INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

ALVEOLATES

PHYLUM CILIATED PROTOZOA, OR CILIATES (CILIOPHORA)

Predominantly free-living marine and freshwater Protozoans; soil dwellers, commensals, and parasites are less common. The phylum comprises approximately 8,000 species. Ciliates are relatively large, ranging from 50 to 300 µm, with some species reaching up to 3 mm in length.

Ciliates differ from other Protozoa by the presence of locomotory Organelles (cilia), nuclear dualism, a complex specialized cortex unique to ciliates, and a specific form of sexual process known as conjugation.

Structure. The body shape of ciliates is extremely diverse (Fig. 31). Swimming species have a streamlined shape: spherical, oval, or rod-like; crawling species are flattened; sessile forms are bell- or trumpet-shaped; they can also have branched bodies, while parasitic species may exhibit various appendages. Externally, The Cell is covered by a cortex (Fig. 32). The cortex contains various structures that collectively maintain the cell's fixed shape. The outer layer of the cortex, the pellicle, is formed by the Plasmalemma and underlying flattened vesicles (alveoli) interconnected in a mosaic pattern. Beneath the pellicle lie the basal bodies (kinetosomes) of the cilia, along with structures associated with each kinetosome: a striated filament directed toward the anterior end of the cell, and bundles of microtubules—one running along the body to its posterior end and the other transversely across the cell. Together, these microfilaments and microtubules form a unique Cytoskeleton found exclusively in ciliates.

Class="center">

Fig. 31. Ciliates (a, b, d–f from Dogiel; c from Poljansky): a - Holophrya; b - Tetrahymena pyriformis; c - Vorticella sp.;

d - Stentor polymorphus; e - Spirostomum ambiguum; f - Stylonychia mytilus; g - Bursaria truncatella:

1 - cytostome; 2 - membranelles; 3 - peristome

Fig. 32. ULTRASTRUCTURE OF THE Ciliate cortex (from Dogiel):

1 - pellicle; 2 - cilium; 3 - trichocyst; 4 - longitudinal microtubules;

5 — transverse microtubules; 6 - kinetosome; 7 - microfilament

Perpendicular to the surface within the ectoplasm are defensive organelles called trichocysts, which "discharge" when irritated, transforming into long threads. They are believed to contain toxic substances that kill enemies or prey. Predatory ciliates possess toxocysts. These appear as capsules with a tubular wall enclosing an inner tube. During hunting, this tube everts outward like a glove finger or is telescopically extended from the capsule, piercing the body of the prey and injecting venom. This paralyzes or kills the prey (other protozoa, rotifers).

The locomotory organelles—cilia—are fundamentally similar to flagella and share the same ultrastructure. Cilia may evenly cover the entire body surface, but they are frequently concentrated in specific areas or grouped to form cirri, membranes, or membranelles. Cirri are tufts or bundles of cilia; membranes are densely packed rows of cilia forming plates; and membranelles are dense rows of cilia (Fig. 31). Cilia beat in a coordinated, oar-like motion. Using cilia, ciliates can swim, crawl along a substrate, or "jump" through the Water Column by pushing off with cirri, as well as sweep food toward the Mouth using membranelles and membranes. The ciliary apparatus associated with feeding is particularly complex in its structure.

For many ciliates, The Development of attachment structures is characteristic at certain stages of their life cycle. For example, in ciliates of the genus Vorticella, an attachment stalk is formed by a specialized organelle called the scopula, which secretes this stalk (Fig. 31, c). In fish parasites such as ciliates of the genus Trichodina, There is a specialized "adhesive disc" formed by protein structures.

The nuclear apparatus of ciliates consists of Two Types of nuclei (nuclear dualism): most commonly, a single large macronucleus and several small micronuclei. The macronucleus is rich in DNA and contains a large amount of nuclear material; it is the site of RNA Synthesis and regulates cellular METABOLISM. Micronuclei are diploid and participate exclusively in the sexual process.

Most ciliates possess a cell mouth (cytostome) and a complex system of digestive organelles (Fig. 30). In some species, the cytostome is located at the anterior end of the cell, usually laterally, at the base of a specialized depression known as the peristome (Fig. 31) and is surrounded by ordinary cilia (Holophrya, Prorodon). In some species, the peristome leads into a narrow channel—the cellular Pharynx, or cytopharynx—which terminates in the endoplasm (Fig. 33).

Fig. 33. Diagram of The structure of Paramecium caudatum:

1 - anterior contractile vacuole; 2 - peristome; 3 - micronucleus;

4 - macronucleus; 5 - cytopharynx; 6 - cytostome; 7 - cilia; 8 - cytoproct; 9 - trichocyst;

10 - posterior contractile vacuole; 11 - food vacuole

The cilia of the peristome and cytopharynx form membranes and membranelles that drive food particles into the mouth. In carnivorous ciliates (such as Didinium), instead of cilia, a specialized apparatus Functions at the cytostome to ingest prey—the toxicyst or rod apparatus (Fig. 34). It consists of hundreds of microtubules forming a tube around the mouth, which provides structural support when engulfing large prey (such as other protozoans, rotifers, etc.). At the bottom of the cytostome, food particles are enclosed by a membrane to form a food vacuole. This vacuole circulates within the ciliate's endoplasm, where the food is gradually digested, while indigestible residues are eliminated through a specialized structure in the pellicle known as the cytopyge (anal pore).

Fig. 34. Didinium nasutum feeding on a paramecium (a) and their rod apparatus (b) (adapted from Dogel)

Suctorian ciliates (Suctoria) lack a cytostome; instead, they capture and suck out their prey using suctorial tentacles containing microtubular tubes (Fig. 35). Parasitic ciliates of the order Astomatida also lack a cytostome.

Fig. 35. Suctorian ciliates:

a - Sphaerophryu magna simultaneously feeding on five ciliates; b - Dendrocometes paradoxus (adapted from Dogel)

Freshwater and frequently marine ciliates possess contractile vacuoles, which serve as osmoregulatory and excretory organelles. In many cases, their structure is elaborated by radially arranged afferent canals featuring dilations (ampullae) that collect excess fluid. Fluid moves from the ampullae into the contractile vacuole and is then expelled to the exterior. This complex is held in a fixed position by microtubular ribbons that originate from the walls of the excretory pore and extend along the ampullae and canals (Fig. 36). Most ciliates possess a cell mouth (cytostome) and a complex system of digestive organelles (Fig. 30). In some species, the cytostome is located at the anterior end of the cell, typically laterally, at the bottom of a specialized depression known as the peristome (Fig. 31), and is surrounded by ordinary cilia (Holophrya, Prorodon). In other species, the peristome leads into a narrow channel—the cytopharynx or cellular pharynx—which terminates in the endoplasm (Fig. 33).

Fig. 36. Diagram of the contractile vacuole structure in Paramecium (from Hausmann): 1 - pore; 2 - microtubular ribbons;

3 - ampulla; 4 - contractile vacuole; 5 - afferent canals

Reproduction. Asexual reproduction occurs via binary fission or budding. During this process, mitosis takes place in both the micronucleus and macronucleus, although in the latter it occurs in a modified, amitotic-like manner. Some organelles are distributed between the daughter Cells, while missing ones are regenerated. In budding, a smaller daughter cell buds off from the larger mother cell (as seen in sessile ciliates and suctorians).

The sexual process—conjugation—is unique to ciliates. Unlike most protozoans, it is not accompanied by The formation of Gametes; instead, it involves the temporary pairing of two individuals and an exchange of nuclear material between them (Fig. 37). Let us examine this process using the slipper animalcule (Paramecium), which possesses one macronucleus and one micronucleus. Two ciliates join along their lateral surfaces, and their peristomes become connected by a cytoplasmic bridge. In each individual, the macronucleus disintegrates, while the diploid micronucleus undergoes Meiosis to produce four haploid nuclei. Three of these disintegrate, and the fourth undergoes mitosis to form a stationary (female) and a migratory (male) nucleus. The partners exchange migratory nuclei, with each fusing with the stationary Nucleus of the opposite cell. This forms a diploid synkaryon. Afterward, the cells separate. Subsequently, through successive mitotic divisions, eight nuclei develop from the synkaryon in each exconjugant. Of these, three degenerate, one becomes the micronucleus, and four become macronuclear anlagen. The micronucleus divides mitotically, accompanied by Cell Division into two, each possessing one micronucleus and two macronuclear anlagen; this process repeats. Ultimately, four cells are produced, each containing one micronucleus and a macronuclear anlage with a diploid set of Chromosomes. Within the macronuclear anlage, the majority of the chromosomes dissolve, and the remaining ones break down into small fragments, most of which also degenerate. This is followed by intensive DNA Synthesis, leading to a multi-hundred- or multi-thousand-fold Amplification of the surviving fragments. Therefore, although the macronucleus contains an enormous amount of DNA (compared to the micronucleus), it cannot be considered polyploid, because it retains only a minor fraction (1.6%) of the complete genome, albeit heavily amplified.

Fig. 37. Successive stages of nuclear apparatus reorganization (a) and its regeneration (b) in Paramecium caudatum during conjugation:

1 - macronucleus; 2 - micronucleus; 3 - synkaryon; R! - reduction division

If a ciliate fails to find a partner for conjugation, it may undergo a sexual process known as autogamy, wherein its own male and female nuclei fuse within a single cell. The Biological Significance of conjugation lies in the exchange of hereditary material between individuals, which drives combinatorial variation. Diversifying the Gene pool of a ciliate population inhabiting a particular environment enhances the organisms' ability to adapt to unfavorable environmental shifts. Another crucial aspect is the regeneration of the macronucleus: successive divisions deplete its DNA content, which would otherwise lead to a decline in GROWTH AND DEVELOPMENT. The life cycle of ciliates involves gametic chromosomal reduction. The phylum Ciliophora is divided into three classes.