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Phylum Ciliophora

The phylum Ciliophora is the largest and the most homogeneous of the principal protozoan groups, and all evidence indicates that they share a com-mon volutionary ancestry. Some 7200 species have been described, and many groups are still not well known.

All possess cilia or compound ciliary structures as locomotor or food-acquiring organelles at some time in the life cycle. Also present is an infraciliary system, composed of ciliary basal bodies, or kinetosomes, below the level of the cell surface and associated with fibrils that run in various direc-tions. Such an infraciliary system may be present at all stages in the life cycle even with marked reduction in surface ciliation. Most ciliates possess a cell mouth, or cytostome. In contrast to the other protozoan classes, ciliates are characterized by the presence of two types of nuclei: one vegetative (the macronuclcus, concerned with the synthesis of RNA as well as DNA) and the other reproductive (the micronucleus, concerned only with the synthesis of DNA). Fission is transverse, and sexual reproduction never involves the formation of free gametes.

Ciliates are widely distributed in both fresh and marine waters and in the water films of soil. About one third of ciliate species are ecto- and endocom-mensals or parasites.

Form and Structure

The body shape is usually constant and in general is asymmetrical; however, radial symmetry with an anterior mouth is probably the primitive condition (Fig. 2-26). Although the majority of ciliates are solitary and free swimming, there are both sessile and colonial forms. The bodies of tintinnids and some heterotrichs, peritrichs, and suctorians are housed within a lorica, a girdle-like encasement, which is either secreted or composed of foreign material cemented together. In the peritrichs the lorica is attached to the substratum, but in many others the lorica is carried about by the organism (Fig. 2-27).

Figure 2-26 Prorodon, a primitive ciliate. (After Faure-Fremict from Corliss.)

Figure 2-27 Tintinnopsis, a marine ciliate with lorica, or test, composed of foreign particles. Note conspicuous membranelles and tentacle-like organelles interspersed between them. (After Faure-Fremiet from Corliss.)

The ciliate body is typically covered by a complex, living pellicle, usually containing a number of different organelles. The pellicular system has been studied in detail in numerous genera, including Paramecium. There is an outer limiting plasma membrane, which is continuous with the membrane, surrounding the cilia. Beneath the outer membrane are closely packed vesicles, or alveoli, which are moderately to greatly flattened (Figs. 2-28 and 2-29). The outer and inner membranes bounding a flattened alveolus would thus form a middle and inner membrane of the ciliate pellicle. Between adjacent alveoli emerge the cilia and mucigenic or other bodies (Fig. 2-29). Beneath the al¬veoli are located the infraciliary system, the kinetosomes, and fibrils. The alveoli contribute to the stability of the pellicle and perhaps limit the permeability of the cell surface (Pitelka, 1970).

Figure 2-28 Section through cilium and pellicle of Colpidium. Note that alveoli are greatly flattened and their inner and outer membranes fused at base of cilium. At top right is an enlarged view of surface and alveolar membranes. At lower right is a cross section of a cilium and surrounding pellicle taken at the level indicated by the dashed line. Note the circle of nine doubled peripheral ciliary fibrils. (After Pitelka.)

Figure 2-29 Pellicular system in Paramecium. (After Ehret and Powers from Corliss.)

The pellicle of the familiar Paramecium has in¬flated kidney-shaped alveoli (Fig. 2-29). The inflated condition and the shape of the alveoli pro¬duce a polygonal space about the one or two cilia that arise between them. Alternating with the alveoli are bottle-shaped organelles, the trichocysts, which form a second, deeper, compact layer of the pellicular system.

Locomotion

The ciliates are the fastest moving of the protozoa. In its beat each cilium performs an effective and a recovery stroke. During the effective stroke the cil-ium is outstretched and moves from a forward to a backward position (Fig. 2-34A and B). In the recovery stroke the cilium is bent over to the right against the body (when viewed from above and looking anteriorly) and is brought back to the for¬ward position in a counterclockwise movement. The recovery position offers less water resistance and is somewhat analogous to feathering an oar. A cilium moves in three different planes in the course of a complete cycle of beat, and the positions have been captured and recorded in scanning electron micrographs of freeze-dried Paramecium (Tamm, 1972).

Figure 2-33 Reconstruction of section of the pellicle of Tetrahymena. Right side is on viewer's left. Abbreviations: kinetodesmos |k); transverse microtubules (tm); postciliary microtubules (pm); longitudinal microtubules (lm); basal microtubules (bm); alveolus (a); cilium (c); epiplasm (c); mitochondrion |m); mucigenic body (mb). (From Allen, R. D, 1967.

Nutrition

Feeding in ciliates parallels, on a microscopic level, feeding in multicellular animals. Typically a distinct mouth, or cytostome, is present, although it has been secondarily lost in some groups. In primitive groups the mouth is located anteriorly, but in most ciliates it has been displaced posteriorly to varying degrees. The mouth opens into a canal or passageway called the cytopharynx, which is separated from the endoplasm by a mem-brane. It is this membrane that enlarges and pinches off as a food vacuole. The wall of the cytopharynx is strengthened with rods (nemades-mata) arranged like the staves of a barrel. Primitively, the ingestive organelles consist only of the cytostome and cytopharynx (Fig. 2-35L), but in the majority of ciliates the cytos¬tome is preceded by a preoral chamber. The preoral chamber may take the form of a vestibule, which varies from a slight depression to a deep funnel, with the cytostome at its base (Fig. 2-35B). The vestibule is clothed with simple cilia derived from the somatic ciliature.

In the higher ciliates the preoral chamber is typically a buccal cavity, which differs from a vestibule by containing compound ciliary organelles in-stead of simple cilia (Fig. 2-35C to F). There are two basic types of such ciliary organelles: the un¬dulating membrane and the membranelle. An undulating membrane is a row of adhering cilia forming a sheet (Fig. 2-36Л and B). A membranelle is derived from two or three short rows of cilia, all of which adhere to form a more or less triangular or fan-shaped plate and typically occur in a series (Figs. 2-27, 2-32D, and 2-36B). Although there is no actual fusion of adjacent cilia in these com¬pound organelles, their kinetosomes and bases are sufficiently close together to produce some sort of structural coupling that causes all of the cilia of a membranelle to beat together.

The free-swimming holozoic species display several types of feeding habits. Some are raptorial, and attack and devour rotifers, gastrotrichs, pro-tozoans, and other ciliates. A smaller number, including Nassula, are herbivorous on algae and diatoms. Many have become specialized for suspension feeding. The oral apparatus of raptorial ciliates is typically limited to the cytostome and cytopharynx.

An interesting group of raptorial ciliates is the aberrant subclass Suctoria, formerly considered a separate class. Free-living suctorians are all sessile and are attached to the substratum directly or by means of a stalk (Fig. 2-37). Cilia are present only in the immature stages. The body bears tentacles, which may be knobbed at the tip or shaped like long spines (Fig. 2-41B). Each tentacle is supported by a cylinder of microtubules and carries special or-ganelles, called haptocysts (Fig. 2-37D). Suctorians feed on other ciliates, and when prey strikes the tentacles, the haptocysts are discharged into the prey body, anchoring it to the tentacles (Figs. 2-37D to F and 2-38). The contents of the prey are then sucked through the tubular tentacle into the suctorian, where they are collected into food vacuoles.

Typically characteristic of suspension feeders is the buccal cavity. Food for suspension feeders con¬sists of any small organic particles, dead or living, particularly bacteria that are suspended in water. Food is brought to the body and into the buccal cavity by the compound ciliary organelles. From the buccal cavity the food particles are driven through the cytostome and into the cytopharynx. When the particles reach the cytopharynx, they collect within a food vacuole.

The order Hymenostomatida—"membrane-mouthed"—contains some of the most primitive suspension feeders. Tetrahymena is a good exam¬ple of such a primitive type (Fig. 2-36B). The cy tostome is located a little behind the leading edge of the body. Just within the broad opening to the buccal cavity are four ciliary organelles—an undulating membrane on the right side of the cham-ber and three membranellcs on the left. The three membranelles constitute an adoral zone of mcm-branelles, which in many higher groups of ciliates is much more developed and extensive.

Figure 2-35 Oral areas of various ciliates. A, In rhabdophorine gymnostomes, such as Coleps, Prorodon, and Didi-nium. B, In a trichostome such as Colpoda, with a vestibule that is displaced from anterior end. C, In a tetrahymenine hymenostome, such as Tetrahymena. D, In a peniculine hymenostome, such as Paramecium. E, In a peritrich, such as Vorticella. F, In a hypotrich, such as Euplotes. (Modified after Corliss, J. O., 1961: The Ciliated Protozoa. Pergamon Press, N.Y.)

Figure 2-36 A, Pleuronema. (After Noland from Corliss.) B, Tetrahymena. (After Corliss, J. O., 1961: The Ciliated Protozoa. Pergamon Press, N.Y.) C, Scanning electron photomicrograph of Uronychia, a marine ciliate, showing the highly developed membranelles. (By Small, E. В., and Marszalek, D. S., 1969: Science, 163: 1064-1065. Copyright 1969, American Association for the Advancement of Science.) D, Buccal organelles of Paramecium. (After Yusa from Man-well.) £, Lacrymaria. (After Conn from Hyman.)

Figure 2-37 A, Four Didinium, raptorial ciliates, attacking one Paramecium. (After Mast from Dogiel.| B, Acineta, a suctorian. (After Calkins from Hyman.| C-F, Suctorian haptocysts and prey cap¬ture. Haptocyst isolated |C| and within tentacle tip (D). Attachment of tentacle to prey (£) and en-gulfmcnt through tentacle (PL (From Sleigh, M. A., 1973: The Bi¬ology of Protozoa, Edward Arnold Publishers, London, p. 64. Based on micrographs of Rudzinska, Bar D E F dele, and Grell.J

Figure 2-38 A colony of the suctonan Heliophrya feeding on Paramecium. Some individuals of Paramecium have just been captured. Others have been ingested to various degrees. (From Spoon et al., 1976: Observations on the behavior and feeding mechanisms of the suctorian Heliophrya erhardi preying on Paramecium. Trans. Am. Micros Soc 95-443-462.|

Reproduction of ciliates (conjugation).

Ciliates differ from almost all other organisms in possessing two distinct types of nuclei—a usually large macronucleus and one or more small micro-nuclei. The micronuclei arc small, rounded bodies and vary in number from l to as many as 20, de¬pending on the species. They are diploid, with little RNA. The micronucleus is a store of genetic material, is responsible for genetic exchange and nuclear reorganization, and also gives rise to the ma-cronuclei. The macronucleus is sometimes called the vegetative nucleus, since it is not critical in sexual reproduction. However, the macronucleus is essential for normal metabolism, for mitotic division, and for the control of cellular differentiation, and it is responsible for the genie control of the phenotype through protein synthesis.

Figure 2-40 Macronuclei (in gray) of various ciliates (micronuclei, in black). A, Euplotes. B, Vorticella. C, Par¬amecium. D, Stentor. (After Corliss, J. О., 1961: The Ciliated Protozoa. Pergamon Press, N.Y.)

One or more macronuclei are present, and they may assume a variety of shapes (Fig. 2-40). The large macronucleus of Paramecium is somewhat oval or bean shaped and is located just anterior to the middle of the body. In Stentor and Spirostomum the macronuclei are long and arranged like a string of beads. Not infrequently the macronucleus is in the form of a long rod bent in different config¬urations, such as а С in Euplotes or a horseshoe in Vorticella. The macronucleus is highly polyploid, the chromosomes having undergone repeated du¬plication following the micronuclear origin of the macronucleus. The macronuclei include numerous nucleoli with RNA.

Asexual reproduction

Asexual reproduction is always by means of binary fission, which is typically transverse. More accu¬rately, fission is described as being homothcto-genic, with the division plane cutting across the ki¬netics—the longitudinal rows of cilia or basal bodies (Fig. 2-41Л). This is in contrast to the symmetrogenic fission of flagellates, in which the plane of division (longitudinal) cuts between the rows of basal granules. Mitotic spindles arc formed only in the division of the micronuclei. Division of the macronuclei is usually accomplished by constriction. When a number of macronuclei arc present, they may first combine as a single body before dividing. The same is true of some forms with beaded or elongated macronuclei.

Modified fission in the form of budding occurs in some ciliate groups, notably the Suctoria. In most members of this subclass the parent body buds off a varying number of daughter cells from the outer surface (Fig. 2-41B); or there is an internal cavity or brood chamber, and the buds form internally from the chamber wall. In contrast to the sessile adults, which lack cilia, the daughter cells, or buds, are provided with several circlets of cilia and are free swimming (Fig. 2-41C). Following a few hours of free existence, the "larva" attaches and assumes the characteristics of the sessile adults.

Although there are no centrioles, the kincto-somes of many ciliates, like the basal granules of flagellates, divide at the time of fission. Furthermore, the kinetosomes play a primary role in the re-formation of organelles. It has been found that all of the organelles can be re-formed providing the cell contains a piece of macronucleus and some kinetosomes. In the more primitive ciliates, in which the cilia have a general distribution over the body surface, the kinetosomes have equal potentials in the re-formation of organelles.

Figure 2-41 A, Homothctogcnic type of fission, in which the plane of division cuts across the kinetics. [After Corliss.) B, Suctorian Ephelota with external buds. (After Noble from Hyman.) C, Detached bud of Dendrocometes. (After Pestel from Hyman.) D, Conjugation in Vorticella. Note the small nonsessilc microconjugant. (After Kent from Hyman.)

However, in the specialized ciliates there is a corresponding specialization of the kinetosomes; only certain ones are involved in the re-formation of new cellular structures during fission. For example, in hypo-trichs such as Euplotes, all of the organelles are re-sorbed at the time of fission, and certain of the kinetosomes on the ventral side of the animal divide to form a special group that is organized in a definite field or pattern. These special "germinal" kinetosomes then migrate to different parts of the body, where they form all of the surface organelles—cirri, peristome, cytopharynx, and other structures.

Sexual reproduction

An exchange of nuclear material by conjugation is involved in sexual reproduction. By apparently random contact in the course of swimming, two sexually compatible members of a particular species adhere in the oral or buccal region of the body. Following the initial attachment, there is degener-ation of trichocysts and cilia (but not kinetosomes) and a fusion of protoplasm in the region of contact. Two such fused ciliates are called conjugants; at-tachment lasts for several hours. During this period a reorganization and exchange of nuclear material occurs (Fig. 2-42Л to F). Only the micron-uclci are involved in conjugation; the macronucleus breaks up and disappears either in the course of or following micronuclear exchange.

The steps leading to the exchange of micronu¬clear material between the two conjugants are fairly constant in all species. After two meiotic divisions of the micronuclei, all but one of them de¬generate. This one then divides, producing two gametic micronuclei that are genetically identical. One is stationary; the other will migrate into the opposite conjugant. The migrating, or "male," nu¬cleus in each conjugant moves through the region of fused protoplasm into the opposite member of the conjugating pair. There the "male" and "female" nuclei fuse with one another to form a "zygote" nucleus, or synkaryon.

Shortly after nuclear fusion the two animals separate; each is now called an exconjugant. After separation, there follow in each exconjugant a varying number of nuclear divisions, leading to the rcconstitution of the normal nuclear condition characteristic of the species. This reconstitution usually, but not always, involves a certain number of cytosomal divisions. For example, in some forms where there is but a single macronucleus and a single micronucleus in the adult, the synkaryon divides once. One of the daughter nuclei forms a micronucleus; the other forms the macronucleus Thus, the normal nuclear condition is restored without any cytosomal divisions.

Figure 2-42 Sexual reproduction in Paramecium caudatum. A to F, Conjugation. В to D, Micronuclei undergo three divisions, the first two of which are meiotic. E, "Male" micronuclei are exchanged. F, They fuse with the stationary micronucleus of the opposite conjugant. G, Exconjugant with macronucleus and synkaryon micronucleus; other mi¬cronuclei have been resorbed. (Modified after Calkins from Wichterman.)

However, in Paramecium caudatum, which also possesses a single nucleus of each type, the synkaryon divides three times, producing eight nuclei. Four become micronuclei and four become macronuclei. The animal now undergoes two cy¬tosomal divisions, during the course of which each of the four resulting daughter cells receives one macronucleus and one micronucleus. In those species that have numerous nuclei of both types, there is no cytosomal division; the synkaryon merely divides a sufficient number of times to produce the requisite number of macronuclei and micronuclei.

In some of the more specialized ciliates, the conjugants are a little smaller than nonconjugating individuals, or the two members of a conjugating pair are of strikingly different sizes. Such dioecious macro - and microconjugants occur in Vorticella (Fig. 2-41D) and represent an adaptation for conjugation in sessile species. The macroconjugant, or "female," remains attached, while the small bell of the microconjugant, or "male," breaks free from its stalk and swims about. On contact with an attached macroconjugant the two bells adhere. A synkaryon forms only in the macroconjugant from one gametic nucleus contributed by each conjugant. However, there is no separation after conjugation, and the little "male" conjugant degenerates. In the Suctoria conjugation takes place between two attached individuals that happen to be located side by side.