is characterized by features of the flagellar apparatus, mitosis, and cytokinesis. The flagella show the 11/5 orientation otherwise characteristic of Ulvophyceae and Pleurastrophyceae, but they are arranged end to end as in the Chlorophyceae. The flagellar root system is asymmetric and includes a rhizoplast that emerges from the base of one flagellum but subsequently associates with a microtubular root from the second basal body. Mitosis studied previously by Pickett-Heaps and Ott in Pedinomonas is closed, unlike in other green algae, and the spindle is persistent. No phycoplast or phragmoplast is formed during cytokinesis. The eyespot of the Pedinophyceae is located at the opposite end of the cell from the flagella and adjacent to the pyrenoid, as in the most primitive members of the Prasinophyceae. Members of the Pedinophyceae lack prasinoxanthin and Mg 2,4D, characteristic of certain other primitive green algae. The primitive green algae include the classes Prasinophyceae and Pedinophyceae. Micromonadophyceae Mattox et Stewart is considered a synonym of Prasinophyceae. Two new orders are established, Pedinomonadales, containing all known members of the Pedinophyceae, and Scourfieldiales, with the single family Scourfieldiaceae fam. nov. and the single genus Scourfieldia.     

MITOSIS IN THE AQUATIC FUNGUS RHIZOPHYDIUM SPHEROTHECA (CHYTRIDIALES)

The structure of centric, intranuclear mitosis and of organelles associated with nuclei are described in developing zoosporangia of the chytrid Rhizophydium spherotheca. Frequently dictyosomes partially encompass the sides of diplosomes (paired centrioles). A single, incomplete layer of endoplasmic reticulum with tubular connections to the nuclear envelope is found around dividing nuclei. The nuclear envelope remains intact during mitosis except for polar fenestrae which appear during spindle incursion. During prophase, when diplosomes first define the nuclear poles, secondary centrioles occur adjacent and at right angles to the sides of primary centrioles. By late metaphase the centrioles in a diplosome are positioned at a 40° angle to each other and are joined by an electron-dense band; by telophase the centrioles lie almost parallel to each other. Astral microtubules radiate into the cytoplasm from centrioles during interphase, but by metaphase few cytoplasmic microtubules are found. Cytoplasmic microtubules increase during late anaphase and telophase as spindle microtubules gradually disappear. The mitotic spindle, which contains chromosomal and interzonal microtubules, converges at the base of the primary centriole. Throughout mitosis the semipersistent nucleolus is adjacent to the nuclear envelope and remains in the interzonal region of the nucleus as chromosomes separate and the nucleus elongates. During telophase the nuclear envelope constricts around the chromosomal mass, and the daughter nuclei separate from each end of the interzonal region of the nucleus. The envelope of the interzonal region is relatively intact and encircles the nucleolus, but later the membranes of the interzonal region scatter and the nucleolus disperses. The structure of the mitotic apparatus is similar to that of the chytrid Phlyctochytrium irregulare.     

THE FINE STRUCTURE OF MITOSIS AND CELL DIVISION IN THE CHRYSOPHYCEAN ALGA OCHROMONAS DANICA1

At the ultrastructural level, cell division in Ochromonas danica exhibits several unusual features. During interphase, the basal bodies of the 2 flagella replicate and the chloroplast divides by constriction between its 2 lobes. The rhizoplast, which is a fibrous striated root attached to the basal body of the long flagellum, extends under the Golgi body to the surface of the nucleus in interphase cells. During proprophase, the Golgi body replicates, apparently by division, and a daughter rhizoplast, appears. During prophase, the 2 pairs of flagellar basal bodies, each with their accompanying rhizoplast and Golgi body, begin to separate. Three or 4 flagella are already present at this stage. At the same time, there is a proliferation of microtubules outside the nuclear envelope. Gaps then appear in the nuclear envelope, admitting the microtubules into the nucleus, where they form a spindle. A unique feature of mitosis in O. danica is that the 2 rhizoplasts form the poles of the spindle, spindle microtubules inserting directly onto the rhizoplasts. Some of the spindle microtubules extend from pole to pole; others appear to attach to the chromosomes. Kinetochores, however, are not present. The nuclear envelope breaks down, except, in the regions adjacent, to the chloroplasts; chloroplast ER remains intact throughout mitosis. At late anaphase the chromosomes come to lie against part of the chloroplast ER. This segment of the chloroplast ER appears to be incorporated as part of the reforming nuclear envelope, thus reestablishing the characteristic nuclear envelope—chloroplast ER association of the interphase cell.     

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.     

Ultrastructure of Draparnaldia glomerata (Chaetophorales, Chlorophyceae) II. Mitosis and cytokinesis

The mitosis and cytokinesis of Draparnaldia glomerata as examined here by transmission electron microscopy are in many aspects similar to those described earlier for other chaetophoralean algae. The standard chaetophoralean model of the mechanism of mitosis/cytokinesis is described in detail. Characteristic in this pattern is the movement of sets of centrioles towards the nuclear poles followed by a proliferation of extranuclear microtubules at prophase, the (partial) fusion of centrioles with the spindle poles at metaphase and anaphase, the simultaneous separation of chromosomes apparently caused by both spindle elongation and shortening of the chromosomal microtubules at anaphase, the expulsion of the centrioles by daughter nuclei and finally the non–persistent spindle at telophase. Cytokinesis takes place by formation of a cell plate associated with phycoplast microtubules. The possible function of the phycoplast in cytokinesis in Draparnaldia is discussed.     

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.     

Cell division in Hymenomonas carterae (Braarud et Fagerland) Braarud (Prymnesiophyceae)

The cell division cycle of Hymenomonas carterae (Braarud et Fagerland) Braarud was investigated at the ultrastructural level. DNA synthesis and cytokinesis occurred during the 8-hour dark period. All organelles, including the flagellar bases were replicated prior to nuclear division. Prophase consisted of a clustering of the chromosomes into distinct groups and the disappearance of the nucleolus. During metaphase there was complete dissociation of the nuclear envelope resulting in the formation of an open spindle containing no major organelles. The metaphase plate formed at right angles to an imaginary line joining the two pairs of flagellar bases. Elongation of the cell and separation of the chromosomes occurred at anaphase. During early telophase the nuclear envelope veformed and was closely associated with the chromosome masses, resulting in the nuclear possessing convoluted profiles. Telophase was characterized by complete break down of spindle fibres, rounding off of the nuclear profiles, reappearance of the nucleolus, emergence of the flagella and the final separation of the two daughter cells.     

Vascular differentiation in the root cortex of peas: Premitotic stages of cytoplasmic reactivation

Summary 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.Abbreviations IF immunofluorescence - IZS interzonal spindle - MT microtubule - MTOC microtubule organizing center - TEM transmission electron microscopy     

FINE STRUCTURE OF MITOSIS AND CLEAVAGE IN FRIEDMANNIA ISRAELENSIS (CHLOROPHYCEAE: CHLOROSARCINACEAE)1

Observations on the ultrastructure of Friedmannia israelensis Chantanachat & Bold revealed the presence of a phycoplast and zoospores with cruciate rootlets. During mitosis, the nuclear envelope partially disintegrates and the basal bodies remain at the cell surface on either side of the developing cleavage furrow. The events during mitosis and cleavage in Friedmannia resemble those reported in the other green algae, Platymonas and Pleurastrum.     

Ultrastructure of meiosis in the hollyhock rust fungus,Puccinia malvacearum II. Metaphase I—Telophase I

Summary The three-dimensional structure of the spindle pole body (SPB) and meiotic spindle during early metaphase I through telophase I inPuccinia malvacearum is analyzed ultrastructurally from serial sections. During early metaphase I the spindle rotates from the perpendicular to a position oblique to the longitudinal axis and parallel to the sagittal plane of the cell. Tubular cisternae are present within the spindle at this stage. The half middle piece (MP) subtends a collateral disc (co-disc) which is inserted eccentrically within each SPB. The SPB, co-disc and half MP at opposite poles are in mirror image. During the transition from early metaphase I to full metaphase I, the spindle orients parallel to the lateral wall of the promycelium and the half MPs are lost. The co-discs partially detach from each discoid SPB and maintain this relation until the end of interphase I. Co-discs become further differentiated as they attach to the subtending sheath-like extension of the nuclear envelope previously occupied by the half MPs. Microvesicles within the nucleoplasm are specific to mid metaphase I. A metaphase plate is absent. The 14 bivalents, which are directly connected to each polar SPB by 2 to 3 kinetochore MTs, are spread over nearly the entire length of the central spindle. The first anaphasic movement involves asynchronous shortening of the kinetochore MTs while the second consists of extensive pole-to-pole elongation. Astral MTs first appear at early metaphase I and become most numerous at anaphase I. An intact nuclear envelope constricts against the central spindle at either end of the interzonal region. Concurrently, centripetal growth of the nuclear envelope under each SPB results in their gradual externalization by the end of telophase I. The sibling nuclei are cut off by constriction of the nuclear envelope at either end of the interzonal region. These meiotic stages inP. malvacearum are compared with those in other basidiomycetes and ascomycetes.     

THE FLAGELLAR APPARATUS OF THE ZOOSPORE OF UROSPORA PENICILLIFORMIS(CHLOROPHYTA)1

The flagellar apparatus of Urospora penicilliformis (Roth) Aresch. is unique, or at least very unusual among green algae. The flagellar axonemes are rigid, and contain wing-like projections. There are no central microtubules in the most proximal part of the axoneme. The transition region contains a series of electron dense transverse lamellae rather than a single septum, and lacks a stellate pattern. There is no cartwheel pattern in the proximal part of the basal bodies. The latter are associated with four different types of fibrous elements: ascending striated fibers that attach to an electron dense plate in the papillar center, lateral striated fibers that parallel microtubular roots, fibrous elements that link adjacent basal bodies, and finally two massive striated fibers that descend into the cell, passing closely along the nucleus (system II fibers, or rhizoplasts). Each of the four microtubular flagellar roots is sandwiched between two system I striated structures. The roots are probably equal; they contain proximally four, and distally up to eight microtubules. Based on the zoospore flagellar apparatus, it is concluded that the multinucleate U. penicilliformis is related to the Ulvaphyceae. Finally, a possible explanation in functional terms is given for the peculiar external morphology and behavior of the zoospore.