Structure and significance of cruciate flagellar root systems in green algae: Female gametes ofBryopsis lyngbyei (Bryopsidales)

The ultrastructure of the flagellar apparatus in the biflagellate female gametes of the green algaBryopsis lyngbyei has been studied in detail. In the flagellum and basal body, microtubule septations occur in some of the B-tubules. The transition region of the flagellum is extremely long (260–290 nm), exhibits a stellate pattern in cross section but lacks the transverse diaphragm. The two basal bodies form an angle of 180° and overlap at their proximal ends. They are connected by a compound non-striated capping plate. Terminal caps associated with the capping plate partially close the proximal end of each basal body. A cruciate flagellar root system with three different types of microtubular roots is present, i. e. the flagellar apparatus does not show 180° rotational symmetry. One root type contains 2 microtubules which are connected to an elaborate cylindrical structure, presumably a mating structure. The opposite root exhibits 3 microtubules over its entire length and is not associated with a cylindrical structure. In their proximal parts both roots are linked to an underlying crescent body. The other two microtubular roots are probably identical and consist of 4 (or 5) microtubules which show configurational changes. These two identical roots insert into the capping plate and link to the inner side (i. e. the side adjacent to the other basal body) of each basal body, whereas the other two roots attach to the outer sides of each basal body. System I striated fibres are probably associated with each of the four roots, while system II fibres have not been observed. The flagellar apparatus of female gametes ofB. lyngbyei shows many unique features but in some aspects resembles that of ulvalean algae. Functional and phylogenetic aspects of cruciate flagellar root systems in green algae are discussed.     

Transformation of the flagella and associated flagellar components during cell division in the coccolithophoridPleurochrysis carterae

Summary Immunofluorescence microscopy, conventional and high voltage transmission electron microscopy were used to describe changes in the flagellar apparatus during cell division in the motile, coccolithbearing cells ofPleurochrysis carterae (Braarud and Fagerlund) Christensen. New basal bodies appear alongside the parental basal bodies before mitosis and at prophase the large microtubular (crystalline) roots disassemble as their component microtubules migrate to the future spindle poles. By prometaphase the crystalline roots have disappeared; the flagellar axonemes shorten and the two pairs of basal bodies (each consisting of one parental and one daughter basal body) separate so that each pair is distal to a spindle pole. By late prometaphase the pairs of basal bodies bear diminutive flagellar roots for the future daughter cells. The long flagellum of each daughter cell is derived from the parental basal bodies; thus, the basal body that produces a short flagellum in the parent produces a long flagellum in the daughter cell. We conclude that each basal body in these cells is inherently identical but that a first generation basal body generates a short flagellum and in succeeding generations it produces a long flagellum. At metaphase a fibrous band connecting the basal bodies appears and the roots and basal bodies reorient to their interphase configuration. By telophase the crystalline roots have begun to reform and the rootlet microtubules have assumed their interphase appearance by early cytokinesis.Abbreviations CR1, CR2 crystalline roots 1 and 2 - CT cytoplasmic tongue microtubules - DIC differential interference contrast light microscopy - H haptonema - HVEM high voltage transmission electron microscopy - IMF immunofluorescence microscopy - L left flagellum/basal body - M metaphase plate - MT microtubule - N nucleus - R right flagellum/basal body - R1, R2, R3 roots 1, 2, and 3 - TEM transmission electron microscopy     

ENTOSIPHON SULCATUM (EUGLENOPHYCEAE): FLAGELLAR ROOTS OF THE BASAL BODY COMPLEX AND RESERVOIR REGION1,2

The flagellar root system of Entosiphon sulcatum (Dujardin) Stein (Euglenophyceae) is described and compared with kinetoplastid and other euglenoid systems. An asymmetric pattern of three microtubular roots, one between the two flagellar basal bodies and one on either side (here called the intermediate, dorsal, and ventral roots), is consistent within the euglenoid flagellates studied thus far. The dorsal root is associated with the basal body of the anterior flagellum (F₁) and lies on the left dorsal side of the basal body complex. Originating between the two flagellar basal bodies, and associated with the basal body of the trailing flagellum (F₂), the intermediate root is morphologically distinguished by fibrils interconnecting the individual microtubules to one another and to the overlying reservoir membrane. The intermediate root is often borne on a ridge projecting into the reservoir. The ventral root originates near the F₂ basal body and lies on the right ventral side of the cell. Fibrillar connections link the membrane of F₂ with the reservoir membrane at the reservoir-canal transition level. A large cross-banded fiber joins the two flagellar basal bodies, and a series of smaller striated fibers links the anterior accessory and flagellar basal bodies. Large nonstriated fibers extend from the basal body complex posteriorly into the cytoplasm.     

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.     

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.     

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.     

COMPARATIVE ANALYSES OF THE DINOFLAGELLATE FLAGELLAR APPARATUS. II. CERATIUM HIRUNDINELLA1

The three-dimensional structure of the flagellar apparatus in the gonyaulacoid dinoflagellate. Ceratium hirundinella var. furcoïdes (Schröder) Hub.-Pest. was determined using serial section electron microscopy. The flagellar apparatus is quite large and consists of several components. The two basal bodies nearly abut at their proximal ends and are separated by an angle of approximately 120° The broad longitudinal microtubular root extends from the cell's left edge of the longitudinal basal body and bends around the sulcal/cingular depression into the cell's left antapical horn. A transverse striated fibrous root is associated with the transverse basal body and a narrow electron dense extension is present along the anterior edge of the transverse basal body. This study revealed severa1 hitherto unreported fibrous components of the flagellar apparatus that link the various microtubular and fibrous components to themselves and to the two striated collars. A large striated fibrous connective links the two striated collars to one another. This fibrous connective is linked to another striated fibrous connective that originates from the longitudinal basal body and lies perpendicular to the longitudinal microtubular root. The readily identifiable and numerous components of the Ceratium flagellar apparatus are comparable to those of other dinoflagellates. The combined presence of well dpveloped striated collars, a striated collar connective, and a basal body angle of approximately 120° indicates that this flagellar apparatus is most like that described for Peridinioid dinoflagellates. Important similarities are also noticeable between this flagellar apparatus and that of Oxyrrhis marina.     

COMPARATIVE ANALYSES OF THE DINOFLAGELLATE FLAGELLAR APPARATUS. I. WOLOSZYNSKIA SP.1

The three-dimensional structure of the flagellar apparatus in Woloszynskia sp. was determined. This recently discovered dinoflagellate possesses two basal bodies that are offset from one another and lie at an angle of approximately 110°. The transverse basal body is associated with a striated fibrous root assemblage that consists of two differently staining fibrous portions with identical striation periodicity. Unlike the transverse striated fibrous roots reported in other dinoflagellates, this assemblage extends to the cell's right beyond the proximal end of the transverse basal body. The striated fibrous root complex is attached to the anterior end of the longitudinal microtubular root by a broad striated fibrous connective. The longitudinal basal body is also associated with the longitudinal microtubular root. The flagellar opening of each emerging axoneme is surrounded by a striated collar. The striated collars are linked to one another by a striated fibrous, striated collar connective. The variations and similarities of the flagellar apparatus and the ventral ridge/striated collar connective in Woloszynskia sp. are compared to similar components in other dinoflagellates.     

ENTOSIPHON SULCATUM (EUGLENOPHYCEAE): FLAGELLAR ROOTS OF THE BASAL BODY COMPLEX AND RESERVOIR REGION1,2

The flagellar root system of Entosiphon sulcatum (Dujardin) Stein (Euglenophyceae) is described and compared with kinetoplastid and other euglenoid systems. An asymmetric pattern of three microtubular roots, one between the two flagellar basal bodies and one on either side (here called the intermediate, dorsal, and ventral roots), is consistent within the euglenoid flagellates studied thus far. The dorsal root is associated with the basal body of the anterior flagellum (F₁) and lies on the left dorsal side of the basal body complex. Originating between the two flagellar basal bodies, and associated with the basal body of the trailing flagellum (F₂), the intermediate root is morphologically distinguished by fibrils interconnecting the individual microtubules to one another and to the over lying reservoir membrane. The intermediate root is often borne on a ridge projecting into the reservoir. The ventral root originates near the F₂ basal body and lies on the right ventral side of the cell. Fibrillar connections link the membrane of F₂ with the reservoir membrane at the reservoircanal transition level. A large cross-banded fiber joins the two flagellar basal bodies, and a series of smaller striated fibers links the anterior accessory and flagellar basal bodies. Large nonstriated fibers extend from the basal body complex posteriorly into the cytoplasm.     

Rearrangement of the flagellar apparatuses and eyespots of isogametes during the fertilization of the marine green alga,Monostroma nitidum (Ulvophyceae,Chlorophyta)

Behaviors of the flagellar apparatuses (flagella, basal bodies, microtubular roots, etc.), mating structures and eyespots of gametes during the fertilization of Monostroma nitidum were studied using field emission scanning electron microscopy and transmission electron microscopy. The biflagellate isogamete (mt⁺ and mt^?) mating structure has a position that is converse between mt⁺ and mt^? gametes relative to the flagellar beat plane and the eyespot. After the adhesion of mt⁺ and mt^? gametes, gamete fusion occurred between the two mating structures. The cell fusion plane expanded to the cell surface as circumscribed by 1s–2d roots in mt⁺ gamete and 1d–2s roots in the mt^? gamete. Two sets of flagellar apparatuses lay side by side in the planozygote and soon become mutually close. The no. 1 basal body of mt⁺ gamete and the no. 2 basal body of mt^? gamete rotated in a counterclockwise direction, as viewed from the cell anterior. Then, the no. 2 basal body of mt⁺ gamete and the no. 1 basal body of mt^? gamete slid into a face to face position. Finally, four flagella and basal bodies exhibited a cruciate arrangement. The basal bodies of the opposing pair (no. 1 and no. 2) were offset in a counterclockwise orientation by the basal body diameter. The 1s and 2d roots of the mt⁺ gamete lay nearly parallel to the 1d and 2s roots of the mt^? gamete, respectively, at the cell fusion plane. Because of the asymmetric localization of the mating structure, association, and subsequent rearrangement of basal bodies and microtubular roots, two eyespots lay on the same side of the planozygote. After the settlement of the planozygote, the flagellar apparatus started to disintegrate in the zygote cytoplasm.     

Basal apparatus behaviour during cellular division (sporulation) in the coccoid green algaChlorosarcina

Summary We studied the basal body cycle (including basal body segregation, duplication, migration, and reorientation) in dividing cells of the colonial coccoid green algaChlorosarcina stigmatica using serial thin sections. Although flagella are lacking, all cells examined possess a rudimentary flagellar apparatus composed of two basal bodies linked by a distal striated fibre, two probasal bodies, and four cruciately arranged microtubular roots (2-4-2-4 type). Basal body segregation occurs at preprophase, during which two half-basal apparatuses (each consisting of one basal body, one probasal body, and a left and a right root) migrate into opposite directions. The segregation axis is defined by the two left roots which remain closely associated during segregation and slide along each other. The segregation axis is parallel to the axis of chromosome separation, and perpendicular to the plane of subsequent cell division. Duplication of basal apparatus components does not occur until telophase when daughter basal apparatuses migrate towards the plane of division. At cytokinesis which is effected by the unilateral ingrowth of a septum, each daughter basal apparatus rotates 90° and becomes associated with the new septum.Abbreviations BA basal (body) apparatus - NBBC nucleus-basal body connector     

ULTRASTRUCTURE OF THE FLAGELLA OF THE COLORLESS PHAGOTROPH PERANEMA TRICHOPHORUM (EUGLENOPHYCEAE).II. FLAGELLAR ROOTS1

Peranema trichophorum (Ehrenberg) Stein, a colorless phagotrophic euglenoid flagellate, has a typically euglenoid microtubular root complement. Striated root components, relatively uncommon in euglenoids, are connected to the basal bodies and to a microtubular root. The flagellar system of Peranema consists of three unequal microtubular roots which extend anteriorly beneath the reservoir membrane, and narrow-band striated roots (periodicity = 29–33 nm) which connect one of the four basal bodies to the movable rodorgan of the feeding apparatus. An inter basal body striated fiber forms a three-way connection between one particular microtubular root, a flagellar basal body, and the striated roots. A striated fibril (periodicity = 18–25 nm), which may be an extension of the striated root system, extends beneath the reservoir membrane. Associated with the striated fibril and the striated roots are cisternae of smooth endoplasmic reticulum.     

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.     

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.     

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.     

Flagellar apparatus ultrastructure inMesostigma viride (Prasinophyceae)

The ultrastructure of the flagellar apparatus ofMesostigma viride Lauterborn (Prasinophyceae) has been studied in detail with particular reference to absolute configurations, numbering of basal bodies, basal body triplets and flagellar roots. The two basal bodies are interconnected by three connecting fibers (one distal fiber = synistosome, and two proximal fibers). The flagellar apparatus shows 180° rotational symmetry; four microtubular flagellar roots and two system II fibers are present. The microtubular roots represent a 4-6-4-6-system. The left roots (1s, 2s) consist of 4 microtubules, each with the usual 3 over 1 root tubule pattern. Each right root (1d, 2d) is proximally associated with a small, but typical multi-layered structure (MLS). The latter displays several layers corresponding to the S₁ (the spline microtubules: 5–7), and presumably the S₂—S₄ (the lamellate layers) of the MLS of theCharophyceae. At its proximal origin (near the basal bodies) each right root originates with only two microtubules, the other spline microtubules being added more distally. The structural and positional information obtained in this study strongly suggest that one of the right roots (1d) ofMesostigma is homologous to the MLS-root of theCharophyceae and sperm cells of archegoniate land plants. Thus the typical cruciate flagellar root system of the green algae and the unilateral flagellar root system of theCharophyceae and archegoniates share a common ancestry. Some functional and phylogenetic aspects of MLS-roots are discussed.Dedicated to Prof. DrLothar Geitler on the occasion of his 90th birthday.     

Flagellar root maps allow speculative comparisons of root patterns and of their ontogeny

A method of mapping the patterns of origin of flagellar roots around basal bodies in two-dimensional diagrams is suggested, making allowance for the varied orientations of members of a pair or quartet of basal bodies in a cell. The method is used to compare flagellar root patterns in a wide range of protistan groups, and appears to demonstrate similarities in many areas. Comparison of such patterns in three published examples shows that during the ontogeny of a basal body it may display first one root pattern and then another, so that the root array of a given basal body is not fixed but changes with the position and role of that basal body in the cell.     

Vischeria stellata (Eustigmatophyceae): ultrastructure of the zoospores,with special reference to the flagellar apparatus

Summary Ultrastructure of the zoospores ofVischeria stellata (R. Chodat ex Poulton) Pascher is investigated, with particular reference to the system of flagellar roots. Microtubular roots and a rhizoplast are present and a model showing their distribution is proposed. Four microtubular roots attach to the basal bodies in a system basically similar to that displayed by the heterokont algae and fungi. The rhizoplast is also similar to that of other heterokont algae. We conclude from these observations that the class Eustigmatophyceae should be placed within the division Heterokontophyta.Abbreviations C chloroplast - B basal body of the emergent flagellum - B' second basal body - E eyespot - F emergent flagellum - FS flagellar swelling - LV lamellate vesicle - M mastigonemes - MTs microtubules - N nucleus - R 1–R 4 microtubular roots - Rh rhizoplast - SB striated band - SV spiral vesicle     

Spirochetes flagellar collar protein FlbB has astounding effects in orientation of periplasmic flagella,bacterial shape,motility, and assembly of motors in Borrelia burgdorferi

Borrelia burgdorferi, the causative agent of Lyme disease, is a highly motile spirochete, and motility, which is provided by its periplasmic flagella, is critical for every part of the spirochete's enzootic life cycle. Unlike externally flagellated bacteria, spirochetes possess a unique periplasmic flagellar structure called the collar. This spirochete‐specific novel component is linked to the flagellar basal body; however, nothing is known about the proteins encoding the collar or their function in any spirochete. To identify a collar protein and determine its function, we employed a comprehensive strategy that included genetic, biochemical, and microscopic analyses. We found that BB0286 (FlbB) is a novel flagellar motor protein, which is located around the flagellar basal body. Deletion of bb0286 has a profound effect on collar formation, assembly of other flagellar structures, morphology, and motility of the spirochete. Orientation of the flagella toward the cell body is critical for determination of wild‐type spirochete's wave‐like morphology and motility. Here, we provide the first evidence that FlbB is a key determinant of normal orientation of the flagella and collar assembly.     

Reconstruction of the flagellar apparatus in Ploeotia costata (Euglenozoa) and its relationship to other euglenoid flagellar apparatuses

The flagellar apparatus of Ploeotia costata Farmer and Triemer was reconstructed using serial sectioning and TEM. The flagellar apparatus is similar to other euglenoids having two flagella arising from basal bodies connected by a striated fiber, and three asymmetrically arranged roots. The flagella emerge subapically from between the two ventral pellicle strips. The dorsal flagellum is 1/2 the body length and actively pulls the cell, while the ventral flagellum is twice the body length and drags along the substrate surface. The ventral and dorsal roots are on the opposite sides of their respective basal bodies, while the intermediate root is associated with the ventral flagellum on the side closest to the dorsal basal body. The dorsal root lines the dorsal side of the reservoir and after giving rise to the dorsal band lines the right side of the reservoir/canal. The ventral and intermediate roots join at the reservoir forming the intermediate-ventral root, which lines the left and ventral sides of the reservoir/canal. There was no evidence of a microtubule-reinforced pocket in P. costata. Comparisons with Ploeotia vilrea, Lentomonas applanatum, and related flagellar apparatuses led to the conclusion that the basic euglenoid flagellar structure is symplesiomorphic but with enough variation to be taxonomically diagnostic.