THE MICROTUBULAR CYTOSKELETON OF AMPHIDINIUM RHYNCHOCEPHALUM (DINOPHYCEAE)1

The sub-thecal microtubular cytoskeleton of Amphidinium rhynchocephalum Anissimowa was investigated using indirect immunofluorescence microscopy and transmission electron microscopy. The majority of sub-thecal microtubules are longitudinally oriented and radiate from one of two sub-thecal transverse microtubular bands that lie adjacent to the anterior and posterior edge of the cingulum.Both transverse bands consist of 3–5 microtubules and are loop shaped with one end adjacent to the cell's right edge of the sulcus and the other end adjacent to the fibrous ventral ridge. The posterior transverse microtubular band (PTB) defines the posterior edge of the cingulum and gives rise to numerous posteriorly directed longitudinal microtubular bundles that consist of 1–3 microtubules per bundle. These bundles end at the posterior end of the cell. The PTB also gives rise to the cingular longitudinal microtubules that underlie the cingular groove and terminate at the anterior transverse microtubular band (ATB). The ATB defines the anterior edge of the cingulum and loops around the base of the epicone. This band gives rise to anteriorly directed longitudinal microtubular bundles that terminate in the small epicone of the cell. The longitudinal microtubular root of the flagellar apparatus is directed posteriorly and lies immediately beneath the theca but is distinct from the subthecal microtubule system. A narrow fibrous ridge is ventrally located to the cell's left between the exit apertures of the transverse and longitudinal flagella. In this position, the ventral ridge lies between and also connects with the anterior and posterior transverse microtubular bands. The ventral ridge is also associated with three microtubules that are distinct from other cytoskeletal microtubules. Our results demonstrate that the majority of sub-thecal microtubules originate from one of two microtubular bands associated with the cingulum. The possible role of the fibrous ventral ridge and its associated microtubules is also discussed.     

ULTRASTRUCTURE OF THE BASAL APPARATUS AND PUTATIVE VESTIGIAL FEEDING APPARATUSES IN A QUADRIFLAGELLATE EUGLENOID (EUGLENOPHYTA)1

The flagellar apparatus and presumptive vestigial feeding apparatuses of a cold-water, photosynthetic, quadriflagellate euglenoid is described. The organism possesses two similar sets of flagella each consisting of one short and one long flagellum. Each pair of flagella is associated with three microtubular roots for a total of six roots in the basal apparatus. At the level of the ventral basal bodies, each intermediate root is nine-membered, while the ventral roots are composed of eight to nine microtubules. Only one of the ventral roots lines the single microtubule reinforced pocket. A four-membered dorsal root attaches to each dorsal basal body, and at the level of the reservoir each gives rise to a dorsal band. An additional bundle of microtubules, not arising from the microtubular roots of the basal apparatus, begins posterior to the basal apparatus as a small group of a few microtubules and extends anteriorly on the right ventral side of the reservoir ending at the canal. At the level of the stigma, the microtubules are organized into a multi-layered bundle that continues to increase in size and eventually splits to form two bundles at the level of the canal. We postulate that these bundles may represent the remnants of a rod-and-vane-type feeding apparatus like that found in many phagotrophic euglenoids.     

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.     

Reconciling the bizarre inheritance of microtubules in complex (euglenid) microeukaryotes

We introduce a hypothetical model that explains how surface microtubules in euglenids are generated, integrated and inherited with the flagellar apparatus from generation to generation. The Euglenida is a very diverse group of single-celled eukaryotes unified by a complex cell surface called the "pellicle", consisting of proteinaceous strips that run along the longitudinal axis of the cell and articulate with one another along their lateral margins. The strips are positioned beneath the plasma membrane and are reinforced with subtending microtubules. Euglenids reproduce asexually, and the two daughter cells inherit pellicle strips and associate microtubules from the parent cell in a semi-conservative pattern. In preparation for cell division, nascent pellicle strips develop from the anterior end of the cell and elongate toward the posterior end between two parent (mature) strips, so that the total number of pellicle strips and underlying microtubules is doubled in the predivisional cell. Each daughter cell inherits an alternating pattern of strips consisting of half of the nascent strips and half of the parent (mature) strips. This observation combined with the fact that the microtubules underlying the strips are linked to the flagellar apparatus created a cytoskeletal riddle: how do microtubules associated with an alternating pattern of nascent strips and mature strips maintain their physical relationship to the flagellar apparatus when the parent cell divides? The model of microtubular inheritance articulated here incorporates known patterns of cytoskeletal semi-conservatism and two new inferences: (1) a multigenerational "pellicle microtubule organizing center" (pMTOC) extends from the dorsal root of the flagellar apparatus, encircles the flagellar pocket, and underpins the microtubules of the pellicle; and (2) prior to cytokinesis, nascent pellicle microtubules fall within one of two "left/right" constellations that are linked to one of the two new dorsal basal bodies.     

Ultrastructure of the mitotic nuclei in Leishmania mexicana ssp. Tridimensional reconstruction of the mitotic spindle and dense plaques

The ultrastructure of mitotic nuclei of the promastigote Leishmania mexicana ssp. was studied by serial thin sections and three-dimensional reconstructions of each divisional stage. At the beginning of nuclear division (equatorial stage), a set of six dense plaques located about the equatorial region of the nucleus and a microtubular spindle develops in the two opposing poles of the nucleus (two sets of polar microtubules). The microtubular mitotic spindle is entirely intranuclear with the nuclear membrane persisting through mitosis. The polar spindle consists of a discrete bundle of about 50 microtubules and the equatorial spindle is formed by about 100 microtubules. The spindle may contain several continuous microtubules, but no microtubular organizing centres were observed in association with the spindle. The plaques and hemiplaques are associated with microtubular bundles; some of the spindle microtubules converge on kinetochore-like plaques. It is suggested that the spindle has a special significance in the physiology of mitosis. The two sets of hemiplaques may guide the separation of the daughter genomes. At the beginning of the elongational stage the mitotic plaques split into halves and each set of half-plaques migrates to one pole. It is concluded that the dense plaques play a kinetochore-like role and thus Leishmania mexicana ssp. may have six chromosomal units. Mitotic events of this species are essentially similar to those of Trypanosoma cruzi.     

Microtubules and the flagellar apparatus in zoospores and cysts of the fungusPhytophthora cinnamomi

Summary A correlated immunofluorescence and ultrastructural study of the microtubular cytoskeleton has been made in zoospores and young cysts ofPhytophthora cinnamomi. Labelling of microtubules using antibodies directed towards tubulin has revealed new details of the arrangement of the flagellar rootlets in these cells, and of the variability that occurs from cell to cell. Most of the variation exists at the distal ends of the rootlets, and may be correlated with differences in cell shape in these regions. The rootlets have the same right and left configuration in all zoospores. The arrangement of the rootlet microtubules at the anterior end of the zoospores raises the possibility that the microtubules on the left hand side of the groove may not comprise an independent rootlet which arises at the basal bodies.The absolute configuration of the flagellar apparatus has been determined from ultrastructural observations of serial sections. In the vicinity of the basal bodies, there is little, if any, variation between individuals, and the structure of the flagellar apparatus is similar to that described for related species of fungi. Two ribbon-like coils surround the central pair of microtubules at the distal tip of the whiplash flagellum, and clusters of intramembranous particles, similar to ciliary plaques, have been found at the bases of both flagella. There are two arrays of microtubules associated with the nucleus in the zoospores. One array lies next to the outer surface of the nuclear envelope, and probably functions in the shaping and positioning of the apex of the nucleus. The nuclear pores in this region are aligned in rows alongside these microtubules. The second array is formed by kinetochore microtubules which extend into a collar-like arrangement of chromatin material around the narrow end of the (interphase) nucleus. During encystment, all flagellar rootlets are internalized when the flagella are detached at the terminal plate. The rootlets arrays are no longer recognizable 5–10 minutes after the commencement of encystment.     

The cytology and ultrastructure of zoospores of Hydrurus foetidus (Chrysophyceae)

The unusual tetrahedral shape of Hydrurus foetidus (Vill.) Trev. zoospores is associated with a complex skeletal system of microtubules extending from a broad flagellar root (up to 19 microtubules) into each of three, pointed anterior processes. The posterior end, also pointed and supported by a separate set of microtubules, contains a single large chloroplast with a prominent posterior furrow containing mitochondrial elements. A large immersed pyrenoid is penetrated by paired thylakoids. There is no eyespot. Numerous large Golgi bodies occur immediately anterior to the nucleus and up to 5–6 contractile vacuoles lie near the cell surface at the anterior end. Two terminally inserted flagella extend from the cell surface, a long one serving for cell locomotion, and the other vestigial with an axonemal pattern of 9+0. The flagellar root system consists of: (1) a thin, striated rhizoplast extending from the basal body of the long flagellum and ramifying over the surface of a conspicuous, anteriorly directed, conical projection of the nucleus; (2) a broad microtubular root which emanates from near the basal body of the long flagellum and appears to function as a MTOC; (3) a compound root, consisting of a striated fiber and two associated microtubules, which runs alongside the basal body of the stubby flagellum before terminating at the cell surface; and (4) a short two-membered microtubular root, also associated with the basal body of the stubby flagellum. Other components of the flagellar apparatus include a large dense body near the proximal end of the basal body of the short flagellum, and a small, dense, core-like structure closely associated with one of its triplet fibers. The flagellar apparatus of H. foetidus is remarkably similar in ultrastructure to that of Chrysonebula holmesii Lund.     

THE FLAGELLAR APPARATUS OF CHRYSOLEPIDOMONAS DENDROLEPIDOTA (CHRYSOPHYCEAE), A SINGLE-CELLED MONAD COVED WITH ORGANIC SCALES1

The flagellar apparatus of Chrysolepidomonas dedrolepidota Peters et Andersen is similar to that of other members of the Ochromonadales, Chrysophyceae. there are four microtubular roots (R_1-4) and a system II fiber (= rhizoplast). the R₁ root consists of three microtubules that nucleate many cytoplasmic microtubules. One compressed band of 10 or more cytoplasmic microtubules is directed black along the R1 root in an anti-parallel direction. The R₂ root consists of one to two microtubules, and it extends toward the distal end of the R₁ root. The R₃ root consists of six (?seven) microtubules near its proximal end. The “a” and “f” microtubules of the R₃ root are under the short flagellum, and the “f” microtubule loops back and under the basal body, extending down to the nucleus. The R₄ root consists of one to two microtubules extending along the left side of the shot flagellum and curving under the short flagellum where it terminates near the “a” microtubule of R₃ Both flagella have a transitional plate and a transitional helix with five gyres. There is a thin, second plate in the basal body at the level of the distal end of the “c” tubules of the basal body triplets. The tripartite flagellar hairs have long lateral filaments but lack short lateral filaments. We compare the flagellar apparatus with that of other members of the Ochromonadales and members of the Hydrurales and Hibberdiales.     

The structure of the flagellar apparatus of the swarm cells ofPhysarum polycephalum

Summary Flagellation ofPhysarum polycephalum amoebae (Myxomycete) involves the formation around the two kinetosomes of a flagellar apparatus leading to a modification in the shape of the amoeba and its nucleus. A tridimensional ultrastructural model of the flagellar apparatus is proposed, based upon observation of the isolated nucleo-flagellar apparatus complex. The flagellar apparatus is composed of a non-microtubular structure (the posterior para-kinetosomal structure), five microtubular arrays and two flagella: a long anterior flagellum and a short flagellum directed backwards. The asymmetry of the flagellar apparatus is due mainly to the presence of the posterior para-kinetosomal structure on the right side of the posterior kinetosome and of the two asymmetrical microtubular arrays 3 and 4. Thus, the flagellar apparatus is right-handed. This asymmetry implies also some spatial constraints on two other microtubular arrays (2 and 5). Except in the case of the microtubular array 1 which links the proximal end of the anterior kinetosome to the nuclear membrane, the number of microtubules of each microtubular array seems to be well defined: 39, 5–6, 7–9, and 2+2 for the microtubular arrays 2, 3, 4, and 5 respectively. All the elements of the nucleo-flagellar apparatus complex are linked either directly or indirectly through bridges. Furthermore, the microtubules which composed the microtubular array 3 are linked through bridges while the microtubules of the microtubular arrays 2, 3, and 4 seem to be linked through a reticulate material. All these spatial relationships lead to a great cohesion of the nucleo-flagellar apparatus complex which appears to be a well defined structure. This suggests thatPhysarum amoebal flagellation can be a promising system to study the morphogenesis of an eucaryotic cell.Abbreviations PIPES Piperazine-N,N-bis [2-ethane-sulfonic acid] - EGTA [Ethylenebis(oxyethylenenitrile)]tetraacetic acid - DMSO Dimethyl sulfoxide     

COMPARATIVE ANALYSES OF THE DINOFLAGELLATE FLAGELLAR APPARATUS. III. FREEZE SUBSTITUTION OF AMPHIDINIUM RHYNCHOCEPHALUM1

The flagellar apparatus of the marine dinoflagellate Amphidinium rhynchocephalum Anissimowa was examined using the techniques of rapid freezing/freeze substitution and serial thin section three dimensional reconstruction. The flagellar apparatus is composed of two basal bodies that are offset from one another and lie at an angle of approximately 150° The transverse basal body is associated with two individual microtubules that extend from the proximal end of the basal body toward the flagellar opening. One of these microtubules is closely appressed to a striated fibrous root that also extends from the proximal base of the transverse basal body. The longitudinal basal body is associated with a nine member microtubular root that extends from the proximal end of the basal body toward the posterior of the cell. The longitudinal microtubular root and the transverse striated fiber are connected by a striated connective fiber. In addition to the microtubules associated with the transverse and longitudinal basal bodies, a group of microtubules originates adjacent to one of the transverse flagellar roots and extends into the cytoplasm. Vesicular channels extend from the flagellar openings to the region of the basal bodies where they expand to encompass the various connective structures of the flagellar apparatus. The possible function and evolutionary importance of these structures is discussed.     

Structure of the cell of the amoeboid flagellate <Emphasis Type="Italic">Thaumatomonas coloniensis</Emphasis> Wylezich et al., 2007 (Thaumatomonadida (Shirkina) Karpov, 1990)

The ultrathin structure of the amoeboid flagellate Thaumatomonas coloniensis Wylezich et al. has been studied. The cell is surrounded by somatic scales forming on the surface of the mitochondria. The heterodynamic flagella emerge from the small flagellar pocket. Both flagella are covered by pineal scales and thin twisted mastigonemes. The kinetosomes lie parallel to each other. The transitional zone of the flagella carries the thin-walled cylinder. The transversal plate of the flagella is above the cell surface. The flagellar root system consists of three microtubular bands and a fibrillar rhizoplast. The vesicular nucleus and Golgi apparatus are of the usual structure. The mitochondria contain tubular cristae. The extrusive organelles (kinetocysts) contain amorphous material and a capsule; they are located in cytoplasm. The capsule consists of a muff and cylinder. Osmiophilic bodies of various shapes contain crystalloid inclusions. The pseudopodia capturing the bacteria emerge from the ventral groove. The groove is armored by the two longitudinal groups of the close situated microtubules. Microbodies and symbiotic bacteria have not been discovered. The resemblance of Th. coloniensis with other thaumatomonads is 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.     

Localization of Tubulin and a Centrin-Homologue in Vegetative Cells and Developing Gametangia of Ectocarpus siliculosus (Dillw.) Lyngb. (Phaeophyceae,Ectocarpales)

The distribution of tubulin and centrin in vegetative cells and during gametogenesis of Ectocarpus siliculosus was studied by immunofluorescence. In interphase cells bundles of microtubules are focused on the centriolar region near the nuclear surface. Some of the bundles ensheath the nucleus while others traverse the cytoplasm in various directions, sometimes reaching the cell cortex. Evaluation of serial optical sections by confocal laser scanning microscopy (CLSM) revealed that the perinuclear and “cytoplasmic” microtubule bundles presumably constitute a single complex. In interphase cells centrin is localized as a single bright spot in the centriolar region. In dividing cells duplication and separation of the microtubular complex and the centrin spot takes place. In post-mitotic cells with two nuclei, the centrioles are located at opposite cell poles, short microtubule bundles emanate from them and partially encompass the nucleus. During gametogenesis a gradual transformation of the vegetative cytoskeleton to the gametic flagellar apparatus occurs.     

Ultrastructure of the amoeboid flagellate <Emphasis Type="Italic">Thaumatomonas zhukovi</Emphasis> Mylnikov et Mylnikov (Thaumatomonadida (Shirkina) Karpov, 1990)

The ultrastructure of the amoeboid flagellate Thaumatomonas zhukovi sp. is presented. The cell is covered by cell body scales that formed on the surface of mitochondria. Capturing bacteria, the pseudopodia emerge from the ventral groove, which is supported by two longitudinal microtubular bands. The heterodynamic flagella emerge from the small flagellar pocket. Both flagella are covered by cone-shaped scales and thin twisted mastigonemes. The transitional zone of the flagella contains a thin-walled cylinder. The transversal plate of the flagella rises above the cell surface. The kinetosomes lie parallel to each other. The flagellar root system consists of three microtubular bands and a fibrillar rhizoplast. The vesicular nucleus and the Golgi apparatus have typical structures. The cytoplasm contains microbodies and food vacuoles. Mitochondria contain tubular cristae. Extrusive organelles (kinetocysts), which contain amorphous material and a capsule, were found in the cytoplasm. The capsule consists of a theca and a cylinder. The resemblance of Thaumatomonas zhukovi to other thaumatomonads is discussed.     

The flagellar apparatus of the golden algaSynura uvella: Four absolute orientations

Summary Flagellated vegetative cells of the colonial golden algaSynura uvella Ehr, were examined using serial sections. The two flagella are nearly parallel as they emerge from a flagellar pit near the apex of the cell. The photoreceptor is restricted to swellings on the flagella in the region where they pass through the apical pore in the scale case and the swellings are not associated with the cell membrane or an eyespot. A unique ring-like structure surrounds the axonemes of both flagella at a level just above the transitional helix. The basal bodies are interconnected by three striated, fibrous bands. Four short (<100 nm) microtubules lie between the basal bodies at their proximal ends. Two rhizoplasts extend down from the basal bodies and separate into numerous fine striated bands which lie over the nucleus. Three- and four-membered microtubular roots arise from the rhizoplasts and extend apically together. As the roots reach the cell anterior, the three-membered root bends and curves clockwise to form a large loop around the flagella; the four-membered root bends anticlockwise and terminates under the distal end of the three-membered root as it completes the loop. There are four absolute orientations, termed Types 1–4, in which the flagellar apparatus can occur. With each orientation type the positions of the Golgi body, nucleus, rhizoplasts, chloroplasts and microtubular roots change with respect to the flagella, basal bodies and photoreceptor. Two new basal bodies appear in pre-division cells, and three short microtubules appear in a dense substance adjacent to each new basal body. Based upon the positions of new pre-division basal bodies, a hypothesis is proposed to explain why there are four orientations and how they are maintained through successive cell divisions.     

Organization of the flagellar apparatus and associate cytoplasmic microtubules in the quadriflagellate alga Polytomella agilis

The organization of microtubular systems in the quadriflagellate unicell Polytomella agilis has been reconstructed by electron microscopy of serial sections, and the overall arrangement confirmed by immunofluorescent staining using antiserum directed against chick brain tubulin. The basal bodies of the four flagella are shown to be linked in two pairs of short fibers. Light microscopy of swimming cells indicates that the flagella beat in two synchronous pairs, with each pair exhibiting a breast-stroke-like motion. Two structurally distinct flagellar rootlets, one consisting of four microtubules in a 3 over 1 pattern and the other of a striated fiber over two microtubules, terminate between adjacent basal bodies. These rootlets diverge from the basal body region and extend toward the cell posterior, passing just beneath the plasma membrane. Near the anterior part of the cell, all eight rootlets serve as attachment sites for large numbers of cytoplasmic microtubules which occur in a single row around the circumference of the cell and closely parallel the cell shape. It is suggested that the flagellar rootless may function in controlling the patterning and the direction of cytoplasmic microtubule assembly. The occurrence of similar rootlet structures in other flagellates is briefly reviewed.     

THE EFFECTS OF COLCHICINE ON SPERMATOGENESIS IN NITELLA

Treatment of Nitella antheridia with colchicine results in various sperm abnormalities, depending upon duration of exposure and subsequent recovery. Early effects of treatment include disappearance of spindle fibers and a cessation of ordered cell wall formation in dividing cells. Sperm released from antheridia treated for 24 hr and allowed to recover for 4–5 days possess branched flagella. After a recovery period of 6–10 days the sperm appear normal; however, following longer recovery periods, the sperm exhibit variations in size and number of flagella. Branched flagella contain a variety of microtubule patterns ranging from branches containing a single microtubule to flagella with an excess of microtubules. Spermatids which differentiate in the presence of colchicine lack flagella and a microtubular sheath. Nuclear contents undergo condensation stages; however, the nucleus as a whole does not undergo the orderly elongation and coiling characteristic of untreated Nitella spermatids. Long-term colchicine treatment followed by a recovery period produces atypical microtubules and microtubular aggregations in the spermatid. The results indicate that colchicine affects not only polymerization of microtubule subunits but also factors responsible for their ordered spatial relationships in the cell. The presence of microtubules is a prerequisite for normal morphological changes during spermiogenesis.     

FINE STRUCTURE OF ZOOSPOROGENESIS, ZOOSPORE GERMINATION, AND EARLY GAMETOPHYTE DEVELOPMENT IN CLADOPHORA SURERA (CLADOPHORALES, CHLOROPHYTA)

The fine structure of zoosporogenesis, zoospore germination, and early gametophyte development in Cladophora surera Parodi et Cáceres were studied. Zoosporogenesis started with simultaneous meiosis in all nuclei of apical initial cells. The resulting haploid nuclei duplicated in turn by successive centric, closed mitoses. Then, each initial cell divided into two short zoosporangia. Numerous vacuoles appeared around each sporic nucleus. The delimitation of uninucleate zoosporocytes occurred by cytokinetic furrows produced by the coalescence of tiny, clear vesicles, without microtubules. Final shape of the zoospore resulted from gradual expulsion of vacuoles from the cell body. Mature biflagellate zoospores exhibited a conspicuous apical papilla containing fine granular globules, the basal apparatus, and a microtubular "umbrella" formed by numerous cortical microtubules that ran backward the length of the cell body. The chloroplast showed a conspicuous eyespot. The zoosporangial wall disorganized at the pore through which the zoospores were liberated. Zoospores settled on a substrate by their anterior papilla secreting an adhesive. Germination involved retraction of the apical papilla, loss of the "umbrella" microtubules and eyespot, and the lateral absorption of the entire flagellar apparatus, i.e. basal apparatus plus axoneme, into the cytoplasm. Early gametophyte development involved the synthesis of a thin, young cell wall, the development of outer peripheral vacuoles, the appearance of the marginal reticulate chloroplast, and the formation of the first central vacuoles derived from abundant endoplasmic reticulum. Close to the plasmalemma ran longitudinally oriented cortical microtubules. Eventually, the germling developed an achlorophylic, elongated rhizoidal portion.     

The Fine Structure of the Colonial Kinetoplastid Flagellate Cephalothamnium cyclopum Stein

Cell structure, cell adhesion, and stalk formation have been examined by electron microscopy in the colonial flagellate, Cephalothamnium cyclopum. Each cell is obconical or spindle-shaped, pointed posteriorly and truncated anteriorly. The cell membrane is underlain by epiplasm 0.1 μm thick in the posterior region, but bands of microtubules support the anterior region which is differentiated into a flagellar pocket, oral apparatus and contractile vacuole. Each of 2 flagella, joined a short way above their bases by an interflagellar connective, has a paraxial rod and mastigonemes. One flagellum is free and is important in food gathering while the other is recurrent and lies in a shallow groove on the ventral cell surface but projects posteriorly into the stalk. The basal bodies of these flagella are bipartite structures connected by a pair of striated rootlets with accessory microtubular fibers. The oral apparatus consists of a funnel-shaped buccal cavity and cytostome. It is supported by helical and longitudinal microtubules and also has nearby striated and microtubular fibers. Possible roles of associated oral vesicles in relation to ingestion are discussed. A reticulate mitochondrion houses a massive kinetoplast which has a fibrillar substructure resembling that of dinoflagellate chromosomes. Adjacent flagellates adhere by laminate extensions of their posterior regions and attach by their recurrent flagella to a communally secreted stalk composed of finely fibrillar material. This study indicates that Cephalothamnium belongs in the order Kinetoplastida, and has many features in common with members of the family Bodonidae.     

Microtubule configurations and post-translational alpha-tubulin modifications during mammalian spermatogenesis

The mechanisms underlying cell cycle progression and differentiation are tightly entwined with changes associated in the structure and composition of the cytoskeleton. Mammalian spermatogenesis is a highly intricate process that involves differentiation and polarization of the round spermatid. We found that pachytene spermatocytes and round spermatids have most of the microtubules randomly distributed in a cortical network without any apparent centrosome. The Golgi apparatus faces the acrosomal vesicle and some microtubules contact its surface. In round spermatids, at step 7, there is an increase in short microtubules around and over the nucleus. These microtubules are located between the rims of the acrosome and may be the very first sign in the formation of the manchette. This new microtubular configuration is correlated with the beginning of the migration of the Golgi apparatus from the acrosomal region towards the opposite pole of the cell. Next, the cortical microtubules form a bundle running around the nucleus perpendicular to the main axis of the cell. At later stages, the nuclear microtubules increase in size and a fully formed manchette appears at stage 9. On the other hand, acetylated tubulin is present in a few microtubules in pachytene spermatocytes and in the axial filament (precursor of the sperm tail) in round spermatids. Our results suggest that at step 7, the spermatid undergoes a major microtubular reordering that induces or allows organelle movement and prepares the cell for the formation of the manchette and further nuclear shaping. This new microtubular configuration is associated with an increase in short microtubules over the nucleus that may correspond to the initial step of the manchette formation. The new structure of the cytoskeleton may be associated with major migratory events occurring at this step of differentiation.