Caenorhabditis elegans

Taxonomic Classification:

Kingdom- Animalia

Phylum- Nematoda

Class- Secernentea

Family- Rhabdiyidae

Genus- Caenorhabditis

Species- Caenorhabditis (see-no-rab-dye-tiss) elegans

Description:

This species of roundworm is the most intensely studied organism in the laboratory. Due to specific features, it's advantageous to the researcher to study this animal. This model organism is small (~1 mm) and free-living in soil environments. It primarily feeds on bacteria and in laboratory settings is fed E. coli because E. coli is inexpensive and grows easily. The entire genome sequence (~20,000 genes) of this organism is known and scientists have manipulated its genes to determine the function of many. A process called "eutely" occurs where the number of cells does not increase, instead the size of cells increase causing growth. In females, there are exactly 959 cells that have been scientifically followed throughout development. The body plan of C. elegans is relatively simple. It is unsegmented and cylindrically shaped with tapered ends. We see bilateral symmetry, mouth and brain at the anterior end, anus at the posterior end, homogenous morphology, and two layers (outer epidermis and inner muscular layer). The intestine is formed from endodermal cells and have a collagen-rich cuticle on the outside for protection that molts four different times during development. Reproduction in C. elegans makes it advantageous to study because of how versatile it is. There are two sexes, hermaphrodite and a male, where the hermaphrodite is a modified female that can produce its own sperm (self-fertilization) or by cross fertilization (mating). In terms of advantageous characteristics, researchers focus on the worms that self-fertilize because a heterozygous female can produce homozygous progeny (genetically identical). A typical life cycle is around three days under optimal conditions, which is very short and is beneficial in experiments where results are needed fast. This species has an offspring of 300 plus progeny.

Figure 1: Adult Hermaphrodite

Locomotion and Feeding Behavior:

Neural circuits are extremely important when it comes to locomotion in the C. Elegans. There are at least four interneuron pairs of longitudinal nerve cords that control forward and backward movement. The characteristic movement of this organism consists of a sinusoidal pattern where we see waves in the body plans from the ventral and dorsal turns. To make this sinusoidal pattern, we see differing contractions of the muscles. If the worm needs to turn ventrally, the ventral muscles will contract while the dorsal muscles are relaxed and the opposite applies for dorsal movement. In Figure 2 below, we see the characteristic movement in waves where given a touch stimulus, the worm will reverse movement in a wave pattern. These contractions are opposed by the force of hydrostatic pressure within the worm. In feeding behavior, a crucial part of their success involves the major chemosensory organs called amphids (located around mouth). With these highly developed sensory organs, C. Elegans are able to sense chemicals with sensory cilia that permeate the cuticle. Each amphid contains twelve sensory neurons. Along with amphids, there are phasmids (located around tail) and inner labial (located around mouth) neurons that are also open to the environment. These sensory organs detect food, danger, and other organisms. Figure 3 shows how developed the organization of the genes within the chemosensory organs are in relation to the ends of the worm. We see grouping of neurons in close proximity to the nerve ring and amphid sensory openings. Also, we see grouping of neurons at the other end of the worm near the phasmid sensory openings. Another huge factor in feeding is the pharynx, which connects the mouth and intestine. This muscular tract has a combination of two motions, pumping and isthmus peristalsis, to feed on bacteria. Pumping involves intaking environmental particles within liquid by sucking it in and then driving out the liquid to keep the particles. Isthmus peristalsis involves peristaltic (wave of contraction) down the pharynx.

Figure 2: Sinusoidal Locomotion

Figure 3: Organization of Chemosensory Organs

A Biomedical Inquisition:

C. elegans has become one of the most important invertebrate animals to study in the laboratory along with the famous Drosophila (that most high school students study in their biology class). Many fields explore this organism including: immunology, toxicology, genetics, neurobiology, pathology, aging, and much more. Current research is being done on drugs in the pharmaceutical industry to test their effect on alleviating or even treating certain diseases. Earlier this year, a study was done on a possible drug called Betulin to alleviate symptoms of Parkinson's Disease using transgenic (crossed DNA) C. elegans models. A characteristic feature of Parkinson's includes a clustering of a protein called alpha synuclein. Using this species, researchers were able to test potencies and physiological effects on the worms to make conclusions about the strength of anti-neurodegeneration. Results supported the possibility of Betulin usage for Parkinson's patients by decreasing the protein (alpha synuclein) clustering and neuron degeneration in specific locations. Another fascinating topic of research currently being done includes studying genes of this species to look further into aging in humans. Using the lifespan of the worm, researchers were able to measure the aging process in humans. Recognition of genes that are key players in regulating lifespan opened up the opportunity to alteration of length of life. Not only did they find that genes play a role in aging, but environmental factors do as well. In addition to neurodegenerative disorders and aging, other research is being done regarding innate immunity (our first line of defense against pathogens). The worms are infected by pathogens, specifically different types of bacterium, fungi, parasites, and viruses. They found that even though there are a significant amount of biochemical pathways shared between humans and this species, there are certain differences when it comes to immunity. In humans, we have cell-mediated immunity (release of complex cells from the host to fight and kill an invasion). C. elegans lacks this and instead have a more simple response with a release of antimicrobial cells to fight the invasion. Another difference involves the mechanism of infection of a pathogen. In humans, pathogens manifest within the body and spread. In C. elegans, they found that this species does not exhibit this behavior. The infections did not internalize. Researchers find this species beneficial in immunity due to the ability to quickly observe the impact of a pathogen infection and further opening up opportunities to test new drugs to kill the pathogen. Although they have a simplistic body plan and small number of cells, C. elegans has rose above and beyond to become one of the most versatile experimental organisms today. Copious amounts of research is being done using these models, including exploring cures for cancer. So, before you conclude that they are too simplistic to reveal discoveries, think again. One day, these worms may impact you directly!

Fun Facts about C. elegans:

• The excretory and respiratory systems are absent in this organism.

  • Circular musculature is absent in this organism (and all of Nematoda).

• High hydrostatic pressure present allows the worm to maintain form during locomotion.

• There are no parasitic forms of this species.

• Males are hardly seen in the wild.

• Storage in liquid nitrogen is used to freeze the worms until further use.

• Although this species is relatively simplistic, we share many pathways and roughly the same amount of genes making it easy to manipulate for studying common human diseases.

  • Their entire life cycle is around two weeks. In Figure 4 below, we see the cycle that includes Dauer larvae, an alternative developmental stage that results from harsh conditions. Their fast development is especially advantageous in the laboratory and in research where results of experiments can be observed quite quickly.

Figure 4: C. elegans Development

Works Cited:

Alberts, Bruce. “Caenorhabditis Elegans: Development from the Perspective of the Individual Cell.” Current Neurology and Neuroscience Reports., U.S. National Library of Medicine, 1 Jan. 1970, www.ncbi.nlm.nih.gov/books/NBK26861/.

Avery, Leon. “C. Elegans Feeding.” Current Neurology and Neuroscience Reports., U.S. National Library of Medicine, 1 Jan. 1970, www.ncbi.nlm.nih.gov/books/NBK116080/.

Bargmann, Cornelia I. “Chemosensation in C. Elegans.” Current Neurology and Neuroscience Reports., U.S. National Library of Medicine, 25 Oct. 2006, www.ncbi.nlm.nih.gov/books/NBK19746/.

Marsh, Elizabeth K and Robin C May. “Caenorhabditis elegans, a model organism for investigating immunity” Applied and environmental microbiologyvol. 78,7 (2012): 2075-81.

Riddle, Donald L. “Introduction: the Neural Circuit For Locomotion.” Current Neurology and Neuroscience Reports., U.S. National Library of Medicine, 1 Jan. 1997, www.ncbi.nlm.nih.gov/books/NBK19982/.

Tsai, Chia-Wen, et al. “Neuroprotective Effects of Betulin in Pharmacological and Transgenic Caenorhabditis Elegans Models of Parkinson’s Disease.” Cell Transplantation, vol. 26, no. 12, Dec. 2017, pp. 1903–1918.

“Using C. elegans for aging research” Invertebrate reproduction & development. vol 59, sup1 (2014): 59-63.