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C.elegans by Payal Shah


Classification/Diagnostic Characteristics

Kingdom: Animalia
Phylum: Nematoda
Class: Secernentea
Order: Rhabditida
Family: Rhabditidae
Genus: Caenorhabditis
Species: C.elegans

This roundworm is amongst the most abundant and diverse animals, even though many of the nematoda species are not yet described. The C. elegans has a transparent, unsegmented, bilaterally symmetrical body that is generally 1 mm in length. the The basic anatomy of the worm is the pharynx, gonads, intestine, mouth, and cuticle integument. As a eumetazoan, it has a nervous system, specialized types of cell junctions, and organized tissues in distinct cell layers.


C. Elegan anatomy
C. Elegan anatomy
(3)(KG)



Relationship to Humans

These worms serve as "model organisms" in labs of geneticists and biologists because the worm is easy to cultivate, matures quickly (about 3 days) and has a fixed number of body cells. Because the organism can be so easily obtained and grown under lab conditions, it's genome has been completely sequenced. It has helped scientists learn more about miRNA and induction.

It is the only organism in which the entire cell linage can be traced. This includes 959 somatic cells, of which 302 are neurons. This has allowed scientists to trace the exact replication and specialization of every single cell in an organism. (7) (AA)

Furthermore, C. elegans have been instrumental in research relating to the nervous system. Because C. elegans only have 302 neurons and John White and many other researchers have studied these worms extensively, much about the C. elegans nervous sytem is known. In fact, each of C. elegans' neurons is named with two or three letter codes. Many types of neurons such as the motor and mechanosensory neurons of C. elegans are currently being studied, and a database called WormAtlas is compiling the morphology, location, and function of each C. elegans neuron. [1] (FZ)

Contains the CED-3 protein that is involved in cell death similar to the cell death in human cells, as well as the CED-4 protein which is involved in the apoptosis pathway also similar to humans.


Habitat and Niche

This particular nematode normally lives in the upper layers of temperate soil. The topsoil of rich farmland often contains 3-9 billion nematodes per acre, and a rotting apple itself can contain up to 90,000 individuals. The organism can also live in a laboratory as it serves as "model organism" for biologists. "Model organisms" are chosen because they are easy to grow and study in a lab environment, so their genetics can be studied because they often exhibit characteristics that represent a larger group.

C. elegans despite being one of the most heavily researched and experimented on organisms in the biology world has had very little investigation into its natural habitat and living conditions. This is probably because C. elegans is readily available in large quantities from a lab and rarely has to harvested form the soil. C. elegans are not easy to find in the soil and may be more easily located inside another organism which is known to have C. elegans in its digestive track, such as a snail. ([2] ) (DA)

Predator Avoidance

A common predator is soil fungi because the soil fungi form constricting rings that perfectly fit the nematode. The fungi forms a three-celled ring, so as the nematode crawls through one of these rings it stimulates the fungus to swell, trapping the worm.

The transparent covering also helps the organism avoid predators.

C. elegans have evolved and adapted in ways that help it escape predators such as soil fungi. A simple touch to the anterior half of the body of C. elegans results in an escape response in which the animal quickly reverses and escapes slow moving predators.[3] (CC)

Nutrient Acquisition

Nematodes exchange nutrients with the environment through the cuticle and the gut which has the thickness of one cell layer. The nutrients then move through the gut by the rhythmic contractions of the pharynx, a highly muscular organ at the front of the worm's body.

The nematode generally feeds on small animals, protists, other roundworms, and on the bacteria that grows on decaying vegetable matter.
Within the soil in which it lives, C.elegans feed on bacteria and fungi and rely on them as a source of nourishment.(14) (TM)

Reproduction and Life Cycle

The life cycle is about three days in which the roundworm goes from a fertilized egg to a adult worm. Also, the worm can reproduce sexually. The adult nematode can be a male or a hermaphrodite, meaning it contains both male and female organs. It lays eggs through a pore on the lower surface of the worm, the vulva. The anchor cell induces (causes) the vulva to form from six cells on the worm's lower surface. There are two molecular signals: the primary inducer and the secondary inducer. The anchor cell secretes a molecule called the LIN-3 protein, which acts as the primary inducer. The primary precursor cell (the one that received greatest concentration of LIN 3 protein) then secretes the secondary inducer. The secondary inducer acts on its neighbors because it is a lateral signal. These molecular switches determine the cell's fate. Each of the six ventral cells has three possible cell fates: they can become a primary vulval precursor cell, a secondary vulval precursor cell or simply an epidermal (skin) cell. Which of the three they become is was determined by which genes the primary and secondary inducers inactivate/ activate.

c. elegans have a life span of 2-3 weeks(HSC)


There are several larval stages, and it passes through 4 juvenile stages. In the fourth, it produces all the sperm, and then begins the production of oocytes.

The most common sexual form of C. elegans is the hermaphrodite (produces both sperm and eggs). Through self-fertilization, this animal can produce about 300 genetically identical offspring (assuming no mutations have occurred). After the production of sperm, oocyte nuclei are produced through meiosis and mature in a syncytium, a mass of multinucleate cytoplasm not separated into multiple cells. Just prior to fertilization, the cytoplasm and nucleus are enclosed by a membrane, and the eggshell is added following fertilization. The fertilized eggs are laid at about the 40-cell stage of development.(3) (JLev)


C. elegans has quite a short life cycle, which is totally dependent on temperature. For example, C. elegans will complete a cycle (egg to egg-producer) in 5.5 days in a 15˚C environment, compared with a 2.5 day cycle at 25˚C. On average, C. elegans will live approximately two weeks. Moreover, C. Elegans can respond to a lack of food by entering a sort of hibernation state called the dauer larval stage. In this interim state, the roundworm is thin and cannot eat. When enough food is available, the worm will return to a normal cycle. (3) (SJ)

C. elegans have six pairs chromosomes. All C. elegans have 5 pairs of autosomes (non sex chromosomes) and 1 pair of sex chromosomes. The sex chromosomes vary depending on whether the individual is a hermaphrodite or a male. Hermaphrodites will have two X chromosomes (XX) as there sex chromosome pair whereas males only have one X chromosome (XO). When there is only one chromosome instead of two it is called the hemizygous state. The hemizygous state can be produced by the loss of an X chromosome or by mating. Because of this state, the males cannot produce offspring on their own, but they can cross-fertilize hermaphrodites. (3). (WSS)

Ce_life_cycle.gif
C. elegans life cycle at 22˚C (SM)
[4]

Growth and Development

While growing, the roundworm sheds its cuticle four times. The nematode fully develops in three days: it goes from a fertilized egg to an adult worm with a nervous system. As an adult worm, it can digest food and reproduce sexually.

Studies show that it can survive with only 10% of it's genes active, this is known as the "minimum genome". Extra genes in the minimal genome encode proteins for cell differentiation, for intercellular communication, and for holding cells together to form tissues. Until recently, biologists considered DNA that wasn't coding for proteins, wasn't tRNA, and wasn't rRNA, "junk", but research on this organism showed that some noncoding regions are transcribed. The noncoding regions are very small, so the tiny RNA molecules are called miRNA.

As discussed in reproduction, the process of the differentiation of the vulval cells shows that much of the nematodes development is controlled by molecular switches that allow a cell to proceed down one of two tracks.

As a protostome, the mouth of the worm typically forms from the blastopore in early development.

Integument

The roundworm is characterized as a ecdysozoan because of it's periodic molting of the exoskeleton. Their unsegmented body has shape because of the thick, multilayered cuticles.

C. Elegans's integument (also known as cuticle) is the outer encasing or protection from the environment of the organism. Specifically, its cuticle is collagenous, which means that it is made of collagen (proteins commonly found in tissues of animals). Its cuticle plays an important role in locomotion, as well, because it attaches to the body wall muscles (8+9) (E.S.S.).

Movement

This roundworm moves by contracting it's longitudinal muscles.

This worm has no discrete joints, so the elasticity in the worm’s body wall and a limited number of muscles will lead to limited change in shape and motion. It moves with sinusoidal undulations of the body, which in simpler terms, means that it bends.5 (JLau)


Sensing the Environment

C. Elegans uses chemosensation to find food, avoid dangerous conditions, mate, and develop properly. It sense chemicals using specific chemosensory neurons that expose its sensory cilia to the environment. (6) (Shwetha)

In the soil C. elegans is exposed to both liquid and airborne chemicals. It detects polar attractants and volatile molecules.
Volatile molecules travel quickly through diffusion and turbulence in the air, whereas polar molecules in the soil tend to diffuse more slowly. Polar molecules are used for short range chemotaxis to bacteria. Where as volatile molecules are used as longer range chemotaxis to food. Chemotaxis is when smaller organisms such as bacteria direct their movement according to certain chemicals in the environment. (10) (BS)
C. Elegans avoids environments with elevated CO2 levels. Its temperature, O2, and salt-sensing neurons all also act as CO2 sensors mediating CO2 avoidance for the organism. AFD thermosensors, or temperature neurons, respond to increasing CO2 with a fall then rise in Ca (2+) levels and show a Ca (2+) spike when CO2 decreases. In addition, BAG O2 sensors and ASE salt sensors, or oxygen and salt neurons, are both activated by CO2 and stay tonically active with persisting high levels of CO2. C. elegans is able to homeostatically minimize exposure to elevated CO2 by using these sensory neurons. (11) (JF)

Gas Exchange

Oxygen is exchanged with the environment in a manner similar to the way nutrients are exchanged. The cuticle and gut serve as mediums for the exchange, and the gut is only one cell layer thick.

Materials are moved through the gut by contraction of the pharynx (CC)

C. Elegans need diffusion to have any forn of gas exchange, however they need to avoid high concentrations of CO2. (12)(ES)


Waste Removal

The excretory system of C. elegans consists of four cells: the pore, the duct, the canal (excretory cell) and the gland cell. This system is responsible for excreting metabolic waste, osmoregulation (maintaining the proper salt concentration in an organism) and excess fluid. C. elegans born without the excretory system will swell to death from the inability to dispose of their fluids.[4] (AY)

Environmental Physiology (Temperature, Water, and Salt Regulation)

Largely ectothermic, nematodes do not internally regulate their temperature; rather, their temperature is dependent on their environment. This dependence often inhibits or encourages the reproductive cycle of a nematode. Further, varying nematodes have differing optimal temperatures, where C. elegans live at best in 25 degree Celsius. (AWC)[5]

Internal Circulation

C. elegans lacks a circulatory system and has no skeletal parts. The worm is small enough that oxygen from the air diffuses through the body.

Chemical Control (i.e Endocrine System)

C. elegans has nuclear hormone receptors which regulate cell action the same way that hormones do in higher organisms. There are a variety of hormones released such as steroids and peptides. C. elegans is similar to Drosophila in its molting cycle (though it does not use ecdysone) and other regulatory functions. (13) (DM)



Review Questions

1. What types of genes is the minimal genome composed of and what percentage of the complete genome does the minimal genome comprise? (BB-V)
2. What is unique about the reproductive system of C.elegans? Does it reproduce asexually or sexually? Explain. (MC)
3. Why does a C. elegan form a three celled ring?(NC)
4. Why are C. elegans so importants to biologists and other scientists? (LC)
5. Explain how waste removal is essential in the survival of C. elegans. (Shwetha)
6. Explain the sensory neural systems that C. elegans utilize to avoid environments with high CO2 concentrations. (BH)


Sources
  1. Hillis, David M. Principles of Life. Sunderland, MA: Sinauer Associates, 2012. Print.
  2. http://blog.neuinfo.org/index.php/news-events/celegansdeepspace
  3. http://avery.rutgers.edu/WSSP/StudentScholars/project/introduction/worms.html
  4. http://www.wormatlas.org/ver1/handbook/excretory.htm
  5. Stephens, Greg J., Bethany Johnson-Kerner, William Bialek, and William S. Ryu. "Dimensionality and Dynamics in the Behavior of C. Elegans." PLOS Computational Biology:. N.p., Apr. 2008. Web. 10 Dec. 2012.
  6. http://www.wormbook.org/chapters/www_chemosensation/chemosensation.html
  7. http://joe.endocrinology-journals.org/content/190/2/191.full
  8. "Caenorhabditis Elegans." Animal Portal. N.p., 2003-2004. Web. 11 Dec. 2012.
  9. Page, Antony P., and Iain L. Johnstone. "The Cuticle." The Cuticle. Worm Book, 2007. Web. 11 Dec. 2012.
  10. http://www.ncbi.nlm.nih.gov/books/NBK19972/
  11. http://www.ncbi.nlm.nih.gov/pubmed/21435556
  12. http://www.gpnc.org/beaver.htm
  13. http://www.wormbook.org/chapters/www_nuclearhormonerecep/nuclearhormonerecep.html
  14. http://avery.rutgers.edu/WSSP/StudentScholars/project/introduction/worms.html
  1. ^ http://thalamus.wustl.edu/nonetlab/ResearchF/elegans.html
  2. ^ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2630584/
  3. ^






    http://www.ncbi.nlm.nih.gov/pubmed/21802299
  4. ^

    http://www.sfu.ca/biology/faculty/hutter/hutterlab/research/Celegans.html
  5. ^





    http://www.biocontrol.entomology.cornell.edu/pathogens/nematodes.html