C.elegans

By Lily Shapiro **Classification/ Diagnostic Characteristics ** //Caenorhabditis elegans// is a member of the nematodes, commonly known as roundworms, which are one of the most abundant of all animal groups, with over 25,000 species distributed throughout the world. //C. elegans// is specifically a type of ecdysozoan within the bilateral invertebrates, and ecdysozoans are classified by an external cuticle that covers the outside of their body. Ecdysozoans in themselves are a part of a group called protostomes (one of two groups), which develop their mouths before the rest of their bodies and are one of the major groups within bilateria- the bilateral organisms. Protostomes also also have a brain surrounding their digestive systems, and a nervous system made up of fused chords. The common ancestor of the protostomes likely had a coelom, or an internal body cavity, but the organisms within this group vary wildly in this characteristic; sometimes the coelom has been highly modified or has disappeared completely. //C. elegans// does not have an actual coelom, but something called a pseudocoelomate, its body cavity is only lined by muscles on the outside, unlike coelomate organisms which have muscle lining on the outside and inside of their body cavities.

**Relationship to Humans ** //C. elegans// is frequently used as a model organism studied by geneticists and developmental biologists because of the fact that its genome has been completely sequenced, and it grows to adulthood in 3 days from a fertilized egg and lives for two or three weeks. The genome of //C. elegans// is 8 times larger than that of yeast and contains 8 times the amount of protein-coding genes for study, but is able to survive in a laboratory with only a minimal portion of these genes. In fact, all 959 somatic cells of //C. elegans// have been traced in scientific experiments. These characteristics make the nematode a prime specimen for laboratory study. Although many nematodes are parasitic and can often infect humans //C. elegans// is an exception.

Similarly to the hydra, //C. elegans// can slow its aging, in the form of inhibiting a similar to insulin aging factor, leading to its inclusion in research on aging. In animal sleep studies, //C. elegans// was shown to be the most primitive organism to exhibit sleep-like behavior. Interestingly enough, samples of //C. elegans// were found to have survived the Space Shuttle Columbia disaster in 2003, leading to further studies of the effects of zero gravity on specimens.

**Habitat and Niche ** These nematodes inhabit temperate environments. They live in upper layers of topsoil unlike other nematodes which frequently spend their lives as parasitic organisms. They often live in mushroom beds and garden soil. They can also thrive in rotting fruit; in fact, up to 10,000 //C. elegans// have been observed to occupy the same rotting fruit. Ecdysozoans often have to thrive in moist environments because of a thin cuticle. The soil also must have a moderate oxygen content. //C. elegans// thrives in cooler climates than some of its closely related species, namely //C. briggsae//.

**Predator Avoidance ** Being in the phylum of ecdysozoa, //C. elegans// has a thick external cuticle surrounding its body, providing the nematode with a certain level of protection in addition to support. The cuticle is secreted by the epidermis underneath, but must be molted, leaving the organism vulnerable for periods of time. In general, nematodes are the predators, and do not require a large amount of protection or tactics to avoid consumption.

**Nutrient Acquisition ** To obtain nutrients and oxygen, //C. elegans// uses its outer cuticle in coordination with its gut, and it is a layer only one cell thick. To move food throughout its body, the pharynx, a muscular organ near the head of the worm, rhythmically contracts, pushing nutrients throughout. //C. elegans// alike to other soil-dwelling nematodes that also are scavengers in the topsoil.

**Reproduction and Life Cycle ** While many parasitic nematodes undergo drastically specialized life cycles in order to spread throughout their hosts and reproduce, //C. elegans// thrives in the soil, and has no need to grow in another organism as that of a parasitic nematode. Nematodes must shed their cuticle four times in their lifetime in order to reach maturity. Nematodes reproduce sexually. //C. elegans// has a very short life cycle of about two to three weeks. The life cycle is comprised of six stages: embryonic, four larval stages (L1-L4), and adulthood. Transitioning from one larval stage to the next, //C. elegans// sheds its outer layer and replaces it with a new one that is specific to a different stage. There are three parts to molting, apolysis, synthesis of a new cuticle, and ecdysis. In apolysis, //C. elegans// breaks off from the obsolete cuticle. //C. elegans// then secreates a new cuticle from the hypodermis. Ecdysis concludes molting, in which the old cuticle is shed from the hypodermis. From one egg to the next egg it usually takes about 3 days. Eggs are fertilized within the adult hermaphrodite and laid in a few hours after at the 40-cell stage. The eggs hatch and proceed through four larval stages and as they reach the end, they tend to have around 300 progeny and then they die. Their life cycle is usually very temperature dependent and under certain temperatures, they have certain fitnesses. As the temperature increases, their life span decreases because they can't sustain under the hot temperatures.



**Growth and Development ** <span style="font-family: Arial,Helvetica,sans-serif;">//C. elegans// is especially useful to scientists because it develops over 3 days from a fertilized egg to a worm that has the ability to digest food, contain a nervous system and reproduce sexually. //C. elegans// simply require the molting of their cuticle in order to grow and mature. The cuticle cannot grow on its own after formation, and so C. elegans requires a new one to grow and must be replaced four times. In early stages of development, however, //C. elegans// forms three layers of cells, as many animals: an outer ectoderm, and inner endoderm (which forms the gut), and a middle layer called the mesoderm. Afterwords they undergo gastrulation, in which a one-cell thick ball inverts, creating an opening to a cavity called the blastopore. Because //C. elegans// is a protostome instead of a deuterostome (both groups are part of bilateria), their mouths develop from this blastopore first, instead of their anus. Later on in development, roundworms appear to become pseudocoelomates, in which they have a body cavity that is filled with fluid in which their internal organs are suspended in, enclosed by muscles from the mesoderm on the outside.

**<span style="font-family: Arial,Helvetica,sans-serif;">Integument ** <span style="font-family: Arial,Helvetica,sans-serif;">Unlike lophotrochozoans, the main group of protostomes, all ecdysozoans have an external cuticle used for protection and support. The cuticle is put in place by the outermost layer of cells- the epidermis which arises from the ectoderm in development. This cuticle must be shed for molting, and in some organisms it forms a rigid exoskeleton, however in //C. elegans// the cuticle is thick and multilayered but still allows worm-like movement throughout the phyla Nematoda. Nematodes shed their cuticle four times in their life cycle. The cuticle of //C. elegans// is primarily made of two proteins: collagen and cuticlin. The epicuticle, or outermost layer of the cuticle, is made constructed of lipids and and other proteins.

**<span style="font-family: Arial,Helvetica,sans-serif;">Movement ** <span style="font-family: Arial,Helvetica,sans-serif;">Even with a multi-layered cuticle, //C. elegans// still retains the wormlike movement that is apparent in all nematodes. To move, they contract their longitudinal muscles, or those lengthwise along their body. Because //C. elegans// is a pseudocoelomate, they have less control over their body movement than coelomate organisms. The structure of this body cavity highly influences movement, and when //C. elegans// compresses its muscles, the fluids in the cavity move from region to region causing them to expand or contract, allowing organisms to move specific body parts.

<span style="font-family: Arial,Helvetica,sans-serif;">A video illustration of the worm-like movement of //C. elegans://

media type="youtube" key="GgZHziFWR7M" width="560" height="315" align="center"

**<span style="font-family: Arial,Helvetica,sans-serif;">Sensing the Environment ** <span style="font-family: Arial,Helvetica,sans-serif;">Without the appendages of other organisms used for sensing, //C. elegans// must rely on its basic ventral nervous system for detecting the environment around it. This nervous system is simply made up of a ring of nervous tissue surrounding the pharynx and dorsal and ventral nerves that run through the length of its body.

**<span style="font-family: Arial,Helvetica,sans-serif;">Gas Exchange ** <span style="font-family: Arial,Helvetica,sans-serif;">Nematodes including //C. elegans// exchange oxygen with their environment through their cuticle and their gut, which is only one cell thick and allows diffusion between the surrounding habitat. <span style="font-family: Arial,Helvetica,sans-serif;">Nematodes do not have a circulatory system, so all gas exchange and excretion occurs by diffusion through the cell wall of the cuticle and the gut.

**<span style="font-family: Arial,Helvetica,sans-serif;">Waste Removal ** <span style="font-family: Arial,Helvetica,sans-serif;">//C. elegans// excretes waste through their anus located at the posterior end (away from their head) of their body attached to their gut, but also have an excretory tube through which waste can exit.

**<span style="font-family: Arial,Helvetica,sans-serif;">Environmental Physiology ** <span style="font-family: Arial,Helvetica,sans-serif;">Many species of ecdysozoans have thin cuticles and are restricted to moist habitats because of their need to diffuse water with their environment, and //C. elegans// requires a constant supply of moisture because of this. Nematodes are ectothermic, and so their temperature is dependent on the environment in which they live in.

**<span style="font-family: Arial,Helvetica,sans-serif;">Internal Circulation ** <span style="font-family: Arial,Helvetica,sans-serif;">//C. elegans// does not have a defined circulatory system, instead a pseudocoel in which the bodily fluids float in, it is unnecessary because of their small, sometime microscopic size (for the most part) and their use of diffusion. These fluids shift with contracting muscles, moving them from one area to another.

**<span style="font-family: Arial,Helvetica,sans-serif;">Chemical Control (Endocrine System) ** <span style="font-family: Arial,Helvetica,sans-serif;">//C. elegans// is one of the few forms of nematodes that contain an endocrine system even though it is considered primitive in evolutionary development. Insulin-like molecules, with the help of DAF-2/insulin(a receptor) and DAF-16/FKHRL1/FOXO(a transcription factor), control stress management and the determination of lifespan in //C. elegans.//

**<span style="font-family: Arial,Helvetica,sans-serif;">Review Questions ** <span style="font-family: Arial,Helvetica,sans-serif;">1. How is the life cycle of //C. elegans// differ from the life cycle of other parasitic nematodes? <span style="font-family: Arial,Helvetica,sans-serif;">2. What differentiates the internal cavity structure of //C. elegans// from other protostomes? <span style="font-family: Arial,Helvetica,sans-serif;">3. How does this internal cavity contribute to fluidity and movement within the organism? <span style="font-family: Arial,Helvetica,sans-serif;">4. Why is //C. elegans// vulnerable to predation at certain instances in its life cycle? <span style="font-family: Arial,Helvetica,sans-serif;">5. Why is //C. elegans// a useful organism for scientific study?

**<span style="font-family: Arial,Helvetica,sans-serif;">Sources ** <span style="font-family: Arial,Helvetica,sans-serif;">http://www.easternct.edu/~adams/images/Celegansmicrograph.jpg <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">http://animaldiversity.ummz.umich.edu/accounts/Caenorhabditis_elegans/ <span style="font-family: Arial,Helvetica,sans-serif;">http://genome.wustl.edu/genomes/detail/caenorhabditis-elegans/ <span style="font-family: Arial,Helvetica,sans-serif;">http://wormclassroom.org/short-history-c-elegans-research <span style="font-family: Arial,Helvetica,sans-serif;">http://avery.rutgers.edu/WSSP/StudentScholars/project/introduction/worms.html <span style="font-family: Arial,Helvetica,sans-serif;">http://www.darwinsgalapagos.com/animals/nematoda_roundworms.htm <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[] <span style="font-family: Arial,Helvetica,sans-serif;">[]

http://animaldiversity.ummz.umich.edu/accounts/Nematoda/