Unit 2: General Characteristics of Nematodes - Subjective Questions

Nematodes are unsegmented, bilaterally symmetrical, triploblastic, pseudocoelomate animals belonging to the phylum Nematoda. They are commonly called roundworms because their bodies are circular in cross-section.

Important terminologies include:

• Nematology: The scientific study of nematodes.
• Phytonematology: The branch of nematology concerned with plant-parasitic nematodes.
• Host: A plant on or within which a nematode feeds and develops.
• Host range: The group of plant species that a nematode can parasitize.
• Stylet: A protrusible, needle-like feeding structure used to puncture plant cells.
• Ectoparasite: A nematode that feeds from outside plant tissues.
• Endoparasite: A nematode that enters and feeds within plant tissues.
• Migratory parasite: A nematode that moves through host tissues while feeding.
• Sedentary parasite: A nematode that establishes a permanent feeding site.
• Infective stage: The developmental stage capable of entering or attacking a host, commonly the second-stage juvenile in many plant parasites.

The general characteristics of nematodes are:

• The body is elongated, cylindrical, unsegmented, and usually tapering at both ends.
• They are bilaterally symmetrical, triploblastic, and pseudocoelomate animals.
• The body is covered by a tough, flexible, non-cellular cuticle that is moulted during development.
• Only longitudinal muscles are present; therefore, movement is typically whip-like or sinusoidal.
• The digestive system is complete, with a mouth, oesophagus or pharynx, intestine, rectum, and anus.
• Plant-parasitic species generally possess a stylet for puncturing plant cells.
• Circulatory and respiratory systems are absent; transport and gas exchange occur through body fluids and the body surface.
• The nervous system consists mainly of a circumpharyngeal nerve ring and longitudinal nerves.
• Most species have separate sexes, although hermaphroditism and parthenogenesis also occur.
• Nematodes normally pass through the egg, four juvenile stages, and adult stage.

A typical plant-parasitic nematode has an elongated, cylindrical body divided into the head, neck, trunk, and tail regions.

• Head region: Bears the mouth opening and sensory structures called amphids. Lip-like structures may surround the mouth.
• Stylet: Located in the anterior region and used to puncture plant cells, inject secretions, and withdraw nutrients.
• Neck region: Contains the oesophagus or pharynx and associated glands.
• Trunk region: Contains most of the intestine and reproductive system. In females, the vulva opens in this region; in males, the reproductive opening is associated with the cloaca.
• Lateral fields: Longitudinal markings on the cuticle that can assist in identification.
• Cuticle: A flexible protective covering with annulations or surface markings in many species.
• Tail region: Varies in shape and length and is important in taxonomy. Male tails may possess a bursa or caudal alae.

Although many nematodes remain vermiform, mature females of sedentary parasites may become swollen, pear-shaped, lemon-shaped, or spherical.

The internal organs of a nematode lie within a fluid-filled pseudocoelom, which functions as a hydrostatic skeleton.

• Body wall: Consists of the cuticle, hypodermis, and longitudinal muscles.
• Digestive system: A complete alimentary canal comprising the mouth, buccal cavity, oesophagus, intestine, rectum, and anus. Plant parasites generally possess a stylet and well-developed oesophageal glands.
• Nervous system: Includes a nerve ring around the oesophagus, dorsal and ventral nerve cords, and sensory organs such as amphids and phasmids.
• Excretory-secretory system: Consists of glandular or tubular structures that regulate body fluids and eliminate metabolic products.
• Reproductive system: Females may have one or two ovaries, oviducts, a uterus, and a vulva. Males possess a testis, vas deferens, seminal vesicle, cloaca, and copulatory spicules.
• Circulation and respiration: Specialized circulatory and respiratory organs are absent. Materials are distributed through pseudocoelomic fluid, while gases diffuse through the body surface.

The nematode body wall consists of the cuticle, hypodermis, and muscle layer.

• The cuticle is a non-cellular, flexible, multilayered outer covering secreted by the hypodermis.
• It protects the nematode from mechanical injury, toxic chemicals, digestive enzymes, and changes in the external environment.
• Surface markings such as annulations, ridges, and lateral fields are useful for taxonomic identification.
• The cuticle provides structural support and helps maintain body shape.
• The hypodermis lies beneath the cuticle and forms longitudinal cords containing nerves and excretory structures.
• The muscle layer consists mainly of longitudinal muscles.
• Pressure from pseudocoelomic fluid acting against the elastic cuticle produces a hydrostatic skeleton.
• Alternate contraction of dorsal and ventral muscles generates the characteristic sinusoidal movement.
• The old cuticle is shed during each moult, permitting growth from one juvenile stage to the next.

The stylet is a protrusible, hollow or partly hollow feeding spear situated in the mouth region of most plant-parasitic nematodes.

A typical stomatostylet has:

• An anterior conus, which penetrates the plant cell wall.
• A central shaft containing a lumen through which secretions and food pass.
• Posterior basal knobs, which provide attachment for protractor muscles.

Its major functions are:

• Penetration: It punctures root and other plant cells.
• Secretion: Oesophageal gland secretions are injected into plant tissues through the stylet.
• Feeding-site formation: Secretions can modify host cells into specialized feeding structures such as giant cells or syncytia.
• Ingestion: The nematode withdraws dissolved cellular contents through the stylet.
• Migration: Repeated puncturing and secretion assist some species in moving through plant tissues.

Stylet form is also taxonomically important. Tylenchid nematodes have a stomatostylet, while Xiphinema and Longidorus possess a longer odontostylet.

The typical nematode life cycle consists of the following stages:

• Egg
• First-stage juvenile, or J1
• Second-stage juvenile, or J2
• Third-stage juvenile, or J3
• Fourth-stage juvenile, or J4
• Adult

The first moult commonly occurs inside the egg, so the J2 hatches. In root-knot and cyst nematodes, J2 is the principal infective stage. It locates a host root, penetrates the tissue, and establishes a feeding site. Four moults occur before adulthood.

Reproduction may occur through:

• Amphimixis: Fertilization involving males and females.
• Parthenogenesis: Development of offspring from unfertilized eggs.
• Hermaphroditism: A single individual produces both male and female gametes.

Eggs may be deposited singly, in gelatinous masses, within plant tissues, or retained inside the female body. Development is strongly influenced by temperature, moisture, host availability, and soil conditions.

Plant-parasitic nematodes can be classified according to their location and movement during feeding.

1. Migratory ectoparasites

• Remain outside the root and move through soil while feeding on epidermal or cortical cells.
• Examples: Xiphinema, Longidorus, Trichodorus, and Paratrichodorus.

2. Sedentary ectoparasites

• Feed from outside the root for an extended period with limited movement.
• Examples: Criconemoides and some Hemicycliophora species.

3. Migratory endoparasites

• Enter plant tissues and move from cell to cell while feeding.
• They generally cause lesions, cell death, and tissue decay.
• Examples: Pratylenchus, Radopholus, and Ditylenchus.

4. Sedentary endoparasites

• Enter the root, establish permanent feeding sites, and become immobile.
• Examples: Meloidogyne, Heterodera, Globodera, and Rotylenchulus.

This classification is important because feeding habit determines symptom development, survival strategy, sampling method, and management approach.

Thus, the principal distinction is the dependence of plant-parasitic nematodes on living plant tissues and their possession of specialized feeding structures.

Meloidogyne species are sedentary endoparasites commonly called root-knot nematodes.

• The infective J2 penetrates a root near the elongation zone and migrates to the vascular region.
• It induces several enlarged, multinucleate giant cells that supply nutrients.
• The female becomes pear-shaped and remains embedded in the root, while the male may become vermiform and leave the tissue.
• Eggs are usually deposited in a gelatinous matrix on or near the root surface.
• Infection produces characteristic root galls or knots, excessive branching, reduced root efficiency, stunting, chlorosis, wilting, and yield loss.
• Important species include M. incognita, M. javanica, M. arenaria, and M. hapla.
• The genus has a very broad host range, attacking vegetables, pulses, oilseeds, cotton, tobacco, ornamentals, fruit crops, and many weeds.
• Severe damage often results when root-knot nematodes interact with wilt-producing fungi and bacteria.

Both groups use J2 as the infective stage and establish permanent feeding sites. However, cyst formation enables Heterodera and Globodera eggs to survive in soil for many years, making management particularly difficult.

Pratylenchus species are known as root-lesion nematodes and are migratory endoparasites.

• Juveniles and adults penetrate roots and move through the cortical tissue.
• They puncture cells with their stylets, feed on their contents, and cause cell death.
• Their movement produces elongated brown or black necrotic lesions.
• Lesions may merge, leading to cortical decay, root pruning, reduced absorption, stunting, chlorosis, and poor crop growth.
• Damaged tissues provide entry sites for fungi and bacteria, producing disease complexes.
• Important species include P. penetrans, P. coffeae, P. thornei, and P. zeae.
• Their host range includes cereals, maize, wheat, coffee, banana, potato, legumes, vegetables, fruit trees, and ornamentals.
• Unlike sedentary endoparasites, all active stages can leave damaged roots and infect new roots.

Their broad host range and ability to persist in roots and soil make crop rotation difficult unless non-host crops are carefully selected.

Rotylenchulus reniformis, the reniform nematode, is a sedentary semi-endoparasite of roots.

• The immature female is the infective stage and is initially vermiform.
• She inserts the anterior portion of her body into the root while the posterior portion remains outside.
• A specialized feeding site is established near vascular tissues.
• After feeding, the female swells into a kidney or reniform shape.
• The male remains vermiform and generally does not feed as an adult.
• Eggs are laid in a gelatinous matrix on the root surface.
• Symptoms include root necrosis, reduced root growth, stunting, chlorosis, wilting, delayed maturity, and yield reduction.
• Major hosts include cotton, pineapple, soybean, cowpea, vegetables, banana, papaya, tea, and several fruit and ornamental crops.
• It is especially important in warm tropical and subtropical soils and can interact with soil-borne pathogens to increase plant damage.

Radopholus similis is called the burrowing nematode and is a migratory endoparasite.

• Females and juveniles enter roots and move through cortical tissues while feeding.
• Their feeding and migration kill cells and produce reddish-brown to black lesions.
• Root lesions enlarge and coalesce, causing extensive root decay and loss of anchorage.
• In banana, severe infection causes a weakened root system and plant toppling, commonly called blackhead disease or toppling disease.
• In citrus, infection is associated with spreading decline.
• Important hosts include banana, plantain, citrus, coconut, avocado, coffee, pepper, ginger, and many ornamentals.
• The nematode can be disseminated through infested planting material, roots, soil, water, and farm equipment.
• Disease severity is often increased by secondary invasion of damaged roots by fungi and bacteria.

Ditylenchus dipsaci is the stem and bulb nematode, a migratory endoparasite that primarily attacks aerial and underground storage tissues.

• It enters plants through stomata, wounds, or young tissues under moist conditions.
• It feeds and migrates between cells, causing separation and breakdown of tissues.
• Symptoms include swollen and distorted stems, shortened internodes, twisted leaves, soft bulbs, tissue browning, cracking, and rotting.
• Infected seedlings may become stunted or die.
• Important hosts include onion, garlic, narcissus, tulip, alfalfa, clover, pea, bean, broad bean, and several weeds.
• The species has many biological races with different host preferences.
• Its fourth-stage juvenile can survive desiccation in a dormant state and persist for long periods in dried plant material, seed, or soil.
• Infested bulbs, seed, transplants, and plant debris are important sources of dissemination.

These genera are migratory ectoparasites that feed on roots from the surrounding soil.

Xiphinema

• Commonly called dagger nematodes and characterized by a long odontostylet.
• Attack grapevine, fruit trees, forest plants, vegetables, and ornamentals.
• Some species transmit nepoviruses; for example, X. index transmits grapevine fanleaf virus.

Longidorus

• Known as needle nematodes and also possess a long odontostylet.
• Feed on cereals, grasses, legumes, vegetables, fruit crops, and woody plants.
• Several species transmit nepoviruses.

Trichodorus and Paratrichodorus

• Commonly called stubby-root nematodes.
• Feed near root tips, causing short, thickened, highly branched root systems.
• Attack potato, tobacco, vegetables, cereals, and many field crops.
• Some species transmit tobraviruses, including tobacco rattle virus.

Their economic importance results from both direct root injury and virus transmission. Even small populations can therefore cause substantial disease and yield loss.

Free-living nematodes contribute significantly to soil decomposition and nutrient cycling.

• Bacterial-feeding nematodes consume bacteria associated with decomposing organic matter.
• Fungal-feeding nematodes graze on fungal hyphae and influence fungal decomposition pathways.
• Nematodes assimilate only part of the nutrients obtained from microorganisms and excrete the excess in mineral forms available to plants.
• They are particularly important in the release of inorganic nitrogen, including ammonium, from microbial biomass.
• Grazing can stimulate microbial turnover and maintain active bacterial and fungal populations.
• Nematode movement through water films redistributes microorganisms within soil pores.
• Their dead bodies and waste products add nutrients to the detrital pool.
• By influencing bacterial and fungal communities, nematodes affect the rate at which organic residues are decomposed.

Therefore, free-living nematodes act as regulators of microbial populations and as intermediaries that transfer nutrients from decomposers to plants and higher trophic levels.

Nematodes occupy several trophic levels in the soil food web and are classified into functional groups according to feeding habit.

• Bacterial feeders regulate bacterial populations and release nutrients immobilized in bacterial cells.
• Fungal feeders influence fungal biomass, decomposition, and nutrient mineralization.
• Plant feeders consume living plant tissues and influence plant productivity and community composition.
• Predatory nematodes feed on other nematodes and small soil animals, helping regulate prey populations.
• Omnivores consume resources from more than one trophic level and contribute to food-web stability.
• Nematodes serve as prey for mites, springtails, predatory fungi, microarthropods, and other soil organisms.
• They transfer energy from microbes and plants to higher consumers.
• Their abundance and community composition respond to disturbance, pollution, tillage, organic amendments, and soil fertility.

Because different nematode groups respond predictably to environmental change, nematode communities are useful bioindicators of soil health and food-web condition.

Direct effects result from nematode feeding and movement:

• Stylet penetration injures cells and removes nutrients.
• Migratory species destroy tissues and produce lesions or necrosis.
• Sedentary species alter host development by forming giant cells, syncytia, galls, or other feeding structures.
• Root injury reduces water and mineral absorption.
• Above-ground symptoms include stunting, chlorosis, wilting, patchy growth, reduced vigour, and yield loss.

Indirect effects arise from interactions with other organisms and physiological stress:

• Wounds provide entry points for fungi and bacteria.
• Nematodes may predispose plants to wilt, root rot, and other diseases.
• Some ectoparasitic species transmit plant viruses.
• Infection changes root exudates and may alter the rhizosphere microbial community.
• Damaged plants become less tolerant of drought, nutrient deficiency, and other environmental stresses.
• Competition for photosynthates occurs because specialized feeding sites act as strong nutrient sinks.

Consequently, nematode damage often exceeds the injury expected from feeding alone and may form complex diseases with other pathogens.

Nematodes have both beneficial and harmful roles, depending on their feeding group and ecological context.

Beneficial roles

• Bacterial and fungal feeders accelerate microbial turnover and nutrient mineralization.
• Predatory and omnivorous nematodes regulate populations of other soil organisms.
• Nematodes form important links between microbes, plants, and higher consumers in the soil food web.
• Their activities can improve nutrient availability and support plant growth.
• Entomopathogenic nematodes help suppress insect pests.
• Community structure provides information about soil disturbance, enrichment, contamination, and ecological maturity.

Harmful roles

• Plant-parasitic nematodes damage roots, stems, leaves, bulbs, seeds, and other plant organs.
• They reduce water and nutrient uptake, leading to stunting, chlorosis, wilting, and yield loss.
• Some transmit plant viruses.
• Their wounds and physiological effects promote infection by fungi and bacteria.
• Persistent eggs, cysts, or dormant juveniles make some species difficult to eliminate.

Thus, nematodes should not be regarded solely as crop pests. A healthy soil contains a diverse nematode community, while damaging populations of plant parasites must be specifically identified and managed.