INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

FLAGELLATES

Flagellates inhabit both freshwater and marine environments; some species have adapted to a parasitic or mutualistic existence within other organisms (symbionts). Different groups of flagellates likely possess diverse (polyphyletic) origins, being united solely by the presence of flagella during their vegetative stages, which may number from one to several hundred. Some species may also persistently or temporarily form pseudopodia.

The size of flagellates ranges from 1-2 µm to several millimeters, and their body shape can be oval, spindle-shaped, or bizarre.

A pellicle forms beneath Cell/30.html">The Plasma Membrane, while certain groups of flagellates feature an outer hemicellulose, chitinous, or gelatinous envelope, or even a hardened shell.

Inside The Cell, a Cytoskeleton develops from microtubules and microfilaments connected to the bases of the flagella.

The flagellar apparatus consists of the flagellum itself, a transition zone, a kinetosome (basal body), and a rootlet system (Fig. 2).

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Fig. 2. Generalized diagram of the flagellar apparatus Structure (after Karpov):

1 - flagellar scales; 2 - mastigonemes; 3 - transition helix; 4 - transverse plate; 5 - transition fibrils; 6 - non-flagellated kinetosome; 7 - fibrillar bridge between kinetosomes; 8 - ribbed rootlet with secondary microtubules; 9 - fibrillar rootlet (rhizoplast); 10 - microtubular rootlet; 11 - flagellated kinetosome; 12 - Free part of the flagellum

The flagellum is externally covered by the Plasmalemma and encloses an Axial Skeleton known as the axoneme, which consists of 11 microtubules: nine peripheral doublets forming a cylinder, and two single microtubules running along the central axis (Fig. 3). In some flagellates, a paraxial rod composed of microfilaments runs parallel to the axoneme. The transition zone marks the point where the flagellum emerges from the cell; it contains a transverse plate and other anchoring structures. The portion of the flagellum embedded in the Cytoplasm is the basal body, or kinetosome, which appears as a cylinder composed of nine peripheral triplets of microtubules. In most species, the free region of the flagellum bears specialized surface structures such as scales or hairs (mastigonemes). The majority of flagellates possess two flagella and, correspondingly, two kinetosomes. Uniflagellated species also typically feature two kinetosomes, one of which is non-flagellated. The rootlet System of the kinetosome may consist of microfilaments or microtubules, functioning as part of the cytoskeleton that not only anchors the flagellum within the cell but also determines its shape and Symmetry. The ULTRASTRUCTURE OF THE flagellar apparatus, particularly its basal region, holds significant diagnostic value and is widely utilized in Taxonomy.

Fig. 3. Flagellar apparatus (after Hausmann, modified): a - diversity of its types;

b - diversity of external Morphology; c - cross-section of a flagellum

Flagellates exhibit diverse nutritional modes, including autotrophy, heterotrophy, and mixotrophy.

Most flagellates reproduce asexually, with nuclear division occurring mitotically and cytoplasmic division proceeding in a meridional plane. Sexual reproduction may take the form of isogamy or anisogamy. The life cycle is accompanied by either gametic or zygotic chromosomal reduction.

Five phyla from this group will be examined: Choanoflagellates (Choanomonada), Euglenoids (Euglenozoa), Polymastigotes (Polymastigota), Opalinids (Opalinata), and Labyrinthomorphs (Labyrinthomorpha).