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.     

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 MITOSIS AND CYTOKINESIS IN KLEBSORMIDIUM MUCOSUM NOV. COMB., FORMERLY ULOTHRIX VERRUCOSA (CHLOROPHYTA)1

A transmission electron microscopy study of dividing cells of Ulothrix verrucosa Lokhorst has provided clear evidence that this species differs in many respects from other Ulothrix Kützing species. These differences include the presence of a microtubular sheath around the prophase nucleus, the complete disintegration of the nuclear envelope coinciding with the proliferation of extranuclear microtubules into the prometaphase nucleus and the intrusion of vacuoles into the interzonal spindle region in between the widely separated telophase nuclei. This necessitates the transfer of Ulothrix verrucosa to the charophycean genus Klebsormidium Silva, Mattox and Blackwell. The new combination Klebsormidium mucosum is proposed. On account of its mitotic pattern, this species can be placed in the (charophycean) evolutionary line towards the higher plants. However, because of its cytokinesis (annular centripetal ingrowth of the plasmalemma) this species probably should be considered as a blind offshoot of this line. It is emphasized that furrowing green algae with a persistent interzonal spindle at telophase (including the presently studied alga) often show an ill-defined cytokinetic microtubular system.     

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

Mitosis 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.     

似金隐藻有丝分裂及胞质分裂的观察

似金隐藻(Cryptomonas chrysoidea)是从青岛附近渤海湾海水中分离得到的一种单细胞藻类。对它的胞质分裂和有丝分裂进行的观察表明,它的胞质分裂在有丝分裂的中期开始,细胞前端沟口处先开始分裂,继而沿纵轴纵沟处形成一收缩沟完成的。似金隐藻的有丝分裂过程中没有染色体和着丝点形成;核膜进入中期时完全消失,纺锤体呈桶状,微管通过染色质团中的通道或直接与染色质团块相联;在后期和末期,两块分开的染色质团十分靠近相应的色素体内质网膜。本文对其分裂过程进行了讨论。     

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.     

TAXOL REDUCES THE RATE OF CYTOKINESIS IN PtK1CELLS

Mitotic PtK₁cells were treated both during mid-anaphase and at furrow initiation with the potent microtubule (MT) stabilizing agent, taxol, to determine the role of MTs in the rate of cytokinetic events. Rates of cytokinesis (μm/min) were measured by changes in furrow diameter. Incubation of PtK₁cells during mid-anaphase with 5 μg/ml taxol slows the rate of cytokinesis by an average of 43%. Instead of furrow initiation to midbody formation taking an average of 10.7 min (1.6 μm/min), furrowing to midbody formation was completed in an average of 19.0 min (0.9 μm/min), which does not include the 7-min period between taxol application in mid-anaphase and furrow initiation. Application of 5 μg/ml taxol to cells at furrow initiation had a reduced effect on decreasing the rate of cytokinesis and midbody formation; furrowing to midbody formation took an average of 14.6 min (1.2 μm/min). These data suggest that delays in the rate of cytokinesis is dependent on the mitotic stage at which taxol is applied. Ultrastructural analysis shows that taxol treatment of anaphase cells prevents midbody formation during early G₁, yet MT number and organization in the furrowed region is not significantly altered from untreated cells. There is little change in the organization and amount of contractile ring microfilaments, yet filaments are also found parallel to midbody MTs. Our results may be explained by the fact that taxol tends to stabilize MTs which probably affects the rate at which they depolymerize in the terminal phases of cytokinesis. Reduction in depolymerization rates of a stable population of MTs could serve to regulate the rate of cytokinesis.     

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 STUDY OF THE SPERMATOZOIDS OF DICHOTOMOSIPHON TUBEROSUS (CHLOROPHYCEAE)1

Spermatozoids of the siphonous green alga Dichotomosiphon tuberosus (A. Br.) Ernst are specialized gametes which differ in many respects from other green algal motile cells, but whose microanatomy nevertheless indicates its chlorophycean affinities. Each cell is anteriorly biflagellate and contains an irregularly shaped nucleus attached to the flagellar bases by a complex support apparatus. There is a single reduced chloroplast in each spermatozoid and numerous (50–100) minute spherical mitochondria, only 0.3 μm diam. These move vigorously in the living cell and when viewed with the light microscope they bear a striking resemblance to bacteria. Rather unexpectedly, no contractile vacuoles could be detected, even though the gametes are naked freshwater cells. Daring spermatogenesis the nucleoli of the vegetative cells disperse and are replaced by a large dense body presumably formed from either nucleolar material or condensed chromatin. The flagellar apparatus includes a cruciate flagellar root system, a feature now known to be characteristic of most green algae, exceptions being those putative ancestors of the higher plants and bryophytes. Discharge of spermatozoids from the antheridia is extremely rapid and the whole process may be finished in 30 sec. The antheridium lacks a pore apparatus, but at maturity bursts open explosively at the apex. Phyletic affinities are discussed and it is concluded that the ultrastructure of the motile cells does not, at this time, support the separation of the siphonous green algae from other green algae into a separate class.     

ULTRASTRUCTURE OF CELL DIVISION AND REPRODUCTIVE DIFFERENTIATION OF MALE PLANTS IN THE FLORIDEOPHYCEAE (RHODOPHYTA): CELL DIVISION IN POLYSIPHONIA1

Cell division in the marine red algae Polysiphonia harveyi Bailey and P. denudata (Dillwyn) Kutzing was studied with the electron microscope. Cells comprising the compact spermatangial branches of male plants were used exclusively because of their small size, large numbers and the ease with which the division planes can be predetermined. Some features characterizing mitosis in Polysiphonia confirm earlier electron microscope observations in Membranoptera, the only other florideophycean algae in which mitosis has been studied in detail. Common to both genera are a closed, fenestrated spindle, perinuclear endoplasmic reticulum, a typical metaphase plate arrangement of chromosomes, conspicuous, layered kinetochores, chromosomal and non-chromosomal microtubules, and nucleus associated organelles (NAOs) known as polar rings (PRs) located singly in large ribosome-free zones of exclusion at division poles in late prophase. However, other features, unreported in Membranoptera, were observed consistently in Polysiphonia. These include the presence of PR pairs in interphase-early prophase cells, the attachment of PRs to the nuclear envelope during all mitotic stages, the migration of a single PR to establish the division axis, a prominent, nuclear envelope protrusion (NEP) at both division poles at late prophase, the prometaphase splitting of PRs into proximal and distal portions, and the reformation of post-mitotic nuclei by the separation of an elongated interzonal nuclear midpiece at telophase. During cytokinesis, cleavage furrows impinge upon a central vacuolar region located between the two nuclei and eventually pit connections are formed in a manner basically similar to that reported for other red algae. Diagrammatic sequences of proposed PR behavior during mitosis are presented which can account for events known to occur during cell division in Polysiphonia. Mitosis is compared with that reported in several other lower plants and it is suggested that features of cell division are useful criteria to aid in the assessment of phylogenetic relationships of red algae.     

ULTRASTRUCTURE OF THE ZOOSPORES AND VEGETATIVE CELLS OF TETRAEDRON AND CHLOROTETRAEDRON (CHLOROPHYCEAE)1

The flagellar apparatus of the zoospores of Tetraedron bitridens Beck-Mannagetta and Chlorotetraedron polymorphum MacEntee, Bold et Archibald includes directly opposed basal bodies, a distal fiber that is elaborated into a ribbed structure to which the continuous striated microtubule-associated component (SMAC) is connected, and partial caps over the proximal end of each basal body. The angle between basal bodies ranges from approximately 25° to 150°. Basal bodies at wider angles are interconnected via their cores. A septum is present in the B-tubule of one basal body triplet in C. polymorphum. Both organisms have four microtubular rootlets arranged in a cruciate pattern. The two X-membered rootlets in a single cell have dissimilar numbers of microtubules. In C. polymorphum there are 5 and 6 microtubules in a 4/1 and 5/1 arrangement. 3/1 and 4/1 rootlets are present in T. bitridens. Zoospores of T. bitridens have a fuzzy coat whereas those of C. polymorphum are naked. Pyrenoids in both species are covered by a continuous starch sheath. Vegetative, interphase cells of C. polymorphum have two centrioles connected by a fiber that are located in depressions in the nuclear envelope. We propose that these two genera may be closely related to Neochloris, and that the coenobial genera Hydrodictyon, Pediastrum and Sorastrum are derived from a Tetraedron-like alga.     

MITOSIS IN DUNALIELLA BIOCULATA (CHLOROPHYTA): CENTRIN BUT NOT BASAL BODIES ARE AT THE SPINDLE POLES

Centrin, a 20 kDa calmodulin-like protein, is located in various basal body-associated fibers in protists. We used indirect immunofluorescence of isolated cytoskeletons or methanol-fixed cells to analyze the distribution of centrin during mitosis of the biflagellate green alga Dunaliella bioculata (Butcher). The distance between the nucleus and the basal apparatus decreased in late interphase, presumably caused by the contraction of the two centrin-containing nucleus–basal body connectors (NBBCs). During prophase, centrin accumulated on the new basal bodies as shown by postembedding immunogold labeling of serial thin sections. The new basal bodies were in close contact with plaque-like structures on the nuclear envelope. In mitotic cells, basal body pairs were separated and positioned at a considerable distance from the poles of the mitotic spindle. At this stage, we observed four separated centrin dots, two associated with the pairs of basal bodies and two located at the spindle poles as shown by double immunofluorescence, including anti-tubulin staining. The latter signals corresponded to an accumulation of centrin between the plasma membrane and the nuclei, indicating that centrin could be involved in mitotic movements of the nuclei. In telophase, centrin was observed along the nuclear surface and one new NBBC developed in each cell half. Our results demonstrate that centrin is present at the acentriolar spindle poles of Dunaliella independently from its localization in the basal apparatus.     

DIVERSITY,LIFE HISTORY,AND ECOLOGY OF TRENTEPOHLIA AND PRINTZINA (TRENTEPOHLIALES,CHLOROPHYTA) IN URBAN HABITATS IN WESTERN IRELAND 1

On the basis of field and culture investigations, five species of the genera Trentepohlia and Printzina were found to occur in urban habitats in western Ireland: Trentepohlia abietina (Flotow) Hansgirg, T. aurea (Linnaeus) Martius, T. iolithus (Linnaeus) Wallroth, T. cf. umbrina (Kützing) Bornet, and Printzina lagenifera (Hildebrandt) Thompson et Wujek. These species formed perennial populations on a variety of substrata. T. abietina occurred on bark of trees; T. cf. umbrina occurred on stone walls; and P. lagenifera grew on several substrata, mainly cement and asbestos sheeting. T. aurea and T. iolithus were found on old concrete and cement walls; in particular, the latter species formed characteristic, extensive, deep‐red patches on many buildings. In culture, best growth and reproduction of these species were observed at 10 and 15° C, 16:8 h light:dark. Both in culture and in the field, reproduction took place by release of biflagellate swarmers behaving as asexual spores, germinating to produce new plants without any evidence of sexual fusion; release of biflagellate swarmers in the field was generally observed in all seasons throughout a whole annual cycle. Confirmation of the occurrence of sexual reproduction in Trentepohlia was not obtained.     

THE ULTRASTRUCTURE OF SCENEDESMUS (CHLOROPHYCEAE). II. CELL DIVISION AND COLONY FORMATION1

Cell division in Scenedesmus is fairly typical of other chlorococcalean genera. The closed spindle has centrioles at polar fenestrae and apparently a series of nuclear divisions precedes cytokinesis. The phycoplast system of cytokinetic microtubules predicts the path of cleavage furrows whose mode of formation is obscure. Before and during cell division, the endoplasmic reticulum invariably accumulates granular material which later, during cytokinesis, appears to he secreted via the golgi bodies. Similar dense granular material then at accumulates outside the forming daughter cells but inside the parental wall, as the latter begins eroding away. By the end of colony formation, the cellulosic parental wall has disappeared, leaving its outer sheath and attached ornamentative features (spines, combs, reticulate or warty layer, etc.) intact as a “ghost.” The spines and combs of new colonies appear to condense out of the extracellular aggregate; their precise mode of formation is obscure. As they form, the daughter cells, having become rearranged within the parental wall, stick to one another apparently at specific sites on their outer surface. A trilaminar (sporopollenin-containing) layer arises first in each cell at these adhesive sites and immediately afterwards, dense material aggregates between the adjacent layers to give rise to the coenobial adhesive. Plaques of the trilaminar layer later appear over the rest of the cell's surface; they grow and fuse so that eventually each cell is enclosed by one continuous Trilaminar Sheath (TLS). While the plaques are forming, another dense layer materializes around the whole coenobium. Depending on the species, this layer turns into either the warty layer, in which instance it is applied directly on to the surface of the TLS except near the coenobial adhesive, or else it becomes the reticulate layer, in which instance it remains entirely separate from the TLS, soon acquiring the complex system of propping spikelets which suspend it from the coenobial surface. When fully farmed, the daughter coenobium is tightly compressed within the parental TLS, with its spines folded lengthwise along the daughter cells. Release of the colony follows a quite explosive rupturing of the parental TLS, and immediately upon release, the daughter colony flattens out and erects its spines.     

MITOSIS AND CELL DIVISION IN CYLINDROCAPSA GEMINELLA (CHLOROPHYCEAE)1

Mitosis and cell division were studied in the green alga Cylindrocapsa geminella Wolle with transmission electron microscopy. Vegetative cells possess a parietal, lobed chloroplast, and a central pyrenoid. Prophase and metaphase nuclei are surrounded by 1–3 layers of perinuclear endoplasmic reticulum. At early prophase a small number of perinuclear microtubules (MTs) are present while at late prophase MTs are concentrated at the presumptive spindle poles. At the same time, MTs begin to appear in the nucleoplasm. Metaphase spindles are diamond-shaped and centric. During telophase, centrioles migrate towards the center of the equatorial zone, presumably guided by a small group of perinuclear MTs. A second system of MTs develops in the equatorial plane, initially consisting of randomly orientated microtubular elements. Later they tend to run in a predominantly radial direction although a common MT focal point or organizing center is not apparent. The two centriole complexes remain at the center of the equatorial plane until well into interphase, facing each other across the newly formed transverse septum. Centrioles are associated with root templates and connecting fibers. The present observations corroborate the view that C. geminella does not form a true filament in the ulotrichalean or chaetophoralean sense, but rather consists of a row of autospores. Its affinity with other “pseudo-filamentous” green algae and the Chlorococcales is discussed. The interpretation of the cytokinetic MTs in C. geminella as a phycoplast appears to be problematic.     

EFFECTS OF DESICCATION AND ILLUMINATION ON PHOTOSYNTHESIS AND PIGMENTATION OF AN EDAPHIC POPULATION OF TRENTEPOHLIA ODORATA (CHLOROPHYTA)1

The photosynthetic rates of Trentepohlia odorata (L.) Martius growing on wall surfaces in Singapore changed throughout the day with a maximum in midmorning and decreasing thereafter during the day. Optimum temperature for photosynthesis was 25° C. Different levels of air humidity also affected photosynthetic rates with low relative humidity reducing the rates and efficiency of photosynthesis. Our results suggested that T. odorata was able to maximize its rate of photosynthesis before photoinhibitory light levels were reached and that its growth might be dependent on high levels of atmospheric relative humidity, which may serve as a source of water supply for the alga.     

EFFECT OF LATRUNCULIN B AND BREFELDIN A ON CYTOKINESIS IN THE BROWN ALGA SCYTOSIPHON LOMENTARIA (SCYTOSIPHONALES,PHAEOPHYCEAE)1

In zygotes of the brown alga Scytosiphon lomentaria (Lyngb.) Link, cytokinesis proceeds by growth of membranous sacs, which are formed by fusion of Golgi vesicles and flat cisternae accumulated at the future cytokinetic plane. It has been reported that depolymerization of actin filaments by latrunculin B does not inhibit mitosis. However, this molecule prevents the formation of the actin plate, which appears at the region of intermingled microtubules from each centrosome just before and during cytokinesis. In this study, zygotes treated with latrunculin B were observed using EM. Remarkably, this reagent inhibited the formation of flat cisternae. Golgi vesicles gathered around the midzone between the two daughter nuclei and fused with the plasma membrane there. As a result, the plasma membrane invaginated, in a complicated manner, into the cytoplasm. However, these invaginations of the plasma membrane never produced a continuous partition membrane. The ultrastructure of zygotes treated with brefeldin A, which prevents Golgi‐mediated secretion, was also examined. Flat cisternae appeared at the future cytokinetic plane, and a new cell partition membrane was formed. However, the partition membrane became thick, because it was filled with amorphous material rather than the normal rigid fibrous material. These results suggested that actin is involved in the formation of flat cisternae, where it is necessary for completion of the new cell partition membrane, and that Golgi vesicles may play an important role in the deposition of cell wall material.     

MITOSIS AND CLEAVAGE DURING ZOOSPOROGENESIS IN SEVERAL COCCOID GREEN ALGAE1

The fine structure of mitosis and cleavage in Axilosphaera vegetata Cox & Deason, Chlorococcum echinozygotum Starr, Chlorosarcinopsis eremi Chantanachat & Bold, Nautococcus mammilatus Korshikoff, N. terrestris Archibald, N. soluta Archibald, Neospongiococcum solitarium Deason, and Tetracystis aeria Brown & Bold was observed. All possess a phycoplast, but differences in basal body position during mitosis were observed. Nautococcus mammilatus differs from the others as protoplast rotation occurs in many cells prior to cleavage.     

SOMATIC CELL FLAGELLAR APPARATUSES IN TWO SPECIES OF VOLVOX (CHLOROPHYCEAE)1

The somatic cell flagellar apparatuses of Volvox carteri f. weismannia (Powers) Iyengar and V. rousseletii G. S. West have parallel or nearly parallel basal bodies which are separated at their proximal ends. The four microtubular rootlets alternate between two and four members, and all are associated with a striated microtubular associated component (SMAC) that runs between the basal bodies. In addition, each half of the flagellar apparatus apparently rotates during development and loses the 180° rotational symmetry characteristic of most unicellular chlorophycean motile cells. All of these features appear necessary for efficient motion of a colony composed of numerous radially arranged cells. However, the structural details of the flagellar apparatuses of these two species differ. The distance between flagella is greater in V. rousseletii than in V. carteri. One distal striated fiber and two proximal striated fibers connect the basal bodies in V. carteri, but both types of fibers are absent from V. rousseletii. In the latter species, a striated fiber wraps around each of the basal bodies and attaches to the rootlets and the SMAC. No such fiber is present in V. carteri. Since the similarities in the flagellar apparatuses can be explained as a result of adaptation for efficient colonial motion in organisms with similar colonial morphology, the differences suggest a wider phylogenetic distance than previously believed.     

A RHIZOPLAST IN CARTERIA RADIOSA (CHLOROPHYCEAE)12

A rhizoplast or rhizoplast-like structure was observed with the electron microscope in Carteria radiosa. The cross-banded structure extends from the proximal end of each of at least 2 of the basal bodies and extends toward, although does not make contact with, the nucleus. The rhizoplast terminates in a ribosome-free area composed of fine granules and microfibrils. This is the first ultrastructural verification of a rhizoplast in a volvocalean alga.