Comprehensive Study Notes on Invertebrate Zoology I: Kingdom Protista and Sub-Kingdom Protozoa

ZOO 211: Invertebrate Zoology I - Course Overview

Course Code: ZOO 211 (Invertebrate Zoology I) / ZOO 212.
Lecturer: Dr D. A. Oyebamiji.

Kingdom Protista Overview

General Characteristics:
* Cellularity: Protists are unicellular organisms.
* Cell Type: They are eukaryotic.
Nutrition and Structural Divisions:
* Algae: Classified as autotrophic (creating their own food) and possess a cell wall.
* Protozoa: Classified as heterotrophic (feeding on organic matter) and lack a cell wall.
Motility: While many species are nonmotile, the majority of protists can move via various mechanisms.
Osmoregulation: Many protists possess contractile vacuoles, which function to remove excessive amounts of water from the cytoplasm.

Sub-Kingdom Protozoa: General Characteristics and Taxonomy

Evolutionary Significance: Protozoans are considered the ancestors of the animal kingdom.
Basic Definition: They are unicellular, and for the most part, microscopic organisms.
Cytoplasmic Organization: As eukaryotic cells, they contain a distinct nucleus as well as complex organelles including the endoplasmic reticulum, golgi apparatus, and mitochondria.
Species Diversity (Levine, 1985):
    * Approximately $65,000$ species of protozoa have been named to date.
    * The majority of these species are free-living.
    * Nearly $7,000$ protozoan species are parasitic, infecting both vertebrate and invertebrate animals.
Human Relevance: Important protozoa include those that infect domesticated animals and birds, as well as zoonotic species (those transmissible from animals to humans).
Etymology: The name is derived from "Proto" (meaning first) and "zoan" (meaning animals), signifying the first animal life to appear in the universe.
Complexity Hierarchy:
* Flagellates: Considered the most primitive form of animal life.
* Ciliates: Regarded as the most highly organized form of protozoa.

Nuclear Structure and Differentiation

General Nuclear traits: Protozoa possess well-defined nuclei enclosed in a nuclear membrane. Though typically unicellular, some species may contain multiple nuclei.
Classification of Nuclei:
    * Vesicular Nuclei:
        * Characterized by a large amount of nucleoplasm and a small amount of chromatin material.
        * Consists of a nuclear membrane binding the nucleoplasm.
        * Endosome (Karyosome): Located centrally. It is devoid of DNA.
        * Nucleolus: Possesses DNA.
        * Chromatin Material: This is Feulgen positive for DNA; it frequently occurs on the inner surface of the nuclear membrane and can appear as strands radiating from said inner surface.
        * Occurrence: Primarily seen in sub-phyla Mastigophora (e.g., Trypanosoma) and Sarcodina (e.g., Entamoeba).
        * Sporozoa Variant: In the sub-phylum Sporozoa, the nucleus possesses a DNA-positive nucleolus (functioning as an intranuclear body) instead of an endosome, with chromatin distributed throughout the nucleus.
    * Compact Nuclei:
        * Characterized by a large amount of chromatin material and a small amount of nucleoplasm.
        * Chromatin is scattered throughout the nucleus.
        * Morphology: Shapes vary and include spherical, oval, cylindrical, sausage-shaped, or horse-shoe-shaped.
Nuclear Distribution Variations:
    * Similar Nuclei: Two identical nuclei are found in Giardia and Hexamita.
    * Dissimilar Nuclei: Seen in Balantidium (Ciliophora).
        * Macro-nucleus: Governs cytoplasmic functions.
        * Micro-nucleus: Functions in sexual reproduction.
    * Functional Differentiation (Trypanosomes):
        * Trophonucleus: Governs the general life functions of the cell.
        * Kinetonucleus: A chromatinic body that regulates the movement of the locomotor organelles.

Cytoplasmic Organization and Organelles

Structural Differentiation: The extra-nuclear part of the cell is the cytoplasm, often divided by a pellicle into two zones:
* Ectoplasm: The outer layer located immediately under the parasite wall; typically homogeneous and hyaline.
* Endoplasm: The inner layer immediately surrounding the nucleus; granular and containing granules, vacuoles, and pigments.
Intracellular Organelles: Includes Golgi apparatus, mitochondria, ribosomes, lysosomes, and other sub-cellular organellae.
Vacuoles:
* Contractile Vacuole: Mainly found in free-living aquatic protozoa. These appear as small, rounded floating vacuoles that coalesce to form a functional unit. Their primary function is osmoregulation and maintaining water balance.
* Food Vacuoles: The site of food particle processing and digestion. They are the seats of enzymatic activity and occur in species such as B. coli and E. histolytica.

Nutrition and Metabolism

Nutritional Types:
    * Holophytic: Synthesis of carbohydrates via chlorophyll carried in chromatophores in the presence of light. Seen in free-living phytoflagellates.
    * Holozoic (Phagotrophic): Utilization of preformed food material. Solid particles are encircled by pseudopodia or taken through a permanent opening called a cytostome. Food is enclosed in food vacuoles where digestive enzymes are secreted.
    * Saprozoic: Absorption of organic food material dissolved in solution via diffusion through the general body surface.
Respiration:
    * Metabolic waste is eliminated via diffusion or contractile vacuoles.
    * Aerobic Respiration: Occurs when oxygen is available. Examples: Malaria parasites, Leishmania, Eimerian oocysts, and some trypanosomes.
    * Anaerobic Respiration: Occurs in the absence of oxygen. Examples: Trichomonas species and intestinal amoebae.

Locomotor Organelles and Movement

Gliding Movement: Observed in Toxoplasma, Sarcocystis, and forms lacking flagella or cilia.
Pseudopodia (Characteristic of Sarcodina): Temporary organelles formed and retracted as needed. They are divided into four types:
    * Lobopoda: Found in Amoeba.
    * Filopodia: Found in Euglypha.
    * Rhizopoda/Reticulopodia: Found in Elphidium.
    * Axopodia: Found in Actinospherium.
Flagella (Characteristic of Mastigophora):
    * Whip-like filamentous protoplasmic extensions arising from basal granules (blepharoplasts).
    * Structure: Composed of a central axial filament called the axoneme, surrounded by a contractile cytoplasmic sheath.
    * Axoneme Configuration: Consists of two central filaments surrounded by $9$ double peripheral filaments (known as the $9+2$ arrangement).
    * Function: Creates currents to aid movement. Some species have flagella attached to the body by an undulating membrane.
Cilia (Characteristic of Ciliates/B. coli):
* Fine, eyelash-like organelles originating from basal granules in the pellicle/ectoplasm.
* Function: Aids in ingestion of food and rotational movement. Movement is described as motor-like and very fast.

Life Cycles and Reproduction

Trophozoite: The vegetative form of parasitic protozoa that feeds, grows, and divides.
General Life Cycle: Often involves a combination of sexual and asexual phases. The cycle can be direct or indirect.
Asexual Reproduction:
    * Binary Fission: The cell divides into two equal daughter cells. The DNA divides first. Example: Amoeba.
    * Sporulation (Multiple Fission): A cell enlarges to form a sporangium. The nucleus divides many times, and daughter nuclei are surrounded by protoplasm bits to form spores. These are covered by a thick wall (cyst). On maturation, the sporangium bursts. Example: Apicomplexa.
    * Budding: A new individual develops from a generative anatomical point on the parent. Buds can be general or restricted to specialized areas. Example: Toxoplasma.
Sexual Reproduction: Involves chromosome reduction (producing haploid gametes).
    * Conjugation: Two individuals fuse temporarily along part of their length for nuclear material exchange, which revitalizes the organism. Example: Balantidium coli.
    * Syngamy: Complete fusion of individuals (gametes) to form a zygote.
        * Isogamy: Fusion of similar gametes (isogametes).
        * Anisogamy: Fusion of dissimilar gametes (anisogametes).
            * Microgamete: Smaller, male gamete.
            * Macrogamete: Larger, female gamete.
        * Macrogamy: Syngamy between fully developed vegetative cells.
        * Microgamy: Syngamy between gametes much smaller than vegetative cells.
    * Gametogony: The process of gamete formation from special cells called gamonts (microgamonts/macrogamonts). This follows an asexual process by sporogony.

Classification of Protozoa

Basis for Classification: Mode of locomotion, and the number and type of nuclei.
Historical Context: Historically based on Levine et al. (1980).
Modern System: Recognizes $7$ phyla based on structure, life history, and physiology: Sarcomastigophora, Apicomplexa, Microspora, Labyrinthomorpha, Ascetospora, Myxozoa, and Ciliophora.
Key Phyla and Examples:
    * Euglenozoa: Euglena viridis, Trypanosoma.
    * Apicomplexa: Plasmodium.
    * Ciliophora: Vorticella, Paramecium.
    * Retortamonda: Giardia.
    * Opalinata: Opalina.
    * Rhizopoda: Amoeba proteus, Entamoeba histolytica, Endolimax nana.
Super Phylum Mastigophora (Zooflagellates):
    * Sub-groups:
        * Zoomastigophora: More than $2$ flagella, no chromoplasts. Example: Trypanosoma.
        * Phytomastigophora: Not more than $2$ flagella, chromoplasts present. Example: Euglena.

Specific Study: Subphylum Euglenida (Class Euglenoidea)

Example Organism: Euglena.
Morphology: Unicellular, spindle-shaped, and bi-flagellated.
Photosynthesis: Contains chloroplasts with pyrenoids used to synthesize paramylon (a starch-like energy storage).
Flagellar System: Two flagella rooted in basal bodies in a reservoir. One flagellum is short, the other is long and visible via light microscopy.
Stigma (Red Eye Spot): Composed of carotenoid pigment granules. It filters sunlight for the paraflagellar body located at the base of the flagellum; it is not itself photosynthetic.
Pellicle: Lacks a cellulose cell wall; instead, has a protein layer supported by microtubule strips spiraling around the cell, providing flexibility and contractility.
Locomotion: Known as euglenoid movement.
Habitat: Freshwater environments.
Osmoregulation: A contractile vacuole is located to one side of the reservoir into which it discharges.
Reproduction: Asexual reproduction via longitudinal binary fission. Cleavage begins at the anterior (front) end and moves toward the posterior (back). Sexual conjugation is rare.

Specific Study: Subphylum Kinetoplasta

General Characteristics: Small, asexual, solitary protists.
Flagellar Structure: One or two flagella arising from an anterior invagination, typically containing a paraxonemal rod.
Mitochondrial Structure: A single, very long mitochondrion extending as a network of branched threads or a loop. It contains discoidal cristae.
The Kinetoplast: A unique, conspicuous organelle located near the flagella kinetosome. It is a modified part of the mitochondrion and contains DNA.
Taxonomic Importance: The size, shape, and position of the kinetoplast are crucial for the taxonomy of trypanosomes and identifying different life cycle stages.