Discocephalus

1. Discocephalus ehrenbergi Dragesco, 1960 (ref. ID; 3119, 3690 original paper, 3846, 7679) reported author and year? (ref. ID; 191)
2. Discocephalus grandis Dragesco, 1954 (ref. ID; 7679) reported year? (ref. ID; 3690)
   See; Marginotricha grandis (Dragesco, 1954) Jankowski, 1978 (ref. ID; 7679)
3. Discocephalus minimus Dragesco, 1968
   See; Prodiscocephalus minimus (Dragesco, 1968) Jankowski, 1979 (ref. ID; 7679)
4. Discocephalus rotatorius Ehrenberg, 1828 (ref. ID; 2316) reported year? (ref. ID; 3690) or Hemprich & Ehrenberg, 1831 (ref. ID; 7679)

Discocephalus ehrenbergi Dragesco, 1960 (ref. ID; 3119, 3690 original paper, 3846, 7679) reported author and year? (ref. ID; 191)

Descriptions

• Cortical spines and protrusions. On the cephalized region of the cell are found 2 kinds of dorsal, cortical protrusions: a series of 11 adoral spines extending anteriorly just above the lapel membranelles and a multipronged, antler-like structure lying just posterior to the collar membranelles. The adoral spines are short (1-2 um) and pointed; each appears closely associated with the leftmost portion of the lapel membranelle. The dorsal antler consists of a main branch (approximately 10-15 um) extending obliquely left and anteriorly; from the main branch extend 4 shorter (2.5 um) branches directed dorsally and approximately 6 intermembranellar branches which run dextrally. Both the adoral spines and the dorsal antler are protargol positive structures.
• Ciliature. Frontal ciliature consists of a U-shaped group of 7-10 hypertrophied transverse cirri plus 2 smaller accessory transverse cirri set within the ventral concavity, 3 right frontal cirri, 1 right anterolateral (migratory) cirrus, several cirri on the cephalized region including 2 malar and a paroral cirrus. The paroral cirrus along with the anteriormost streak II cirrus form the 2 large anterior cirri typical of the genus. Somatic ciliature comprises marginal cirri, caudal cirri and dorsal bristles. Left marginal cirri are divided into 2 groups: an anterior set of 2 hypertrophied cirri plus a posterolateral set of 13-14 membranelle-like cirri located within a cortical depression. Right marginal cirri are absent. On the dorsal surface are 6 rows of long cilia (7-8 um in length) that extend onto the cephalized region. Four sets of caudal cirri are present at the posterior right border of the dorsal surface: the first (posteriormost) is composed 4 cirri; the second is composed of 3 cirri; the third is composed of 2 cirri; the fourth consists of a single cirrus. Although this is the arrangement of caudal cirri usually observed, individuals of some populations possess only 3 sets of caudal cirri. These sets are arranged in a series (from posterior to anterior) of 3, 2, 1 or 3, 3, 2. Buccal ciliature comprises approximately 18 lapel (ventral) membranelles, approximately 6 collar (dorsal) membranelles (with a distinctly longer cilia) and a paroral and endoral membrane. The paroral membrane consists of 2 rows of cilia that emerge from below the right buccal overture (the left border of the peristomial lobe), extending anteriorly to a buccal cleft. Here the paroral membrane differentiates into a tongue-like ciliary oganelle, the peristomial cirrus. The deeper endoral membrane is a single row of cilia. (ref. ID; 7679)

• Ultrastructure:
  • Cytoskeleton. The cytoskeleton of D. ehrenbergi is extensive and complex. There are 2 separate parts to the cytoskeleton: a posterior portion, dominated by a large central microtubular trunk and an anterior portion within the cephalized region of the cell. The posterior cytoskeleton includes microtubules originating on the peripheral fibrillar matrix of cirri. The central cytoskeletal trunk is formed by the union of anterior microtubular bundles from each transverse cirrus. Branches from the anterior cytoskeleton extend to frontal cirri, collar membranelles, the dorsal antler, and both membranes of the paroral apparatus. Small connectors extend from a paroral branch to attach to the right base of each lapel membranelle; on the left of each lapel membranelle are the adoral spines. Thin rootlets (submembranellar fibers) descend posteriorly into the cytoplasm from each membranelle at the region of the adoral spine. A postmembranellar fiber runs posteriorly from near the adoral spines to subtend the proximal membranelles, then curves anteriorly to join with the paroral branch of the cytoskeleton. All components of the cytoskeleton in D. ehrenbergi are composed of microtubules.
  • Cirri. The cirral bases of D. ehrenbergi are variously shaped packets of kinetosomes ranging from parallelogram arranged sets to irregular polygon shaped sets. A fibrillar sheath surrounds each cirrus distally near the cell surface; it then descends to encircle the kinetosomes proximally as the pripheral cirral matrix. Nine interkinetosomal connectives (continuous with the peripheral fibrillar matrix) join neighboring kinetosomes: a right anterior (RA), left anterior (LA), left oblique (LO), left (L), left posterior (LP), posterior (P), right posterior (RP), right (R), and right oblique (RO). Connectives LO, RO and P anastomize between kinetosomes forming a recticulate pattern that is repeated throughout the cirral base. Two additional connectors, left and right postkinetosomal connectors, located on each side of the postciliary microtubules, link each posteriormost cirral kinetosome with the peripheral fibrillar matrix. Cirral kinetosomes can be as few as 12 in some frontal cirri, to greater than 100 in transverse cirri. Rows of transverse cirri kinetosomes are oriented at approximately 40 degrees to the longitudinal axis of the cell, while in other frontal cirri the angle is increased to nearly 100 degrees. This kinetosomal alignment (ranging from acute to obtuse arrays) can be explained by the degree of rotation of procirri after initial streak formation during morphogenesis. Transverse microtubular ribbons arise from near triplet number 4 of kinetosomes bordering the anterior left edge of each cirrus; these ribbons appear to extend directly toward the pellicle. Postciliary microtubular ribbons originate from the posteriormost cirral kinetosomes, then extend ventrally and posteriorly, presumably to contribute to a posterior microtubular bundle. In addition to these complete microtubular ribbons, single microtubules are present at triplet number 9 (a remnant postciliary microtubular ribbon) of internal cirral kinetosomes. The internal postciliary microtubules of transverse cirri extend posteriorly between kinetosomes to contribute to a posterior microtubular bundle.
  • Buccal apparatus. The membanelles are paramembranelles; each comprises 4 rows of kinetosomes: the first (posteriormost) and second are of equal length and are the longest rows, the third row is shorter (having 4-5 fewer kinetosomes than rows 1 and 2), and the fourth row (anteriormost) is shortest with only 3-4 kinetosomes per row. Membranelles of the lapel region have more kinetosomes per row than proximal or distal membranelles. Transverse microtubular ribbons are associated with triplet 4 of each kinetosome of row 4 and those kinetosomes of row 3 that are not bordered by row 4 kinetosomes. Postciliary microtubular ribbons arise from only row 1 membranellar kinetosomes; these ribbons course laterally within the intermembranellar ridge to contribute to the formation of the postmembranellar fiber that extends along the left and posterior borders of the zone of membranelles. Nematodesmal microtubules descend from the base of membranellar kinetosomes into the cytoplasm where they coalesce with other components of the microtubular cytoskeleton. The paroral membrane is a longitudinal row of kinetosomal pairs (polystichomonade); this membrane lies just below the right buccal overture, extends anteriorly, then ends at the buccal cleft. It is the shorter of the 2 oral membranes. Within the buccal cleft, the anterior end of the paroral membrane is differentiated into a separate unit; the short (7-8 kinetosmal pairs) peristomial cirrus observed in S.E.M. Whereas the posterior part of the paroral membrane protrudes laterally from the right buccal wall, the peristomial cirrus extends dorsoventrally from the buccal cleft. Furthermore, the posterior paroral row is membrane-like (without a peripheral fibrillar matrix), while the differentiated anterior segment is surrounded by electron dense material -hence appearing more cirrus-like. Postciliary microtubules are present on the right membrane border; transverse microtubules are present on the left border of the paroral membrane. Transverse microtubular ribbons are also associated with the left kinetosomes of the peristomial cirrus; these microtubules extend directly toward the pellicle. The longer, single row of kinetosomes of the endoral membrane is deep within the buccal cavity. It arises dorsal to the paroral membrane, extending from just posterior of the perostomial cirrus, along the buccal wall to the level of the proximalmost membranelle (well beyond the posterior extend of the paroral membrane). This membrane (a stichomonade) protrudes dorsally from the ventral wall of the buccal cavity - a position which results in the endoral kinetosomes being oriented at 180 degrees relative to the kinetosomes of the paroral membrane. Thus, each endoral kinetosome appears as a clockwise cartwheel (as viewed from outside the cell). This rotation does not affect the position of postciliary and transverse microtubular ribbons: postciliary microtubules originate from the right, transverse microtubules arise from the left of each endoral kinetosome. The transverse microtubules radiate from each kinetosome to anastomose with other components of the cytoskeleton. The postciliary microtubules are directed posteriorly in an overlapping series. Packets of membrane (pharyngeal discs) line the right side of the buccal cavity near the cytostome.
  • Cortical spines. The adoral spines and dorsal antler observed in S.E.M. are membrane-bound bundles of microtubules that are continuous with the anterior cytoskeleton and, in the antler, the nematodesmal microtubules of the paramembranelles. Most microtubules comprising the dorsal antler are arranged linearly along the longitudinal axis of the structure. Some microtubules, especially those located beside the antler membrane, are arranged perpendicularly to the longitudinal axis of the structure.
  • Dorsal bristle complex. Each of the dorsal bristle rows of D. ehrenbergi consist of a series of kinetosomal pairs - each pair is positioned 60 degrees to the longitudinal axis of the cell. The anterior kinetosome of most doublets bears a long cilium (7 um), the posterior kinetosome is non-ciliated. In some bristle complexes on the cephalized region of the cell, however, the anterior kinetosome bears a short (2-3 um) cilium. Both kinetosomes lie within a cortical pit; whereas the cell membrane extends into the pit and onto the cilium, the alveolar membrane extends only to the pit wall. Alveolar plates, as observed in Euplotes (Hausmann and Kaiser 1979) and Certesia (Wicklow, unpublished), are absent. Just below the cell membrane a band of dense fibrillar material surrounds the distal portion of each kinetosomal pair. From this distal band descend a series of fibrillar ribs that curve proximally toward the kinetosomes where they join a second band. Radiating fibrillar arms then link this second band to the kinetosomal triplets, thereby completing a basket-like framework around each bristle pair. At the anterior end of this basket, near the cell surface, is a small outpocket. Two desmoses connect the kinetosomes of each pair: a thick left desmose between triplet 9 of the anterior kinetosome and triplets 4 and 5 of the posterior kinetosome, and a thin right desmose between triplet 2 of the anterior kinetosome and triplet 3 of the posterior kinetosome. The left side of each bristle complex is bordered by a dense mass of fibrillar material; nematodesmal microtubules originate from this fibrillar patch as well as from the base of the bristle kinetosomes. Although some nematodesmal microtubules run posteriorly, most radiate toward the left side of the cell. A transverse microtubular ribbon, consisting of 5 microtubules, originates at triplets 4 and 5 of the anterior kinetosome. Two postciliary microtubular ribbons are associated with each bristle complex: a single postciliary microtubule originates near triplet 9 of the anterior kinetosome and a series of 3 postciliary microtubules arise from triplet 9 of the posterior kinetosome. Both transverse and postciliary microtubular ribbons extend dorsally toward the pellicle. A kinetodesmal fiber is associated with triplets 6 and 7 of the posterior kinetosome; it runs slightly anteriorly at a sharp angle toward the pellicle.
  • Cytoplasm. Within the ribosome rich cytoplasm are scattered over 100 macronuclei. Fewer micronuclei, containing dense chromatin material, are also present as well as vesicles of the Golgi complex and dense lipid inclusions. Groups of rod-shaped endosymbiotic bacteria (approximately 1.3 um in length, approximately 0.3 um in width) occur throughout cytoplasm. Two membrane systems surround each symbiont: an outer membrane (or cell wall) and an inner cytoplasm membrane. (ref. ID; 7679)
• Morphogenesis: Cortical morphogenesis during cell division in D. ehrenbergi occurs in one latitudinal zone; from this zone develop buccal, frontal, and somatic ciliature for both proter and opisthe daughter cells. The first morphogenetic event observed within this zone is the formation of an oral primordium (OP) at the cell surface. Later, in close association with the enlarging OP, an additional lacelike network of primordia arises: the frontal primordia (FP) (subsequently developing into an oblique series of 7-9 frontal streaks) and, beside the right edge of the OP, an undulating membrane primordium (UMP). From the UMP differentiate the paroral and endoral membranes; a paroral cirrus develops from the anterior end of the paroral membrane. While the OP differentiates membranelles (in a posteriad direction), a cortical invagination (the future buccal cavity) is formed with the membranelles and endoral membrane within, the paroral membrane along the right edge. A cortical ridge between the membranelles and the paroral membrane represents the future right buccal overture. Meanwhile, the frontal streaks, developing within cortical grooves, split into proter and opisthe frontal fields and the parental paroral apparatus begins to dedifferentiate. Four oblique ranks of procirri differentiate from each of these frontal fields: transverse cirri compose the first (posteriormost) rank; 2 accessory transverse cirri compose the second rank; 1 migratory, 3 midfrontal and 2 malar cirri compose the third rank; 3 right frontal cirri and 1 anterior frontal cirrus compose the fourth rank. At the same time as the collar membranelles are curving toward the cell's right, the frontal cirri move anteriorly, also curving to the right, until they are disposed in a longitudinal series. The migratory cirrus is positioned on the right lateral surface, just posterior to the distal membranelles. Only the transverse cirri appear to move posteriorly as they assume a U-shaped configuration. Microtubules are formed at the anterior edge of each transverse cirrus; these grow anteriorly, then eventually enlarge and unite to form the major component of the posterior cytoskeleton. When the frontal streaks are still in a single field and the membranelles have begun to differentiate within a cortical invagination, a left marginal cirral primordium (MP) appears. It arises in close association with the posterior marginal cirrus, proliferates posteriorly, then divides into anterior (proter) and posterior (opisthe) marginal anlagen. From each of these MP streaks differentiate 2 kinds of cirri: the pair of hypertrophied anterior cirri, and the posterolateral series of membranelle-like cirri. Although originating from the same within-row primordium, these 2 sets of cirri differ in position, kinetosomal arrangement, and presumably, function. The six dorsal kineties arise by within-row development. In addition to dorsal bristles, caudal cirri differentiate from the 4 rightmost dorsal bristle primordia. The 4 caudal cirri are actually 4 sets of cirri. The first (posteriormost) set comprises 4 cirri - one from each of the 4 rightmost kineties; the second set consists of 3 cirri from the 3 rightmost kineties; the third set consists of 2 cirri from the 2 rightmost kineties; the fourth is a single cirrus arising from the rightmost kinety. The parental ciliature begins to be disassembled and resorbed during late development, after cytokinesis is underway. Only the parental membranelles are retained (partially redifferentiated) by the proter. (ref. ID; 7679)

Behavior and Ecology

Sampling site

Discocephalus rotatorius Ehrenberg, 1828 (ref. ID; 2316) reported year? (ref. ID; 3690) or Hemprich & Ehrenberg, 1831 (ref. ID; 7679)

Descriptions