Biology 2

Sister Phyla: Explain why Nematoda and Arthropoda are sister phyla, focusing on their shared characteristic of ecdysis.
Human Impact of Nematoda: Discuss the impact of Nematoda on humans, both detrimental (parasitic diseases) and beneficial (research models, biocontrol agents).
Hygiene Hypothesis: Explain the Hygiene Hypothesis and analyze why it has not been widely accepted, including its criticisms and alternative explanations.
Subphyla of Arthropoda: Describe the 5 subphyla of Arthropoda, focusing on how their appendages have been modified into new structures for diverse functions.

Metazoa: Refers to animals as a whole, characterized by multicellularity and heterotrophy.
Eumetazoa: Defined by the presence of true specialized tissues and often, organs.
Bilateria:
- Characterized by bilateral symmetry and being triploblastic (three germ layers: ectoderm, mesoderm, endoderm).
- Includes Ecdysozoa and Lophotrochozoa, both major clades of protostomes.
Ecdysozoa:
- A superphylum of protostomes that grow by molting (ecdysis) an external cuticle.
- Contains both pseudocoelomates (like Nematoda) and coelomates (like Arthropoda).
Protostomia:
- Development where the blastopore forms the mouth first.
- Includes Arthropoda (arthropods) and Nematoda (roundworms), among others.
Deuterostomia:
- Development where the blastopore forms the anus first.
- Comprises groups like Chordata and Echinodermata.

Animals in the Ecdysozoa group possess an external cuticle that serves as an exoskeleton, providing protection, structural support, preventing desiccation, and offering points for muscle attachment.
The process of ecdysis refers to the periodic molting of this rigid outer structure, which is necessary for growth. During molting, the animal is vulnerable to predators and environmental stresses.

Commonly known as roundworms; characterized by a one-way complete digestive tract (mouth and anus).
Their body plan includes a pseudocoelom, which functions as a hydrostatic skeleton aiding movement.
Lack specialized circulatory and respiratory systems; gas exchange occurs via diffusion across the body wall.
Contains at least 100,000 species, yet only 20,000 species have been described so far, indicating vast undiscovered diversity.
Highly abundant soil inhabitants, with densities of up to 1 billion individuals per hectare in fertile soil.
Functions within the Soil Food Web:
- Root-feeding nematodes impact plant shoots and roots by damaging cells and altering nutrient uptake, often reducing crop yields.
- Serve as decomposers, predators of bacteria and fungi, and prey for arthropods and other soil organisms, playing crucial roles in nutrient cycling.

Beneficial Effects:
- Caenorhabditis elegans: First multicellular organism to have its entire genome sequenced ( $2n = 12$ chromosomes).
- Widely used in research as a model organism to study genetics, development, neurobiology, and aging.
- Some nematodes are used as biocontrol agents against insect pests in agriculture.
Detrimental Effects:
- Heartworms ( Dirofilaria immitis): Transmitted by mosquitoes, common in dogs and other canids, especially in regions like the Okanagan in Canada, leading to severe cardiopulmonary disease.
- Trichinella spiralis: Acquired through consumption of poorly cooked pork or wild game, leading to trichinellosis in humans, characterized by muscle pain, fever, and digestive issues.
- Ascaris lumbricoides: The largest intestinal nematode in humans, common in areas with poor sanitation, causing malnutrition, abdominal pain, and intestinal blockage.

Proposed in 1989 by David Strachan: Suggests a correlation between decreased exposure to pathogens/parasites (especially during childhood) and increased rates of asthma, allergies, and autoimmune diseases in developed nations.
Interaction of helminths with the immune system:
- Helminths may modulate the host immune response by promoting regulatory T cells (Treg cells) and anti-inflammatory cytokines, thereby decreasing excessive inflammation and allergic reactions.
Criticisms of the Hygiene Hypothesis:
- Lack of direct causal relationships consistently established within epidemiological studies due to confounding variables.
- Alternative hypotheses exist regarding urbanization, altered diets, antibiotic overuse, reduced microbiome diversity, and environmental triggers as contributing factors to immune dysregulation.
- Therapeutic nematode infection (e.g., Necator americanus, human hookworm) is being studied for potential benefits for conditions like Celiac disease and Type 2 diabetes, although results are still preliminary.

Major defining features of Arthropoda:
- Bilateral symmetry in their body plan.
- Segmented body plan allowing for specialized functions in different regions.
- Hard exoskeleton made from chitin, providing protection and support.
- Jointed appendages that allow for a variety of precise movements and diverse functions.
Subphyla of Arthropoda:
1. Trilobita: Now extinct; significant in the fossil record, characterized by a three-lobed body, head (cephalon), thorax, and tail (pygidium).
2. Chelicerata: Includes spiders, scorpions, mites, ticks, and horseshoe crabs; characterized by chelicerae (pincer-like or fangs for feeding) and pedipalps (sensory, grasping, or reproductive functions).
3. Myriapoda: Centipedes and millipedes belong here. Centipedes typically have one pair of legs per segment and are carnivorous, while millipedes have two pairs of legs per segment and are detritivorous.
4. Crustacea: Includes crabs, lobsters, barnacles, shrimp, and isopods; most utilize gills for respiration and possess biramous (branched) appendages; dominant in aquatic environments.
5. Hexapoda: Comprising insects which show the most diversity among living organisms, characterized by three distinct body regions (head, thorax, abdomen), three pairs of legs attached to the thorax, and typically one or two pairs of wings.

Key Features of appendage modification in different subphyla:
- Different Hox gene expressions are crucial for controlling the development and identity of segments and their associated appendages along the anterior-posterior body axis, leading to variations in limb morphology and function.
- Examples of appendage functions include:
- Antennae for sensory perception (touch, taste, smell) in most arthropods (Crustacea, Hexapoda, Myriapoda).
- Chelipeds (claws) for defense, aggression, and prey capture in crustaceans (e.g., crabs, lobsters).
- Swimmerets for swimming, creating currents for respiration, and holding eggs in female crustaceans.
- Mouthparts: Highly varied, from mandibles (chewing) in myriapods, crustaceans, and insects, to stylets (piercing-sucking) in some insects, and chelicerae for grasping/piercing in chelicerates.
- Wings: Modified thoracic appendages in many insects, enabling flight and promoting dispersal and colonization of diverse habitats.

Understanding these foundational biological concepts is critical for studying animal phyla, particularly the ecological and evolutionary significance of Nematoda and Arthropoda in relation to human health, agricultural productivity, and environmental dynamics