LECTURE NOTES IN ZOOLOGY

Module 1. ZOOLOGY AS A SCIENCE. SUBKINGDOM PROTOZOA

Content Module 1.2. Subkingdom Protozoa. General Characteristics

Topic 1.2.5. Phylum Myxozoa

General characteristics of the phylum. Origin, evolution, paleontological records, modern diversity, geographical distribution, and their role in nature and human life. Overview of major taxonomic groups essential for human life and the functioning of natural ecosystems. Class Myxosporea.

Exclusively parasitic lifestyle. Multinucleated vegetative stages—amoeboid plasmodia, nuclear differentiation into vegetative (somatic) and generative nuclei; formation of multicellular spores. Two classes: Myxosporea and Actinosporea, which are parasites of oligochaete worms and sipunculans.

Class Myxosporea. Parasites of Fishes and certain cold-blooded vertebrates. Intracellular parasitism (fish roe). Over 1,000 species of myxosporeans. Complex Morphology. A shell consisting of two or more Valves, polar or stinging capsules, and an amoeboid germ (sporoplasm) with two haploid nuclei. Spores are fusiform, lenticular, or cigar-shaped, sometimes with three to six valves, and may be star-shaped. Rounded, pyriform, or oval structures. A vacuole is located within the capsule cavity, containing a spirally coiled polar filament with one free end. Spores are highly resistant. Myxosoma

cerebralis (affecting trout and other salmonids) can remain viable on the bottom of a dried-up reservoir for up to 12 years—resistant even to boiling in sulfuric acid. Autogamy and restoration of diploidy. Multiple mitotic Divisions of the Nucleus result in a multinucleated plasmodium containing vegetative (somatic) and generative nuclei. The plasmodium moves using lobopodia; Nutrition occurs via pinocytosis or microvilli, utilizing Enzymes and parietal Digestion. Phagocytosis of host Cells. External and internal budding. Pansporoblast—mitotic division into eight diploid nuclei, two of which cease division and transform into somatic nuclei that regulate vegetative Functions. The remaining six nuclei undergo Meiosis (single-step meiosis) yielding 12 haploid nuclei (grouped into sets of 6 to form sporoblasts lacking somatic nuclei). Valve formation takes place. The nuclei are expelled into the spore cavity and degenerate. Two cells form a germ with two haploid nuclei. In some coelozoic species possessing small plasmodia with one or two vegetative nuclei and one or two generative nuclei, the generative nuclei directly transform into sporoblasts giving rise to one or two spores; whereas in large histozoic species, hundreds or even thousands of spores are produced.