THE FLAGELLAR APPARATUS OF THE ZOOSPORE OF UROSPORA PENICILLIFORMIS(CHLOROPHYTA)1

The flagellar apparatus of Urospora penicilliformis (Roth) Aresch. is unique, or at least very unusual among green algae. The flagellar axonemes are rigid, and contain wing-like projections. There are no central microtubules in the most proximal part of the axoneme. The transition region contains a series of electron dense transverse lamellae rather than a single septum, and lacks a stellate pattern. There is no cartwheel pattern in the proximal part of the basal bodies. The latter are associated with four different types of fibrous elements: ascending striated fibers that attach to an electron dense plate in the papillar center, lateral striated fibers that parallel microtubular roots, fibrous elements that link adjacent basal bodies, and finally two massive striated fibers that descend into the cell, passing closely along the nucleus (system II fibers, or rhizoplasts). Each of the four microtubular flagellar roots is sandwiched between two system I striated structures. The roots are probably equal; they contain proximally four, and distally up to eight microtubules. Based on the zoospore flagellar apparatus, it is concluded that the multinucleate U. penicilliformis is related to the Ulvaphyceae. Finally, a possible explanation in functional terms is given for the peculiar external morphology and behavior of the zoospore.     

FINE STRUCTURE OF THE FLAGELLAR APPARATUS OF UROGLENA AMERICANA (CHRYSOPHYCEAE)1

The three-dimensional structure of the flagellar apparatus of Uroglena americana Calkins (Uroglenopsis americana [Calkins] Lemmerman) was determined using serial section reconstruction. The three microtubular rootlet systems (R₂, R₃, and R₄) follow the general pattern found in other chrysophytes. The R₂ rootlet originates between the basal bodies of the mastigoneme-bearing long flagellum (F₁) and the short smooth flagellum (F₂) and is attached to the former by a fibrous connection. The R₃ rootlet system originates as a trough-shaped band of six microtubules spanning the distance between the proximal ends of the F₁ and F₂ basal bodies. The six-membered rootlet splits into two parts (designated R₃ and R₃) which circle the depression from which the F₂ flagellum emerges in counter-clockwise direction. These two rootlets pass beneath the F₂ basal body and descend into the cell alongside the chloroplast. The R₄ rootlet originates in fibrous material which passes diagonally over the F₂ basal body, forms a clockwise loop about three-quarters of the way around the depression, and ends in the cytoplasm. In place of a typical chrysophyte R₁ rootlet, U. americana has a different array of microtubules attached to the F₁ basal body which we have designated the descending rootlet (DR). This rootlet is a hairpin-shaped structure lying just below the surface of the cell; its longitudinal axis is predominantly parallel to the longitudinal axis of the cell. The DR resembles the bypassing rootlet which occurs in phaeophyte zoospores. Other chrysophytes may possess rootlets which are similar to the DR found in Uroglena.     

FINE STRUCTURE OF THE SPERMATOZOID OF ZAMIA WITH SPECIAL REFERENCE TO THE FLAGELLAR APPARATUS

The locomotor apparatus of the spermatozoid of Zamia integrifolia consists of numerous flagella having the typical 9 + 2 substructure connected through basal bodies to a spiral band of complex structure. Basal bodies have a fine structure somewhat resembling that found in algae, mosses, and ferns, but they are much longer. They are composed of a circle of 9 double fibers just beneath the plasma membrane, changing to 9 doublets interconnected by fibrils in a star-pattern, giving over to a centriolar type of 9 triplet fibers embedded in an electron-dense layer of the spiral band, and ending in a “cartwheel” configuration. A system of microtubules arranged in a spiral, secondary to the flagellated spiral, is thought to underlie the plasma membrane in flagellated regions. It is suggested that this system accounts for “euglenoid” movements of the sperm. Other details of cellular fine structure are described.     

FINE STRUCTURE OF THE FLAGELLAR APPARATUS OF DINOBRYON CYLINDRICUM (CHRYSOPHYCEAE)1

The three-dimensional structure of the flagellar apparatus of Dinobryon cylindrioum Imhof. (UTEX no. LB 2266) was determined using serial section reconstruction. Four microtubular rootlet systems (R₁, R₂, R₃, and R₄)and a rhizoplast are present, following the general pattern found in other chrysophytes. The R₁ rootlet, containing seven microtubules, originates at the basal body of the long flagellum that bears mastigonemes (F₁). The R₁ rootlet forms an arc which curves in clockwise direction (when viewed from the anterior end of the cell) approximately halfway around the pit from which the short smooth flagellum (F₂) emerges. Numerous microtubules cascade from the exterior-facing side of this rootlet to the tail of the cell. The R₂ rootlet originates between the F₁ and F₂ basal bodies, is attached to the F₁ basal body by a fibrous connection, and forms a clockwise arc above the R₁ rootlet. This rootlet extends approximately one quarter of the way around the pit. The R₃ rootlet system originates as a trough-shaped band of six microtubules spanning the distance between the proximal ends of the F₁ and F₂ basal bodies. The six-membered rootlet splits into two parts, designated R₃ and R₃. Both parts circle the pit in counter-clockwise direction, pass beneath the F₂ basal body, and descend into the cell alongside the chloroplast. The R₄ rootlet originates in fibrous material, passes diagonally over the top of the F₂ basal body, forms a clockwise loop at least three quarters of the way around the pit to the interior of the R₃ and R₃ rootlets, and ends in the cytoplasm. Similarities of rootlet origins and other details of the flagellar apparatus of D. cylindricum with those of other heterokont organisms reinforce the idea that these organisms are phylogenetically related.     

FLAGELLAR APPARATUS,EYESPOT AND BEHAVIOR OF MICROTHAMNION KUETZINGIANUM (CHLOROPHYCEAE) ZOOSPORES1,2,3

The flagellar apparatus of Microthamnion kuet-zingianum Naegeli differs from, that of Chlamydomonas reinhardtii Dangeard in that the zoospores can autonomously orient their basal bodies for different types of swimming behavior, including forward, and backward progression with, stationary intervals. Reorientation of the basal regions of the flagella and of the basal bodies were documented by cinefilms and by stroboscopic and electron micrographs. Even when the flagella. were sheared off, the remaining stubs (containing the basal bodies) were capable of being reoriented, by the organism. Thus the mechanism of basal body reorientation cannot reside in the 9 + 2 flagellar shaft. Rather, the reorienting process involves a shortening or lengthening of the distal fiber and of the plasma membrane region overlying an anterior papilla. In their helical and spiral motions, the zoospores trace complicated, but surprisingly regular curves. Such motion might result from the inherent 3-dimensional structure and beat of the flagella. The eyespot has an invariable, highly asymmetric location within the cell in direct proximity with a specific microtubular band (MT_E), but nevertheless may occur in either the anterior or posterior region of the chloroplast. Further, multiple eyespots may occur along the same side of MT_E. This observation is consistent with the discovery (in Fucus sperm) that microtubules serve to align individual eyespot granules in eyespot-ontogeny. By this means the position of the eyespot within a cell could well be determined.     

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

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

FINE STRUCTURE OF THE MALE AND FEMALE GAMETES OF ATRACTOMORPHA PORCATA (SPHAEROPLEACFAE,CHLOROPHYTA) WITH EMPHASIS ON THE DEVELOPMENT AND ABSOLUTE CONFIGURATION OF THE FLAGELLAR APPARATUS1

Gametogenesis in Atractomorpha porcata Hoffman was initiated b the synchronous mitotic division of nuclei within a multinucleate gametangium. Uninucleate gametes were subsequently produced following two series of cytokinetic divisions. The first series involved the formation of phycoplast microtubules (phycoplastic cytokinesis), whereas the second series did not (nonphycoplastic cytokinesis). Centrioles were connected by a rudimentary striated distal fiber by the time they migrated to the planes of division preceding the first series of cytokinetic division. These first divisions produced binucleate gametocytes. A well-developed flagellar apparatus lay near the cell surface in close proximity to each nucleus of the gametocyte prior to the second series of cytokinetic divisions that produced the uninucleate gametes. As seen in apical view, the paired basal bodies were directly opposed, with no lateral displacement of their longitudinal axes. In lateral view, the paired basal bodies diverged from one another at an angle of 130–180° (female) or 170–180° (male) and were connected by an arched, distal striated fiber about 670–750 nm long and 600 nm at its widest part. Four electron-opaque, pyramid-shaped lateral bodies flanked the basal bodies in close contact with their undersurfaces. The flagellar roots demonstrated a cruciate arrangement, with s = 6–9 over 1 (female gametes) or 7–10 over 1 (male gametes) microtubules and d= 2 microtubules. In male gametes, one of the multistranded roots was located close to the eyespot, and a second system of cytoskeletal microtubules was detected internally. Based on gamete ultrastructure, Atractomorpha porcata appears to be the most undifferentiated member of the genus.     

FINE STRUCTURE OF BIFLAGELLATE ZOOSPORES OF ASTEROCOCCUS SUPERBUS (TETRASPORALES, CHLOROPHYCEAE), INCLUDING THE ABSOLUTE CONFIGURATION OF THE FLAGELLAR APPARATUS

The ultrastructure of zoospores of Asterococcus superbus (Cienk.) Scherffel was studied to provide ultrastructural data relevant to the systematic position of the genus. Our results demonstrated that the motile cells of A. superbus were similar to those of the tetrasporalean algae, such as Tetraspora sp. and Tetrasporidium javanicum Moebius . The flagellar apparatus of A. superbus had the same clock-wise orientation of basal bodies and the V-shaped alignment of basal bodies as Tetraspora cylindrica (Wahlb.) Ag. and T. lubrica (Roth) Ag., but differed by having rhizoplasts . The motile cells of A. superbus displayed chlamydomonadal ultrastructure, similar to Chlamydomonas reinhardtii Dangeard , including the absolute configuration of the flagellar apparatus. The pyrenoid matrix in A. superbus, however , showed a large lateral invagination occupied by chloroplast stroma, a characteristic that has never been observed in Chlorophyta.     

ELECTRON MICROSCOPE OBSERVATIONS ON THE FLAGELLAR APPARATUS OF BRYOPSIS MAXIMA (CHLOROPHYCEAE)1

The flagellar apparatus in male gametes of the siphonaceous green alga, Bryopsis maxima Okamura, was studied and compared with that of other green biflagellate cells. The proximal portions of two basal bodies are connected by a single striated proximal band, unique among the biflagellate reproductive cells of green algae studied. Anterior to the flagellar bases is a pair of distal bands different from the single structure in other biflagellate cells. These bands which arise from the distal portion of each basal body, extend upward in the papilla and curve down toward the lower edges of the basal bodies. They seem to have no direct association with each other. Two pairs of distinct flagellar roots, one consisting of 3–5 microtubules and the other of a partially striated fiber of undetermined numbers of microtubules, diverge from the basal body region and extend towards the cell posterior. Their component microtubules are disorganized into single or smaller groups midway over the cell length. The uniqueness of the flagellar apparatus is briefly discussed.     

COMPARATIVE ANALYSIS OF THE FINE STRUCTURE OF YOUNG AND ADULT INDIVIDUALS OF DUNALIELLA SALINA (POLYBLEPHARIDACEAE,CHLOROPHYCEAE) WITH EMPHASIS ON THE FLAGELLAR APPARATUS1

Individuals of Dunaliella salina (Dunal.) Teod. change their shape during ontogenesis. Here we describe the fine structure of this species with emphasis on distinctions between young and adult individuals. The cell coat is present at early stages of cell development and may be synthesized by vesicles of nuclear membrane-associated endoplasmic reticulum. Scanning electron microscopical observations show differences in the surface pattern of the cell coat in young and adult cells. The nucleus of young cells is more or less spherical, whereas that of adult cells is pyriform. The Golgi apparatus is positioned immediately under the basal bodies and consists of three dictyosomes in young cells and six to eight dictyosomes in adult cells. The flagellar apparatuses of young and adult cells have a 1/7 o'clock (i.e. clockwise) displacement of basal bodies and are grossly similar, but there are subtle differences between specific components. Two non-axonemic basal bodies (1′, 2′) appear in a plane perpendicular to that determined by the flagella-bearing basal bodies (1, 2). The cruciate microtubular rootlet system has a 4–2–4–2 alternation pattern. In adult cells, rhizoplasts emerge from each terminal body and run parallel to the four rootlets.     

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.     

THE FLAGELLAR APPARATUS OF HYMENOMONAS CORONATA (PRYMNESIOPHYTA)1

The ultrastructure of Hymenomonas coronata Mills was reinvestigated to determine the microarchitecture of the flagellar apparatus. Cell morphology and flagellar apparatus structure are very similar to those of Pleurochrysis. Some important variations occur. First, a crystalline root (= compound root) is absent on microtubular root 1. Second, a two-stranded microtubular root emanates at a right angle from microtubular root 2. Third, a fibrous root emanates from the dorsal region between the basal bodies and extends to the cell's right, paralleling microtubular root 3. These similarities and variations in flagellar apparatus characters are discussed in reference to known variations in the Prymnesiophyta.     

THE FLAGELLAR APPARATUS OF CHILOMONAS PARAMECIUM (CRYPTOPHYCEAE) AND ITS COMPARISON WITH CERTAIN ZOOFLAGELLATES1

The major components of the internal flagellar apparatus of Chilomonas paramecium Ehr. are two large microtubular roots and a striated root paralleled by three microtubules. The two microtubular roots overlap at the basal bodies. One microtubular root follows a curved path in the anterior of the cell, and the other extends straight to the posterior passing through a groove in the nucleus. The striated root extends laterally from the basal bodies. Except that it is smaller, the posteriorly directed root bears a strong resemblance to the axostyle of oxymonads. The overall arrangement and structure of the flagellar roots is similar to the pelta, axostyle and costa of trichomonads and the pelta and axostyle of oxymonads, groups of mitochondrion-less, largely parasitic or symbiotic protozoans. An affinity between cryptomonads and oxymonads or trichomonads would have many phylogenetic implications, some of which are discussed.     

THE FLAGELLAR APPARATUS OF OXYRRHIS MARINA (PYRROPHYTA)1

The three-dimensional structure of the flagellar apparatus in the dinoflagellate Oxyrrhis marina has been reinvestigated and found to consist of several previously unknown components and component combinations that appear strikingly similar to those of some gymnodinoid taxa. The flagellar apparatus of this dinoflagellate is asymmetric and extremely complex consisting of a longitudinal and a transverse basal body that gives rise to eight structurally different components. The only posteriorly directed component is the large microtubular root that consists of 45–50 microtubules at its origin and is attached proximally to a perpendicularly oriented striated fibrous component. Arising from each basal body, two striated fibrous roots with different periodicities extend to the cell's left. A single stranded microtubular root with associated electron dense material emanates from the transverse basal body and also extends to the cell's left. A striated fibrous connective arises from the longitudinal basal body and extends toward the cell's right ventral surface and terminates near the sub-thecal microtubular system. A compound root consisting of microtubules and electron dense material also originates from the longitudinal basal body and extends ventrally into the anterior region of the tentacle. Structural similarities between the parallel striated fibrous roots of Oxyrrhis and Polykrikos are discussed as are flagellar apparatus similarities among other gymnodinoid dinoflagellates. A diagrammatic reconstruction of the Oxyrrhis flagellar apparatus is also presented.     

THE FLAGELLAR APPARATUS OF GYMNODINIUM SP. (DINOPHYCEAE)1

The detailed structure of the flagellar apparatus has been determined in a small dinoflagellate of the genus Gymnodinium. Although diminutive, this dinoflagellate possesses a complex flagellar apparatus consisting of a posteriorly directed microtubular root, a transverse striated fibrous root, several striated fibrous connectives that attach the basal bodies to one another as well as to the different roots, and a conspicuous non-striated fibrous connective that directly links the posteriorly directded microtubular root with the extended lobe of the nucleus. This represents the second discovery of a nuclear connective linked to the flagellar apparatus in the Dinophyceae but is the first report to elucidate the spatial relationships of the connective with the flagellar apparatus and the cell. A detailed diagrammatic reconstruction is provided and the similarities between these flagellar apparatus features are compared with those known for other dinoflagellates. Additionally, the structure and displacement of the nuclear connective are compared with nuclear connectives described in other protists.     

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.     

ABSOLUTE CONFIGURATION OF THE FLAGELLAR APPARATUS OF BIFLAGELLATE ZOOSPORES OF ENTEROMORPHA FLEXUOSA SSP. PILIFERA (ULVOPHYCEAE,CHLOROPHYTA)1

The absolute configuration of the flagellar apparatus of biflagellate zoospores of Enteromorpha flexuosa (Wulfen ex Roth.) J. Agardh ssp. pilifera (Kütz.) Bliding was determined. Viewed from the anterior of the cell, the flagellar apparatus shows 180° rotational symmetry with a counter-clockwise absolute orientation of its components. In longitudinal sections, the posteriorly directed basal bodies form an angle of about 170°–180° to one another. A reduced striated distal fiber connects the two basal bodies. The cruciate microtubular rootlet system has a 4–2–4–2 alternation pattern. Striated microtubule-associated components (SMACs or system I-fibers) and rhizoplasts (or system II fibers) accompany the two-membered rootlets. Striated bands connect the proximal sheaths with the four-Membered rootlets. The bilobate terminal caps do not completely cover the proximal ends of the basal bodies. This is the first ultrastructural study of biflagellate zoospores in a member of the Ulvales.     

STRUCTURE AND SIGNIFICANCE OF THE CRYPTOMONAD FLAGELLAR APPARATUS. I. CRYPTOMONAS OVATA (CRYPTOPHYTA)1

The absolute configuration of the flagellar apparatus in Cryptomonas ovata has been elucidated and found to be similar to that reported for Chilomonas paramecium. Variations apparent in the flagellar apparatus of Cryptomonas ovata include the presence of striations in the mitochondrion associated lamella, a rhizostyle which does not bear wing-like extensions from the microtubules and does not lie close to the nucleus, and a striated fibrous anchoring structure associated with one basal body which has not hitherto been described. The flagellar apparatus also includes a four stranded microtubular root which traverses into the anterior dorsal lobe of the cell, a striated fibrous root which is associated with a five stranded microtubular root, and a two stranded Cr root. The homologous nature of these roots to those in the larger cryptomonads is discussed in relation to the apparent reduction in flagellar apparatus size and complexity among the smaller cryptomonads. A diagrammatic reconstruction of the flagellar apparatus of Cryptomonas ovata is also presented.     

THE FLAGELLAR APPARATUS OF CHRYSOCHROMULINA SP. (PRYMNESIOPHYCEAE)1

The flagellar apparatus of an undescribed species of Chrysochromulina Lackey that bears “eyelash” scales is reconstructed. The transitional region consists of two transitional plates each with an axosome, with no stellate pattern between them. Fine osmiophilic rings lie between the flagellar membrane and the outer doublets in the transitional region. The two jagella and the haptonema are inserted in a subapical depression that is lined ventrally by a spine-like projection formed by one of the parietal chloroplasts. The angles of insertion are similar to those of some other Chrysochromulina species in that both the haptonema and the right basal body lie at an extreme angle to the left basal body. The connectives of the apparatus consist of a striated distal band with a dorsal extension to the R1 and a ventral extension overlying the R2, a striated distal accessory band, an auxiliary connective from the right basal body to the adjacent ventral chloroplast, a well-developed intermediate band, two striated proximal bands, and a striated proximal accessory band. Of the microtubular roots in this Chrysochromulina species, three are associated with the left side of the cell (an R1 of 8+3; a small crystalline compound root, the R1C, associated with the R1; an R2 of three micro-tubules), and two are associated with the right basal body (an R3 of 2/2 microtubules with which the single-stranded R4 converges to form a 2/2+1 and then a 2/3 tiered arrangement). Comparisons are drawn with other species in the genus and related genera, particularly Prymne-sium.     

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.