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.     

THE SEXUAL LIFE CYCLES OF PFIESTERIA PISCICIDA AND CRYPTOPERIDINIOPSOIDS (DINOPHYCEAE)1

Sexual life cycle events in Pfiesteria piscicida and cryptoperidiniopsoid heterotrophic dinoflagellates were determined by following the development of isolated gamete pairs in single‐drop microcultures with cryptophyte prey. Under these conditions, the observed sequence of zygote formation, development, and postzygotic divisions was similar in these dinoflagellates. Fusion of motile gamete pairs each produced a rapidly swimming uninucleate planozygote with two longitudinal flagella. Planozygotes enlarged as they fed repeatedly on cryptophytes. In <12 h in most cases, each planozygote formed a transparent‐walled nonmotile cell (cyst) with a single nucleus. Zygotic cysts did not exhibit dormancy under these conditions. In each taxon, dramatic swirling chromosome movements (nuclear cyclosis) were found in zygote nuclei before division. In P. piscicida, nuclear cyclosis occurred in the zygotic cyst or apparently earlier in the planozygote. In the cryptoperidiniopsoids, nuclear cyclosis occurred inthe zygotic cyst. After nuclear cyclosis, a single cell division occurred in P. piscicida and cryptoperidiniopsoid zygotic cysts, producing two offspring that emerged as biflagellated cells. These two flagellated cells typically swam for hours and fed on cryptophytes before encysting. A single cell division in these cysts produced two biflagellated offspring that also fed before encysting for further reproduction. This sequence of zygote development and postzygotic divisions typically was completed within 24 h and was confirmed in examples from different isolates of each taxon. Some aspects of the P. piscicida sexual life cycle determined here differed from previous reports.     

ULTRASTRUCTURE OF THE DINOFLAGELLATE PERIDINIUM CINCTUM F. OVOPLANUM. II. LIGHT AND ELECTRON MICROSCOPIC OBSERVATIONS ON FERTILIZATION

Fertilization in Peridinium cinctum f. ovoplamtm has been investigated at both the light and electron microscopic levels. Gamete formation occurs when vegetative cells are placed into nitrogen deficient media. The majority of gametes observed possess thin thecal plates; however, some are naked. Gametes have few chloroplasts as compared to vegetative cells, numerous membrane bounded storage bodies, many starch grains, and chromosomes which appear slightly unwound. Gamete fusion is observed to peak 7–10 days after inoculation into nitrogen deficient media. Fusion occurs in an area of the sulcus devoid of reticulate thecal plates at or adjacent to the flagellar pores. A fertilization tube is formed and proceeds to widen along the sulcus. Karyogamy occurs within the fertilization tube before plasmogamy is completed. The resulting planozygote is a two walled structure containing two longitudinal flagella. It enlarges over a 2-week period giving rise to the hypnozygote.     

Behavior of flagella and flagellar root systems in the planozygotes and settled zygotes of the green alga Bryopsis maxima Okamura (Ulvophyceae,Chlorophyta) with reference to spatial arrangement of eyespot and cell fusion site

Behaviors of male and female gametes, planozygotes and their microtubular cytoskeletons of a marine green alga Bryopsis maxima Okamura were studied using field emission scanning electron microscopy, high‐speed video microscopy, and anti‐tubulin immunofluorescence microscopy. After fusion of the biflagellate male and female gametes, two sets of basal bodies lay side by side in the planozygote. Four long female microtubular roots extended from the basal bodies to the cell posterior. Four short male roots extended to nearly half the distance to the posterior end. Two flagella, one each from the male and female gametes, become a pair. Specifically, the no. 2 flagellum of the female gamete and one male flagellum point to the right side of the eyespot of the female gamete, which is located at the cell posterior and which is associated with 2s and 2d roots of the female gamete. This spatial relationship of the flagella, microtubular roots, and the eyespot in the planozygote is retained until settlement. During forward swimming, the planozygote swings the flagella backward and moves by flagellar beating. The male and female flagella in the pair usually beat synchronously. The cell withdraws the flagella and becomes round when the planozygote settles to the substratum 20 min after mixing. The axoneme and microtubular roots depolymerize, except for the proximal part and the basal bodies. Subsequently, distinct arrays of cortical microtubules develop in zygotes until 30 min after mixing. These results are discussed with respect to the functional significance of the spatial relationships of flagellar apparatus‐eyespot‐cell fusion sites in the mating gametes and planozygote of green algae.     

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.     

The life history of the toxic marine dinoflagellate Protoceratium reticulatum (Gonyaulacales) in culture

Asexual and sexual life cycle events were studied in cultures of the toxic marine dinoflagellate Protoceratium reticulatum. Asexual division by desmoschisis was characterized morphologically and changes in DNA content were analyzed by flow cytometry. The results indicated that haploid cells with a C DNA content occurred only during the light period whereas a shift from a C to a 2C DNA content (indicative of S phase) took place only during darkness. The sexual life cycle was documented by examining the mating type as well as the morphology of the sexual stages and nuclei. Gamete fusion resulted in a planozygote with two longitudinal flagella, but longitudinally biflagellated cells arising from planozygote division were also observed, so one of the daughter cells retained two longitudinal flagella while the other daughter cell lacked them. Presumed planozygotes (identified by their longitudinally biflagellated form) followed two life-cycle routes: division and encystment (resting cyst formation). Both the division of longitudinally biflagellated cells and resting cyst formation are morphologically described herein. Resting cyst formation through sexual reproduction was observed in 6.1% of crosses and followed a complex heterothallic pattern. Clonal strains underwent sexuality (homothallism for planozygote formation and division) but without the production of resting cysts. Ornamental processes of resting cysts formed from the cyst wall under an outer balloon-shaped membrane and were fully developed in <1 h. Obligatory dormancy period was of ∼4 months. Excystment resulted in a large, rounded, pigmented, longitudinally biflagellated but motionless, thecate germling that divided by desmoschisis. Like the planozygote, the first division of the germling yielded one longitudinally biflagellated daughter cell and another without longitudinal flagella. The longitudinal biflagellation state of both sexual stages and of the first division products of these cells is discussed.     

VEGETATIVE REPRODUCTION AND SEXUAL LIFE CYCLE OF THE TOXIC DINOFLAGELLATE GYMNODINIUM CATENATUM FROM TASMANIA,AUSTRALIA1

The toxic, chain-forming dinoflagellate Gymnodinium catenatum Graham was cultured from vegetative cells and benthic resting cysts isolated from estuarine waters in Tasmania, Australia. Rapidly dividing, log phase cultures formed long chains of up to 64 cells whereas stationary phase cultures were composed primarily of single cells (23-41 pm long, 27-36 pm wide). Vegetative growth (mean doubling time 3-4 days) was optimal at temperatures from 14.5-20° C, salinities of 23-34% and light irradiances of 50-300 μE·m^?2·s^?1. The sexual life cycle of G. catenatum was easily induced in a nutrient-deficient medium, provided compatible opposite mating types were combined (heterothallism). Gamete fusion produced a large (59-73 μm long, 50-59 μm wide) biconical, posteriorly biflagellate planozygote (double longitudinal flagellum) which after several days lost one longitudinal flagellum and gradually became subspherical in shape. This older planozygote stage persisted for up to two weeks before encysting into a round, brown resting cyst (42-52 μm diam; hypnozygote) with microreticulate surface ornamentation. Resting cysts germinated after a dormancy period as short as two weeks under our culture conditions, resulting in a single, posteriorly biflagellate germling cell (planomeiocyte). This divided to form a chain of two cells, which subsequently re-established a vegetative population. Implications for the bloom dynamics of this toxic dinoflagellate, a causative organism of paralytic shellfish poisoning, are discussed.     

ULTRASTRUCTURE OF CEPHALEUROS VIRESCENS (CHROOLEPIDACEAE; CHLOROPHYTA). IV. ABSOLUTE CONFIGURATION ANALYSIS OF THE CRUCIATE FLAGELLAR APPARATUS AND MULTILAYERED STRUCTURES IN THE PRE- AND POST-RELEASE GAMETES

Transmission electron microscopy of pre-release and post-release biflagellate gametes of Cephaleuros virescens has produced comparative data on these cells and on the detailed absolute arrangement of the flagellar apparatus. In all major respects including the presence of two multilayered structures (MLS's) the closely compacted, non-motile but mature pre-release gametes are similar to the mature, free swimming post-release gametes. The elongated shape of the free-swimming gametes differs from the more compact form of the pre-release gametes, but does not reflect a major difference in the arrangement of internal components. The flagella are bilaterally keeled and each keel contains a cylindrical element. Each flagellar base is encircled by a densely staining collar of modified plasmalemma at the point of entry into the apical papilla. The equal anterior flagella enter the papilla from opposite sides; their basal bodies are parallel and overlapping. Each terminates in a densely staining terminal cap. No capping plate is present. Each basal body is associated both with a three-layered MLS, the anterior layer of which becomes a lateral microtubular spline of 2 to 8 microtubules, and with an additional medial compound root of two layers of microtubules (2 over 4 or 5). Both the compound microtubule root and the spline may acquire additional microtubules as they extend distally in close proximity to mitochondria and the plasmalemma. No striated roots, or rhizoplasts, have been observed. Two densely staining plaques are associated with the plasma membrane at specific anterior sites and may be comparable to the presumptive mating structures seen in other green algal motile cells. The reversed bilateral symmetry of the cells produces two possible arrangements of the flagellar apparatus, namely, a 11/5 (or left-handed) arrangement or a 1/7 (or right-handed) arrangement. Only 11/5 cells have been found. Despite the presence of distinct multilayered structures, some aspects of the gametes of Cephaleuros quite closely resemble the cruciate motile cells of algae now regarded by some authors as typical of Ulvophyceae, sensu Stewart and Mattox.     

LIFE CYLE AND SEXUALITY OF THE FRESHWATER RAPHIDOPHYTE GONYOSTOMUM SEMEN (RAPHIDOPHYCEAE)1

Previously unknown aspects in the life cycle of the freshwater flagellate Gonyostomum semen (Ehrenb.) (Raphidophyceae) are described here. This species forms intense blooms in many northern temperate lakes, and has increased in abundance and frequency in northern Europe during the past decades. The proposed life cycle is based on observations of life cycle stages and transitions in cultures. Viable stages of the life cycle were individually isolated and monitored by time‐lapse photography. The most common processes undertaken by the isolated cells were: division, fusion followed by division, asexual cyst formation, and sexual cyst formation. Motile cells divided by two different processes. One lasted between 6 and 24 h and formed two cells with vegetative cell size and with or without the same shape. The second division process lasted between 10 and 20 min and formed two identical cells, half the size of the mother cell. Planozygotes formed by the fusion of hologametes subsequently underwent division into two cells. Asexual cyst‐like stages were spherical, devoid of a thick wall and red spot, and germinated in 24–48 h. Heterogamete pairs were isogamous, and formed an angle of 0–90° between each other. Planozygote and sexual cyst formation were identified within strains established from one vegetative cell. The identity of these strains, which was studied by an amplified fragment length polymorphism analysis, was correlated with the viability of the planozygote. Resting cyst germination was described using cysts collected in the field. The size and morphology of these cysts were comparable with those formed sexually in culture. The excystment rate was higher at 24°C than at 19 or 16°C, although the cell liberated during germination (germling) was only viable at 16°C. The placement of G. semen within the Raphidophyceae family was confirmed by sequence analysis of a segment of the 18S ribosomal DNA.     

INTERACTIVE EFFECTS OF SALINITY AND TEMPERATURE ON PLANOZYGOTE AND CYST FORMATION OF ALEXANDRIUM MINUTUM (DINOPHYCEAE) IN CULTURE1

The factors regulating dinoflagellate life‐cycle transitions are poorly understood. However, their identification is essential to unravel the causes promoting the outbreaks of harmful algal blooms (HABs) because these blooms are often associated with the formation and germination of sexual cysts. Nevertheless, there is a lack of knowledge on the factors regulating planozygote‐cyst transitions in dinoflagellates due to the difficulties of differentiating planozygotes from vegetative stages. In the present study, two different approaches were used to clarify the relevance of environmental factors on planozygote and cyst formation of the toxic dinoflagellate Alexandrium minutum Halim. First, the effects of changes in initial phosphate (P) and nitrate (N) concentrations in the medium on the percentage of planozygotes formed were examined using flow cytometry. Second, two factorial designs were used to determine how salinity (S), temperature (T), and the density of the initial cell inoculum (I) affect planozygote and resting‐cyst formation. These experiments led to the following conclusions: 1. Low P/N ratios seem to induce gamete expression because the percentage of planozygotes recorded in the absence of added phosphate (‐P) was significantly higher than that obtained in the absence of added nitrogen (‐N), or when the concentrations of both nitrogen and phosphate were 20 times lower (N/20 + P/20). 2. Salinity (S) and temperature (T) strongly affected both planozygote and cyst formation, as sexuality in the population increased significantly as salinity decreased and temperatures increased. S, T combinations that resulted in no significant cyst formation were, however, favorable for vegetative growth, ruling out the possibility of negative effects on cell physiology. 3. The initial cell density is thought to be important for sexual cyst formation by determining the chances of gamete contact. However, the inoculum concentrations tested did not explain either planozygote formation or the appearance of resting cysts.     

Biology and morphology of freshwater rapacious flagellate <Emphasis Type="Italic">Colponema</Emphasis> aff. <Emphasis Type="Italic">loxodes</Emphasis> Stein (Colponema,Alveolata)

The biology, morphology, and ultrastructure of the freshwater rapacious flagellate Colponema aff. loxodes, which attacks bodonids and chrysomonads, are studied. The flagellate is characterized by three-membrane alveolar pellicle, vesicular nucleus, two heterodynamous flagella, two microtubule bands which armor the longitudinal groove, and mitochondria with tubular cristae. Toxicysts (thread-organelles) are found in the cytoplasm. The posterior flagellum is characterized by the proximal fold. Micropores are completely absent. After being caught, the prey is taken into the longitudinal groove. Vegetative swimming cells are present in the life cycle. No reproduction or latent cysts are found. The taxonomical position of Colponema aff. loxodes is discussed in comparison with other colponemids and protists.     

CELL DIVISION IN THE SCALY GREEN FLAGELLATE HETEROMASTIX ANGULATA AND ITS BEARING ON THE ORIGIN OF THE CHLOROPHYCEAE

H. angulata is a scale-covered, asymmetrical green unicell with two laterally attached, anisokont flagella. In recent years it has been classified in the Prasinophyceae. The flagellar apparatus replicates, and the cell begins to cleave at the side opposite the flagella before the nucleus can be perceived to be in prophase. The flagellar apparatuses separate, and the extra-nuclear development of the spindle occurs from the regions occupied by rhizoplasts. Rhizoplasts or partial rhizoplasts lie at the flat metaphase spindle poles. By metaphase, the cell has already elongated to the extent that it is nearly twice as long as at interphase. The spindle and the cell itself elongate greatly during anaphase with a concomitant further separation of the flagellar pairs. Although the interzonal spindle persists during cytokinesis as in charophycean algae, H. angulata is similar in flagellar scale morphology and other characteristics to the chlorophycean Platymonas, which has a collapsing interzonal spindle at telophase, a phycoplast, and a wall-like theca, which develops by the fusion of small stellate scales. It is hypothesized that the collapsing telophase spindle and phycoplast evolved in green flagellates similar to Platymonas, in which cell and spindle elongation became restricted by a cell wall that evolved from stellate scales similar to those in Heteromastix. Such walled flagellates are then visualized as having eventually given rise to Chlamydomonas and to the entire range of chlorophycean algae with phycoplasts. It is pointed out that the hypothesis has a number of implications by which its validity could be judged when sufficient information becomes available.     

IMMUNOLOCALIZATION OF CENTRIN IN OXYRRHIS MARINA (DINOPHYCEAE)1

A new polyclonal antibody was raised against centrin isolated from the flagellate green alga Spermatozopsis similis (Chlorophyta; anti-SSC). It stains by immunofluorescence and immunoelectron microscopy well-known reference systems for centrin like the nucleus–basal body connectors in Chlamydomonas reinhardtii (Chlorophyta) and the system II fibers (rhizoplasts) of Scherffelia dubia (Chlorophyta). In addition, it recognizes in immunoblots a single 20-kDa protein in isolated cytoskeletons of Spermatozopsis similis and Tetraselmis striata (Chlorophyta) as well as purified centrin isolated from Tetraselmis striata. Using this antibody, centrin was localized in whole cells and isolated cytoskeletons of Oxyrrhis marina Dujardin (Dinophyceae) by immunofluorescence and immunogold electron microscopy. In the flagellar apparatus of O. marina, five different structures were antigenic. Four short fibers (connectives 1–4) link the basal bodies to the four major fibrous flagellar roots, which do not cross-react with anti-centrin. The most prominent of the labeled structures (connective 5), a crescent-shaped fiber, extends from the flagellar canal of the transverse flagellum along the base of the tentacle to the flagellar canal of the longitudinal flagellum, interconnecting the distal parts of the microtubular roots/bands in the basal apparatus. For most of its length, it underlies and is connected to a transversely oriented subamphiesmal microtubular band. In immunoblot analyses, anti-SSC recognizes only a single 20-kDa protein in cytoskeletons of O. marina. Functional and phylogenetic aspects of centrin-containing structures in dinoflagellates are discussed.     

COMPARATIVE ULTRASTRUCTURE OF ZOOSPORES WITH PARALLEL BASAL BODIES FROM THE GREEN ALGAE DICTYOCHLORIS FRAGRANS AND BRACTEACOCCUS SP.

The chlorococcalean algae Dictyochloris fragrans and Bracteacoccus sp. produce naked zoospores with two unequal flagella and parallel basal bodies. Ultrastructural features of the flagellar apparatus of these zoospores are basically identical and include a banded distal fiber, two proximal fibers, and four cruciately arranged microtubular rootlets with only one microtubule in each dexter rootlet. In D. fragrans, each proximal fiber is composed of two subfibers, one striated and one nonstriated, and each sinister rootlet is composed of five microtubules (4/1), decreasing to four away from the basal bodies. In Bracteacoccus sp., each proximal fiber is a single unit, the sinister rootlets are four (3/1) or rarely five (4/1) microtubules, and each basal body is associated with an unusual curved structure. The basic features of the flagellar apparatus of the zoospores of these two algae resemble those of Heterochlamydomonas rather than most other chlorococcalean algae that have equal length flagella, basal bodies in the V-shape arrangement, and clockwise absolute orientation. It is proposed that these algae with unequal flagella and parallel basal bodies have a shared common ancestry within the green algae.     

SEXUAL PROCESSES IN THE LIFE CYCLE OF GYRODINIUM UNCATENUM (DINOPHYCEAE): A MORPHOGENETIC OVERVIEW1

Sexual processes in the life cycle of the dinoflagellate Gyrodinium uncatenum Hulburt were investigated in isolated field populations. Morphological and morphogenetic aspects of gamete production, planozygote formation, encystment, excystment, and planomeiocyte division are described from observations of living specimens, Protargol silver impregnated material and scanning electron microscope preparations. The sexual cycle was initiated by gamete formation which involved two asexual divisions of the vegetative organism. Gametes were fully differentiated following the second division and immediately capable of forming pairs. Either isogamous or anisogamous pairs were formed by the mid-ventral union of gametes. Gametes invariably joined with flagellar bases in close juxtaposition. Complete fusion of gametes required ca. 1 h, involved plasmogamy followed by karyogamy and resulted in a quadriflagellated planozygote. Planozygotes encysted in 24–48 h to yield a hypnozygote capable of overwintering in estuarine sediments. Hypnozygotes collected from sediment in late winter readily excysted upon exposure to temperatures above 15°C. A single quadriflagellated planomeiocyte emerged from the cyst and under culture conditions divided one to two days later. The four flagella were not evenly distributed at the first division and both bi- and tri-flagellated daughter cells were formed.     

DISTINCTIVE LECTIN LABELING OF THE FLAGELLAR APPARATUS OF TETRASELMIS (PRASINOPHYCEAE) AND DUNALIELLA (CHLOROPHYCEAE)1

Fluorescent labeling of the flagellar apparatus of Tetraselmis (Prasinophyceae) and Dunaliella (Polyblepharidaceae, Chlorophyceae) were successfully performed using fluorescein isothiocyanate–labeled lectins from Arachis hypogaea and Glycine maxima. These lectins specifically bound to the flagella and kinetosome of the cell but did not bind to the cell surface. Lectin binding on the flagellar apparatus remained constant under different culture media, temperatures, irradiances, cell division cycle, and culture aging. All the Tetraselmis and Dunaliella analyzed (five species, 20 clones) showed intense labeling of the flagellar apparatus. In contrast, no other species analyzed (46 clones of 25 species from four classes) showed binding to their flagellar apparatus. After the lectin treatment, many cells remained alive, and they were able to swim with the flagellar apparatus intensely labeled. The lectin binding indicates that the flagella and kinetosome of Tetraselmis are rich in Gal and GalNH₂ moieties and that the flagella of Dunaliella are rich in Gal and GalNAc moieties. Apparently, this feature seems to be specific to these species.     

ULTRASTRUCTURE AND PHYLOGENETIC RELATIONSHIPS OF CHAETOPELTIDALES ORD. NOV. (CHLOROPHYTA,CHLOROPHYCEAE)1

Vegetative cells and zoospores of Hormotilopsis gelatinosa Trainor & Bold, H. tetravacuolaris Arce & Bold, Planophila terrestris Groover & Hofstetter, and Phyllogloea fimbriata (Korchikov) Silva were examined by transmission electron microscopy. All cells had pyrenoids traversed by cytoplasmic channels. Zoospores were quadriflagellate and had essentially cruciate flagellar apparatuses. Scales were present on free-swimming zoospores. These features are essentially identical to those of Chaetopeltis sp. and are dissimilar to those of other described green algae. The new order Chaetopeltidales is created to accommodate the genera Chaetopeltis, Hormotilopsis, Planophila sensu Groover & Hofstetter, Phyllogloea, Dicranochaete, and Schizochlamys, organisms previously scattered among the orders Tetrasporales, Chloro-coccales, Chlorosarcinales, and Chaetophorales. Members of the order are closely related to the ancestral chlorophycean flagellate genus Hafniomonas, may be ancestral with respect to other Chlorophyceae, and may also be closely related to the ulvophycean order Ulotrichales.     

Desmosomes and hemidesmosomes in the flagellate Crithidia fasciculata

Summary The flagellum of the trypanosomatid flagellate Crithidia fasciculata expands asymmetrically as it emerges from the reservoir. Where the flagellar memhrane approaches the membrane lining the reservoir, desmosomes are found. These structures are arranged in several slightly curved lines and have many features in common with vertebrate desmosomes.In cultures, the flagellates stick to each other by their flagella and form rosettes. In these bundles of cells, probable sites of adhesion between flagella, or between flagella and pieces of debris, are marked by a dense filamentous tract which passes posteriorly along the flagellum and by a thick band lying just below the flagellar membrane. It is suggested that similar adhesions are found in the insect host where the flagellate attaches itself to the gut wall.     

DEVELOPMENT OF THE FLAGELLAR APPARATUS AND FLAGELLAR POSITION IN THE COLONIAL GREEN ALGA PLATYDORINA CAUDATA (CHLOROPHYCEAE)1

The ultrastructure of the flagellar apparatus in pre-inversion and inversion stages of Platydorina resembles that of Chlamydomonas in having 180° rotational symmetry and clockwise absolute orientation. Basal bodies are in a “V” configuration and connected by one distal and two proximal fibers. Alternating two- and four-membered microtubular rootlets are cruciately arranged. During maturation, the basal bodies rotate and separate, and 180° rotational symmetry is lost. Simultaneously, each proximal fiber detaches from one of the functional basal bodies, and the distal fiber detaches from both. The mature apparatus has widely separated and nearly parallel basal bodies. Flagellar orientation in Platydorina is completed just after inversion and a flattening of the colony called intercalation, resulting in the pairs of flagella of neighboring cells extending from the colony in opposite directions in an alternating fashion. Flagellar orientation and separated basal bodies minimize the interference between the flagella of neighboring cells. Basal bodies and rootlets of the two intercalated halves of a colony rotate, resulting in the effective strokes of the flagella of every cell being towards the colonial posterior. The flagella of each cell beat with an effective stroke in the direction of the two inner rootlets. The flagella have an asymmetrical ciliary type beat. The rotated, separated, and parallel basal bodies, together with the nearly parallel rootlets probably are adaptations for movement of this colonial volvocalean alga. The flagellar apparatus in immature stages of Platydorina lends support to the suggestion that the alga has evolved from a Chlamydomonas-like ancestor.     

A FLAVIN-LIKE AUTOFLUORESCENT SUBSTANCE IN THE POSTERIOR FLAGELLUM OF GOLDEN AND BROWN ALGAE1

An autofluorescent substance occurs in the flagella of flagellate cells of the golden and brown algae. It is localized only in the posterior (short) flagellum and could not be detected in the anterior (long) one. It showed maximum fluorescence emission at 515–520 nm upon excitation of 440 nm; therefore, it is considered to be a flavin. This substance is distributed widely among flagellate cells of golden and brown algae irrespective of their nature (vegetative cells, zoospores, gametes, or sperm). It is absent, however, in some brown algal zoospores and sperm which lack an eyespot and flagellar swelling and are considered to lack phototaxis. Because the flagellar swelling in the posterior flagellum is a presumptive photoreceptor for phototaxis in these groups, it is suggested that the flavin located in the posterior flagellum acts as a photoreceptor pigment in phototaxis.