Introduction
Nematodes are a group of worms. They occur naturally and are very hard to detect visually. These are common soil pests that affect plants. The soil at low levels contains numerous Nematodes. Nematodes can enter the farm through infected transplants. They are parasites of both plants and animals and attack the insects also. However, they cause severe damage to plants. But not all Nematodes are harmful to the plants. Some play an essential role in nutrient recycling.
Commonly known as roundworms, they are unsegmented vermiform pests. They are free-living organisms. Sometimes they enter the plant to extract nutrients from the root cell. They stress tolerance of the plant. Plants abundant with water and nutrients can tolerate nematode attacks. Once they are present in the soil, It is almost impossible to eliminate.
History
In 1758, Carolus Linnaeus described some nematode genera (such as Ascaris), then included in the taxon of worms, Vermes. The name of the group Nematoda, also called “nematodes", originally was defined by Karl Rudolphi in 1808. The term came from Nematoidea, defined from Ancient Greek. It was later treated as family Nematodes by Burmeister in the year 1837.
Characteristics of Nematodes
Following are the Nematodes Characteristics
1.Tubular in appearance. It has an elongated and thin body (hair-like).
2.The alimentary canal is distinct, but the head and tail are not visually different.
3.The majority of these are tiny and can be microscopic.
4.They are free-living organisms.
5.They reproduce sexually. They produce amoeboid sperm cells.
6.They have a nervous system.
7.They are parasites of both plants and animals.
8.They have cuticles that moult periodically.
Classification of Nematodes
Approximately 15000 species of Nematoda have been identified till now. Some nematodes live in the roots of plants; some spend their life inside the plants. These are not plant-specific. Following are the main three classes of Nematodes that has been classified further in subclasses and subclasses:
Kingdom : Animalia
Nematodes are multicellular eukaryotic organisms like other organisms (animals, plants, most algae, fungi, Metazoa, and protists) in the kingdom Animalia. Their cells contain a nucleus and other organelles. They obtain nutrients from organic sources, just like other organisms.
Phylum : Nematoda
Nematodes, also known as roundworms, make up the phylum Nematoda.
1.Class Rhabditea
a.Parasite Rhabditea
b.Free-Living Rhabditea
c.Rhabditis
d Tylenachia
2.Class Enoplea
a.Enoplia
b.Dorylaimia
3.Class Chromadorea
a.Chrimadoria
1.Class : Rhabditea
Class Rhabditea has both free-living and parasitic nematodes. The majority is of parasite nematodes in this class. Rhabditea Free-living feeds on bacteria as a source of energy. They can be found in between soil particles as well as in water.
General Characteristics of Rhabditea
They are unsegmented.
They Possess a cylindrical body.
They are tapered at either end.
They possess a cuticle and hypodermis.
Adult species have intestines, gonads and pharynx.
They have invaginated cuticles with nerves.
Rhabditea is Further Classified into Two Subclasses
Parasitic Rhabditea
Parasitic Rhabditea nematodes examples are Ascaris species, Enterobius species (e.g. human pinworm), Necator, and Wuchereria species. These species cause many serious diseases in human beings. This species is common in the tropics.
Free-Living Rhabditea Nematodes
They are found in temperate environments. They live in bacteria-rich habitats such as compost to obtain nutrients. They depend on other insects for transport from one location to another. Caenorhabditis Elegans worm is the best example of Free-Living Rhabditea Nematodes.
Further Classification is as Follows
Rhabditis - They have well-developed Phasmids and poorly developed invaginated cuticles with nerves called aphids.
Tylenchia - Found in plants often in the form of parasites.
2.Class : Enoplea
Enoplea makes up the phylum Nematoda. These ancestrally diverge nematodes. Some examples of Enoplea nematodes are Trichuris, Diotyphyme, and Diotyphyme.
Enoplea Nematodes Characteristics:
Cylindrical or bottle-shaped oesophagus.
Well-developed Amphids.
The simple excretory system made up of a few ventral or glandular cells.
Do not live in marine environments.
Possess teeth-like structures.
Subclasses of Enoplea Nematodes
Enoplia - They have oval or pouch-like amphids, cylindrical oesophagus, and smooth bodies.
Dorylaimida - The majority of this species is a free-living organism. They can be predators or omnivores. Some, like Trichinella, exist as parasites.
3.Class : Chromadorea
This class can be found in a broader range of habitats as it consists of about four distinct lineages. This class also has both free-living and parasitic members. Chromadorea is smaller in size. But they are higher in number in their habitats as they reproduce at a higher rate.
Characteristics
Three esophageal glands.
Spiral aphids.
Pore-like amphid of Chromadorea.
Possess annulated cuticles.
Glandular of tubular excretory systems.
Sub-class: (i) Chromadoria
Control Nematoda
The infected plants by nematodes cannot grow well, are paler than normal, often dwarfed, and may wilt in the heat of the day, small leaves. These symptoms can be misunderstood with symptoms of nutrient deficiency. It has been noticed that infected plants can look healthy while growing in fertile soil or during cool weather.
The growth of plants with nematodes will lead to a larger population of it. Try to inspect the roots of plants before placing them into the farm. Because if nematodes are present in the soil, they are almost impossible to abolish altogether, but the damage to plants due to them can be reduced.
Phylogeny
The phylogenetic relationships of the nematodes and their immediate relatives from the Metazoans family still remain unresolved. They were assigned to the group Ecdysozoa during the 1990s together with moulting animals (such as arthropods).
However, they were identified quite succinctly with their closest relatives of Nematodes evidently by the morphological characters and molecular phylogenies. They are placed in a sister taxon to the parasitic Nematomorpha, which form Nematoda.
The Scalidophora and Nematoda from the clade Cycloneuralia, but with some disagreement between the available morphological and molecular pieces of evidence. Cycloneuralia, with the validity of the available data, often makes the rank of superphylum.
Anatomy
Nematodes are about 5 to 100 µm thick and 0.1 to 2.5 mm long. They could be from the microscopic range to as much as 5 cm, while some could be even larger, reaching over 1m in length. The body has ridges, rings, bristles, or other distinctive structures.
The radially symmetric head of a nematode is relatively distinct, while the rest of the body is bilaterally symmetrical, having sensory bristles protruding outwards around the mouth. The mouth, which often bears teeth, has either three or six lips. The caudal gland is often found at the tip of the tail.
The epidermis is a single layer of cells covered by thick collagenous cuticles. The cuticle is of a complex structure and has distinct layers. Beneath the epidermis, a layer of longitudinal muscle cells is found. The relatively rigid cuticle with the muscles forms a hydroskeleton. Projections originate from the underlines of muscle cells towards the nerve cords in which nerve cells normally extend fibres into the muscles.
Digestive System
In carnivorous species, the oral cavity is lined with cuticles strengthened by ridges. The mouth often includes a sharp stylet to thrust into its prey. The stylet could be hollow and be used for sucking liquids from plants or animals.
Digestive glands are found in the pharynx, producing enzymes that start to break down the food. The stomach is absent, with the pharynx connecting directly to an intestine that forms the main gut.
This produces further enzymes to absorb the available nutrients with the help of its single-cell thick lining. The rectum is further lined by tiny cuticles to expel the waste generated through the anus just below and at the tail tip.
Excretory System
Nematodes excrete nitrogenous waste in the form of ammonia through the body wall, while salts are excreted by osmoregulation. In many marine nematodes, there are renette glands present. These glands are responsible to excrete salt through a pore present on the animal’s ventral side. The transverse duct opens into a common canal connecting the excretory pore in mostly other nematodes.
Nervous System
Four peripheral nerves are found to run along the length of the body on the dorsal, ventral, and lateral surfaces. The ventral nerve is the largest, while the dorsal nerve is responsible for motor control and the lateral nerves are for sensory actions.
The nervous system contains cilia which are all nonmotile with a sensory function. The body of nematodes is shielded with numerous sensory bristles to provide the touch sense. Two small pits or 'amphids' have nerve cells and chemoreception organs.
Agriculture and Horticulture
Depending on its species, a nematode might be useful or detrimental to plant health. Two categories of nematodes are the predatory ones, which kill garden pests, while the pest nematodes (root-knot nematode) attack plants.
There are vectors spreading plant viruses between crop plants. Eelworms or plant-parasitic nematodes often attack leaves and buds. Parasitic nematodes can be managed by the rotation of plants of nematode-resistant species.
Natural antagonist such as the fungus Gliocladium roseum is used as a treatment method. Chitosan produces plant defence responses to destroy parasitic growth of nematodes on roots of various crops such as soybean, corn, sugar beet etc., without harming beneficial nematodes in the soil. However, soil steaming is an effective method to kill nematodes and eliminate both harmful and beneficial soil microbiota.
Caenorhabditis elegans
Caenorhabditis elegans is a free-living nematode (roundworm), about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model organism.
Biology
C. elegans is unsegmented, vermiform, bilaterally symmetrical, with a cuticle integument, four main epidermal cords and a fluid-filled pseudocoelomate cavity. Members of the species have many of the same organ systems as other animals. In the wild, they feed on bacteria that develop on decaying vegetable matter.
C. elegans has both a hermaphrodite sex, and a very rare male population, which makes up 0.05% of the total C. elegans on average. The basic anatomy of C. elegans includes a mouth, pharynx, intestine, gonad, and collagenous cuticle. Males have a single-lobed gonad, vas deferens, and a tail specialized for mating. Hermaphrodites have two ovaries, oviducts, spermatheca, and a single uterus.
C. elegans eggs are laid by the hermaphrodite. After hatching, they pass through four larval stages (L1-L4). When crowded or in the absence of food, C. elegans can enter an alternative third larval stage called the dauer state.
Dauer larvae are stress-resistant and do not age. Hermaphrodites produce all their sperm in the L4 stage (150 sperm per gonadal arm) and then switch over to producing oocytes. The sperm are stored in the same area of the gonad as the oocytes until the first oocyte pushes the sperm into the spermatheca (a kind of chamber where the oocytes become fertilized by the sperm).
The male can inseminate the hermaphrodite, which will use male sperm preferentially (both types of sperm are stored in the spermatheca). When self-inseminated the wild-type worm will lay approximately 300 eggs. When inseminated by a male, the number of progeny can exceed 1,000.
At 20°C, the laboratory strain of C. elegans has an average life span of approximately 2–3 weeks and a generation time of approximately 4 days. Hermaphrodites can mate with males or self-fertilize.
C. elegans has five pairs of autosomes and one pair of sex chromosomes. Sex in C. elegans is based on an X0 sex-determination system. Hermaphrodite C. elegans have a matched pair of sex chromosomes (XX); the rare males have only one sex chromosome (X0).
Life Cycle
Life Cycle of C. elegans. Animals increase in size throughout the four larval stages, but individual sexes are not easily distinguished until the L4 stage. At the L4 stage, hermaphrodites have a tapered tail and the developing vulva (white arrowhead) can be seen as a clear half circle in the center of the ventral side.
The males have a wider tail (black arrowhead) but no discernable fan at this stage. In adults, the two sexes can be distinguished by the wider girth and tapered tail of the hermaphrodite and slimmer girth and fan-shaped tail (black arrowhead) of the male.
Oocytes can be fertilized by sperm from the hermaphrodite or sperm obtained from males through mating. The dauer larvae are skinnier than all of the other larval stages. Photographs were taken on Petri dishes (note the bacterial lawns in all but the dauer images). Bar 0.1 mm.
Laboratory uses
C. elegans is studied as a model organism for a variety of reasons. Strains are cheap to breed and can be frozen. When subsequently thawed they remain viable, allowing long-term storage. Because the complete cell lineage of the species has been determined, C. elegans has proven especially useful for studying cellular differentiation.
From a research perspective, C. elegans has the advantage of being a multicellular eukaryotic organism that is simple enough to be studied in great detail. The developmental fate of every single somatic cell (959 in the adult hermaphrodite; 1031 in the adult male) has been mapped out.
These patterns of cell lineage are largely invariant between individuals, in contrast to mammals where cell development from the embryo is more largely dependent on cellular cues. In both sexes, a large number of additional cells (131 in the hermaphrodite, most of which would otherwise become neurons), are eliminated by programmed cell death (apoptosis).
In addition, C. elegans is one of the simplest organisms with a nervous system. In the hermaphrodite, this comprises 302 neurons whose pattern of connectivity has been completely mapped out, and shown to be a small-world network.
Research has explored the neural mechanisms responsible for several of the more interesting behaviors shown by C. elegans, including chemotaxis, thermotaxis, mechanotransduction, and male mating behavior. Unusually, the neurons fire no action potentials.
A useful feature of C. elegans is that it is relatively straightforward to disrupt the function of specific genes by RNA interference (RNAi). Silencing the function a gene in this way can sometimes allow a researcher to infer what the function of that gene may be.
The nematode can either be soaked in (or injected with) a solution of double stranded RNA, the sequence of which is complementary to the sequence of the gene that the researcher wishes to disable. Alternatively, worms can be fed on genetically transformed bacteria which express the double stranded RNA of interest.
C. elegans has also been useful in the study of meiosis. As sperm and egg nuclei move down the length of the gonad, they undergo a temporal progression through meiotic events. This progression means that every nucleus at a given position in the gonad will be at roughly the same step in meiosis, eliminating the difficulties of heterogeneous populations of cells.
The organism has also been identified as a model for nicotine dependence as it has been found to experience the same symptoms humans experience when they quit smoking.
As for most model organisms, there is a dedicated online database for the species that is actively curated by scientists working in this field. The WormBase database attempts to collate all published information on C. elegans and other related nematodes. A reward of $5000 has been advertised on their website, for the finder of a new species of closely related nematode. Such a discovery would broaden research opportunities with the worm.
The genome
C. elegans was the first multicellular organism to have its genome completely sequenced. The finished genome sequence was published in 1998, although a number of small gaps were present (the last gap was finished by October 2002).
The C. elegans genome sequence is approximately 100 million base pairs long and contains approximately 20,000 genes. The vast majority of these genes encode for proteins but there are likely to be as many as 1,000 RNA genes. Scientific curators continue to appraise the set of known genes, such that new gene predictions continue to be added and incorrect ones modified or removed.
In 2003, the genome sequence of the related nematode C. briggsae was also determined, allowing researchers to study the comparative genomics of these two organisms.
Work is now ongoing to determine the genome sequences of more nematodes from the same genus such as C. remanei , C. japonica and C. brenneri . These newer genome sequences are being determined by using the whole genome shotgun technique which means that the resulting genome sequences are likely to not be as complete or accurate as C. elegans (which was sequenced using the 'hierarchical' or clone-by-clone appoach).
The official version of the C. elegans genome sequence continues to change as and when new evidence reveals errors in the original sequencing (DNA sequencing is not an error free process). Most changes are usually minor, adding or removing only a few base pairs (bp) of DNA. E.g. the WS169 release of WormBase (December 2006) lists a net gain of 6 bp to the genome sequence. Occasionally more extensive changes are made, e.g. the WS159 release of May 2006 added over 300 bp to the sequence.
Nematode evolution
It has been shown that a small number of conserved protein sequences from sponges are more similar to humans than to C. elegans. This suggests that there has been an accelerated rate of evolution in the C. elegans lineage. The same study found that several phylogenetically ancient genes are not present in C. elegans.
C. elegans scientists
In 2002, the Nobel Prize for Medicine was awarded to Sydney Brenner, H. Robert Horvitz and John Sulston for their work on the genetics of organ development and programmed cell death (PCD) in C. elegans. The 2006 Nobel Prize in Physiology or Medicine was awarded to Andrew Fire and Craig, for their discovery of RNA interference in C. elegans.
Because all research into C. elegans essentially started with Sydney Brenner in the 1970's, many scientists working in this field share a close connection to Brenner (they either worked as a post-doctoral or post-graduate researcher in Brenner's lab or in the lab of someone who previously worked with Brenner).
Because most people who worked in his lab went on to establish their own worm research labs, there is now a fairly well documented 'lineage' of C. elegans scientists. This lineage was recorded in some detail at the 2003 International Worm Meeting and the results were stored in the Wormbase database.
C. elegans in the media
C. elegans made news when it was discovered that specimens had survived the Space Shuttle Columbia disaster in February 2003.
