Lecture 6 - Nematodes

Phylogenetic Tree and Animal Classification

  • The phylogenetic tree in this material is based on the works of Nielsen et al. [23] and Howard et al. [24].

Major Taxonomic Groups

  • Xenacoelomorpha

  • Bilateria

    • Nephrozoa

    • Protostomia

    • Originated around 610 million years ago (mya)

    • Ecdysozoa

      • Includes:

      • Loricifera

      • Priapulida

      • Scalidophora

      • Cryptovermes

      • Spiralia

      • Includes Kimberella

      • Deuterostomia

      • Includes:

        • Kinorhyncha

        • Nematoda

        • Nematoida

        • Nematomorpha

        • Tardigrada

        • Panarthropoda

          • Onychophora

          • Antennopoda

          • Arthropoda

Ecdysozoa

  • History: Clade proposed in 1992.

  • One of the two major protostome groups; the other being Spiralia.

  • Defining Feature:

    • Presence of a cuticle consisting of three or more layers:

    • Exocuticle: Proteinaceous layer.

    • Endocuticle: Collagen or chitin-based layer.

    • Chitin is present in all ecdysozoans at some stage of their life cycle.u

  • The cuticle undergoes molting known as ecdysis, which is under hormonal control, specifically regulated by the hormone ecdysone.

  • Characteristics:

    • Lack of locomotory or external epithelial cilia.

Ecdysozoan Characteristics

  • Reproduction: Most ecdysozoans produce amoeboid sperm.

  • Cleavage: No spiral cleavage is observed.

  • Systems:

    • Respiratory and Circulatory Systems: Only found in Onychophorans and Arthropods.

  • Biodiversity: Comprises 8 phyla and accounts for 83% of known animal diversity.

Eumetozoa Classification

  • Parazoa: Includes Porifera and Placozoa.

  • Mesozoa: Includes taxa such as Orthonectida.

  • Eumetozoans:

    • Radiata:

    • Includes Ctenophora and Cnidaria.

    • Bilateria:

    • Protostomia:

      • Ecdysozoa:

      • Cycloneuralia:

        • Includes Scalidophora (Kinorhynchs, Loricifera, Priapulids).

      • Nematoida: Includes Nematoda and Nematomorpha.

      • Panarthropoda:

        • Comprises Arthropods, Tardigrades, Onychophorans, and Lopopods.

      • Lophotrochozoa:

      • Spiralia:

        • Platyzoa: Includes Platyhelminthes and gastrotrichs.

        • Gnathifera: Includes Rotifera and Acanthocephala.

        • Trochozoa: Includes Mollusca, Annelida, Nemertea, and Sipunculida.

        • Lophophorata: Includes Brachiopoda, Bryozoa, Entoprocta, and Phoronida.

    • Deuterostomia:

      • Ambulacraria:

      • Xenacoelamorpha: Includes Acoela and other taxa.

      • Echinodermata and Hemichordata.

      • Chordata: Includes chordates, tunicates, and cephalochordates.

    • Disputed Group: Chaetognatha.

Nematoda (Roundworms)

  • Classification: Known as Pseudocoelomates.

  • Body Structure:

    • Covered by an elastic, protective cuticle.

  • Abundance: Highly abundant but primarily small-sized species.

  • Habitat/Behavior: Many species are either free-living or parasitic.

    • Example: Root-feeding nematodes.

    • Illustrative examples include:

    • Uninfected Ant

    • Ant Infected by Nematodes

Physiology of Nematoda

  • Gas Exchange: Occurs across the body wall.

  • Nutrient Transport: Nutrients diffuse through the body from the gut.

  • Locomotion: Movement is achieved by contracting lateral muscles.

  • Model Organism: Caenorhabditis elegans is a significant model organism in research focusing on genetics, development, and aging.

Examples of Nematodes

  • Plant Hosts:

    • Affected Plants:

    • Cotton

    • Tomatoes

    • Soybeans

Different Forms of Nematodes

  • Life Stage Forms:

    • Rhabditiform

    • Filariform

    • Hookworm: Adult forms are typically found in the intestine.

    • Ascaris lumbricoides:

    • Pinworms: Notably encountered in human infections.

    • Trichinella spiralis: A parasitic nematode.

Health Reminder

  • Always remember to cook your pork! to avoid parasitic infections from Trichinella spiralis and similar pathogens.

Historical Context

  • Guinea worms (Dracunculus):

    • Reference to a 9th-century engraving showcasing the impact of this parasitic infection on humans.

Microfilariae and Other Species

  • Microfilariae:

    • Wuchereria bancrofti: An important health-related parasite.

  • Loa loa:

    • Documented through photomicrography by Dr. Mujib.