Model of the bacterial flagellar motor: response to varying viscous load.

A model of bacterial flagellar drive by cytomembrane streaming is described and applied to experiments of bacterial propulsion under varying viscous load. The theory predicts a linear dependence of the reciprocal propulsion velocity on the viscosity of the suspension medium, if the velocity of cytomembrane streaming far from the basal body of the flagella is assumed independent of the external viscosity. The theoretical predictions are in good agreement with experiments on free-swimming and on tethered bacteria. From comparison of the theory with experiments, the surface viscosity of the bacterial cytomembrane is evaluated for different bacterial species and turns out to be in the range observed experimentally on lipid monolayers.     

Independent analysis of the flagellum surface and matrix proteomes provides insight into flagellum signaling in mammalian-infectious Trypanosoma brucei

The flagellum of African trypanosomes is an essential and multifunctional organelle that functions in motility, cell morphogenesis, and host-parasite interaction. Previous studies of the trypanosome flagellum have been limited by the inability to purify flagella without first removing the flagellar membrane. This limitation is particularly relevant in the context of studying flagellum signaling, as signaling requires surface-exposed proteins in the flagellar membrane and soluble signaling proteins in the flagellar matrix. Here we employ a combination of genetic and mechanical approaches to purify intact flagella from the African trypanosome, Trypanosoma brucei, in its mammalian-infectious stage. We combined flagellum purification with affinity-purification of surface-exposed proteins to conduct independent proteomic analyses of the flagellum surface and matrix fractions. The proteins identified encompass a broad range of molecular functionalities, including many predicted to function in signaling. Immunofluorescence and RNA interference studies demonstrate flagellum localization and function for proteins identified and provide insight into mechanisms of flagellum attachment and motility. The flagellum surface proteome includes many T. brucei-specific proteins and is enriched for proteins up-regulated in the mammalian-infectious stage of the parasite life-cycle. The combined results indicate that the flagellum surface presents a diverse and dynamic host-parasite interface that is well-suited for host-parasite signaling.     

Food uptake and fine structure of Cryothecomonas longipes sp. nov., a marine nanoflagellate incertae sedis feeding phagotrophically on large diatoms

Cryothecomonas longipes Schnepf and Kühn sp. nov. is a colourless biflagellate organism, 9–14 μm long and 7–9 μm wide when not filled with food vacuoles. It was detected in the North Sea, feeding with pseudopodia on diatoms. It penetrates the host shell, while the main body of the flagellate remains outside the frustule. Cells are covered with a multilayered theca. The pseudopodium protrudes through a preformed slit in the theca. Each flagellum also emerges through a pit in which the theca forms a funnel of complex structure that girdles each flagellum. The anterior flagellum is 9–15 μm long and oriented forward; the ventral flagellum, posteriorly directed, is 20–24 μm long and bears fine hairs. The flagellar roots consist of microtubules that emerge at satellites around the basal bodies and run along the flagellar pits. In addition, the ventral flagellum is accompanied by a band of six microtubules. It is proximally attached to a small fibrillar band, which interconnects the basal bodies. Cryothecomonas longipes has two or three types of extrusomes which pierce the theca when discharged. Their mode of discharge is discussed. Microbody-like vesicles containing small tubules are closely associated with older digestion vacuoles. Cryothecomonas longipes is compared with other species of the genus and a diagnosis is given. Received: 4 March 1999 / Received in revised form: 28 July 1999 / Accepted: 30 August 1999     

Polar Flagellar Motility of the Vibrionaceae

Polar flagella of Vibrio species can rotate at speeds as high as 100,000 rpm and effectively propel the bacteria in liquid as fast as 60 μm/s. The sodium motive force powers rotation of the filament, which acts as a propeller. The filament is complex, composed of multiple subunits, and sheathed by an extension of the cell outer membrane. The regulatory circuitry controlling expression of the polar flagellar genes of members of the Vibrionaceae is different from the peritrichous system of enteric bacteria or the polar system of Caulobacter crescentus. The scheme of gene control is also pertinent to other members of the gamma purple bacteria, in particular to Pseudomonas species. This review uses the framework of the polar flagellar system of Vibrio parahaemolyticus to provide a synthesis of what is known about polar motility systems of the Vibrionaceae. In addition to its propulsive role, the single polar flagellum of V. parahaemolyticus is believed to act as a tactile sensor controlling surface-induced gene expression. Under conditions that impede rotation of the polar flagellum, an alternate, lateral flagellar motility system is induced that enables movement through viscous environments and over surfaces. Although the dual flagellar systems possess no shared structural components and although distinct type III secretion systems direct the simultaneous placement and assembly of polar and lateral organelles, movement is coordinated by shared chemotaxis machinery.     

Gliding movement in Peranema trichophorum is powered by flagellar surface motility

A colorless euglenoid flagellate Peranema trichophorum shows unique unidirectional gliding cell locomotion on the substratum at velocities up to 30 micro m/s by an as yet unexplained mechanism. In this study, we found that (1) treatment with NiCl(2) inhibited flagellar beating without any effect on gliding movement; (2) water currents applied to a gliding cell from opposite sides caused detachment of the cell body from the substratum. With only the anterior flagellum adhering to the substratum, gliding movement continued along the direction of the anterior flagellum; (3) gentle pipetting induced flagellar severance into various lengths. In these cells, gliding velocity was proportional to the flagellar length; and (4) Polystyrene beads were translocated along the surface of the anterior flagellum. All of these results indicate that a cell surface motility system is present on the anterior flagellum, which is responsible for cell gliding in P. trichophorum.     

Chondrocyte calcium signaling in response to fluid flow is regulated by matrix adhesion in 3-D alginate scaffolds

The interaction between chondrocytes and their surrounding extracellular matrix plays an important role in regulating cartilage metabolism in response to environmental cues. This study characterized the role of cell adhesion on the calcium signaling response of chondrocytes to fluid flow. Bovine chondrocytes were suspended in alginate hydrogels functionalized with RGD at concentrations of 0–400 μM. The hydrogels were perfused and the calcium signaling response of the cells was measured over a range of fluid velocities from 0 to 68 μm/s. Attachment to RGD-alginate doubled the sensitivity of chondrocytes to flows in the range of 8–13 μm/s, but at higher fluid velocities, the contribution of cell adhesion to the observed calcium signaling response was no longer apparent. The enhanced sensitivity to flow was dependent on the density of RGD-ligand present in the scaffolds. The RGD-enhanced sensitivity to flow was completely inhibited by the addition of soluble RGD which acted as a competitive inhibitor. The results of this study indicate a role for matrix adhesion in regulating chondrocyte response to fluid flow through a calcium dependent mechanism.     

STUDIES OF SPERMATOGENESIS IN THE HEPATICAE V. BLEPHAROPLAST DEVELOPMENT IN MARCHANTIA POLYMORPHA

Coaxial centrioles and a microtubule organizing center (MTOC) constitute each centrosome in spermatid mother cells of Marchantia polymorpha. During cell division the centrosome separates at its midregion and the two centrioles undergo a planar rotation that brings them to lie somewhat staggered and nearly parallel with their proximal ends embedded in osmiophilic granular material similar in appearance to that of the MTOC. Microtubules of the multilayered structure (MLS) arise in this material below the posterior centriole and parallel to its long axis. The rotation of centrioles and the initiation of S₁ tubules below the posterior centriole determine polarity of the incipient blepharoplast. Lower MLS strata are formed under the anterior centriole by the compaction of granular, osmiophilic matrix. Formation and growth of S₂ vertical lamellae occur at the left front edge of the MLS in association with MTOC-like matrix localized near the cell membrane. The MLS enlarges to about 0.4 μm wide by 0.6 μm long and is ovoid in outline except for a short distal projection underlying the posterior centriole. Subsequently the lamellae are transformed into homogenous, osmiophilic matrix that contributes directly to the expansion of all MLS strata including microtubules. The stratum of lamellae is interpreted as a planar MTOC subject to morphogenetic control. Each of the four strata grows proximally while the tapering distal projection lengthens beneath the posterior basal body. Dense matrix above the MLS, apparently elaborated by the S₂ layer, is organized into cartwheel and triplet components of the basal bodies’ proximal extensions. Organization of triplet tubules proceeds from proximal to distal toward preexisting triplets. Osmiophilic matrix contributes to the formation of microtubule keels and osmiophilic crests and may serve as a cementing material that stabilizes the spatial relationships of blepharoplast components. After full expansion of the MLS’ lower strata, the S₂ layer is reorganized into lamellae. Flagellar growth in Marchantia is postulated to involve a process whereby subunits or their precursors are elaborated by the MLS, translocated to the distal end of the flagellum and incorporated into the axonemal tubules. When MLS microtubules elongate to form a long, narrow band, the distal half of the S₂ layer is again in the osmiophilic matrix state.     

THE FINE STRUCTURE AND MODE OF ATTACHMENT OF THE SHEATHED FLAGELLUM OF VIBRIO METCHNIKOVII

The sheathed flagellum of Vibrio metchnikovii was chosen for a study of the attachment of the flagellum to the bacterial cell. Normal and autolysed organisms and isolated flagella were studied by electron microscopy using the techniques of thin sectioning and negative staining. The sheath of the flagellum has the same layered structure as the cell wall of the bacterium, and in favourable thin sections it appears that the sheath is a continuation of the cell wall. After autolysis the sheath is usually absent and the core of the flagellum has a diameter of 120 A. Electron micrographs of autolysed bacteria negatively stained with potassium phosphotungstate show that the core ends in a basal disc just inside the plasma membrane. The basal disc is about 350 A in diameter and is thus considerably smaller than the "basal granules" described previously by other workers.     

The Structure and Origin of Mastigonemes in Ochromonas minute and Monas sp.*

Structure, function, and development of mastigonemes (flagellar hairs) of 2 chrysophycean flagellates were examined with light and electron microscopy in whole mount and sectioned preparations. Mastigonemes of both organisms are identical, consisting of a tapered base 0.25–0.3 μm long, maximum width of 0.03 μm; a hollow shaft 0.85 μm × 23 nm; and 2 types of laterally projecting filaments. Two rows of mastigonemes are attached to the long flagellum, one on each side in the same plane as the central pair of microtubules. One row is composed of single mastigonemes while the other bears them in “tufts.” The primary mastigonemal attachment is on the flagellar membrane. Developmental sequences as supported by electron micrographs and kinetic studies demonstrate the intracellular location of promastigonemes during reflagellation, colchicine-inhibited reflagellation, and release from inhibition. The promastigonemes first appear in the peri-nuclear space in association with the outer nuclear membrane and several dozen may accumulate there. These may pinch off as bundles and move into the cytoplasm, or if mastigonemes are being utilized rapidly by the cell, the promastigonemes are channeled a few at a time from the perinuclear space into the Golgi apparatus where some structural modifications are made. The mastigonemes are then transported in Golgi-derived secretory-type vesicles to the cell surface near the base of the growing flagellum where the vesicle membrane fuses with the plasma membrane and the mastigonemes become extracellular, although the membrane association is retained. The origin of the asymmetric arrangement of mastigonemes on the flagellum is discussed.     

A post‐translational,c‐di‐GMP‐dependent mechanism regulating flagellar motility

Elevated levels of the second messenger cyclic dimeric GMP, c‐di‐GMP, promote transition of bacteria from single motile cells to surface‐attached multicellular communities. Here we describe a post‐translational mechanism by which c‐di‐GMP initiates this transition in enteric bacteria. High levels of c‐di‐GMP induce the counterclockwise bias in Escherichia coli flagellar rotation, which results in smooth swimming. Based on co‐immunoprecipitation, two‐hybrid and mutational analyses, the E. coli c‐di‐GMP receptor YcgR binds to the FliG subunit of the flagellum switch complex, and the YcgR–FliG interaction is strengthened by c‐di‐GMP. The central fragment of FliG binds to YcgR as well as to FliM, suggesting that YcgR–c‐di‐GMP biases flagellum rotation by altering FliG‐FliM interactions. The c‐di‐GMP‐induced smooth swimming promotes trapping of motile bacteria in semi‐solid media and attachment of liquid‐grown bacteria to solid surfaces, whereas c‐di‐GMP‐dependent mechanisms not involving YcgR further facilitate surface attachment. The YcgR–FliG interaction is conserved in the enteric bacteria, and the N‐terminal YcgR/PilZN domain of YcgR is required for this interaction. YcgR joins a growing list of proteins that regulate motility via the FliG subunit of the flagellum switch complex, which suggests that FliG is a common regulatory entryway that operates in parallel with the chemotaxis that utilizes the FliM‐entryway.     

Composition of the antennal nerve of Homarus americanus at different points along the flagellum

The antennal flagellum nerve of Homarus americanus was investigated as to structure, number and size of axons, and propagation velocity. Frequency distributions of axon diameters, evaluated at four equidistant levels on the flagellum, ranged with continuity between 0.25 and 14.7 µm, with maximum at 0.5-1.5 µm. Axons 0.5-1.5 µm in diameter were more abundant at the distal level, indicating sensory specialization near the tip. The total axon numbers increased from about 9000 at the distal level to about 45 000 at the base. Axons of different size followed different patterns of increase in number from tip to base; these patterns were examined in relation to structural features of the flagellum, and to hypotheses of association with known or unidentified receptors. Propagation velocities were distributed with continuity, in the range between 3.39 and 0.24 m/s; velocity-diameter correspondences were outlined.     

Polar flagellar motility of the Vibrionaceae.

Polar flagella of Vibrio species can rotate at speeds as high as 100,000 rpm and effectively propel the bacteria in liquid as fast as 60 microm/s. The sodium motive force powers rotation of the filament, which acts as a propeller. The filament is complex, composed of multiple subunits, and sheathed by an extension of the cell outer membrane. The regulatory circuitry controlling expression of the polar flagellar genes of members of the Vibrionaceae is different from the peritrichous system of enteric bacteria or the polar system of Caulobacter crescentus. The scheme of gene control is also pertinent to other members of the gamma purple bacteria, in particular to Pseudomonas species. This review uses the framework of the polar flagellar system of Vibrio parahaemolyticus to provide a synthesis of what is known about polar motility systems of the Vibrionaceae. In addition to its propulsive role, the single polar flagellum of V. parahaemolyticus is believed to act as a tactile sensor controlling surface-induced gene expression. Under conditions that impede rotation of the polar flagellum, an alternate, lateral flagellar motility system is induced that enables movement through viscous environments and over surfaces. Although the dual flagellar systems possess no shared structural components and although distinct type III secretion systems direct the simultaneous placement and assembly of polar and lateral organelles, movement is coordinated by shared chemotaxis machinery.     

Composition of the antennal nerve of Homarus americanus at different points along the flagellum

The antennal flagellum nerve of Homarus americanus was investigated as to structure, number and size of axons, and propagation velocity. Frequency distributions of axon diameters, evaluated at four equidistant levels on the flagellum, ranged with continuity between 0.25 and 14.7 µm, with maximum at 0.5-1.5 µm. Axons 0.5-1.5 µm in diameter were more abundant at the distal level, indicating sensory specialization near the tip. The total axon numbers increased from about 9000 at the distal level to about 45 000 at the base. Axons of different size followed different patterns of increase in number from tip to base; these patterns were examined in relation to structural features of the flagellum, and to hypotheses of association with known or unidentified receptors. Propagation velocities were distributed with continuity, in the range between 3.39 and 0.24 m/s; velocity-diameter correspondences were outlined.     

Flagellar gyration and midpiece rotation during extension of the acrosomal process of Thyone sperm: how and why this occurs

The midpiece of Thyone sperm contains a large mitochondrion and a centriolar pair. Associated with one of the pair, i.e., the basal body of the flagellum, are satellite structures which apparently anchor the flagellar axoneme to the mitochondrion and to the plasma membrane covering the midpiece. Immediately before and as the acrosomal process elongates, the flagellum and the midpiece begin to rotate at 1-2 rotations per second even though the head of the sperm, by being firmly attached on its lateral surfaces to the coverslip, does not rotate at all. This rotation is not observed in the absence of flagellar beating whose frequency is much greater than that of its gyration. To understand how the midpiece rotates relative to the sperm head, it is first necessary to realize that in Thyone the flagellar axoneme projects at an acute angle to the principal axis of the sperm and is bent towards one side of this axis. Thus movement of the flagellum induces the sperm to tumble or yaw in solution. If the head is stuck, the midpiece will rotate because all that connects the sperm head to the midpiece is the plasma membrane, a liquid-like layer. A finger-like projection extends from the proximal centriole into an indentation in the basal end of the nucleus. In contrast to the asymmetry of the flagellum, this indentation is situated exactly on the principal axis of the sperm and, along with the finger-like projection, acts as a biological bearing to maintain the orderly rotation of the midpiece. The biological purpose of flagellar gyration during fertilization is discussed.     

SPERMIOGENESIS IN THE PULMONATE SNAIL,EUHADRA HICKONIS III. FLAGELLUM FORMATION*

The formation of the flagellum in the spermatid of the Japanese land snail, Euhadra hickonis, is introduced by the appearance of a central indentation in the differentiated posterior side of the spherical nucleus early in spermiogenesis. One centriole moves to this part of the cell, changes in several structural respects and acquires a short-lived “centriole adjunct”. At first it lies tangential to the nuclear surface as it begins to induce formation of the flagellar axoneme; then it turns so that its proximal end fits into the deepening nuclear indentation (“implantation fossa”). Cytoplasmic tubules appear to mediate this shift in direction. Internal changes in the centriolar components begin as it initiates formation of the axoneme, and continue throughout spermiogenesis. First, a dense “cap” forms at its proximal end, the microtubular triplets become doublets and a pair of singlets occupies the center of the complex. All these microtubules extend from the dense cap and are continuous with those of the axoneme. As the basal body (modified centriole) becomes set in the implantation fossa, the material of the centriole adjunct forms 9 strands, which are continuous with the peripheral coarse fibers when these develop. The microtubular doublets of the basal body are visible for a short time between the fiber strands; in the mature spermatozoon they are found embedded in the basal body portions of the coarse fibers in a degenerated form. Posterior to the basal body, however, they separate from the inner sides of the striated coarse fibers and become the doublets of the axoneme. The proximal part of the elongating axoneme lies in a posterior extension of the cell, in which glycogen particles and mitochondria are conspicuous. As the mitochondria unite into a sheath tightly surrounding the axoneme, the structure of their cristae changes to form a paracrystal-line “mitochondria derivative”, which consists of many layers close to the nucleus and progressively fewer posteriorly. Outside of this “primary sheath”, more modified mitochondria unite to form a “secondary sheath” of paracrystalline lamellae which encloses a compartment, filled with glycogen particles, that extends in a low-pitched helix nearly to the end of the flagellum. In the late spermatid, microtubules become arranged at regular intervals around the nucleus and secondary sheath of the flagellum for a short period while the remaining cytoplasm and spermatid organelles such as the Golgi complex are being discarded. The flagellum of the mature spermatozoon is 250–300 μm in length, tapering gradually from a diameter of ca 1 μm just behind the nucleus to less than 0.3 μm at its tip, as the result of reduction in the amount of stored glycogen, the number of paracrystalline lamellae and the diameter of the peripheral fibers.     

DEVELOPMENT OF THE FLAGELLAR APPARATUS OF NAEGLERIA

Flagellates of Naegleria gruberi have an interconnected flagellar apparatus consisting of nucleus, rhizoplast and accessory filaments, basal bodies, and flagella. The structures of these components have been found to be similar to those in other flagellates. The development of methods for obtaining the relatively synchronous transformation of populations of Naegleria amebae into flagellates has permitted a study of the development of the flagellar apparatus. No indications of rhizoplast, basal body, or flagellum structures could be detected in amebae. A basal body appears and assumes a position at the cell surface with its filaments perpendicular to the cell membrane. Axoneme filaments extend from the basal body filaments into a progressive evagination of the cell membrane which becomes the flagellum sheath. Continued elongation of the axoneme filaments leads to differentiation of a fully formed flagellum with a typical "9 + 2" organization, within 10 min after the appearance of basal bodies.     

The pattern of acropetal and basipetal cytoplasmic streaming velocities in Chara rhizoids and protonemata, and gravity effect on the pattern as measured by laser-Doppler-velocimetry

 The spatial pattern of acropetal and basipetal cytoplasmic streaming velocities has been studied by laser-Doppler-velocimetry (LDV) in the positively gravitropic (downward growing) rhizoids of Chara globularis Thuill. and for the first time in the negatively gravitropic (upward growing) protonemata. The LDV method proved to be precise and yielded reproducible results even when tiny differences in velocities were measured. In the apical parts of the streaming regions of both cell types, acropetal streaming was faster than basipetal streaming. Starting at the apical reversal point of streaming, the velocity increased basipetally with the distance from that point and became fairly constant close to the basal reversal point; subsequently, the velocity decreased slightly acropetally as the apical reversal point was again approached. There was no change in velocity at the basal reversal point. However, at the apical reversal point there was an abrupt decrease in velocity. The pattern of the ratio of acropetal to basipetal streaming velocity (VR) was a function of the relative distance of the site of measurement from the apical reversal point rather than a function of the absolute distance. Upon inversion of the rhizoids, the VR decreased on average by 3.8% (±0.4%), indicating that the effect of gravity on the streaming velocity was merely physical and without a physiological amplification. Rhizoids that had developed on the slowly rotating horizontal axis of a clinostat, and had never experienced a constant gravity vector, were similar to normally grown rhizoids with respect to VR pattern. In protonemata, the VR pattern was not significantly different from that in rhizoids although the direction of growth was inverse. In rhizoids, oryzalin caused the polar organization of the cell to disappear and nullified the differences in streaming velocities, and cytochalasin D decreased the velocity of basipetal streaming slightly more than that of acropetal streaming. Cyclopiazonic acid, known as an inhibitor of the Ca²⁺-ATPase of the endoplasmic reticulum, also reduced the streaming velocities in rhizoids, but had slightly more effect on the acropetal stream. It is possible that the endogenous difference in streaming velocities in both rhizoids and protonemata is caused by differences in the cytoskeletal organization of the opposing streams and/or loading of inhibitors (like Ca²⁺) from the apical/subapical zone into the basipetally streaming endoplasm. Received: 4 October 1999 / Accepted: 4 November 1999     

Role of sodium bioenergetics in Vibrio cholerae

The ability of the bacterium to use sodium in bioenergetic processes appears to play a key role in both the environmental and pathogenic phases of Vibrio cholerae. Aquatic environments, including fresh, brackish, and coastal waters, are an important factor in the transmission of cholera and an autochthonous source. The organism is considered to be halophilic and has a strict requirement for Na(+) for growth. Furthermore, expression of motility and virulence factors of V. cholerae is intimately linked to sodium bioenergetics and to each other. Several lines of evidence indicated that the activity of the flagellum of V. cholerae might have an impact on virulence gene regulation. As the V. cholerae flagellum is sodium-driven and the Na(+)-NQR enzyme is known to create a sodium motive force across the bacterial membrane, it was recently suggested that the increased toxT expression observed in a nqr-negative strain is mediated by affecting flagella activity. It was suggested that the V. cholerae flagellum might respond to changes in membrane potential and the resulting changes in flagellar rotation might serve as a signal for virulence gene expression. However, we recently demonstrated that although the flagellum of V. cholerae is not required for the effects of ionophores on virulence gene expression, changes in the sodium chemical potential are sensed and thus alternative mechanisms, perhaps involving the TcpP/H proteins, for the detection of these conditions must exist. Analyzing the underlying mechanisms by which bacteria respond to changes in the environment, such as their ability to monitor the level of membrane potential, will probably reveal complex interplays between basic physiological processes and virulence factor expression in a variety of pathogenic species.     

Reflective properties of different eyespot types in dinoflagellates

The reflective properties of different types of dinoflagellate eyespots were investigated using confocal laser scanning microscopy in the epireflection contrast mode. Although the eyespots studied differed with respect to localization (cytosol or plastid) and organization of the globule layer(s), all types effectively absorbed and reflected blue-green laser light (principal lines of 488/514 nm). The relative orientation of the eyespot surface towards the light source strongly influenced the reflective properties. Maximal reflection occurred when the eyespot surface was approximately perpendicular to the light source and rapidly decreased at increasing angles of light incidence. Horizontal and vertical optical sectioning of live and fixed cells resolved differences in the reflection patterns. Focusing of reflected light on the basal portion of the longitudinal flagellum was observed for the cytosolic eyespot of Glenodinium sp. and the triple membrane-bounded eyespot of Peridinium foliaceum, presumably a vestige of a host plastid. This flagellum is thought to be mainly involved in mediating orientational movement responses. In contrast, the reflection patterns obtained from the eyespot of Woloszynskia pascheri, which represents the third and most commonly observed dinoflagellate eyespot type within a plastid, point to only minor focusing. Reflection signals could be followed a considerable distance into the sulcus in all cases, indicating that in dinoflagellate eyespots, irrespective of the presumed receptor location (plasma membrane overlying the eyespot and/or the basal part of the longitudinal flagellum), back reflection of non-absorbed light can enhance the excitation probability of the photoreceptor(s). Such a combined reflection/absorption screen allows maximal contrast modulation and will, in conjunction with the specialized geometry of the dinoflagellate eyespots, increase the directionality of these eyespot aparatuses considerably.     

Tritrichomonas foetus: Fine Structure of Freeze-Fractured Membranes1

ABSTRACT. Freeze-fracture techniques reveal differences in fine structure between the anterior three flagella of Tritrichomonas foetus and its recurrent flagellum. The anterior flagella have rosettes of 9–12 intramembranous particles on both the P and E faces. The recurrent flagellum lacks rosettes but has ribbon-like arrays of particles along the length of the flagellum, which may be involved in the flagellum's attachment to the cell body. This flagellum is attached to the membrane of the cell body along a distinct groove that contains few discernible particles. Some large intramembranous particles are visible on the P face of the cell body membrane at the point where the flagellum emerges from the cell body. The randomly distributed particles on the P and E faces of the plasma membrane have a particle density of 919/μm² and 468/μm² respectively, and there are areas on both faces that are devoid of particles. Freeze-fracture techniques also reveal numerous fenestrations in the membrane of the Golgi complex and about 24 pores per μm² in the nuclear. membrane.