Reticulopodial Network Research Articles

Interactions of Globobulimina Auriculata with Nematodes: Predator Or Prey?

Abstract Studies of carnivorous behaviour of benthic foraminifers are rare and mostly focused on laboratory experiments. Controlled experiments have shown that some agglutinated and intertidal species prey on meio- to macrofaunal metazoans. Here we present observations of the behaviour of specimens of the infaunal benthic foraminiferal species, Globobulimina auriculata and G. turgida, made within several hours of collection from ∼117 m depth in the Alsbäck Deep of the Gullmar Fjord, Sweden. We observed live nematodes within the tests of G. auriculata. Video observations recorded over a 17-hour period showed a G. auriculata specimen with a living nematode whose tail appeared to be entangled within the foraminifer's reticulopodial network. The nematode eventually coiled around the foraminifer's aperture and became much less active, though ingestion into the foraminifer's test was not documented. If these observations indicate feeding by G. auriculata, they differ from previous observations of predation by Ammonia tepida, which utilised external reticulopodial activity to extract the soft tissue of its prey. An alternative interpretation of the video observations, consistent with the observations of the live nematodes inside G. auriculata, was that the nematode was attempting to prey upon the foraminifer. The G. turgida specimens, in contrast, relatively quickly surrounded themselves in soft sediment spheres commonly seen in deposit-feeding foraminifers, and were never observed with nematodes within their tests. We speculate that these contrasting feeding strategies might reduce competition and facilitate the coexistence of these two globobuliminid species.

Chapter 11 In Vivo Manipulation of Internal Cell Organelles

Publisher Summary This chapter discusses the in vivo manipulation of internal cell organelles. Optical tweezers-based techniques offer novel opportunities for subcellular manipulation and the study of cell processes that require precise positioning of cell organelles. The chapter describes some basic techniques for the intracellular manipulation of cell organelles using the giant freshwater amoeba, Reticulomyxa, and the green alga, Spirogyra, as model systems. Both cell models afford damage-free manipulation of mitochondria, particles, or membrane tubes, thus providing insights into the forces required to move organelles. The experiments on cell process formation in Reticulomyxa and on motor forces in Spirogyra were performed with the optical tweezers setup. The giant amoeba Reticulomyxa possesses a central cell body and a peripheral feeding network of fine cytoplasmic strands that extend several centimeters from the cell body. To measure the forces driving single mitochondria along microtubules within fine strands of the reticulopodial network, a mitochondrium simply has to be trapped with a force greater than the molecular motor force.

Taxonomic studies on the Chlorarachniophyta. II. Generic delimitation of the chlorarachniophytes and description of Gymnochlora stellata gen. et sp. nov. and Lotharella gen. nov.

SUMMARYA new chlorarachniophytan alga, Gymnochlora stellata Ishida et Y. Hara gen. et sp. nov., has been isolated from Anae Island in Guam. It is a green, star‐shaped, unicellular, amoeboid organism with several filopodia that do not form a reticulopodial network. Neither zoospores nor walled coccoid cells have been observed throughout the life cycle. The chloroplast ultrastructure is similar to those of described species; however, the pyrenoid matrix, which is invaded by many tubular structures originating from the inner membrane of the chloroplast envelope, is unique. A classification system is proposed for the Chlorarachniophyta. In this system, the ultrastructural features of the pyrenoid and the location of the nucleomorph in the periplastidial compartment are used as generic criteria, while the morphological features of the vegetative cells and life cycle patterns are used for species criteria. The described species, except for Cryptochlora perforans Calderon‐Saenz et Schnetter, are also reassessed under the new system, and consequent nomenclatural requirements for the genus Chlorarachnion are dealt with in this paper. The taxonomic rank of a previously described species, Chlorarachnion globosum Ishida et Y. Hara, is elevated and Lotharella globosa (Ishida et Y. Hara) Ishida et Y. Hara gen. nov. et comb. nov. is proposed.

Toxic foraminifera: Innocent until proven guilty

Reticulopods of benthic and planktic foraminifera have been periodically reported to possess the ability to narcotize, paralyze or even kill larger prey organisms by means of toxins. This paper critically examines the historical development of the idea of toxicity in foraminifera and provides new observational data on prey capture in the benthic species Glabratella ornatissima. Data presently available provide only minimal evidence for the presence of toxins in foraminifera. Most available evidence suggests that foraminiferal prey capture is in large part mediated by molecular cell-surface receptors (e.g. carbohydrates, glycoproteins) present in the extracellular matrix of adhesive pseudopodia. A survey of available data indicates that the majority of experimentally studied foraminifera exhibit the capability to effectively capture motile prey organisms, including in at least some cases organisms that exceed the predator body size. This suggests that the reticulopodial networks of many foraminiferal species may function as effective traps for a variety of motile organisms and that carnivory in foraminifera may be more widespread than previously assumed.

Spatially separated classes of microtubule bridges in the reticulopodial network of Allogromia: Evidence for a dynein-like ATPase in the filopodial cytoskeleton

Summary The microtubular bundles within lysed reticulopodial networks of Allogromia laticollaris comprise regions of tightly paralleled microtubules alternating with regions of less well ordered microtubular arrays, exhibiting a more wavy appearance. Both regions are characterized by at least two different classes of intermicrotubular crosslinks in mutually exclusive domains: While structural MAPs (MAP2) predominantly occur in the loosely interconnected microtubular regions, periodically arranged bridges resembling ciliary dyneins in size and arrangement are restricted to the highly ordered areas. The occurrence of a dynein-like motor protein was demonstrated by immunolabelling with an antibody against the 14S-subunit of the ciliary dynein-ATPase. Since in the immunofluorescence, the microtubular bundles stain in a punctate pattern consistent with the discontinuous distribution of the regularly arranged globular crossbridges seen in the tightly paralleled microtubular arrays by electron microscopy, it is suggested that they may represent dynein-like motors.

Direct evidence for the occurrence of simple microtubule organizing centers (MTOCs) and for end-to-end joining of microtubules with modified ends in the reticulopodia of Allogromia laticollaris (foraminifera)

The partial disassembly of filopodial microtubules into helical filaments, respectively the reassembly from paracrystals of exactly registered tubulin helices was recognized as a special adaptation to the highly dynamic reticulopodial movements in the foraminifer Allogromia laticollaris [4, 16]. Electron microscopic investigations of partially extracted or completely lysed filopodial strands revealed some additional adaptations to the local tubulin demands within the widely spread, rapidly changing reticulopodial networks. As probable foci for the initiation of additional microtubules from pools of tubulin during the netto growth phase, small (40 × 60 nm) cube-shaped, electron dense bodies were discovered which could act as mobile MTOCs at very distant sites from the metabolically active cell body within the reticulopodial system. The microtubule ends, proximally as well as distally to the filopodial origins, are characterized by a closed appearance because of a terminal cap which is continuous with the microtubule wall, whereas some other microtubules appear as the usual open cylinders. It is suggested that the closed ends, which occur either freely or decorated by the cube-shaped, electron dense bodies, may play a role in the de novo initiation of microtubules as well as in the transformation to the helical state or vice versa. Points of discontinuity within single longitudinally cut microtubules can be observed directly by electron microscopy and correspond to end-to-end contacts of closed MT-terminations. These junctions probably represent initial contact points of an annealing process between adjacent shorter MT-segments. Microtubule annealing occurs rapidly in vitro [14, 15] and could explain the elongation rates observed during filopodial outgrowth.

The ultrastructure of the plasmodial protist Leucodictyon marinum Grell

Leucodictyon marinum Grell, obtained from tide pools near the Shimoda Marine Research Center (Japan), is a meroplasmodium with flattened cell bodies, enclosed in loricae, and a common reticulopodial network. The cell bodies have a single, excentrically located nucleus which contains a central nucleolus, several heterochromatic areas and a filamentous rodlet of unknown function. Plastids are lacking. The mitochondria have tubular cristae. Extrusomes, resembling discobolocysts, do occur, but are rarely met with. Many vacuoles with a homogeneous content indicate that the final part of digestion takes place in the cell bodies. The reticulopodia are slender threads, but show dilatations, caused by vesicles which transport food to the cell bodies. The reticulopodia contain microtubules. There are two types of cytokinesis: Equal division leads to enlargement of the meroplasmodium, unequal division to zoosporo-genesis. The development of the two flagella of the zoospore is described. The flagellar root system consists only of a short fibre connecting both basal bodies and some microtubules running to the plasmalemma. The difficulty to classify the recently investigated species Reticulosphaera socialis, Chlorarachnion reptans and Leucodictyon marinum is discussed. If the "reticulo-meroplasmodial complex" is a synapomorphy, indicating their affinity, they may derive from an early offshoot of the ancestral Heterokontophyta and can scarcely be affiliated to one of the established classes.

The ultrastructure of Reticulosphaera socialis grell (Heterokontophyta)

Reticulosphaera socialis GRELL, a protist from the Caribbean coast of Mexico, has a life cycle consisting of a benthic and a pelagic phase. The benthic phase is the dominating part. It is represented by the meroplasmodium, a cellular association which consists of the spherical cell bodies and a common reticulopodial network. Each cell body is surrounded by an ampulla-like wall, whose narrow part also protects the reticulopodial root connecting the cell body with the remaining network. A single nucleus with nucleolus is situated in the middle of the cell body. A centriole is attached to the nuclear envelope. In addition to the usual organelles (mitochondria, Golgi complexes, plastids), the cytoplasm includes many vesicles and a variable number of bacteria. The latter are probably parasites. The reticulopodial strands serve for capturing and digesting diatoms. Apart from the phagocytized food included in digestive vacuoles, the reticulopodia contain only mitochondria, bacteria and vesicles of different size. Bundles of microtubules extend in a longitudinal direction. The meroplasmodium grows by binary fission of its cell bodies. Mitosis is characterized by single centrioles at opposite poles of the spindle and partial dissolution of the nuclear envelope. At cytokinesis, which begins with an unequal division or budding, one of the daughter nuclei passes from the major part into the minor part. Later on, an equalization occurs, in which the minor part becomes supplied with cellular material by its reticulopodial root. When the diatoms become sparse and light intensity is sufficient, some of the cell bodies form clusters of yellow cells which carry out photosynthesis. Their plastids, also observable in life, show the ultrastructure typical for heterokont algae. The lamellae consist of three thylakoids. Girdle lamellae are lacking. Each plastid is surrounded by two membranes, but remnants of two additional membranes may also be present. Under moderate illumination the meroplasmodium disintegrates into single cells, the so-called transitional cells. They resemble amoebae and transform into the pelagic cells which float in the sea water. Although the pelagic cells look like small heliozoans, their ultrastructural analysis has shown that they have two flagella, inserted at an oblique angle to each other. Both flagella possess a "transitional helix". Whether they are of different length could not be clarified. It is certain, however, that they are heteromorphic: One flagellum bears a basal swelling, associated with a single layer of osmiophilic droplets (eyespot) in a nearby plastid. With respect to the root fiber system, it is of interest that a rhizoplast is lacking. The pelagic cell has only two plastids, associated with the nucleus. Except at the edge of the eyespot, both plastids have girdle lamellae. They show remnants of a periplastidial compartment. The flagellar and photoreceptor apparatus support the assumption that Reticulosphaera socialis has to be classified with the heterokont algae (Heterokontophyta). The color of the photosynthesizing cells and the lack of a rhizoplast during mitosis of the cell bodies and in the root fiber system of the pelagic cells suggest an affiliation to the Xanthophyceae (Tribophyceae).

Anzeichen sexueller Fortpflanzung bei dem plasmodialen Protisten Chlorarachnion reptans Geitler / Indications of Sexual Reproduction in the Plasmodial Protist Chlorarachnion reptans Geitler

Abstract The algal protists Reticulosphaera socialis Grell and Chlorarachnion reptans Geitler are “meroplasmodia”: They consist of separate cell bodies and a comm on reticulopodial network by means of which they capture and digest food organisms, for instance diatoms. When food becomes exhausted, the meroplasmodium disintegrates sooner or later into single cells. In Chlorarachnion reptans shortage of food may lead directly to globular resting cells with a distinct cell wall. In other cases disintegration is delayed. The plasmodial cells, still connected by reticulopodial strands, move about and join with single resting cells to form pairs. After pair formation they lose the reticulopodial connections, round off and form a cell wall of their own. The cells which rested from the beginning on start growing, whereas the cells which searched for them do not. It is probable that the nuclei of the latter are transferred into the growing cells so that synkarya are formed. If this the case, pair formation is a fertilization process and the growing cells are zygotes. A single zygote, isolated and transferred on a “lawn” of diatoms, gives rise to a plasmodium whose cells display both sexual types again.

Direct observation of microtubule dynamics in Reticulomyxa: Unusually rapid length changes and microtubule sliding

AbstractMicrotubule dynamics has been studied extensively in vitro, but comparatively little information is available on the in vivo behavior of microtubules. Here we report on the assembly, disassembly, and sliding of microtubules in the giant freshwater amoeba, Reticulomyxa. We have found that treating the cell with 0.25% trypsin induces the rapid formation of exceedingly flat areas within the reticulopodial network, allowing for the direct observation of microtubule behavior by DIC optics and computer‐enhanced video microscopy. In flattened areas, microtubule sliding occurs at rates of between 1 and 6.5 μm/sec. The average rate of microtubule assembly is 1.6 μm/sec, while microtubule disassembly takes place at about 4 μm/sec and can reach up to 19.5 μm/sec. We also observed many cases where a microtubule forms a hairpin loop and eventually breaks, resulting in bidirectional disassembly from the point of breakage. Our observations demonstrate sliding of cytoplasmic microtubules in vivo. The high rates of microtubule assembly/disassembly in this cell type are difficult to reconcile with conventional views of association and dissociation processes at microtubule ends and suggest unconventional mechanisms for the growth and shrinkage of microtubules.

Reticulomyxa: a new model system of intracellular transport.

Reticulomyxa is a large multinucleated freshwater protozoan that provides a new model system in which to study intracellular transport and cytoskeletal dynamics. Within the cell body and reticulopodial network, rapid, visually striking saltatory organelle motility as well as bulk cytoplasmic streaming can be readily observed. In addition, the cytoskeletal elements within these strands undergo dynamic splaying and fusing rearrangements, which can be visualized by video-enhanced light microscopy. A reactivatable lysed cell model has been developed that appears to preserve, and therefore permits examination of, these three forms of motility in a more controlled environment. Individual organelle movements are microtubule-based and have similarities to, but also differences from, the recently described kinesin-based transport. This lysed cell model can be further manipulated to provide native, ordered, completely exposed networks of either microtubules or microfilaments, or a combination of both, and thus may serve as a versatile motility assay system in which to examine the movement of exogenously added isolated organelles or latex beads.

Cytoskeletal architecture and motility in a giant freshwater amoeba, Reticulomyxa.

Reticulomyxa is a large, multinucleated freshwater protozoan with striking intracellular transport. Cytoplasmic streaming and saltatory movements of individual organelles (at rates of up to 25 micron/sec) are observed within the naked cell body and the extensive reticulate peripheral network of fine cytoplasmic strands. As demonstrated by video-enhanced light microscopy, individual organelles move only when associated with cytoskeletal linear elements. The linear elements are composed of mixed colinear bundles of microtubules and actin filaments, which form the backbone of the reticulopodial network. The constant branching, sprouting, and fusion of network strands suggest unique membrane properties and an unusually dynamic cytoskeleton. The electrophoretic mobility of Reticulomyxa tubulins and the lack of crossreactivity with several antibodies known to react with many plant and animal tubulins suggest that they may differ from other tubulins more widely than might be expected. Reticulomyxa's large size, the rapidity and pervasiveness of the two forms of transport, and the simple and ordered cytoskeleton make the organism well suited for future studies on the mechanisms of intracellular transport.

Immunocytochemical Identification of Tubulin Containing Paracrystals in Allogromia Reticulopods

The marine protozoan Allogromia sp, strain NF Lee extends an elaborate reticulopodial network (RN) which contains an elongate microtubule-(MT)-based cytoskeleton. The MTs are located primarily within cytoplasmic fibrils which are visible by light microscopy (LM) in highly flattened or “two dimensionalized” reticulopodia. It was shown previously that allogromiid RNs withdraw in response to hypertonic Mg2+-seawater. An ultrastructural analysis of this phenomenon indicated that large patches of paracrystalline (PC) material, composed of helical filament aggregates, form concomitant with a decrease in MT number. Similar large patches of PC aggregates are also found in juvenile Allogromia before they extend a RN, which disappear during RN formation. Finally, PC aggregates are occasionally seen near microtubules in normal untreated RNs. Thus there is circumstantial evidence to propose that PC aggregates in Allogromia represent an intermediate form of tubulin; however, more definitive biochemical or immunocytochemical data is not available.

Surface transport properties of heticulopodia: Do intracellular and extracellular motility share a common mechanism?

The reticulopodial networks of the foraminiferan protozoans Allogromia sp., strain NF, and A. laticollaris display rapid (up to 11 μm/second) and bidirectional saltatory transport of membrane surface markers (polystyrene microspheres). Electron microscopy shows that microspheres adhere directly to the reticulopodial surface glycocalyx. A videomicroscopic analysis of this phenomenon reveals that microsphere movement is typically independent of pseudopod extension/withdrawal and that particles of different sizes and surface properties display similar motile characteristics. The motile properties of surface-associated microspheres appear identical to those of saltating intracellular organelles. Indeed, in some instances the surface-attached microspheres appear transiently linked in motion to these underlying organelles. Our observations suggest that, in reticulopodia, surface transport of microspheres and intracellular transport of organelles are driven by a common mechanism.

Multiple fission in Allogromia sp., strain NF (Foraminiferida): release, dispersal, and ultrastructure of offspring.

The release, dispersal, and ultrastructure of juveniles arising through multiple fission in the benthic foraminiferan Allogromia sp., strain NF (Lee & Pierce, 1963) has been examined by light and electron microscopy. An extensive reticulopodial network participates in the dispersal of fully differentiated young as they emerge from the fragmented parental test. During the earliest stages of release, offspring are of two classes--aroused and unaroused. Unaroused juveniles, which have not extended pseudopods, attach externally to the network and are transported bidirectionally along its surface. Aroused juveniles, which have extended pseudopods and are in protoplasmic continuity with the network, move quickly to the periphery of the network. Within 24 h, juveniles establish a communal "feeding reticulum" in which dispersed individuals are in protoplasmic continuity with neighbors via a common reticulopodial network. At the ultrastructural level, the cell body cytoplasm of unaroused juveniles contains numerous patches of a paracrystalline material, which disappears as their pseudopodia are extended to join the communal feeding reticulum. This paracrystalline material therefore appears to be a temporary reservoir of precursors required for pseudopod construction.

Studies on the motility of the foraminifera. II. The dynamic microtubular cytoskeleton of the reticulopodial network of Allogromia laticollaris.

Lamellipodia have been induced to form within the reticulopodial networks of Allogromia laticollaris by being plated on positively charged substrata. Video-enhanced, polarized light, and differential interference contrast microscopy have demonstrated the presence of positively birefringent fibrils within these lamellipodia. The fibrils correspond to the microtubules and bundles of microtubules observed in whole-mount transmission electron micrographs of lamellipodia. Microtubular fibrils exhibit two types of movements within the lamellipodia: lateral and axial translocations. Lateral movements are often accompanied by reversible lateral associations between adjacent fibrils within a lamellipodium. This lateral association-dissociation of adjacent fibrils has been termed 'zipping' and 'unzipping'. Axial translocations are bidirectional. The axial movements of the microtubular fibrils can result in the extension of filopodia by pushing against the plasma membrane of the lamellipodia. Shortening, or complete withdrawal, of such filopodia is accomplished by the reversal of the direction of the axial movement. The bidirectional streaming characteristic of the reticulopodial networks also occurs within the lamellipodia. In these flattened regions the streaming is clearly seen to occur exclusively in association with the intracellular fibrils. Transport of both organelles and bulk hyaline cytoplasm occurs bidirectionally along the fibrils.

Fully correlative LM and HVEM analysis of motile fibrils and particles in the reticulopods of Allogromia sp. (protozoa, foraminiferida)

The subcellular mechanism underlying the bidirectional transport of organelles in the reticulopods of foraminiferans is poorly understood. Applications of urea and colchicine suggest a role for microtubules (MT) in mediating this transport. Allen et al. have shown that intracellular transport is effected only when organelles are in contact with motile fibrils, visible by LM, and imply that such fibrils are composed of microtubules. We recently suggested that the bidirectional transport of latex particles attached to the extracellular surface of reticulopods may also be mediated by these fibrils. Since motile fibrils are an essential component of the transport mechanism, we sought to determine their ultrastructural composition and the identity of the organelles they transport.Specimens were mounted in microperfusion chambers as described previously. Either intact organisms or isolated but functional reticulopodial networks (as illustrated here, fig 1A) may be employed. Addition of lym latex microspheres allows simultaneous study of surface motility and intracellular transport, and also provides reference marks for correlation of LM/HVEM images.

In Vivo Microtubule Assembly/Disassembly States in Reticulopods of Allogromia SP

Ultrastruetural changes occur during retraction of the reticulopodial network(RN) of the marine foraminiferan Allogromia sp.(strain N.F. Lee) after induction by varied experimental stimuli. The normally active RN (Fig. 1) emerges from the cell body(CB) and radiates over long distances, exhibiting bidirectional streaming of particles throughout. The normal, untreated RN cytoskeleton contains microtubules(MT). 5nm filamente(F), and helical filaments(HF), which are probably derived from MT by uncoiling. Helical filaments may aggregate to form paracrystals(PC). We originally, and others subsequently have proposed tubulin packaging to facilitate rapid transport and/or storage. Retraction in Allogromia is a state in which rapid transport of cytoskeletal proteins is essential. We have evidence that suggests packing of helical filaments may fulfill this role. See fig. 2.

Ultrastructural Observations on Induced Retraction of the Reticulopodial Network of Allogromia SP. (Foraminifera)

Allogromia sp. (strain N.F. Lee) provides a convenient model system for studying intracellular transport. We have been examining ultrastructural changes which occur in the reticulopodial network (RN) and cell body (CB) of Allogromia at various stages in the retraction response stimulated by 0.4M MgCl2 solution. As retraction proceeds, microtubules (MT) disassemble and form areas of regularly packed cytoplasm (TP-tigroid patterning) resembling vinca alkaloid induced paracrystals of tubulin. TP is first seen in the RN, and later in the CB. TP consists of dark filamentous material 12.5 nm in width separated by a light spacing of 8 nm when plane of section gives a perfect repeat pattern (arrow-fig. 6). TP may fill the entire profile of a reticulopod and in early stages of retraction is seen in close juxtaposition to MT. In the CB, TP occurs in patches 0.3-2.0 μm in diameter. TP is seen with great frequency in retracting Allogromia, but also, it can be seen occasionally in animals with fully extended RN.

Feeding and Digestion in Allogromia (Foraminifera)

Foraminifera (“forams”) are important to micropaleontology as stratigraphic indicators and within this context the literature is extensive. Comparatively, biological studies are few. The striking bidirectional cytoplasmic streaming in the reticulopo-dial network (RPN) of some forams has been of primary interest. Only recently has their potential as a model system for other aspects of cellular transport been recognized. The naked foraminiferan Allogromia sp. (N.F.Lee), is particularly well suited for such studies. We have found that by plating these forams on coverslips treated with protamine sulfate (1.5 mg/ml) the flexible theca flattens, thus exposing the cell body contents and complex patterns of cytoplasmic streaming to the light microscope (LM). Our aim is to take advantage of the ease with which correlative LM and EM techniques can now be applied to the study of function and transport of components in both the cell body (CB) and RPN.