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Protistology 6 (3), 139—146 (2010)
Protistology
Ultrastructural and functional aspects of static and motile systems in two taxa of the Alveolata: Dinoflagellata and Ciliata
Klaus Hausmann and Norbert Hulsmann
Division ofProtozoology, Institute of Biology/Zoology, Free University of Berlin, Berlin, Germany
Summary
In this synopsis we review some, for a long time well known, ultrastructural and functional aspects of static and motile systems in dinoflagellates and ciliates, i. e., alveolar (=amphiesmal) plates, contraction-elongation and coiling phenomena, piston activities, cell contractions, peduncle and tentacle feeding. It is our effort to point to the existence of a rich source of observations on structural features and cognitions on functions in unicellular organisms in the literature which are in danger to be forgotten.
Key words: dinoflagellates, ciliates, alveolar/amphiesmal plates, body contraction/ elongation, peduncle/tentacle feeding
Several decades ago the idea that dinoflagellates and ciliates might have a common root was of more heretical nature. The time was not ripe to accept such a possibility, although there had already been ultrastructural und cytochemical observations that certain ciliates possess plates in their alveoli, as most of the dinoflagellates do in their amphiesmata. Motile phenomena in dinoflagellates such as cell contraction or piston contraction and elongation were never discussed in the context of stalk or body contraction and their similarity to the elongation of ciliates; even such similar processes as feeding by a peduncle and feeding by tentacles or by cytopharyngeal baskets were not properly compared.
Today it is widely accepted that the phylum Alveolata includes protist taxa such as Dinoflagellata,
Ciliophora, Apicomplexa as well as Perkinsozoa. Using structural and molecular methods, it has been convincingly shown that these protistan taxa originate from a common ancestor (Hausmann et al., 2003). The name-giving structures are subplasmalemmal alveoli/amphiesmata. These flattened vacuoles are arranged in a mosaic-like pattern and nearly always exhibit a species-specific arrangement. A first review on the general relationships between flagellates and ciliates was published by Lee and Kugrens in 1992.
Alveolar (=amphiesmal) plates
In armoured dinoflagellates, the amphiesmata contain so called thecal plates (Fig. 1) which consist of a non-cellulosic glucan (Nevo and Sharon, 1969; Dodge and Crawford, 1970) but not of proteins.
© 2010 by Russia, Protistology
i A pm
■ B
Fig. 1 . A — The armoured dinoflagellate Peridinium tabulatum with thecal plates (tp); B — plasmamembrane (pm) underlaid by amphiesmata (am) which contain amphiesmal (= alveolar = thecal) plates (ap) (from Hausmann, 2008).
Alveolar plates are relatively seldom found in ciliates (Fig. 2) and normally the alveoli are electron lucent. Alveolar plates of ciliates do not consist mainly of polysaccharides. At least two different varieties of plates are known: In the alveoli of e.g. Coleps, the plates are composed of a polysaccharide matrix incrusted by calcium phosphocarbonate (Faure-Fremiet et al., 1968). In contrast, the plates in Euplotes consist mainly of protein with a fine polysaccharide coating (Bohm and Hausmann 1981). Such plates are thought to give the cortex of dinoflagellates and ciliates rigidity and thus also protection (Nobili, 1967; Walker, 1975; Hausmann and Kaiser, 1979).
CONTRACTION-ELONGATION-MECHANISMS IN FLAGELLATES
Besides this more structural, static similarity between dinoflagellates and ciliates, a highly dynamic phenomenon can be found in these two taxa, i.e., a contraction-elongation-mechanism in certain species of both groups which is astoundingly similar, if not identical.
The basis for the following comparison is the motile behaviour of a species of the dinoflagellate genus Erythropsidinium (Erythropsis). These flagellates, which are currently the focus of attention in the context of phylogenetics (Gomez, 2008), have been the subject of interest for a long time due to a spectacular organelle complex (an eye-like stigma) termed ocelloid (Fig. 3). It is a rather intricate structure which resembles the compound eye of arthropods in several respects (Greuet, 1967, 1968, 1987). Although there is thus far no experimental evidence at all that this structure is involved in photoreception, this rather amazing organelle has attracted a lot of attention. Perhaps for this reason, however, another significant structure of these
Fig. 2. Alveolar plates in the ciliate Euplotes vannus.
A — silverline system showing the regular arrangement of alveoli (light microscopy); B — alveolar plates inside the alveoli (electron microscopy);
C — 3D-reconstruction of the pellicle. ialm - inner alveolar membrane, oalm - outer alveolar membrane, ap - alveolar plates, pm - plasmamembrane (from Hausmann et al., 2003).
dinoflagellates has been somewhat neglected: Its highly dynamic posterior end, called tentacle or piston.
This tube-like cell extension can elongate to more than 10fold of its shortest stage (Fig. 3). The elongation-contraction cycle is a very rapid process which lasts only about one second in some species. The cell performs this movement continuously except for occasional short interruptions. Although it has been speculated that this piston is somehow involved in cell locomotion (Greuet, 1987; Gomez, 2008), there is no convincing prooffor this explanation thus far.
When studying this movement in detail by frame-by-frame analysis of video sequences (Hausmann, 2006), it becomes obvious that the elongation and the contraction of the tentacle differ from each other clearly and unambiguously: elongation takes twice or three times as long as the extremely rapid contraction (Fig. 3).
But what might be the cell biological purpose of this activity be? To answer this question, it
Fig. 3. Erythropsidinium pavillardi. A — habitus (n - nucleus, o - ocelloid, p — piston); B — illustration of the elongation-contraction cycle of the piston, based on a one second lasting video sequence (from Hausmann, 2006); C — 3D-reconstruction of the ultrastructure of the piston of Erythropsis (er - endoplasmic reticulum, mi -mitochondrion, mtl - microtubular lamellae, myo - myonemes) (after Greuet).
may be helpful to look for comparable activities in other flagellates. So far nothing has been published for free-living flagellates which could be compared directly with the piston movement of Erythropsidinium. Noctiluca scintillans, a hete-rotrophic, unarmoured (i.e. no plates in the amphi-esmata) dinoflagellate, possesses likewise a tentacle used for food acquisition. However, this appendage moves in a completely different way, slowly swinging to and from, and cannot be compared with the tentacular behaviour of Erythropsidinium (Greuet, 1987).
Coiling of elongated organelles in flagellates
AND CILIATES
A comparable rapid contraction of a long cell process can be observed in the haptonema of several prymnesiomonad (haptophyte) flagellates (Leadbeater, 1971). However, in corresponding cases the cell process does not shorten during the contraction; the structure retains its length and only coils or bends (Fig. 4) (Kawachi et al., 1991; Inouye and Kawachi, 1994; Nakayama et al., 2005). The mechanism of this movement is still unclear.
Fig. 4. Fig. 4. Chrysochromulina parva with uncoiled Fig. 5. Elongated (A) and coiled stalk of the ciliate
(A, B), coiling (C) and coiled haptonema (D). f - Vorticella (B) (courtesy of Peter Sitte, Freiburg,
flagellum, h -haptonema (from John et al., 2002). Germany).
At this point one should have a look at other representatives of Alveolata and examine whether haptonema-like contractions might occur here or not. The first candidates are the ciliates; the stalk contractions of peritrich ciliates such as Vorticella or Carchesium immediately come to mind (Amos, 1972; Allen, 1973a, 1973b; Amos et al., 1975). However, this is the same situation as it is for the haptonema, namely coiling instead of shortening (Fig. 5), and cannot be compared with piston contractions.
CONTRACTION-ELONGATION-MECHANISMS IN CILIATES
Looking further for contraction activities in ciliates, the body contraction of heterotrichs, for instance of species belonging to Stentor, might reflect a comparable situation. This contraction-elongation system was studied in great detail in the 1970s (Bannister and Tatchell, 1968, 1972; Huang and Pitelka, 1973). It was demonstrated that microtubular lamella, myonemes (bundles of 4 nm filaments), and ER cisternae, which are arranged in a highly organized, characteristic spatial pattern, are the cellular constituents responsible for the contraction and elongation of these ciliates. This situation is true in general for contractile heterotrichs such as Spirostomum ambiguum (Hawkes and Holberton, 1974, 1975) and Eufolliculina uhligi (Fig. 6) (Mulisch et al., 1981).
It has been shown that the myonemes are responsible for the rapid (mere milliseconds) contraction of the cells. During contraction, these filaments change their conformation and thicken to 10-12 nm, becoming shorter and tubular. This conformational alteration does not require ATP, but it is highly influenced by the concentration of calcium. This suggests that the ER cisternae adjacent to the myonemes serve as a site of calcium sequestration.
The elongation of the ciliate (which lasts several seconds) is caused by an active gliding of the microtubular lamellae relative to each other, possibly with dynein serving as a motor protein.
Piston activity in warnowiid dinoflagellates
What is the ultrastructure of Erythropsidinium's piston? In an early study, Greuet (1967) described the constituents of the tentacle in great detail. The nature and arrangement of the cellular structures involved in the contraction-elongation process in Erythropsidinium are the same as found for heterotrich ciliates: microtubules, myonemes and ER cisternae (Fig. 6). It is not surprising that the phenomena are also the same: Extremely rapid contraction and relatively slow extension of the piston.
The piston activity of the dinoflagellate Erythropsidinium has never been discussed in articles on the contractile cortex of the heterotrich ciliates which
Fig. 6. The ciliate Stentor sp. elongated (A) and contracted (B) (courtesy of Heinz Schneider, Landau, Germany); C — 3D-reconstruction of the cortex of Stentor coeruleus. er - endoplasmic reticulum, mtl - microtubular lamellae, myo - myoneme (after Huang and Pitelka).
were published somewhat later. Obviously the chasm between dinoflagellates and ciliates seemed so deep that none of the zoologically oriented ciliatologists assumed that it was worthwhile to discuss the situation in flagellates perhaps they were not even aware of the ultrastructural details of botanical unicellular organisms. Nowadays we are in a somewhat better situation. Hopefully!
Cell contraction in leptodiscinae
DINOFLAGELLATES
There are several other motility phenomena known from protists (Cachon and Cachon, 1982). One example is the body contraction e.g. of the marine Leptodiscinae, a group of marine dinoflagellates. These anteroposteriorly flattened flagellates are able to contract rapidly (Fig. 7). The structures responsible for this contraction consist of a layer of parallel filaments located beneath the cell membrane of certain parts of the body (Cachon and Cachon, 1984). The filaments are of non-actin type. They appear helically coiled and doubly twisted, and form tubular structures when contracted. ER
Fig. 7. The leptodiscin dinoflagellate Cymbodinium elegans. A — body contraction; B — reconstruction of the cell region responsible for the body contraction (O in A); al — alveolus (= amphiesma), er - endoplasmic reticulum, mi - mitochondrion, myo - myoneme, pm - plasmamembrane (after Cachon and Cachon).
cisternae traverse the filamentous layer regularly (Fig. 7). According to the cell biological tenor of this study, the authors discuss pleasantly and correctly that this situation is remarkably similar to that of the stalks of the peritrich ciliates. However, they make no statement about the phylogenetic significance and implications of these results. The time was not yet ripe for this aspect.
Feeding by a peduncle in dinoflagellates
= FEEDING BY A TENTACLE IN SUCTORIAN CILIATES?
The structure and mode of function of suctorian tentacles was studied in great detail during the 1960s (Rudzinska, 1965; Bardele, 1972). The main constituents of a tentacle are microtubules arranged helically in two concentric, tube-shaped arrays, actin-like microfilaments and different types of vesicles. The following scenario is proposed to occur during food uptake by suctorians (Fig. 8) (Rudzinska, 1965; Bardele, 1974; Tucker, 1974): Prey organisms which happen to come into contact with their haptocysts (type of extrusomes) become
Fig. 8. Feeding mechanism of a suctorian ciliate (A — C). al - alveolus, fv - food vacuole, mt - microtubules, pm - plasmamembrane, pr - prey, t - tentacle (from Hausmann et al., 2003).
stuck at the knob-like ends of involved tentacles. The cell membrane of the captured prey is ruptured at the contact zone. Vesicles then stream upward (centrifugally) along the peripheral tube of the tentacle and fuse with the membrane of the tentacle tip, providing the membrane material necessary for phagocytosis. Concomitantly, the membrane of the tip invaginates into the inner tube of the tentacle and prey cytoplasm rapidly streams down with it. When the invaginating membrane reaches the end of the inner tube, food vacuoles are pinched off. Arm-like bridges — most possibly dyneins — connecting the tentacle microtubules with the invaginating membrane are thought to be involved in the generation of the motive force for ingestion. A genuine suction act — implicating the generation of pressure differences — is highly unlikely (Rudzinska, 1973).
In our opinion, the structural and functional basis of the ‘peduncle feeding’ visible in certain heterotrophic dinoflagellates, which was studied in detail almost one decade later (Schnepf et al., 1985), is highly similar, if not the same, as in ‘tentacle feeding’ in suctorian ciliates (Fig. 9) in that in both cases the host plasmalemma is pierced by the feeding organelle but not ingested as a membrane. As a consequence of this process there is only one membrane, namely that of the food vacuole, between the cytoplasm of the predator and the ingested cytoplasm, in dinoflagellates as well as in ciliates.
However, the situation in dinoflagellates has been interpreted speculatively in a way that cannot be compared with the interpretation accepted for suctorians. Moreover, a new, somewhat delusive terminus for peduncle feeding has been created without convincing experimental proof: myzocy-tosis (i.e. phagocytosis by sucking) (Schnepf and Deichgräber, 1984; Schnepf and Elbrächter, 1992). Despite the fact that more then ten years ago a suction activity in this type of phagocytosis was ruled out (Rudzinska, 1973), this new terminus has unfortunately been accepted and adopted by botanists and protistologists, including the taxonomic designation Myzozoa for the taxon comprising dinoflagellates and apicomplexans (Cavalier-Smith and Chao, 2004).
It must be mentioned that in none of the feeding organelles, peduncle nor tentacle, has an actual suction, i.e. the creation of low pressure inside the feeding cell, been demonstrated. Consequently, both termini, suctorians as well as myzocytosis, are misleading and should be substituted by neutral terms.
In closing, it should be mentioned that the cytopharyngeal baskets which are used by nasso-phorean ciliates for phagocytosis (Tucker, 1968, 1972, 1978; Hausmann and Peck, 1978, 1979) are structurally and functionally the same as the tentacles of suctorians and thus, in our opinion, as the peduncles of dinoflagellates.
Fig. 9. Peduncle feeding in a dinoflagellate. f -flagellum, fv - food vacuole, mt - microtubules, n - nucleus, p - peduncle, pr - prey, pu - pusule (after Schnepf et al.).
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Address for correspondence: Klaus Hausmann. Division of Protozoology, Institute of Biology/Zoology, Free University of Berlin, D - 14195 Berlin, Germany, e-mail: [email protected]
