View It

 Abstract

This activity is divided into two separate lab sessions. In the first session students will become familiar with the morphology, behavior, and life cycle of normal wild-type C. elegans, the N2 strain. (Note: If time allows, it is interesting for students to also view behavioral mutants. A list of mutants is available at Amanda Kahn's UCSF site, http://bargmann.ucsf.edu/~amanda.) In the second session students will expose N-2 worms to two different concentrations of ethyl alcohol and observe movement behavior.

Content Information Important for the Teacher:
C. elegans in brief

C. elegans is a free-living soil nematode. Its approximately 1mm. body is composed of 959 somatic cells, including approximately 300 neurons. The body is essentially a tube (the cuticle), with two smaller tubes inside it - the pharynx and the reproductive system (which takes up most of the volume.)

C. elegans moves by flexing and relaxing four logitudinal bands of muscles comprised of some 81 muscle cells. This alternating between flexing and relaxing creates an easily recognizable sine-curve movement. When cultured on plates, the worms leave a "track" in the agar which can be measured, if desired.

C. elegans has a primitive nervous system which consists of sensory neurons in the head which are capable of responding to taste, smell, touch, and temperature. These neurons communicate with a "nerve ring" in the anterior of the body, which in turn communicates with a nerve cord running dorsally along the body. The nerve ring functions roughly like a mammalian brain, receiving signals from the sensory neurons and sending signals to the muscles via the nerve cord.

C. elegans is usually hermaphroditic, with two X chromosomes, or a genotype of XX. Nondisjunction will occasionally result in a male, who has only 1 X chromosome, or a genotype of XO. (Note: exposure of hermaphrodites to a 10% solution of ethanol greatly increases the rate of nondisjunction, resulting in an abnormally high number of male offspring. This is useful if setting up genetic crosses.) C. elegans progresses through four stages to adulthood (mating of male and hermaphrodite), lays eggs, and produces a new generation in a cycle of approximately four days at classroom temperature. (more mating images, 1 and 2)

C. elegans is easily cultured on petri dishes with a nutrient medium which has been seeded with E. coli, its preferred food. The dishes can be kept at room temperature and require no special handling, other than normal precautions agains contamination by foreign bacteria or mold. (C. elegans is remarkably tolerant to mold, less so to foreign bacteria.) C. elegans has a dauer, or "spore-like" state and may be kept on a parafilmed plate for up to three months at room temperature. It will revive and resume its life cycle when placed on a clean seeded plate in a procedure known as "chunking" (explained in the next section.) Therefore, it is simple to keep a culture of C. elegans going without constant picking to new plates - one can simply let a culture "starve out" (go into dauer state due to lack of food) and keep it until a new culture of live animals is needed. C. elegans should be handled using a "pick" made from a Pasteur pipette (directions follow in the next section) and a small piece of wire, preferably platinum, although fine gauge nichrome wire will do. The worms can be easily seen and "picked" using a standard classroom dissecting microscope. The light from a standard classroom compound microscope is too harsh - exposure for more than a few seconds will kill the animal.

Additional information about obtaining, culturing, handling, and disposing of C. elegans is available from Amanda Kahn's website in her "C. Elegans 101: A Worm Breeder's Primer" at http://bargmann.ucsf.edu/~amanda.

Type of Lab

This lab is a socially relevant, inquiry-based, constructivist learning activity. Students are to observe animal behavior after exposure to a known substance of abuse, infer the effects of the substance on the nervous and muscular function of the organism, and extrapolate the effects of the substance on human nervous and muscular system function. Student observations and inferences should lead to student-generated questions about nervous system function, muscular function, and the specific effects of other substances of abuse on these and other systems.

Instructional Strategies Used

Lecture, demonstration, discussion, co-operative group work, research design, hands on use of scientific research methods.