C. elegansAmy Cui
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(SL 13)

Classification/Diagnostic characteristics

Caenorhabditis elegans is a nematode, or roundworm; the species exhibits all characteristics of nematodes. The phylum Nematoda is comprised of nematodes (roundworms), which are unsegmented organisms shaped by thick, multilayered cuticles. The cuticle is shed four times as the nematode grows. Nematodes show bilateral symmetry, and their anatomy classifies them as pseudocoelomates. Pseudocoelomates’ bodies comprise of the gut (the endoderm), the pseudocoel (the cavity), the muscle (the mesoderm), internal organs, and the ectoderm. There are over 25,000 identified species of nematodes, but it is predicted that there may be a few million total species. Many nematodes are microscopic, but some can grow to be as long as nine meters long; while some are predators and feed on protists and small animals such as other roundworms, some are parasites. Nematodes may successfully live in various environments, ranging from upper soil layers and bottoms of lakes to marine sediments. Caenorhabditis elegans is a model organism ideal for the research of geneticists and developmental researchers because it is easy to cultivate; C. elegans has a fixed number of body cells and a 1-mm long transparent body. Currently, its genome is completely sequenced.


Domain: Eukarya
Kingdom: Animalia
Phylum: Nematoda
Class: Secernentea
Order: Rhabditida
Family: Rhabditidae
Genus: Caenorhabditis
Species: Caenorhabditis elegans
(JM 12)

Relationship to humans

MicroRNA (miRNA) were first discovered in C. elegans and recently been applied to studies in chronic diseases such as cancer. The function of miRNA is to target mRNA where translation, a final step in protein synthesis, is being inhibited so that the miRNA can degrade the mRNA. During early stages of breast cancer, cancer cells cause a distinctive pattern of miRNA cells to appear in the serum of the patient's blood, and miRNA are now being investigated as markers for cancer which are otherwise undetectable.
Additionally, it shares with human beings nervous and digestive systems as well as the ability to reproduce sexually.

Nematodes may be parasitic in humans. The most common parasitic nematodes include hookworms, ascarids, pinworms, and whipworms. The percentage of nematode species that act as parasites to humans is small, but around 60 species are known to be parasitic. Most parasitic nematodes involve infection of the intestine, and nematodes comprise the largest population of worm-related diseases in humans. (AC) (18)

Habitat and niche

Caenorhabditis elegans commonly occupies rich soils or other environments in which nutrients are rich.

Caenorhabditis elegans is found primarily in microorganisms rich anthropogenic (originated from humans) environments such as compost piles, mushroom beds, and garden soil. Caenorhabditis elegans has been found in places as diverse as Europe, North America, Asia, North Africa, and Australia as well as aquatic environments. However, genetic testing has shown that Caenorhabditis elegans has moved very long distance (usually through a human medium), rendering its original natural environment unknown. These roundworms can also be found inside the digestive track of living animals such as snails, moths, and slugs. Scientists believe that the Caenorhabditis elegans prefer environments with an oxygen content of around 6% and avoid environments where the oxygen falls below 2% or above 12%. (SF)(6)

Predator avoidance

As a nematode, C. elegans is generally preyed upon by nematophagous fungi. Nematophagous fungi generally use a constricting ring, which acts as a trap and rapidly inflates inwards due to a gentle friction caused by a nematode, such as C. elegans. As a result, the nematode is captured and its cuticle is penetrated by the hyphae of the nematophagous fungi, which then digests the nematode. However, C. elegans has a touch response to help avoid this predation. When C. elegans senses a tightening noose, it quickly reverses itself and ceases its exploratory head movements. This anterior touch response results in an average escape rate of 81 +/- 6% for C. elegans with nematophagous fungi. Other nematodes that have reduced anterior touch responses have an average escape rate of 43 +/- 8%. Tyramine coordinates the anterior touch response in C. elegans and greatly increases the chances of survival. (5) (RS)
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The approach, entry, and escape of C. elegans (5)


Nutrient acquisition/Gas exchange

As their cuticle and gut are one cell layer thick, C. elegans as well as all other nematodes are able to exchange oxygen and nutrients with the environment through their skin.

At the head of the nematode is a mouth opening that connects to a pharynx, or throat, that pulls food in and contracts to move it through the gut, which is a long cavity lacking any muscles. The food digested by nematodes is not distributed specifically through the cells but is rather located all along the body cavity. (8) Many nematodes are carnivorous and feed on creatures smaller than themselves, including other nematodes. Others feed on phytoplankton, plant roots, dead organic matter, or bacteria and fungi that are themselves feeding on decomposing materials. (9) (CM)


Reproduction and life cycle

The adult C. elegans is hermaphroditic, containing both male and female reproductive organs, and reproduces sexually. Sexual reproduction involves the joining of gametes, haploid sex cells, to form offspring. The egg-laying process is done through the vulva on the lower (ventral) surface of the worm. Anchor cells in this region induce the vulva to form six cells on the this surface during development, and molecular signals create the primary and secondary inducers. Each of the six formed cells has the potential to become either a primary vulval precursor cell, a secondary vulval precursor cell, or an epidermal cell. The induction process involves the activation and inactivation of individual sets of genes through signal transduction, and the process shows that many aspects of development are controlled by molecular switches which allow a single cell to proceed down one of several possible paths.



Life Cycle of C. Elegan (1)
Life Cycle of C. Elegan (1)


Male C. elegans mate every 10 minutes or so. A male moves towards a hermaphrodite until his sensory-laden tale touches his mate. When he reaches the hermaphrodite, he does a series of steps explained in the figure below (10). (SP)
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C. elegans eggs are fertilized within the adult hermaphrodite and laid a few hours afterward. Eggs hatch and go through 4 larval stages, each ending with molting. When animals reach adulthood, they produce about 300 offspring each, and live a total of about 2 weeks.
These organisms can adopt an alternative life form, called the dauer larval stage, if plates are too crowded or if food is scarce. Dauer larvae are thin and can move, but their mouths are plugged and they cannot eat. They can remain viable for a surprising three months. Dauer larvae can roam around for months and then reenter the L4 stage when they encounter a food source and live for 15 more days. (11) (NU)


Growth and development

The transparent body of a Caenorhabditis elegans completely develops three days after the egg is fertilized.


C.elegans begins as a fertilized egg which then undergoes cell cleavage. During the proliferation stage of embryogenesis, a total of 558 essentially undifferentiated cells form the small worm that is enclosed in an egg. During the next stage of embryogenesis, morphogenesis occurs, and the cells are differentiated without additional cell divisions. The embryo elongates and takes the basic shape of an animal with a front, back and fully differentiated tissues. After the eggs hatch, the cells continue to divide and go through the larval stages separated by molts. This only occurs if the larva is fed after hatching. If not, the larva can last about six days without food and then it will die. Some c.elegans go through the dauer larva state, which occurs when conditions aren't favorable for the larva to continue growing. During the dauer state feeding stops and movement is slowed. This period only ends when the larvae experiences favorable conditions. After the final molt, the c.elegans goes into its adult stage and can begin to produce its own eggs. All of development only takes three days. (1) (LK)

Integument

Like all nematodes, C. elegans is an unsegmented organism which is covered by thick, cuticles. Over the course of its development, the organism will shed its multilayered cuticles several times. The cuticles of C. elegans are transparent, which greatly eases the observation of the organism's internal functions.
Nematodes are ecdysozoa, which are members of Kingdom Animalia which have skeletons comprised of cuticle. Cuticle is thin and is composed of organic materials and allows ecdysozoa to be flexible. Because minerals are not involved in the formation of cuticle, ecdysozoa do not need joints to be completely elastic, whereas animals with mineral skeletons require joints to exhibit the same flexible nature. Steroids control the ecdysis, or period shedding, of the cuticle. (AC) (16)


Movement

C. elegans is a free-moving species, and its motility is accounted for by its pseudocoel. The pseudocoels of nematodes, fluid-filled spaces in which many organs are suspended, are lined with the mesoderm while internal organs are unlined. As fluids are relatively incompressible, the contraction of the muscles surrounding the pseudocoel causes the fluids to shift to various parts of the body, thus moving specific body parts. As do all nematodes, C. elegans moves by contracting its longitudinal muscles, which span its body.

Sensing the environment

An Adult C. elegans hermaphrodite has 302 neurons that belong to two independent nervous systems: a somatic nervous system and a pharyngeal nervous system. The somatic nervous system is large and has around 282 neurons and it has neuronal support cells. The pharyngeal nervous system is small and has around 20 neurons.The two systems can communicate through a single pair of Ring/Pharynx interneuron or RIP. The somatic nervous system has neurons and their processes are usually positioned between the hypodermis and the body wall muscle. These neurons and their processes share a basal lamina with the hypodermis that isolates them from the muscles. In the pharyngeal neurons lie directly among the pharyngeal muscles and are not separated from them by basal lamina. The neurons in the hermaphrodite have been assigned to 118 distinct classes to their topology and synaptic connection patterns.The cell bodies of most neurons cluster are in the ganglia in the tail or head. C. elegans explores its environment and finds favorable surroundings by chemotaxis, thermotaxis, and aerotaxis. It escapes from harmful stimuli by avoidance behaviors. The perception of environment, including mechanical stimuli, chemicals, noxious substances, oxygen levels, temperature, etc. is done through 24 sensillar organs and various sensory neuron that perform sensory functions. Some nonsensillar neurons can perform sensory functions like oxygen sensation too. Each sensillum has ciliated endings of one or more neurons and usually two types of glia. The two types of glia would be the socket cells and the sheath cells. Most sensilla are located in the head, except for posterior derids and phasmids. Through the function C. elegans can navigate through thermal, chemical, and oxygen gradients by modulating the probability of its turning behavior and its speed. A gentle touch of the body is sensed by six touch receptor neurons. The touch response circuit involves 6 interneurons and 69 motor neurons. Harsh touches can be responded to without any touch receptor neurons. This response can be eliminating by killing the PVD neurons (2). (SM)

Waste removal

C. elegans remove waste through the use of four cells: the pore, duct, canal and the gland cell. This excretory system removes metabolic waste and excess fluids. This system is also responsible for maintaining the organism's salt concentration (See figure below). The canal cell functions in part as a kidney, in the sense that it helps regulate osmolarities of solutions. It excretes saline fluid via the duct and pore in order to maintain the animal's salt balance (osmoregulation) and probably to remove metabolites. (SM, 3)
external image exc1.jpg

Environmental physiology

Temperature regulation - Like all nematodes, C. Elegans are ectothermic; they do not have an internal temperature regulating system.
Salt regulation - C. Elegans are osmoconformers, meaning that their internal solute level is the same as their surroundings.
(SL 14, 15)

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The nematode population thrives in warmer soil temperatures during late summer and early autumn months. (AC) (17)


Internal circulation

As a very simple microorganism, C. elegans does not have any sort of internal circulatory system. It uptakes all gases through its external cell membranes then transports them through diffusion throughout its body. C. elegans can afford not to have a circulatory system because its small size provides for a high surface area to volume ratio, allowing it to take in a sufficient amount of oxygen to perform cellular respiration. (4) (YR)

Chemical control


C. Elegans can attain and regulate chemicals through diffusion, the spreading of particles from areas of different concentrations. There are two essential types of chemicals which frequently pass through this organism in its environment: water-soluble chemicals, and volatile chemicals (unpredictable chemicals that evaporate readily into gaseous form). While both are essential for the survival of the organism, volatile chemicals diffuse more efficiently. (7) (SR)

Review Questions


1. What is the relevance of C. elegans to modern-day biotechnology? (S.S)
2. What does the C. elegans reveal about the evolution of the digestive tract? (RG)
3. If the
4. If humans and C. elegans have similar nervous and digestive systems, and they have also led to some advanced research with cancer, could more parallels possibly be made? What other applications to humans today could C. elegans contribute, or have they reached their potential with the current research found? (AG)
5. What four cells can C. elegans use to remove waste? What is this excretory system also responsible for monitoring? (AC)
6. What does cell formation on the vulva during reproduction show about the mechanisms of molecular switches in development? (AC)

Cited Sources:

1. "Introduction to C.Elegans Anatomy." Wormatlas. Wormatlas, n.d. Web. 18 Nov. 2013.
2. "Hermaphrodite - Nervous System - General Overview." Hermaphrodite - Nervous System - General Overview. N.p., n.d. Web. 24 Nov. 2013.
3. http://www.wormatlas.org/ver1/handbook/excretory.htm
4. http://www.sfu.ca/biology/faculty/hutter/hutterlab/research/Celegans.html
5. Maguire, Sean M., Christopher M. Clark, John Nunnari, Jennifer K. Pirri, and Mark J. Alkema. "The C. Elegans Touch Response Facilitates Escape from Predacious Fungi." Current Biology 21.15 (2011): 1326-330. NCBI. U.S. National Library of Medicine, 28 July 2011. Web. 17 Nov. 2013.
6."Ecology of Caenorhabditis Species*." //Ecology of Caenorhabditis Species//. N.p., n.d. Web. 25 Nov. 2013.
(SF)
7. http://www.ncbi.nlm.nih.gov/books/NBK19972/
8. "The Roundworms." //Introduction to the Nematoda//. University of California Museum of Paleontology, n.d. Web. 21 Nov. 2013.
9. Ward, Paul. "Nematoda - Nematodes - Round Worms." Nematoda. N.p., n.d. Web. 21 Nov. 2013.
10. C."Our Scientists." Howard Hughes Medical Institute, 2013. Web. 22 Nov. 2013.
11. "Introduction to C. Elegans." //Introduction to C. Elegans//. Waksman Student Scholars, n.d. Web. 24 Nov. 2013.
12. "Caenorhabditis Elegans." //Animaldiversity.ummz.umich.edu///. University of Michigan Museum of Zoology, n.d. Web. 25 Nov. 2013.
13.http://www.bbsrc.ac.uk/upload/080221_sarid_nematode.jpg
14.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2619641/
15.http://www.biocontrol.entomology.cornell.edu/pathogens/nematodes.html
16. "The Molting Animals." Introduction to the Ecdysozoa. University of California Museum of Paleontology, n.d. Web. 28 Nov. 2013.
17. "Sting Neolaimus Longicaudn Turf." APSnet. The American Phytopathological Society, 2013. Web. 01 Dec. 2013.
18. Hökelek, Murat. "Nematode Infections ." Medscape. WebMD LLC, n.d. Web. 01 Dec. 2013.