Nemathelminthes: Life Cycle | Zoology Optional Notes for UPSC PDF Download

Life Cycle of Nematodes: Stages, Egg, and Embryonic Development

Egg Stage:

• Nematodes start their life cycle as eggs.
• These oval-shaped eggs have three protective layers: an external protein layer (secreted by the uterus wall), a middle chitinous layer (the true shell secreted by the egg itself), and an inner lipid layer.
• The composition of the chitin in the eggshell varies among nematode species.

Embryonic Development:

• Adult female nematodes lay eggs, initiating the embryonic development process.
• During embryogenesis, the egg undergoes cleavage, forming two primary cells: the first somatic cell (S1) and the parental germinal cell (P1).
• Further cleavages lead to the formation of various cell types.
• The S1 blastomere gives rise to ectodermal cells, while P1-derived cells contribute to the development of gonads.
• The mesodermal and endodermal tissues are also produced from specific cells.

Hatching (Ecdysis):

• Hatching is the process by which juveniles emerge from the egg.
• It can be triggered by host-related stimuli or occur under normal environmental conditions.
• Some nematodes release juveniles from cysts, which is considered an emergence rather than hatching.

Moulting (Eclosion):

• After hatching, juvenile nematodes resemble adults but differ in body size and gonad development.
• Growth is associated with moulting, which typically occurs four times, marking five distinct stages of development.
• During moulting, the entire cuticle, including various structures, is shed.
• Growth tends to be more pronounced after the final moult, and moulting occurs during the earlier part of the growth curve.

Stimulus for Moulting:

• Neurosecretory cells in nematodes are stimulated to produce secretions that activate glands responsible for producing enzymes or hormones initiating moulting.
• The stimulus for moulting can be complex and influenced by factors such as the host, temperature, pH, and soil salt content.
• Cuticular and hypodermal structures, like hemizonids, serve as receptors for the moulting stimulus.

Growth and Development:

• Nematodes have four juvenile stages before reaching adulthood.
• The duration of each juvenile stage can vary.
• Gonad development begins in the first juvenile stage before hatching.
• Nematodes have flexibility in terms of their feeding sites within host plants, affecting their development and physiology.

In summary, the life cycle of nematodes involves complex developmental stages, moulting, and growth, which are influenced by various environmental factors and host interactions. Understanding these stages is crucial for comprehending nematode biology and their impact on plants and animals.

Root-Knot Nematodes (Meloidogyne spp.)

• Root-knot nematodes are sedentary endoparasites that primarily infest underground plant parts.
• The eggs are encased in a gelatinous matrix, often protruding from host tissues.
• A single egg mass can contain around 200 to 300 oval eggs, making it larger than the female's body.
• The life cycle begins with eggs, which are usually one-celled and deposited by the female.
• Embryonic development commences shortly after deposition, leading to the formation of two, four, eight cells, and so on.
• The embryo and the first-stage larva remain within the egg but are not highly active.
• Following the first moult, the second-stage infective juvenile develops within the egg.
• Larval hatching occurs under suitable physical conditions, independent of host root exudates.
• Second-stage larvae emerge from the egg and are found free in the soil.
• These juveniles infect new host root tissue, primarily in the meristematic zone (behind the root tip).
• After establishing a feeding site in the pericycle, they become sedentary.
• Three more moults take place, resulting in female nematodes with a spherical body embedded in the host tissue.

Feeding and Nurse Cells:

• During feeding, larvae secrete substances that pierce the cell wall, leading to cell enlargement in the vascular cylinder and increased cell division in the pericycle.
• The nematode's feeding stimulates the development of a nurse cell system, often referred to as a "syncytium" or "giant cell."
• These multinucleate cells contain dense cytoplasm and enlarged nuclei, are metabolically active, and are essential for the nematode's nourishment.

Adult Male Development and Parthenogenesis:

• Larvae that develop into adult males are initially parasitic and undergo three moults.
• They eventually leave the host, adopting a worm-like form, to approach females for copulation.
• Parthenogenesis, or reproduction without males, is common among root-knot nematodes.
• It typically takes around 30 days for a mature female to develop, with variations depending on host species and environmental factors like temperature and soil type.

Cyst Nematodes (Globodera spp. and Heterodera spp.)

• Second-stage larvae usually penetrate the root just behind the growing point and undergo rapid growth.
• Three moults occur within the host over approximately 5-6 weeks.
• Cysts, which are initially white and packed with eggs, become visible on the root surface after death. Over time, the body wall hardens into a tough, brown covering.
• The cysts detach from the root and fall into the soil.
• Larval emergence from cysts often responds to root exudates from host plants, with optimal emergence conditions involving a temperature increase after a period of low temperature.
• Cysts release eggs over a period of 3-4 years, with about 50% of the eggs remaining viable each year.
• Cyst nematodes generally have one generation per year.

Nematode Multiplication and Host Interaction:

• The texture of the soil influences nematode multiplication, favoring light-textured soils for the migration of second-stage juveniles.
• Nematodes induce the formation of nurse cells, but gall formation is less distinct in cyst nematodes compared to root-knot nematodes.
• Cyst nematodes often create a single syncytium per nematode, while root-knot nematodes may involve one or more nematodes in a syncytium.
• Syncytia formed by cyst nematodes have relatively small nuclei compared to the enlarged nuclei in syncytia caused by root-knot nematodes.
• There are differences in the characteristics of these syncytia, including the presence of abundant mitochondria, golgi bodies, protoplasts, and dense endoplasmic reticulum.
  

Citrus Nematode (Tylenchulus semipenetrans)

• The citrus nematode is a sedentary semi-endoparasite of citrus tree roots.
• Females are commonly found attached to thick and stunted rootlets, surrounded by soil particles held in place by a gelatinous mucus secreted by the female.
• The eggs are laid in groups by young females with their head and neck buried in the root cortex.
• Larvae hatch from eggs in approximately 12-14 days.
• Mature males develop within a week after three moults, with one moult occurring within the egg.
• Second-stage female larvae take about 14 days to locate the host root and feed on epidermal cells until they are ready to moult.
• Fourth-stage females and young females appear around 21 days after entering roots.
• Egg-laying occurs approximately 40 days later.
• The entire cycle from egg to adult takes 6 to 8 weeks at 25°C, with reproduction occurring without males.

Nurse Cells and Population Impact:

• Citrus nematodes create a feeding zone known as a "nurse cell."
• These cells consist of discrete parenchyma cells with a single nucleus, located in the root cortex.
• Unlike other nematodes, syncytium is not formed in the case of citrus nematodes.
• The nematode's feeding results in necrosis, but the damage does not typically extend to the stellar region of the root.
• The population of citrus nematodes is closely linked to the decline of citrus trees, with severe infestations in sandy loam soil.
  

Reniform Nematode (Rotylenchulus reniformis)

• The adult female of the reniform nematode is an obligate, sedentary, semi-endoparasite that feeds on roots, while the males are non-parasitic.
• This nematode species is bisexual, and reproduction occurs through amphimixis (the fusion of gametes from two parents).
• The reniform nematode has a unique life cycle.
• Newly hatched second-stage larvae have a well-developed stylet but do not feed. They go through three successive moults to become young females and adult males.
• Young females actively feed on the cortical cells of root tissue, eventually becoming partially or completely embedded in the root.
• After three days of feeding, a slight swelling of the posterior body becomes visible.
• Eight days later, the female deposits eggs in a gelatinous matrix outside the root tissue.
• When these eggs come into contact with water, they promptly begin to hatch.
• The entire life cycle is completed in approximately 25 days, provided that the young females have immediately found a suitable host.

Nurse Cell System:

• The reniform nematode, as a semi-endoparasite with a sedentary nature, induces a specialized nurse cell system for a continuous food supply.
• This system involves the expansion of cell walls at the feeding site, partial dissolution of cell walls, fusion of neighboring cell protoplasts, and the formation of a multinucleate syncytium.
• These syncytia are mainly confined to the pericycle, while other pericycle cells are stimulated metabolically but remain discrete and uninucleate.
  

Burrowing Nematode (Radopholus similis)

• In the burrowing nematode, females and all juvenile stages are infective, while males are non-parasitic and morphologically degenerate, lacking a stylet.
• Penetration typically occurs near the root tip.
• Nematodes penetrate roots within 24 hours, and the cells around the penetration site turn brown.
• Once inside the roots, nematodes take up an intercellular position in the cortical parenchyma, where they feed on the cytoplasm of nearby cells, causing cavities that may merge to form tunnels.
• Burrowing nematodes do not enter the central vascular tissues (the stellar portions) of the root.
• The nematode's life cycle is completed within 24 to 30 days, within a temperature range of 21 to 32°C.
• Females lay an average of 4 to 5 eggs for two weeks within infested tissues.
• Eggs hatch after 8 to 10 days, and the juvenile stages are completed in 10 to 13 days.
• A lower soil temperature, sufficient soil moisture, and the availability of fresh, tender roots promote population growth of the burrowing nematode.