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.     

A STUDY OF CELL WALL AND FLAGELLA FORMATION DURING CELL DIVISION IN THE SCALY GREEN ALGA,SCHERFFELIA DUBIA (CHLOROPHYTA)1

The thecate green flagellate Scherffelia dubia (Perty) Pascher divides within the parental cell wall into two progeny cells. It sheds all four flagella before cell division, and the maturing progeny cells regenerate new walls and flagella. By synchronizing cell division, we observed mitosis, cytokinesis, cell maturation, flagella extension, and cell wall formation via differential interference contrast microscopy of live cells and serial thin‐section EM. Synthesis of thecal and flagellar scales is spatially and temporally strictly separated. Flagellar scales are collected in a pool during late interphase. Before prophase, Golgi stacks divide, flagella are shed, the parental theca separates from the plasma membrane, and flagellar scales are deposited on the plasma membrane near the flagellar bases. At prophase, Golgi bodies start to synthesize thecal scales, continuing into interphase after cytokinesis. During cytokinesis, vesicles containing thecal scales coalesce near the cell posterior, forming a cleavage furrow that is initially oriented slightly diagonal to the longitudinal cell axis but later becomes transverse. After the progeny nuclei have moved into opposite directions, resulting in a “head to tail” orientation of the progeny cells, theca biogenesis is completed and flagellar scale synthesis resumes. Progeny cells emerge through a hole near the posterior end of the parental theca with four flagella of about 8 μm long. The precise timing of flagellar and thecal scale synthesis appears to be an evolutionary adaptation in a scaly green flagellate for the thecal condition, necessary for the evolution of the phycoplast and thus multicellularity in the Chlorophyta.     

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.     

CELL DIVISION AND COLONY FORMATION IN THE GREEN ALGA COELASTRUM (CHLOROCOCCALES)1

Multinucleate cells of Coelastrum undergo precisely directed cytokinesis, guided by phycoplast microtubules, to form a number of uninucleate daughter cells which subsequently adhere to form characteristically patterned aggregates. As there is no movement of the daughter cells relative to one another before their adhesion, the disposition of cells in daughter colonies reflects the pattern of cytokinesis of parent cells. Centrioles lie at the poles of the mitotic nuclei which are partially enclosed by a perinuclear envelope of endoplasmic reticulum. The centrioles disappear at the time of cytokinesis of the parental cell and apparently reform de novo once the daughter cells have acquired a cell wall following their adhesion. The trilaminar layer of cell wall, often termed the pectic layer, does not stain with ruthenium red and resists acetolysis suggesting that it contains sporopollenin rather than pectin.     

DETECTION OF A GROUP I (IE) FUNGAL INTRON IN THE GREEN ALGAL GENUS UROSPORA (ULVOPHYCEAE)1

A group IE intron at position 989 (Escherichia coli numbering) in the 18S rRNA gene is described for the first time from the green algae based on primary and secondary structure analysis of the intron. The intron occurred in Urospora neglecta Kornmann, U. wormskioldii (Mertens) Rosenvinge, and in an undescribed taxon closely allied with U. wormskioldii. The Urospora intron was 397 or 399 bp in length, except for one variant, which was 652 bp in length. The latter intron appeared to be a chimera, consisting of a conspecific part, a segment more closely related to another species, and a region of unknown origin. A search of data bases containing introns indicated that the green algal intron is homologous with other 989 introns, all occurring in fungi, which may be the source of the Urospora intron. The introduction of this intron into the Urospora lineage has occurred within the last 14 million years, based on estimates of the earliest divergence of the genus from other members of the family Acrosiphoniaceae, all of which lack 18S introns.     

PHYLOGEOGRAPHIC STUDIES IN THE TROPICAL SEAWEED CLADOPHOROPSIS MEMBRANACEA (CHLOROPHYTA,ULVOPHYCEAE) REVEAL A CRYPTIC SPECIES COMPLEX1

The seaweed Cladophoropsis membranacea (Hofman Bang ex. C. Agardh) Børgesen is a widely distributed species on coral reefs and along rocky coastlines throughout the tropics and subtropics. In a recent population‐level survey openface>1600 individuals with eight microsatellite loci, a number of isolates from biogeographically disjunct locations could not be amplified for any of the loci. Nonamplifiable and amplifiable isolates co‐occurred within the Canary Islands, Cape Verde Islands, and in the Caribbean. These unexpected results led to question whether or not C. membranacea is a single species. Phylogenetic relationships were evaluated using rDNA ITS1 and ITS2 sequence comparisons from 42 isolates sampled from a subset of 30 of the 66 locations. Four well‐supported clades were identified. Sequence divergence within clades was <1%, whereas between‐clade divergence was 2%–3%. Intraindividual variation was extremely low with no effects on the analysis. A strong, but imperfect, correspondence was found between ITS clades and amplifiable microsatellite loci. It is concluded that C. membranacea consists of three cryptic species. Using Pacific isolates as an outgroup, the most basal clade included the Central Canary Islands, Cape Verde, and Bonaire (Caribbean) isolates and thus spanned the widest latitude. Two derived sister clades consisted of a southern transtropical group stretching across the SE Caribbean to the Cape Verde Islands and African coast (but not the Canary Islands) and a NE‐Canary Island‐Mediterranean clade that also included the Red Sea. The detection of overlapping biogeographic distributions highlights the importance of ecotypic differentiation with respect to temperature and the importance of shifting sea surface isotherms that have driven periodic extinctions and recolonizations of the Canary Islands—a crossroads of marine floral exchange—since the last glacial maximum.     

DIURNAL CELL DIVISION REGULATED BY GATING THE G1/S TRANSITION IN ENTEROMORPHA COMPRESSA (CHLOROPHYTA)1

The cell‐cycle progression of Enteromorpha compressa (L.) Nees (=Ulva compressa L.) was diurnally regulated by gating the G₁/S transition. When the gate was open, the cells were able to divide if they had attained a sufficient size. However, the cells were not able to divide while the gate was closed, even if the cells had attained sufficient size. The diurnal rhythm of cell division immediately disappeared when the thalli were transferred to continuous light or darkness. When the thalli were transferred to a shifted photoperiod, the rhythm of cell division immediately and accurately synchronized with the shifted photoperiod. These data support a gating‐system model regulated by light:dark (L:D) cycles rather than an endogenous circadian clock. A dark phase of 6 h or longer was essential for gate closing, and a light phase of 14 h was required to renew cell division after a dark phase of >6 h.     

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.     

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.     

ASSESSING THE RELATIONSHIPS OF SOME COCCOID GREEN LICHEN ALGAE AND THE MICROTHAMNIALES (CHLOROPHYTA) WITH 18S RIBOSOMAL RNA GENE SEQUENCE COMPARISONS1

Eight complete nuclear-encoded small-subunit ribosomal RNA (18S rRNA) gene sequences were determined for four genera of the Microthamniales (Pleurastrophyceae) and for Gloeotilopsis planctonica Iyengar & Philipose (Ulvophyceae, Ulotrichales) to investigate evolutionary relationships within the Microthamniales and the taxonomic position of this order within the green algae. Phylogenies inferred from these data revealed specific relationships at the level of genera and species that disagree with those inferred from vegetative cell morphology but agree with those inferred from motile cell characters. The rRNA phylogenies provide even better resolution than that gained from morphology alone. The coccoid lichen alga Trebouxia spp. is specifically related to other coccoid lichen and soil algae (i.e. Myrmecia biatorellae Boye-Petersen and Friedmannia israelensis Chantanachat & Bold), forming the “Lichen Algae Group,” an evolutionarily distinct lineage within the Microthamniales. Trebouxia is a paraphyletic and Pleurastrum a polyphyletic genus in rRNA phylogenies. In contrast to previous hypotheses based on morphology, Pleurastrum terrestre Fritsch & John is not closely related to Trebouxia but occupies an isolated position within the Microthamniales. The filamentous alga Microthamnion kuetzingianum is not ancestral to coccoid members of the Microthamniales but is closely related with the coccoid Fusochloris perforata (Lee & Bold) Floyd, Watanabe & Deason. The Microthamniales are inferred to be an array of independent lineages that radiate nearly simultaneously and may also include some autosporic coccoid taxa previously classified in the Chlorophyceae. Monophyly of the Microthamniales could not be demonstrated unequivocally. In contrast to a hypothesis based on ultrastructure, the Microthamniales are evolutionarily distinct from the Ulotrichales. The latter are ancestral to the radiation of the Microthamniales and the Chlorophyceae in the rRNA phylogenies.     

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.     

THE EFFECT OF HYDROXYUREA AND FLUORODEOXYURIDINE ON CELL CYCLE EVENTS IN THE CHLOROCOCCAL ALGA SCENEDESMUS QUADRICAUDA (CHLOROPHYTA)1

The effect of hydroxyurea and 5-fluorodeoxyuridine (FdUrd) on the course of growth (RNA and protein synthesis) and reproductive (DNA replication and nuclear and cellular division) processes was studied in synchronous cultures of the chlorococcal alga Scenedesmus quadricauda (Turp.) Bréb. The presence of hydroxyurea (5 mg·L^?1)from the beginning of the cell cycle prevented growth and further development of the cells because of complete inhibition of RNA synthesis. In cells treated later in the cell cycle at the time when the cells were committed to division, hydroxyurea present in light affected the cells in the same way as a dark treatment without hydroxyurea; i. e. RNA synthesis was immediately inhibited followed after a short time period by cessation of protein synthesis. Reproductive processes including DNA replication to which the commitment was attained, however, were initiated and completed. DNA synthesis continued until the constant minimal ratio of RNA to DNA was reached. FdUrd (25 mg·L^?1) added before initiation of DNA replication in control cultures prevented DNA synthesis in treated cells. Addition of FdUrd at any time during the cell cycle prevented or immediately stopped DNA replication. However, by adding excess thymidine (100 mg·L^?1), FdUrd inhibition of DNA replication could be prevented. FdUrd did not affect synthesis of RNA, protein, or starch for at least one cell cycle. After removal of FdUrd, DNA synthesis was reinitiated with about a 2-h delay. The later in the cell cycle FdUrd was removed, the longer it took for DNA synthesis to resume. At exposures to FdUrd longer than two or three control cell cycles, cells in the population were gradually damaged and did not recover at all.     

STRUCTURE AND ABSOLUTE CONFIGURATION OF THE FLAGELLAR APPARATUS IN THE ISOGAMETES OF BATOPHORA (DASYCLADALES,CHLOROPHYTA)1

The overall appearance of the flagellar apparatus in the isogametes of Batophora oerstedii. J. Ag. is most like that which occurs in motile cells of the Ulvophyceae. Like other Ulvophyceae, the basal bodies overlap and are arranged in the 11/5 configuration, microtubular roots are arranged in a cruciate pattern and system II striated fibers are present. The basal body connective which generally lacks striation in the Ulvophyceae is clearly different in Batophora, being composed of two large non-striated halves which connect to the anterior surface of each basal body and are then connected to one another by a distinctly fibrous centrally striated region. This variation in the basal body connective and the presence of two posteriorly directed system II striated fibers is clearly different from homologous structures reported in siphonous green algae of the Caulerpales. Based upon these variations and similarities among flagellar apparatus components in siphonous green algae, it is suggested that the Dasycladales and Siphonodadales are more closely related to one another than to the Caulerpales.     

MITOSIS AND CELL DIVISION IN HYDRURUS FOETIDUS (CHRYSOPHYCEAE)1

Mitosis and cell division have been examined ultrastructurally in the vegetative cells of Hydrurus foetidus (Vill) Trev. and found to resemble that of Ochromonas in two important aspects. First, the rhizoplast acts as the spindle organizing body and second, the spindle elongates considerably during anaphase. It differs from Ochromonas in that there is no movement of the basal bodies and flagella towards the poles. Moreover, the nuclear envelope remains relatively intact throughout early stages of mitosis, with gaps developing at the poles during prophase to permit entry of spindle microtubules. Disruption of the nuclear envelope does not occur in the equatorial plane until late anaphase. The spindle persists into telophase and is bent towards the posterior of the cell by the ingrowing edge of the cleavage furrow. Persistence of the spindle and lack of Ochromoms-type cell elongation may be related to the constricting presence of the sheath during cell division—a completely different strategy to that adopted by the green algae under conditions of similar constraint.     

MITOSIS AND CYTOKINESIS IN SESSILE SPORANGIUM OF TRENTEPOHLIA AUREA (CHLOROPHYCEAE)1

An ultrastructure study of mitosis and cytokinesis in the sessile sporangium of Trentepohlia aurea (L.) Mart, was made to clarify the phylogenetic position of the alga. Mitosis was closed and centric at late anphase with cytokinesis involving the production of cleavage membranes by dictyasames between the numerous, well-separated daughter nuclei. Neither phycoplast nor phragmoplast microtubules were observed during cytokinesis. The lack of phycoplast microtubules and the presence of multilayered structures in flagellated cells suggest Trentepohlia is phylogenetically related to those green algae thought to have given rise to the land plants. The primitive type of mitosis and the lack of microbodies suggest that the ancestors of Trentepohlia may have branched off from this line relatively early.     

TWO SNOW SPECIES OF THE QUADRIFLAGELLATE GREEN ALGA CHLAINOMONAS (CHLOROPHYTA,VOLVOCALES): ULTRASTRUCTURE AND PHYLOGENETIC POSITION WITHIN THE CHLOROMONAS CLADE1

The quadriflagellate snow alga Chlainomonas Christen, distributed in New Zealand and North America, has several unusual structural attributes. A process assumed to be cytokinesis involves extrusion of protoplasm from the parent through a narrow canal, C. kolii (J. T. Hardy et Curl) Hoham produces a net‐like outer envelope rather than a cell wall, and the flagellar basal apparatus of Chlainomonas consists of two semi‐independent pairs of basal bodies. Structural connections between basal body pairs appear minimal, but a connecting system different from that observed in other genera exists within each pair. Phylogenetic analysis using rbcL sequences places Chlainomonas in the Chloromonas clade, other known members of which are all biflagellate. Chlainomonas is split into two robust lineages, with New Zealand collections sharing an origin with northern North American collections. Although the quadriflagellate condition is regarded as ancestral in the Chlorophyceae, we speculate—based on ultrastructural and molecular data presented here—that Chlainomonas represents a derived form that has arisen from fusion of two ancestral biflagellate cells. Other explanations (for example, that Chlainomonas represents a diploid form of a biflagellate species) are remotely possible but are presently at odds with extensive observations of field material. Improvements in techniques for experimental manipulation of these sensitive cryophiles will be required to fully characterize their structure and progress our understanding of their biology.     

THE FLAGELLAR APPARATUS OF ENTOCLADIA VIRIDIS MOTILE CELLS,AND THE TAXONOMIC POSITION OF THE RESURRECTED FAMILY ULVELLACEAE (ULVALES,CHLOROPHYTA)1

The flagellar apparatuses of the quadriflagellate zoo-spores and biflagellate female gametes of the marine chaetophoracean alga Entocladia viridis Reinke are significantly different from those of algae belonging to Chaetophoraceae sensu stricto, but closely resemble those of ulvacean genera. These differences permit the taxonomic reassignment of certain marine chaetophoracean genera and an evaluation of the flagellar apparatus features used to characterize the class Ulvophyceae. Critical features of the zoospore include arrangement of the four basal bodies into an upper and a lower pair with the proximal ends of the upper basal bodies overlapping, terminal caps, proximal sheaths connected to one another by striated bands, and a cruciate microtubular rootlet system having a 3-2–3-2 alternation pattern and striated microtubule-associated components that accompany the two-membered rootlets. An indistinct distal fiber occurs just anterior to the basal bodies, and is closely associated with the insertion into the flagellar apparatus of the three-membered rootlets. The flagellar apparatus demonstrates 180° rotational symmetry, and its components show counterclockwise absolute orientation when viewed from above. Newly described features include the prominently bilobed structure of the terminal caps on the upper basal body pair, and the presence of both a granular zone and an additional single microtubule anterior to each of the four rootlets, an arrangement termed the “stacked rootlet configuration.” Rhizoplasts were not observed and are presumed to be absent. The gamete is identical, except for the absence of the lower basal body pair and the presence of an electron-dense membrane associated structure that resembles the mating structure found in Ulva gametes. These findings, correlated with life history data, sporangial and gametangial structure and developmental patterns, chloroplast pigment arrays, and vegetative cell ultrastructural features, compel the removal of Entocladia viridis and similar members of the marine Chaetophoraceae to a separate family, the Ulvellaceae. The latter is referred to the order Ulvales of the Ulvophyceae. The counterclockwise absolute orientation of components, and terminal caps, may be the most consistent flagellar apparatus features of ulvophycean green algae, while variations in other features previously considered diagnostic for the Ulvophyceae may serve instead to identify discrete lineages within this class.     

EVOLUTIONARY RELATIONSHIPS AMONG MULTIPLE MODES OF CELL DIVISION IN THE GENUS NANNOCHLORIS (CHLOROPHYTA) REVEALED BY GENOME SIZE, ACTIN GENE MULTIPLICITY, AND PHYLOGENY

A single cell divides to multiply, but not all cells follow the same pattern of division. We studied cell division in seven strains from six species belonging to the genus Nannochloris Naumann and classified their modes of cell division into three types: binary fission ( N. bacillaris Naumann), budding ( N. coccoides Naumann), and autosporulation resulting in the formation of two to four daughter cells ( N. maculata Butcher, N . sp. SAG 251-2, N. atomus Butcher CCAP 251/7 and SAG 14.87, and N. eucaryotum [Wilhelm et al.] Menzel and Wild). To determine the evolutionary relationships among these multiple modes of cell division, we investigated the strains' genome sizes, copy number of actin genes, and phylogeny. The genome sizes were determined by counter-clamped homogeneous electric fields electrophoresis and fluorimetry. The genomes are very small and range from 12.6 Mbp ( N. maculata ) to 47.4 Mbp ( N. atomus SAG 14.87). The genomes of Nannochloris species seem to be among the smallest for free-living eukaryotes. Nannochloris bacillaris (binary fission), N. coccoides (budding), Nannochloris sp. (two-cell type of autosporulation), and N. eucaryotum (multicell type of autosporulation) contain a single actin gene, whereas N. maculata (two-cell type of autosporulation) and two strains of N. atomus (two-cell type of autosporulation) contain two actin genes. This suggests that the actin gene was duplicated in this eukaryote, which has a very small genome. Phylogenetic analyses of partial actin gene sequences suggest that autosporulation is the ancestral mode of cell division.     

COLCHICINE-INDUCED ALTERATIONS IN COLONY DEVELOPMENT IN EUDORINA ELEGANS (VOLVOCALES,CHLOROPHYTA)1

Colonies of Eudorina elegans Ehrenberg were treated with a 0.2% colchicine solution for periods of up to 48 h, and a number of alterations were observed in dividing colonies. Nuclear alterations were observed after 20 min of treatment, due to the inhibition of spindle microtubule polymerization. This inhibition resulted in increased ploidy levels, and permanent diploid colonies were obtained. The inhibition of cytoplasmic microtubule polymerization resulted in a number of structural alterations including: unequal cytokinesis of plakeal cells, the partial or complete inhibition of cytokinesis (30 min treatment), production of “stellate” cells (90 min treatment), and the subsequent formation of extra-cytoplasmic particles around the plakeal cells (3 h treatment). A possible cytoskeletal function of peripherally orient ed microtubules, and the role of the phycoplast microtubules is discussed. In addition, colchicine treatment caused an inhibition of inversion (60 min treatment), an increase in golgi-associated vesicles, and an excessive production of colonial envelope material (3 h treatment). The latter resulted in the formation of flattened Gonium-like colonies. The process of inversion is discussed in light of the above results. Chloroplast microtubules, however, were unaffected by colchicine treatment.     

FINE STRUCTURE OF CELL DIVISION IN PYRAMIMONAS PARKEAE NORRIS AND PEARSON (CHLOROPHYTA,PRASINOPHYCEAE)1

Cell division in Pyramimonas parkeae is described and compared with some other green algae. The first indication of mitosis is division of the chloroplast, accompanied by growth of a prominent microbody, followed by replication of the 4 basal bodies. Also closely timed with this is the replication of the Golgi and other components of the basal body complex. Two basal body complexes separate, each taking a position at either pole of the nucleus which has migrated to a characteristic position just beneath the plasmalemma of a broadened and flattened flagellar pit. Cytokinesis is accomplished by the fusion of ducts and vesicles with the simultaneous release of scales to the newly formed exterior. Cells swim throughout division.