G2-PHYLUM-NEMATODA

Phylum Nematoda

Topic’s Focus

  • Distinction of different phyla in terms of classification, phylogeny, and organization.

  • Clear understanding of differentiation between Phylum Platyhelminthes and Phylum Nematoda.

  • Encouragement for students to share learnings and broaden their knowledge on the topic.

What is a Phylum?

  • Defined as a taxonomic rank above class and below kingdom.

  • Acceptable by the International Code of Nomenclature, applicable universally to all living organisms.

  • Third in the taxonomic hierarchy, following domain and kingdom.

Phylum Nematoda

  • Nematodes are either parasitic or free-living worms capable of shedding their external cuticle to grow.

History of Phylum Nematoda

  • Name "Nematoda" from Ancient Greek (nêma, nêmatos, "thread"), defined by Karl Rudolphi in 1808.

  • Originally included in Vermes by Linnaeus in 1758; promoted to phylum by Nathan Augustus Cobb.

Nematode Reproduction

  • Characterized by sexual and asexual methods, along with various developmental stages.

Sexual Reproduction in Nematodes

  • Common among many species, involving distinct male and female individuals.

  • Males possess specialized structures (spicules) for sperm transfer during copulation.

  • Male deposits sperm into the female's spermatheca.

  • In species like Caenorhabditis elegans, females can be hermaphrodites (self-fertilization).

  • Gonochoristic species like Caenorhabditis remanei require mating for fertilization.

Asexual Reproduction in Nematodes

  • Occurs in species like Helicotylenchus pseudorobustus, where reproduction happens without fertilization (parthenogenesis).

  • Results in entirely female populations capable of laying fertilized eggs.

Stages of Nematode Reproduction

  1. Mating

    • Male and female find each other; males use spicules for sperm transfer.

  2. Fertilization

    • Sperm fertilizes oocytes in females, occurring immediately or later.

  3. Egg Development

    • Fertilized eggs develop in a protective shell.

  4. Egg Laying

    • Mature eggs expelled through the female's vulva.

  5. Hatching

    • Juvenile larvae hatch from eggs after an incubation period.

  6. Juvenile Development

    • Juveniles undergo several molts before maturity (four stages L1 to L4).

  7. Maturity

    • Reached once juveniles are adults, capable of reproduction.

Important Characteristics of Nematoda

  • Bilaterally symmetrical and triploblastic body.

  • Cylindrical shape with tissue-level organization.

  • Possession of a pseudocoelom (body cavity).

  • Distinct alimentary canal with a mouth and anus.

  • Sexual dimorphism present.

  • Lacking circulatory and respiratory systems.

  • Free-living or parasitic; parasitic types can cause diseases in hosts.

  • Internal fertilization, sexual reproduction.

  • Periodic molting of cuticle.

  • Synctical epidermis with dorsal or ventral nerve cords.

  • Longitudinal body-wall muscles and amoeboid sperm cells.

  • Chemosensory organs (e.g., amphids) located on lips.

Digestive System of Nematodes

  • Complete digestive system: food enters through mouth and exits through anus.

Organs of the Digestive System

  1. Stomodeum (Foregut)

    • Initial part for ingestion.

  2. Midgut

    • Chemical breakdown and nutrient absorption; secretes digestive enzymes.

  3. Proctodeum (Hindgut)

    • Final waste processing area before expulsion through anus.

  • Lacks stomach; digestion occurs directly in the intestine.

  • Pseudocoelom acts as a hydrostatic skeleton for support and nutrient transport.

Feeding Strategies

  • Bacterial Feeders: Important for soil nutrient cycling.

  • Fungal Feeders: Role in controlling fungal populations.

  • Predatory: Feed on other small organisms (including nematodes).

  • Plant Parasitic: Feed on plant roots, creating agricultural damage.

  • Animal Parasitic: Infect animals with disease potential.

Representative Species of Nematodes

  1. Ascaris lumbricoides (Humans)

    • Human roundworm; effects include malnutrition and anemia.

    • Life cycle involves migrating from intestines to lungs and back.

  2. Ascaris suum (Swine/Pigs)

    • Adult worms can produce up to 2 million eggs daily; significant agricultural impact.

  3. Trichuris trichiura (Human)

    • Whipworm causing gastrointestinal issues; also associated with malnutrition.

  4. Trichuris vulpis (Canine/Dogs)

    • Inflicts gastrointestinal problems; eggs expelled in dog feces.

  5. Hookworms

    • Ancylostoma duodenale and Necator americanus; transmitted through skin contact with contaminated soil.

  6. Pinworms (Enterobius vermicularis)

    • Primarily affects children; intense itching due to egg laying near anal area.

  7. Root-Knot Nematodes

    • Cause "galls" on plant roots, widely impacts agriculture.

  8. Cyst Nematodes

    • Major crop pests; significant economic impact worldwide.

  9. Root Lesion Nematodes

    • Damage plant roots, causing nutrient deficiencies in plants.

  10. Sting Nematodes

  • Feeding leads to stunted plant growth; thrive in moist sandy soils.

Locomotion of Nematodes

  • The body wall made of cuticle and longitudinal muscle cells allows whip-like motion.

Classes of Phylum Nematoda

I. Class Rhabditea

  • Includes free-living and parasitic nematodes. Most are parasites. II. Class Enoplea

  • Comprised of ancestrally divergent cylindrical nematodes. III. Class Chromadorea

  • Characterized by three esophageal glands among other physical traits.