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.     

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

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

DEVELOPMENT OF THE FLAGELLAR APPARATUS AND FLAGELLAR ORIENTATION IN THE COLONIAL GREEN ALGA GONIUM PECTORALE (VOLVOCALES)1

Vegetative cells of Gonium pectorale have a fine structure similar to that of Chlamydomonas. In addition, three zones comprise an extracellular matrix; a fibrillar sheath and tripartite boundary surround individual cells, and a fragile capsule zone surrounds the entire colony. Cytokinesis is accomplished by a phycoplast and cleavage furrow. The flagellar apparatus of the immature vegetative cell of this colonial alga is similar to that of Chlamydomonas, but the basal bodies are slightly separated at their proximal ends. The four microtubular rootlets alternate between two and four members. During development, the basal bodies become further separated and nearly parallel. The distal fiber is stretched, but it remains attached to both basal bodies. At maturity, the basal bodies of peripheral cells of the colony have rotated in opposite directions on their longitudinal axes resulting in a displacement of the distal fiber to one side, an asymmetrical orientation of the rootlets and loss of 180° rotational symmetry. Central cells remain similar to Chlamydomonas in that basal bodies do not rotate, rootlets are cruciate, the distal fiber remains medially inserted and 180° rotational symmetry is conserved. A “pin-wheel” configuration of flagellar pairs and the orientation of parallel rootlets toward the colony perimeter probably accounts for the rotation of the colonies during forward swimming. In addition, these ultrastructural features support the traditional placement of G. pectorale as an intermediate between the unicellular Chlamydomonas and the more complex colonial volvocalean genera.     

A TEST OF TWO POSSIBLE MECHANISMS FOR PHOTOTACTIC STEERING IN VOLVOX CARTERI (CHLOROPHYCEAE)

We tested two competing models that could explain how differential flagellar activity leads to phototactic turning in spheroids of Volvox carteri f. weismannia (Powers) Iyengar. In one model, turning results from the flagella of anterior cells in the lighted and shadowed hemispheres beating at different frequencies. In a competing model, turning results from a change in beat direction in these flagella. Both models successfully explain phototactic steering under constant illumination, but they make different predictions when colonies are exposed to abrupt changes in light intensity. If turning is due to control of flagellar beat frequency, both progression and rotation rates will change in the same direction and with similar magnitudes. If spheroid turning is due to a change in flagellar beat direction, a decreased rate of progression will accompany an increased rate of rotation and vice versa. We used video-microscopy to observe the behavior of positively phototactic V. carteri spheroids exposed to 10× step-up and step-down stimuli. After a step-up stimulus, spheroids slow their progression and rotation by equal amounts. No significant changes are reported in these parameters after the reciprocal step-down response. These observations are consistent with the variable flagellar frequency model and inconsistent with the variable flagellar direction model for phototactic turning. Switching the direction of light stimulus by 180° results in reorientation of positively phototactic spheroids. The kinetics of this reorientation did not precisely match the predictions of either model.     

The flagellar apparatus of the scaly green flagellatePyramimonas obovata: Absolute configuration

Summary The flagellar apparatus of the quadriflagellate scaly green algaPyramimonas obovata has been studied in detail and the absolute configuration of the flagellar apparatus has been determined. The flagellar root system is cruciate (4-2-4-2-system). 18 major basal body associated fibrous structures connect the four basal bodies with each other. Each basal body is linked to an adjacent basal body by a unique set of connecting fibres, i.e., the flagellar apparatus does not exhibit 180° rotational symmetry. The flagellar apparatus ofPyramimonas obovata is compared with that of quadriflagellate motile cells of theChlorophyceae sensu Stewart andMattox and the phylogenetic relationships are discussed.     

Ultrastructure of the flagellar apparatus inPyramimonas octopus (Prasinophyceae)

Summary While green algae usually lack one of the outer dynein arms in the axoneme, flagella of the octoflagellated prasinophytePyramimonas octopus possess dynein arms on all peripheral doublets. The outer dynein arm on doublet no. 1 is modified, and additional structures are associated with doublets no. 2 and 6. The flagellar scales are asymmetrically arranged. Thus the two rows of thick flagellar hairscales are displaced towards doublet no. 6,i.e., in the direction of the effective stroke of each flagellum. The underlayer of small scales includes two nearly opposite double rows scales, arranged in the longitudinal direction of the flagellum. The hairscales emerge from these rows. The double rows are separated on one side by 9, on the other by 11 rows of helically arranged scales. The central pair of microtubules twists, but the axoneme itself (represented by the 9 peripheral doublets), does not seem to rotate. The flagella are arranged in two groups, showing modified 180° rotational symmetry. The effective strokes of the two central flagella are exactly opposite, while the other flagella beat in six intermediate directions.     

Flagellar waveform and rotational orientation in a Chlamydomonas mutant lacking normal striated fibers

The Chlamydomonas mutant vfl-3 lacks normal striated fibers and microtubular rootlets. Although the flagella beat vigorously, the cells rarely display effective forward swimming. High speed cinephotomicrography reveals that flagellar waveform, frequency, and beat synchrony are similar to those of wild-type cells, indicating that neither striated fibers nor microtubular rootlets are required for initiation or synchronization of flagellar motion. However, in contrast to wild type, the effective strokes of the flagella of vfl-3 may occur in virtually any direction. Although the direction of beat varies between cells, it was not observed to vary for a given flagellum during periods of filming lasting up to several thousand beat cycles, indicating that the flagella are not free to rotate in the mature cell. Structural polarity markers in the proximal portion of each flagellum show that the flagella of the mutant have an altered rotational orientation consistent with their altered direction of beat. This implies that the variable direction of beat is not due to a defect in the intrinsic polarity of the axoneme, and that in wild-type cells the striated fibers and/or associated structures are important in establishing or maintaining the correct rotational orientation of the basal bodies to ensure that the inherent functional polarity of the flagellum results in effective cellular movement. As in wild type, the flagella of vfl-3 coordinately switch to a symmetrical, flagellar-type waveform during the shock response (induced by a sudden increase in illumination), indicating that the striated fibers are not directly involved in this process.     

Comparative studies on physiological responses to phosphorus in two phenotypes of bloom-forming <Emphasis Type="Italic">Microcystis</Emphasis>

Toxic Microcystis blooms frequently occur in eutrophic water bodies and exist in the form of colonial and unicellular cells. In order to understand the mechanism of Microcystis dominance in freshwater bodies, the physiological and biochemical responses of unicellular (4 strains) and colonial (4 strains) Microcystis strains to phosphorus (P) were comparatively studied. The two phenotype strains exhibit physiological differences mainly in terms of their response to low P concentrations. The growth of four unicellular and one small colonial Microcystis strain was significantly inhibited at a P concentration of 0.2 mg l⁻¹; however, that of the large colonial Microcystis strains was not inhibited. The results of phosphate uptake experiments conducted using P-starved cells indicated that the colonial strains had a higher affinity for low levels of P. The unicellular strains consumed more P than the colonial strains. Alkaline phosphatase activity in the unicellular strains was significantly induced by low P concentrations. Under P-limited conditions, the oxygen evolution rate, F _v/F _m, and ETR _max were lower in unicellular strains than in colonial strains. These findings may shed light on the mechanism by which colonial Microcystis strains have an advantage with regard to dominance and persistence in fluctuating P conditions. Handling editor: L. Naselli-Flores     

Lidocaine,a local anesthetic,reversibly inhibits cytoplasmic Streaming inVallisneria mesophyll cells

Summary Lidocaine, which like other local anesthetics is known to inhibit intracellular transport in animal cells, was tested for its effect on the rotational cytoplasmic streaming in the mesophyll cells of the aquatic plantVallisneria. The drug caused reversible inhibition of cytoplasmic streaming in a dose dependent manner within the 2–20 mM range; higher concentrations resulted in permanent cessation of all cytoplasmic motion. Upon recovery following replacement of the normal bathing medium, cytoplasmic rotation was always resumed in the direction of the original movement exhibited by a given cell. The lidocaine effect was virtually independent of the ionic composition of the incubation medium, but it was markedly affected by the external pH; acidic conditions (pH 6) largely prevented the inhibition of streaming, whereas an alkaline environment (pH 8) accelerated both the onset of the effect and the recovery upon removal of the anesthetic. On the basis of these results and findings in other systems, it is suggested that lidocaine acts through interference with mechanisms that regulate cytoplasmic streaming, rather than with the motile apparatus or the supply of metabolic energy.Abbreviation APW artificial pond water     

Basal bodies and associated structures are not required for normal flagellar motion or phototaxis in the green alga Chlorogonium elongatum

The interphase flagellar apparatus of the green alga Chlorogonium elongatum resembles that of Chlamydomonas reinhardtii in the possession of microtubular rootlets and striated fibers. However, Chlorogonium, unlike Chlamydomonas, retains functional flagella during cell division. In dividing cells, the basal bodies and associated structures are no longer present at the flagellar bases, but have apparently detached and migrated towards the cell equator before the first mitosis. The transition regions remain with the flagella, which are now attached to a large apical mitochondrion by cross-striated filamentous components. Both dividing and nondividing cells of Chlorogonium propagate asymmetrical ciliary-type waveforms during forward swimming and symmetrical flagellar-type waveforms during reverse swimming. High-speed cinephotomicrographic analysis indicates that waveforms, beat frequency, and flagellar coordination are similar in both cell types. This indicates that basal bodies, striated fibers, and microtubular rootlets are not required for the initiation of flagellar beat, coordination of the two flagella, or determination of flagellar waveform. Dividing cells display a strong net negative phototaxis comparable to that of nondividing cells; hence, none of these structures are required for the transmission or processing of the signals involved in phototaxis, or for the changes in flagellar beat that lead to phototactic turning. Therefore, all of the machinery directly involved in the control of flagellar motion is contained within the axoneme and/or transition region. The timing of formation and the positioning of the newly formed basal structures in each of the daughter cells suggests that they play a significant role in cellular morphogenesis.     

Multiple Origins of Colonial Green Flagellates from Unicells: Evidence from Molecular and Organismal Characters

Phylogenetic hypotheses generated from cladistic analysis of organismal and molecular data are shown to be generally congruent and/or complementary for comparisons of unicellular and colonial green algae in the Chlorophyceae. Cladistic analysis of organismal character data corroborates the alliance of colonial Stephanosphaera with unicellular Haematococcus (Haematococcaceae sensu Smith), inferred from previous studies of nuclear-encoded rRNA sequence data. The organismal data also support monophyly of the colonial Volvocaceae (sensu Smith). Alliances of other unicellular taxa, including those ascribed to the "Euchlamydomonas" Hauptgruppe (sensu Ettl), are not resolved by organismal characters principally because the structure of the data is skewed to shared ancestral characters (symplesiomorphies) and unique characters (autapomorphies) which define individual taxa only. Reanalysis of rRNA sequence data, with additional sequence data for critical taxa, does not support monophyly of the colonial Volvocaceae (sensu Smith). However, these data are weak in the support of the alternate hypothesis of nomnonophyly. In contrast, relationships among most unicellular flagellates are unambiguously resolved by the molecular data. Although the failure of the sequence data to resolve relationships among colonial flagellates appears to be due to a sampling of conservative sequences, an ancient, rapid radiation event or taxon sampling bias may also be contributing to the ambiguity problem. Results from analysis of a combined data set (organismal and molecular) are generally consistent with the inferences of the organismal character data regarding the colonial flagellates and are also consistent with the inferences of the sequence data regarding the unicellular taxa.     

The fine structure of motile cells in the generaUlvaria andMonostroma,with special reference to the taxonomic position ofMonostroma oxyspermum (Ulvophyceae,Chlorophyta)

The zoospores and isogametes ofUlvaria obscura var.blyttii, the isogametes ofMonostroma bullosum, and the anisogametes ofM. grevillei have a flagellar apparatus with counterclockwise absolute orientation and terminal caps, and therefore belong to theUlvophyceae. On the basis of the absence or presence of body scales and the morphologies of certain flagellar apparatus components,Ulvaria obscura var.blyttii is retained in theUlvales, whileM. bullosum, M. grevillei andM. oxyspermum are referred to theUlotrichales. Differences in scale morphology, certain flagellar apparatus components, and early thallus ontogeny support the transfer ofM. oxyspermum to the genusGayralia. Mating structures and their positional relationships within the cell are described from the gametes examined. A plasmalemma-associated plaque that may be a degenerate mating structure occurs in someG. oxysperma motile cells.     

The Vfl1 Protein in Chlamydomonas localizes in a rotationally asymmetric pattern at the distal ends of the basal bodies

In the unicellular alga Chlamydomonas, two anterior flagella are positioned with 180 degrees rotational symmetry, such that the flagella beat with the effective strokes in opposite directions (Hoops, H.J., and G.B. Witman. 1983. J. Cell Biol. 97:902-908). The vfl1 mutation results in variable numbers and positioning of flagella and basal bodies (Adams, G.M.W., R.L. Wright, and J.W. Jarvik. 1985. J. Cell Biol. 100:955-964). Using a tagged allele, we cloned the VFL1 gene that encodes a protein of 128 kD with five leucine-rich repeat sequences near the NH(2) terminus and a large alpha-helical-coiled coil domain at the COOH terminus. An epitope-tagged gene construct rescued the mutant phenotype and expressed a tagged protein (Vfl1p) that copurified with basal body flagellar apparatuses. Immunofluorescence experiments showed that Vfl1p localized with basal bodies and probasal bodies. Immunogold labeling localized Vfl1p inside the lumen of the basal body at the distal end. Distribution of gold particles was rotationally asymmetric, with most particles located near the doublet microtubules that face the opposite basal body. The mutant phenotype, together with the localization results, suggest that Vfl1p plays a role in establishing the correct rotational orientation of basal bodies. Vfl1p is the first reported molecular marker of the rotational asymmetry inherent to basal bodies.     

The flagellar apparatus structure inMicrospora (Chlorophyceae) confirms a close evolutionary relationship with unicellular green algae

The spatial configuration of the flagellar apparatus of the biflagellate zoospores of the green algal genusMicrospora is reconstructed by serial sectioning analysis using transmission electron microscopy. Along with the unequal length of the flagella, the most remarkable characteristics of the flagellar apparatus are: (1) the subapical emergence of the flagella (especially apparent with scanning electron microscopy); (2) the parallel orientation of the two basal bodies which are interconnected by a prominent one-piece distal connecting fiber; (3) the unique ultrastructure of the distal connecting fiber composed of a central tubular region which is bordered on both sides by a striated zone; (4) the different origin of the d-rootlets from their relative basal bodies; (5) the asymmetry of the papillar region which together with the subapical position of the basal bodies apparently cause the different paths of corresponding rootlets in the zoospore anterior; (6) the presence of single-membered d-rootlets and multi-membered s-rootlets resulting in a 7-1-7-1 cruciate microtubular root system which, through the different rootlet origin, does not exhibit a strict 180° rotational symmetry. It is speculated that the different basal body origin of the d-rootlets is correlated with the subapical implant of flagella. It is further hypothesized that in the course of evolution the ancestors ofMicrospora had a flagellar papilla that has migrated from a strictly apical position towards a subapical position. Simultaneously, ancestral shift of flagella along the apical cell body periphery has taken place as can be concluded from the presence of an upper flagellum overlying a lower flagellum in the flagellar apparatus ofMicrospora. The basic features of the flagellar apparatus of theMicrospora zoospore resemble those of the coccoid green algal generaDictyochloris andBracteacoccus and also those of the flagellate green algal genusHeterochlamydomonas. This strengthens the general supposition thatMicrospora is evolutionarily closely related to taxa which were formerly classified in the traditionalChlorococcales.     

FLAGELLAR APPARATUS STRUCTURE IS SIMILAR BUT NOT IDENTICAL IN VOLVULINA STEINII,EUDORINA ELEGANS,AND PLEODORINA ILLINOISENSIS (CHLOROPHYTA): IMPLICATIONS FOR THE “VOLVOCINE EVOLUTIONARY LINEAGE”1

The colonial and multicellular members of the Volvocales can be arranged in order of increasing size and complexity as the “volvocine series.” This series is often assumed to reflect an evolutionary progression. The flagellar apparatuses of previously examined algae are not consistent with a simple lineage. The flagellar apparatuses of Astrephomene gubernaculifera Pocock, Gonium pectorale Müller, Platydorina caudata Kofoid, Volvox rousseletii G. S. West, and V. carteri f. weismannia (Powers) Iyengar differ from one another, and there is no apparent progression inflagellar apparatus features from the simple to complex colonial forms. We examined the flagellar apparatuses of Volvulina steinii Playfair, Eudorina elegans Ehr., and Pleodorina illinoisensis Kofoid and found them to be similar to one another. The basal bodies are connected by a distal fiber that is offset to the anti side of the cell. Two microtubular rootlets originate on the inside of the basal bodies and extend toward the syn side. The other two rootlets are oriented perpendicular to the first two and are anti-parallel to each other. A coarsely striated component underlies the four-membered rootlets and extends to the basal bodies. A proximal fiber complex connects the two basal bodies. This complex consists of a branched striated component on the cis side of each basal body. One part extends toward the anti side of the cell, while the other extends into a fibrous component that runs between basal bodies. An additional structure extends in the anti direction from the trans side of each basal body. A fibrous component extends past one basal body in all four species. This component goes past the trans basal body in Volvulina steinii and the cis basal body in E. elegans and P. illinoisensis. The flagellar apparatuses of these organisms are similar to those of G. pectorale and Volvox carteri but different from the other colonial volvocalean algae examined. The algae examined in this study plus G. pectorale and V. carteri probably share a common evolutionary history that postdates the transition from the unicellular to colonial habit. Such a shared evolutionary history is a requirement of the volvocine hypothesis. However, we have not observed progressive changes in the flagellar apparatus correlated with increasing cell number, differentiation, and sexual specialization. Thus, it is possible, but not certain, that G. pectorale, Volvulina steinii, E. elegans, P. illinoisensis, and Volvox carteri may form part of a volvocine lineage.     

Analysis of motility parameters from paddlefish and shovelnose sturgeon spermatozoa

Ninety to 100% of paddlefish Polyodon spathula were motile just after transfer into distilled water, with a velocity of 175 μm s^-1, a flagellar beat frequency of 50 Hz and motility lasting 4–6 min. Similarly, 80–95% of shovelnose sturgeon Scaphirhynchus platorynchus spermatozoa were motile immediately when diluted in distilled water, with a velocity of 200 μm s^-1, a flagellar beat frequency of 48 Hz and a period of motility of 2–3 min. In both species, after sperm dilution in a swimming solution composed of 20 mM Tris–HCl (pH 8·2) and 20 mM NaCl, a majority of the samples showed 100% motility of spermatozoa with flagella beat frequency of 50 Hz within the 5 s following activation and a higher velocity than in distilled water. In such a swimming medium, the time of motility was prolonged up to 9 min for paddlefish and 5 min for sturgeon and a lower proportion of sperm cells had damage such as blebs of the flagellar membrane or curling of the flagellar tip, compared with those in distilled water. The shape of the flagellar waves changed during the motility phase, mostly through a restriction at the part of the flagellum most proximal to the head. A rotational movement of whole cells was observed for spermatozoa of both species. There were significant differences in velocity of spermatozoa between swimming media and distilled water and between paddlefish and shovelnose sturgeon.     

Reconstruction of the flagellar apparatus and microtubular cytoskeleton inPyramimonas gelidicola (Prasinophyceae,Chlorophyta)

Summary The absolute configuration of the flagellar apparatus inPyramimonas gelidicola McFadden et al. has been determined and shows identity withP. obovata, indicating that they are closely related. Comparison with the flagellar apparatus of quadriflagellate zoospores from the more advancedChlorophyceae suggest thatPyramimonas may be a primitive ancestral form. The microtubular cytoskeleton has been examined in detail and is shown to be unusual in that it does not attach to the flagellar apparatus. Cytoskeletal microtubules are nucleated individually, and this is interpreted as an adaptation to the methods of mitosis and scale deployment. In view of the primitive nature of these processes, it is proposed that this type of cytoskeletal organization may represent a less advanced condition than that of the flagellar root MTOCs (microtubule organizing centers) observed in theChlorophyceae.     

ULTRASTRUCTURE OF THE FLAGELLAR APPARATUS OF PYROBOTRYS (CHLOROPHYCEAE)1

The flagellar apparatus of Pyrobotrys has a number of features that are typical of the Chlorophyceae, but others that are unusual for this class. The two flagella are inserted at the apex, but they extend to the side of the cell toward the outside of the colony, here designated as the ventral side. Four basal bodies are present, two of which extend into flagella. Four microtubular rootlets alternate between the functional and accessory basal bodies. In each cell, the two ventral rootlets are nearly parallel, but the dorsal rootlets are more widely divergent. The rootlets alternate between two and four microtubules each. A striated distal fiber connects the two functional basal bodies in the plane of the flagella. Two additional, apparently nonstriated, fibers connect the basal bodies proximal to the distal fiber. Another striated fiber is associated with each four-membered rootlet near its insertion into the flagellar apparatus. A fine periodic component is associated with each two-membered rootlet. A rhizoplast-like structure extends into the cell from each of the functional basal bodies. The arrangement of these components does not reflect the 180° rotational symmetry that is usually present in the Chlorophyceae, but appears to be derived from a more symmetrical ancestor. It is suggested that the form of the flagellar apparatus is associated with the unusual colony structure of Pyrobotrys.     

Ultrastructure of the flagellar apparatus inPleurochrysis (classPrymnesiophyceae)

Summary The ultrastructure of the flagellar apparatus of aPleurochrysis, a coccolithophorid was studied in detail. Three major fibrous connecting bands and several accessory fibrous bands link the basal bodies, haptonema and microtubular flagellar roots. The asymmetrical flagellar root system is composed of three different microtubular roots (referred to here as roots 1,2, and 3) and a fibrous root. Root 1, associated with one of the basal bodies, is of the compound type, constructed of two sets of microtubules,viz. a broad sheet consisting of up to twenty closely aligned microtubules, and a secondary bundle made up of 100–200 microtubules which arises at right angles to the former. A thin electron-dense plate occurs on the surface of the microtubular sheet opposite the secondary bundle. The fibrous root arises from the same basal body and passes along the plasmalemma together with the microtubular sheet of root 1. Root 2 is also of the compound type and arises from one of the major connecting bands (called a distal band) as a four-stranded microtubular root and extends in the opposite direction to the haptonema. From this stranded root a secondary bundle of microtubules arises at approximately right angle. Root 3 is a more simple type, composed of at least six microtubules which are associated with the basal body. The flagellar transition region was found to be unusual for the classPrymnesiophyceae. The phylogenetic significance of the flagellar apparatus in thePrymnesiophyceae is discussed.