Stomatogenic events accompanying binary fission in Blepharisma.

Stomatogenesis was studied in the heterotrich ciliate Blepharisma japonicum stained with protargol. During binary fission not only is a new oral apparatus made for the posterior daughter, but the already existing oral apparatus of the parent cell is reorganized, i.e., partially disassembled and then subsequently reassembled to provide a functional feeding apparatus for the anterior daughter cell. These morphogenetic events, requiring 2 1/2 to 3 hr, are complete by the time the anterior and posterior daughters separate. In preparation for division, an oral anlage is formed by the rapid proliferation of kinetosomes along 4-5 stomatogenic kinetics directly subtending the cytostome. This field of randomly oriented kinetosomes ultimately gives rise to the feeding apparatus of the posterior daughter cell. Early in division, the oral anlage separates into 2 longitudinal fields of kinetosomes: one is destined to give rise to the undulating membrane and the other forms the adoral zone of membranelles. Shorly after the anlage is established posterior to the cytostome, reorganization of the existing functional mouth is initiated. The morphologic changes associated with this dedifferentiation-redifferentiation sequence lead to the formation of an oral apparatus for the anterior daughter and cannot be distinguished from those characteristically seen during physiologic reorganization.     

Stomatogenic Events Accompanying Binary Fission in Blepharisma*

SYNOPSIS. Stomatogenesis was studied in the heterotrich ciliate Blepharisma japonicum stained with protargol. During binary fission not only is a new oral apparatus made for the posterior daughter, but the already existing oral apparatus of the parent cell is reorganized, i.e. partially disassembled and then subsequently reassembled to provide a functional feeding apparatus for the anterior daughter cell. These morphogenetic events, requiring 21/2 to 3 hr, are complete by the time the anterior and posterior daughters separate. In preparation for division, an oral anlage is formed by the rapid proliferation of kinetosomes along 4–5 stomatogenic kinetics directly subtending the cytostome. This field of randomly oriented kinetosomes ultimately gives rise to the feeding apparatus of the posterior daughter cell. Early in division, the oral anlage separates into 2 longitudinal fields of kinetosomes: one is destined to give rise to the undulating membrane and the other forms the adoral zone of membranelles. Shortly after the anlage is established posterior to the cytostome, reorganization of the existing functional mouth is initiated. The morphologic changes associated with this dedifferentiation-redifferentiation sequence lead to the formation of an oral apparatus for the anterior daughter and cannot be distinguished from those characteristically seen during physiologic reorganization.     

The influence of fission line expression on the number and positioning of oral primordia in the cdaA1 mutant of Tetrahymena thermophila.

During cytokinesis, furrowing creates new boundaries for daughter cells. Following a shift to a restrictive temperature, cells of the temperature-sensitive cell-division-arrest (cdaA1) mutant of Tetrahymena thermophila complete development of the oral apparatus for the prospective posterior daughter cell before becoming arrested in cytokinesis. When maintained under weak restrictive conditions (35 degrees C), some of the chains were arrested prior to the start of fission line formation (D-shaped chains), whereas others manifested rudimentary unilateral furrowing on the ventral side (B-shaped chains). In their second cell cycle following the temperature shift, the D-shaped chains usually formed only one oral primordium, at a position highly correlated with the length of the entire chain. The B-shaped chains always produced two separate oral primordia, located at irregular positions anterior and posterior to the division furrow, often close to the posterior oral apparatus produced during the first cycle. These results suggest that the formation of the fission line sets a reference boundary to assess the number of oral primordia and influence their position, that appear during subsequent morphogenetic episodes. They also indicate that, during cell division cycles, pre-existing oral apparatuses do not strongly inhibit the formation of new oral apparatuses in their close vicinity.     

The influence of fission line expression on the number and positioning of oral primordia in the cdaA1 mutant of Tetrahymena thermophila

During cytokinesis, furrowing creates new boundaries for daughter cells. Following a shift to a restrictive temperature, cells of the temperature-sensitive cell-division-arrest (cdaA1) mutant of Tetrahymena thermophila complete development of the oral apparatus for the prospective posterior daughter cell before becoming arrested in cytokinesis. When maintained under weak restrictive conditions (35°C), some of the chains were arrested prior to the start of fission line formation (D-shaped chains), whereas others manifested rudimentary unilateral furrowing on the ventral side (B-shaped chains). In their second cell cycle following the temperature shift, the D-shaped chains usually formed only one oral primordium, at a position highly correlated with the length of the entire chain. The B-shaped chains always produced two separate oral primordia, located at irregular positions anterior and posterior to the division furrow, often close to the posterior oral apparatus produced during the first cycle. These results suggest that the formation of the fission line sets a reference boundary to assess the number of oral primordia and influence their position, that appear during subsequent morphogenetic episodes. They also indicate that, during cell division cycles, pre-existing oral apparatuses do not strongly inhibit the formation of new oral apparatuses in their close vicinity. © 1992 Wiley-Liss, Inc.     

Molecular subdivision of the cortex of dividing Tetrahymena is coupled with the formation of the fission zone.

In contrast to a mitotic-spindle-associated bipolar cytokinesis, the cytokinesis of polarized ciliates is preceded by a reorganization of the cortex into dual metameric patterns for prospective daughter cells and then separated by a transverse fission line. This study concerns relations between the generation of cortical metamery and the formation of the fission line in an amicronuclear (i.e., without mitotic spindle) ciliate, Tetrahymena pyriformis. The fission line appears in the division of T. pyriformis as a transverse line formed by equatorial gaps in the meridional ciliary rows, with the second oral structure (OA2) formed posterior to it. It was found that the metamery of cortical morphogenesis is expressed by the appearance of increased MPM2 antibody binding in dividing cells in an apical area and posterior to the fission line gaps, including patterned changes of this binding in both oral apparatuses (OA1 and OA2), and by a reciprocal decrease of binding of an anti-epiplasm antibody. These tested antigens are localized to different cortical structures, but in predividing cells both uniformly show formation of the fission line contrast of labeling. A serine/threonine kinase inhibitor, 6-dimethylaminopurine (6-DMAP), was applied to dividing T. pyriformis at specific stages: (1) if 6-DMAP was added to early dividing cells, it prevented cells from initiating cytokinesis. (2) If 6-DMAP was added to cells at stages close to the physiological transition point of cell division, it yielded either (i) a partial formation of the fission line on the ventral side, combined with modified growth of undivided cortex adjacent to the fission line, with abnormal cytokinesis, or (ii) variable anterior displacement of the complete fission line, which contracted slowly but uniformly. (3) If 6-DMAP was applied during cytokinesis, it did not delay cell division, but daughter cells become abnormal and underwent an incomplete oral reorganization. These results suggest that the generation of metamerism in the cortex of T. pyriformis involves differentiation of the asymmetric fission zone. At least four stage-dependent 6-DMAP-sensitive effects jointly control the progress of cell division and the mutual spatial relations between the generation of metamery and the appearance, completeness, and position of the fission zone in the cortex of polarized T. pyriformis.     

The cleavage program in the 2d cell lineage of Tubifex embryos

Early development in clitellate annelids is characterized by a highly stereotyped sequence of unequal, spiral cleavages. Cell 2d (i.e., the second micromere of the D quadrant) in the oligochaete Tubifex tubifex also undergoes an evolutionarily conserved sequence of cell division to produce four bilateral pairs of ectodermal teloblasts that act as embryonic stem cells. This study was conducted to characterize each of the 15 rounds of cell division that occur in the 2d cell lineage in this clitellate. After its occurrence, cell 2d undergoes three rounds of highly unequal divisions, giving off the first smaller daughter cell toward the posterior right of the larger daughter cell, the second cell toward the posterior left, and the third cell toward the anterior side of the cell; the larger daughter cell that results from the third division (i.e., the great-granddaughter cell of 2d) then divides equally into a bilateral pair of NOPQ proteloblasts. Cell NOPQ on either side of the embryo undergoes 11 rounds of cell division, during which ectoteloblasts N, Q, and O/P are produced in this order. After its appearance, NOPQ undergoes highly unequal divisions twice cutting off the smaller cells toward the anterior end of the embryo and then divides almost equally into ectoteloblast N and proteloblast OPQ. After its appearance, OPQ undergoes highly unequal divisions twice giving off the first smaller cell toward the anterior and the second smaller cell toward the posterior of the embryo and then divides almost equally into ectoteloblast Q and proteloblast OP. Finally, OP undergoes highly unequal division four times after its birth budding off the smaller cells toward the anterior and then cleaves equally into ectoteloblasts O and P. In the unequally dividing cells of the 2d cell lineage, the mitotic apparatus (MA), which forms at the cell's center, moves eccentrically toward the cortical site where the smaller cell will be given off. The moving MA is oriented perpendicular to the surface it approaches, and its peripheral pole becomes closely associated with the cell cortex. In contrast, the MA involved in the equal divisions remains in the cell center throughout mitosis. The key features of the cleavage program in the 2d cell lineage are discussed in light of the present observations. The mechanical aspects of unequal cleavage in the 2d cell lineage and the modes of specification of MA orientation are discussed. A comparison of the cleavage mode in the 2d cell lineage is also performed among six selected clitellate annelid species.     

Effect of alteration in the global body plan on the deployment of morphogenesis-related protein epitopes labeled by the monoclonal antibody 12G9 in Tetrahymena thermophila

We have employed monoclonal antibodies to reinvestigate the janus mutants of the ciliate Tetrahymena thermophila, which cause reversal of circumferential polarity on the dorsal surface of the cell. This reversal brings about frequent ectopic expression of ventral cortical landmarks, such as a "secondary" oral apparatus, on the dorsal surface. The principal antibody employed, FXXXIX-12G9, immunolabels both transient cortical structures not directly associated with basal bodies (the fission line and the postoral meridional filament) and more permanent structures (apical band and oral crescent) that are associated with basal bodies. 12G9-immunolabeling of janus cells has revealed additional phenotypes, including disorder of ciliary rows. Further, this labeling has shown that the postoral meridional filament is often expressed and the apical band is frequently interrupted on the mid-dorsal surface of janus cells irrespective of whether or not these cells express a "secondary" oral apparatus. Of the permanent structures revealed by 12G9 immunofluorescence, modifications of the oral crescent (OC) are associated with prior modifications in the development of basal body-containing structures in the secondary oral apparatus. The formation of the apical band (AB) is also commonly abnormal in janus cells; analysis of specific abnormalities shows that the AB depends both on its initiation at a specific site near the anterior basal body of apical basal body couplets and on the normal location of these couplets just posterior to the fission line. We also have uncovered an intriguing difference in the reactivity of apical-band filaments to the 12G9 antibody in the two non-allelic janus mutants (janA1 and janC2) that we have investigated. Taken together, our observations indicate that the formation of new cellular structures at division depends both upon pre-existing cytoskeletal structures and upon the positional information provided by large-scale cellular polarities.     

The filaments supporting ciliary primordia and fission furrow in the hypotrich ciliateParaurostyla weissei, revealed by the monoclonal antibody 12G9: Studies on wild-type and ciliary hypertrophy mutant

Summary The unique monoclonal antibody FXXXIX 12G9 obtained againstTetrahymena cortices was used to label cytoskeletal structures related to basal body proliferation inParaurostyla weissei. The antibody binds to an amorphous material interconnecting basal bodies in compound ciliary structures: dorsal units, cirri and membranelles in interfission cells, and filamentous structures supporting the primordia of ciliary structures and fission line in dividing cells. The antibody visualized meridional filaments preceding proliferation of new basal bodies in the oral primordium and structures accompanying all developing ciliary primordia. It congregated in differentiating new procirri and membranelles, whereas another population of transient meridional structures accompanied the final distribution of new structures. A meridional filament connecting transverse cirri with the oral apparatus, marking the future stomatogenic meridian, persisted in both division products until completion of cell elongation. The fission line was found to originate from an anterior extension of the pre-oral filament toward the parental oral structures. It then encircled the cell's midbody demarcating the boundary between daughter cells; two additional circumferential structures bordering the anterior and posterior ends of differentiating division products participate in formation of the new poles. They disappear after separation of daughter cells and completion of resorption of parental ciliature. In the enhanced multi-left-marginal mutant expressing gross hyperduplication of basal bodies, the location of the 12G9 antigen corresponded to that in wild-type cells. The sequence of formation of meridional filaments in the mutant was found to be altered. The filaments in the left lateral domain preceded the formation of the preoral filament, yet the temporal pattern of basal body assembly was not modified. The fission line, as in wild-type cells, originated in connection with the oral primordium. We conclude that the nucleation of the filamentous structures bearing the 12G9 antigen and the basal body assembly occur by independent mechanisms reading the same cell cycle signals. We suggest that the 12G9-antigen-bearing protein might be similar to septins: involved in signaling the position of the oral primordium and the fission line and functioning in establishing and maintaining the asymmetric cortical domain characteristics.Abbrevations AZM zone of adorai membranelles - bb basal bodies - CC caudal cirri - FC frontal cirri - Fmf frontal meridional filament - FTV the primordia of fronto-ventro-transverse cirri - LD, RD dorsal rows of bristle units - LM, RM left or right marginal cirral row - OA oral apparatus - OP primordium of the adoral membranelles - pLM, pRM primordium of the left or right marginal cirri - pLD, pRD primordia of the left or right dorsal bristle rows - pUM primordium of the undulating membranes - TC transverse cirri - UM undulating membranes - VC ventral cirral rows     

Role of intercellular contacts in generating an asymmetric mitotic apparatus in the Tubifex embryo

The 2-cell stage embryo of Tubifex is composed of a smaller cell, AB, and a larger cell, CD. At the second cleavage, the CD-cell divides unequally. The mitotic apparatus (MA) involved in this division is organized asymmetrically: the MA pole to be segregated to a smaller cell is flattened and truncated, and associated with the anterior cortex facing the AB-cell, while the other pole is symmetric and located more centrally. The present study was undertaken to elucidate the mechanism that generates asymmetry in the MA organization in CD-cells. When CD-cell nuclei, which are normally located near the anterior cortex, were displaced toward the posterior end of the cell (i.e. opposite AB-cells) by centrifugation, MA assembled ectopically there, and were bilaterally symmetric in organization. Similar symmetric MA were formed in isolated CD-cells, which divided more equally than intact cells. This equality of cell division was dramatically reduced if the anterior surface of isolated CD-cells formed contact with other cells, such as AB-, C- and 4D-cells. The MA that formed in these reconstituted embryos were asymmetric in organization; one MA pole was always found to be truncated and apposed to the cortical site at the cell contact. Symmetric MA were also observed in cytochalasin-treated embryos. Together with the finding that one of the MA poles is physically attached to the anterior cortex of the intact CD-cell, these results suggest that factors generating asymmetry in the spatial organization of MA poles reside at the anterior cortex of the CD-cell and that this cortical mechanism is dependent upon cell contacts.     

The Binary Fission of Lacrymaria olor (O.F.M., 1786).

SUMMARY. Observations on binary fission of Lacrymaria olor show that it is a transverse fission. It involves probable intranuclear division of the micronucleus. Both micro- and macronucleus elongate in preliminary stages. Each is ultimately divided as cytoplasmic constriction cuts the spindle fibers of the former, and the connecting, nucleoplasmic thread of the latter.
Surging movements of cytoplasm after fission elongate the daughter organisms and move new nuclei to normal, central sites. The anterior proboscis of the posterior daughter regenerates suddenly, complete with coronal cilia. Metachronal waves along ciliary meridians, strongly reversed on the posterior daughter, cause an oscillating movement which pulls the two apart, except for a slender, pellicular thread, ultimately severed. Until broken, this thread connects the rear tip of the anterior animal to the forward end of the proboscis of the posterior one. The organism is semi-quiescent, with proboscis retracted (except spasmodically) throughout fission. After fission the anterior animal quickly begins feeding movements and soon swims away. The posterior animal requires about half an hour before being able to begin feeding and swimming movements.
The anterior contractile vacuole of the original animal becomes the primarily active vacuole of the anterior daughter; the posterior one that of the posterior daughter. Missing vacuoles are regenerated by the daughters in about one hour after fission. The division process requires about one hour for completion at 22.4°C.     

Morphology and morphogenesis of a freshwater ciliate, Epistylis chlorelligerum Shen, 1980 (Ciliophora, Peritrichia)

An oligohalobic peritrichous ciliate, Epistylis chlorelligerum Shen, 1980, was collected from a ditch in Hangzhou, China. The morphology, oral infraciliature, and morphogenesis of the species were studied using living and protargol-impregnated specimens. Zooids of E. chlorelligerum are 160-230 × 50-60 μm in vivo, and characterized by green-colored endoplasm containing symbiotic algae. The oral infraciliature presents a well-developed filamentous reticulum linked to the circular fiber of the cytostome; the outer two rows of P3 extend adstomally over P1 and usually enfold it. During binary fission, one daughter cell inherits most part of the old buccal apparatus and the reorganized haplokinety and germinal kinety (Hk' and G'), and new buccal apparatus of the other daughter cell is mostly developed from the original germinal kinety (G) and haplokinety (Hk): new peniculi 2, 3 (2P2, 2P3), new haplokinety (2Hk), and new germinal kinety (2G) are formed from G, while the new peniculus 1 (2P1) and its peristomial extention (2Pk) originate from Hk. The epistomial membrane can be observed until the two sets of buccal apparatus begin to separate from each other.     

The dynamics of filamentous structures in the apical band, oral crescent, fission line and the postoral meridional filament in Tetrahymena thermophila revealed by monoclonal antibody 12G9

The ciliate Tetrahymena thermophila possesses a multitude of cytoskeletal structures whose differentiation is related to the basal bodies - the main mediators of the cortical pattern. This investigation deals with immunolocalization using light and electron microscopy of filaments labeled by the monoclonal antibody 12G9, which in other ciliates identifies filaments involved in transmission of cellular polarities and marks cell meridians with the highest morphogenetic potential. In Tetrahymena interphase cells, mAb 12G9 localizes to the sites of basal bodies and to the striated ciliary rootlets, to the apical band of filaments and to the fine fibrillar oral crescent. We followed the sequence of development of these structures during divisional morphogenesis. The labeling of the maternal oral crescent disappears in pre-metaphase cells and reappears during anaphase, concomitantly with differentiation of the new structure in the posterior daughter cell. In the posterior daughter cell, the new apical band originates as small clusters of filaments located at the base of the anterior basal bodies of the apical basal body couplets during early anaphase. The differentiation of the band is completed in the final stages of cytokinesis and in the young post-dividing cell. The maternal band is reorganized earlier, simultaneously with the oral structure. The mAb 12G9 identifies two transient structures present only in dividing cells. One is a medial structure demarcating the two daughter cells during metaphase and anaphase, and defining the new anterior border of the posterior daughter cell. The other is a post-oral meridional filament marking the stomatogenic meridian in postmetaphase cells. Comparative analysis of immunolocalization of transient filaments labeled with mAb12G9 in Tetrahymena and other ciliates indicates that this antibody identifies a protein bound to filamentous structures, which might play a role in relying polarities of cortical domains and could be a part of a mechanism which governs the positioning of cortical organelles in ciliates.     

Physiological Reorganization and Post-Traumatic Regeneration in Stylonychia mytilus: Reflections on Developmental Constraints and the Evolutionary Origin of Alternative Modes of Asexual Morphogenesis

We investigated development of cortical ciliature in Stylonychia mytilus during starvation-induced physiological reorganization, and during regeneration following amputation of the anterior part of the cell. Cortical reorganization in the two processes is generally similar. The posterior part of the adoral zone of membranelles is resorbed and replaced with newly assembled membranelles. The pre-existing set of ventral cirri and dorsal bristles is entirely resorbed and replaced with new ones. Regenerants exhibit posterior displacement of the frontal-ventral-transverse cirri primordium and the undulating membrane primordium, and recruit basal bodies from ectopic locations for the development of these ciliature. This illustrates flexibility in the initiation site of ciliary primordia, and opportunism in utilizing building blocks. Such morphogenetic versatility of hypotrichs provides the basis for the operation of a global control of pattern formation, which governs cortical reorganization in dividers, and additionally, in the absence of the prerequisites for binary fission, alternative modes of cortical development such as physiological reorganization or regeneration. These considerations suggest that the three processes are homologous and that physiological reorganization and regeneration have evolved from binary fission. In physiological reorganization and regeneration, the micro- and macronuclei reorganize to resemble that in binary fission; these nuclear events are considered evolutionary relics of the nuclear development of binary fission. Tetrahymena also exhibits such morphogenetic flexibility; stomatogenesis is under global control, so that asexual cells can replace its oral apparatus without undergoing binary fission. Paramecium , on the other hand, adopts a more rigid strategy in relying heavily on pre-existing structures for morphogenetic cues; this could have imposed constraints in the exploration of alternative modes of asexual development.     

The Dynamics of Filamentous Structures in the Apical Band, Oral Crescent, Fission Line and the Postoral Meridional Filament in Tetrahymena thermophila Revealed by Monoclonal Antibody 12G9

The ciliate Tetrahymena thermophila possesses a multitude of cytoskeletal structures whose differentiation is related to the basal bodies the main mediators of the cortical pattern. This investigation deals with immunolocalization using light and electron microscopy of filaments labeled by the monoclonal antibody 12G9, which in other ciliates identifies filaments involved in transmission of cellular polarities and marks cell meridians with the highest morphogenetic potential. In Tetrahymena interphase cells, mAb 12G9 localizes to the sites of basal bodies and to the striated ciliary rootlets, to the apical band of filaments and to the fine fibrillar oral crescent. We followed the sequence of development of these structures during divisional morphogenesis. The labeling of the maternal oral crescent disappears in pre-metaphase cells and reappears during anaphase, concomitantly with differentiation of the new structure in the posterior daughter cell. In the posterior daughter cell, the new apical band originates as small clusters of filaments located at the base of the anterior basal bodies of the apical basal body couplets during early anaphase. The differentiation of the band is completed in the final stages of cytokinesis and in the young post-dividing cell. The maternal band is reorganized earlier, simultaneously with the oral structure.The mAb 12G9 identifies two transient structures present only in dividing cells. One is a medial structure demarcating the two daughter cells during metaphase and anaphase, and defining the new anterior border of the posterior daughter cell. The other is a post-oral meridional filament marking the stomatogenic meridian in postmetaphase cells. Comparative analysis of immunolocalization of transient filaments labeled with mAb12G9 in Tetrahymena and other ciliates indicates that this antibody identifies a protein bound to filamentous structures, which might play a role in relying polarities of cortical domains and could be a part of a mechanism which governs the positioning of cortical organelles in ciliates.     

Further evidence for cortical control over replication of the macronucleus and basal bodies in Stentor

This paper presents further evidence that the cortex controls macronuclear replication and basal body production during the cell cycle of Stentor. At the onset of cell division, basal body production occurs on the ventral side of the cell to form an oral primordium; this structure develops slowly into the oral apparatus destined for the posterior daughter cell. Meanwhile, a series of morphological changes in the macronucleus (coalescence, elongation, nodulation) doubles the number of nodes in preparation for division. When a cell undergoing oral development is grafted to a morphostatic cell of equal size, oral development is usually induced in the morphostatic component and the two members of the graft complex eventually become synchronized with respect to macronuclear morphology. However, primordium induction and nuclear synchronization usually do not occur when the 2 members of the graft complex are separated by cortical discontinuities which heal gradually, even though the graft components demonstrably share a common endoplasm throughout the experiment. These results suggest that the cell surface may control the replication of organelles such as the macronucleus and basal bodies which are normally kept “in step” with the cell cycle in such a way that they are not lost or reproduced too frequently.     

Oral Apparatus Structure in the Carnivorous Macrostomal Form of Tetrahymena vorax

The structure of the oral apparatus in the carnivorous macrostomal form of Tetrahymena vorax has been investigated using serial thin sections and preparations of isolated oral apparatuses. The cilia of the oral apparatus are organized into an undulating membrane that borders the right and part of the posterior margin of the buccal cavity and three membranelles that project from plateaus on the anterior surface. Each membranelle consists of one short row and two longer rows of hexagonally packed kinetosomes. The organization of the microtubules of the oral ribs is identical to that in the T. vorax microstomal cell type. However, the first oral rib originates near the first kinetosome at the anterior end of the undulating membrane. The fine filamentous reticulum that underlies part of the oral ribs in the macrostomal cell type is not striated, unlike the reticulum in the microstomal form. A band of filaments similar to the fine filamentous reticulum extends around the anterior margin of the large cytostomal opening that occupies most of the posterior part of the oral cavity. The single row of microtubules along the left side of the oral cavity and cytostome also has filaments associated with it. A major difference between the microstomal and macrostomal forms in the structure of the oral apparatus is in the oral connectives. The macrostomal cell type contains only a single cross-connective that joins the three membranelles and the anterior portion of the undulating membrane. The posterior or peripheral connective between the posterior ends of membranelles one and two and the posterior end of the undulating membrane is absent.     

Reconciling the bizarre inheritance of microtubules in complex (euglenid) microeukaryotes

We introduce a hypothetical model that explains how surface microtubules in euglenids are generated, integrated and inherited with the flagellar apparatus from generation to generation. The Euglenida is a very diverse group of single-celled eukaryotes unified by a complex cell surface called the "pellicle", consisting of proteinaceous strips that run along the longitudinal axis of the cell and articulate with one another along their lateral margins. The strips are positioned beneath the plasma membrane and are reinforced with subtending microtubules. Euglenids reproduce asexually, and the two daughter cells inherit pellicle strips and associate microtubules from the parent cell in a semi-conservative pattern. In preparation for cell division, nascent pellicle strips develop from the anterior end of the cell and elongate toward the posterior end between two parent (mature) strips, so that the total number of pellicle strips and underlying microtubules is doubled in the predivisional cell. Each daughter cell inherits an alternating pattern of strips consisting of half of the nascent strips and half of the parent (mature) strips. This observation combined with the fact that the microtubules underlying the strips are linked to the flagellar apparatus created a cytoskeletal riddle: how do microtubules associated with an alternating pattern of nascent strips and mature strips maintain their physical relationship to the flagellar apparatus when the parent cell divides? The model of microtubular inheritance articulated here incorporates known patterns of cytoskeletal semi-conservatism and two new inferences: (1) a multigenerational "pellicle microtubule organizing center" (pMTOC) extends from the dorsal root of the flagellar apparatus, encircles the flagellar pocket, and underpins the microtubules of the pellicle; and (2) prior to cytokinesis, nascent pellicle microtubules fall within one of two "left/right" constellations that are linked to one of the two new dorsal basal bodies.     

Pole reversal and the development of cell asymmetry during division in cryptomonad flagellates

Summary A unique form of cell division is reported for the cellsKomma caudata andCryptomonas ovata (Cryptophyceae). During cytokinesis, the posterior tail-like region of each daughter cell develops from the anterior region of the parental cell. This process, termed pole reversal, involves a major realignment in overall cell polarity as well as alterations to cytoplasmic and surface components. Pole reversal may be a consequence of flagellar apparatus transformation and reorientation during division, and pole reversal may facilitate the development of the asymmetric cell shape in daughter cells.     

Origin and Morphogenetic Movements of the Pores of the Contractile Vacuoles in Paramecium qurelia

The position of the contractile vacuoles in non-dividing Paramecium aurelia, as judged by that of the pores, is relatively constant both laterally and longitudinally, as is the distance between them. Some variability of the distance between the pores persists from the recent fission among the smaller animals of normal shape. At the onset of fission two new vacuoles appear, one anterior to each of the old vacuoles; all the vacuoles must undergo relocation because their mean positions now are somewhat removed from that in the non-dividing animals. The pores of the front daughter are too close together: the anterior pore is too far back, and the posterior is too far forward; likewise the pores of the hind daughter are too close together: the anterior too far forward as well as the posterior. The relocation does not begin until shortly before separation of the daughters and is practically complete by the time the daughters have assumed their normal length/width ratio after separation. A neuroneme connects the pore to one of the basal granules adjacent to its place of origin, now far removed.
Occasionally more than one new pore appears at the usual time and place during very early fission; these are very close together and usually serve a single vacuole. However, most of them fail to survive the relocation following this and the next fission, so they are found in the posterior position very rarely. But, in some individuals they may survive and serve a single or separate vacuoles. In a certain clone of Paramecium aurelia multiple pores and vacuoles are rather frequent; these are, of course, the result of the tendency to produce multiple pores and vacuoles at the time of fission.     

Tip Transformation in Tetrahymena: A Morphogenetic Response to Interactions Between Mating Types*

SYNOPSIS. In Tetrahymena thermophila subline B, a morphogenic alteration of the anterior end of cells of mating types III and VII results from a cellular interaction which precedes and is a prerequisite for pairing. Cell pairing begins 1 h after starved cells of complementary mating type are mixed. The 1 h-long lag period is characterized by an actinomycin D-sensitive inductive interaction in the first 30 min, followed by a maturation period. Tip transformation begins during the maturation period and continues after pairing. Scanning electron microscopy of deciliated cells reveals ridges which form a chevron meeting in a midline seam between the oral apparatus and the anterior tip. During transformation, the seam broadens until the ridged surface is completely smooth. Melding of the ridges also occurs at the tip of the cell resulting in its blunted appearance. Cells of complementary mating types join in the region of this modified surface, which eventually becomes a specialized cell junction perforated by cytoplasmic bridges. Thus, pursuant to an inductive interaction the structure of the tip of cells is modified in anticipation of the pairing event. Rate of cell pairing might be limited by rate of tip transformation.