Model Organism Week: Getting to Know Your Worms (C. elegans)

What is so impressive about microscopic nematodes as a biological model? Sidney Brenner first answered that question in his 1974 paper “The Genetics of Cenorhabitis elegans” where he introduced the nematode as an ideal model for studying genetics. The purpose of this blog is to enlighten the community about C. elegans and their contribution to science.

The longer I do research in other fields, the more I long for the unique advantages of the little worms. Granted they are not the system of choice when modeling complex human diseases, but they have a lot to offer in determining the eccentricities of cellular, genetics, and developmental processes.

Reasons why C. elegans are great model organisms

1. Short generation time: between 3 days-1 week depending on temperature.
2. Stocks can be frozen: the eggs survive the freezing process so stocks don’t have to be continuously propagated.
3. Very easy to propagate and recover: a square of agar can be cut out of starved plates and placed upside down on new agar plates, recovering the strain. This method can also be used to propagate strains.
4. Very cheap to maintain: essentially all you need for C. elegans research is a microscope and an incubator, no expensive animal house costs.
5. Temperature sensitive: the worms grow at different speeds depending on the temperature. See life cycle.
6. Present no biohazard: not much paperwork and no IRB necessary. Agar plates do not require special disposal.
7. The first complex organism to have its genome sequenced: therefore it’s the most annotated.
8. The fate of all cells are known: mostly all cells have been traced through development, making C. elegans a great developmental model.
9. Simple organisms with complex structures: such as a nervous system, digestive system, and extensive germline.
10. Very easy to do forward genetic screens: see genetics section.
11. Plentiful morphological markers: see mapping section.
    

    Picture taken from Jan Kammenga, Wageningen University

12. RNAi was discovered in C. elegans and can be incorporated by feeding. The Arliger lab has created an extensive library of RNAi constructs in bacteria. These can be grown up and fed to the worms.
13. Embryos and adults are translucent. Easy to GFP labeled structures and other phenotypes.
14. Balancers for each chromosome exist. Recessive sterile and lethal mutations can be stably propagated and easily detected. Also many balancers are marked via GFP.
15. The community is very fun. It is easy to collaborate and the regional meetings are a blast!
16. There are great resources. I cannot think of another model organism with such user friendly resources, see references.
17. Hermaphroditic, self mating allows for efficient generation of recessive mutants
18. Large broad size: for easy screening and large N value. Single Hermaphrodite has about 300 progeny.
19. They are not cute and fuzzy. One does not feel remorse for harming a worm as one might when working with more complex organisms. They do not make noise and they do not bite.

Drawbacks

1. No site directed mutagenesis, due to only one crossover per chromosome. There are a couple ways around this. Forward genetics screens are very easy, see genetic section. Also transgenic constructs can be either injected or bombarded into worms.
2. Researchers have to argue for funding. It is sometimes difficult to argue for the applicability of research human drug and disease.
3. Plates can become contaminated by fungus, other bacteria, or mites. Low levels of contamination can be countered by chunking of good regions. Massive contamination can be eliminated by successive rounds of bleaching.

What are C. elegans?

C. elegans are microscopic, translucent worms that have become one of the major model organisms in science. While C. elegans are close cousins to parasitic nematodes, they appear to be harmless to higher organisms and reside primarily in the soil subsisting on E. coli². C. elegans have a 100mb genome spread over 6 chromosomes. C. elegans are primarily XX hermaphrodites, capable of self-fertilization, with the occasional XO male that arise through spontaneous deletion of one X (also may be obtained through crosses). Adult hermaphrodites are ~1mm in length, and have massive brood sizes of ~300 progeny. Even though these worms are a relatively simple organism, they contain many complex structures including, a complete nervous system, gut, and an extensive germline.

Life Cycle

The life cycle of C. elegans varies depending on certain environmental conditions. In ideal conditions, upon hatching juvenile C. elegans proceed through 4 different larval stages as seen in figure². If resources are scarce, such as in conditions of over crowding, juvenile C. elegans enter a dauer in which they become elongated and limit their consumption of resources until conditions improve. Temporal control of the C. elegans life cycle is temperature sensitive. At 25ºC a generation time is about 3-4 days, where as it extends to almost a week at 16ºC. Some mutations in C. elegans are also temperature sensitive, only showing a phenotype at either 16ºC or 25ºC.

Diagram from Jorgensen and Mango, Nature Reviews Genetics 3:356-369; graph from M. Blaxter.

Germline and Mating

The germline of the adult hermaphrodite in composed of two gonad arms that produce the developing oocytes. These oocytes pass through the spermatheca where they become fertilized. Fertilized embryos collect in the uterus until laid. Since the worms are translucent, pregnant females are easily observed. Male C. elegans have only one gonad arm which produced sperm. Upon mating with the hermaphrodite, the sperm from the male C. elegans proceeds to the spermatheca where it exhibits a competitive advantage over the originating sperm, therefore cross progeny occur more frequently than self progeny². A self-fertilizing hermaphrodite has about 300 progeny.

Figure taken from the Hansen lab homepage, University of Calgary

Genetics

Why C. elegans are my favorite genetic model

All model organisms have their advantages, or else they would not be selected as a suitable model. C. elegans harbor several unique advantages when it comes to studying genetics. C. elegans were the first complex organism to have its genome sequenced. Therefore the C. elegans genome is the most highly annotated and probably best understood⁴.

Forward Genetic Screen

C. elegans are a very robust tool studying forward genetics. Mutations can be easily induced by many methods, though EMS (ethyl methane sulfonate), ionizing radiation, and transposon hopping are the main three⁴. The self fertilizing capabilities, the short generation times, and the abundance of progeny allow for quick isolation of phenotypes of interest. Newly isolated strains are then often out-crossed by mating with wildtype males in order to reduce background mutations. These strains can then be frozen for later recovery, another advantage unique to C. elegans.

Mapping

Thousands morphological markers have been identified can easily be used to determine linkage^1,5. Most are morphological and behavioral in nature. Many identified in Brenner’s introductory paper are still commonly used. That paper also describes in great detail how ideal C. elegans are for mapping. Another very useful mapping tool discovered in C. elegans is Snip-SNP mapping, which combines using morphological marker and SNPs between two different types of wildtype worms (mainly Ns Bristol and Hawaii). This technique allows for more precise mapping. Many balancers for each of the 6 chromosomes have been constructed; which allows for easy detection and propagation of many recessive sterile and lethal mutations. For more detail, see the mapping section of wormbook.

Resources on C.elegans

This is not a complete list of resources, just those that I have found very helpful over the years. All of them and many others can be accessed via wormbase http://www.wormbase.org/.

Wormbase is the primary resource for C. elegans research. It is the best online database available (blows NCBI and flybase out of the water). This resource alone makes working with any other model system truly irritating.

The genetics of Caenorhabditis Elegans by Sydney Brenner– This is the paper that started it all. It is truly amazing what Dr. Brenner and colleagues accomplished. The mutation details and mapping schemes are incredible. This is a “must read” for any C. elegans researcher or anyone interested in genetics.

Wormbook; the great minds of the C. elegans community banded together to create this resource to C. elegans research. It is very helpful, though it does not give a good introduction to the model organism.

Worm Breeders Gazette; this is a new thing on wormbase, I have not used it, but it sounds interesting.

Wash U. SNP site – a very useful site for Snip-SNP mapping

Worm Atlas – Another great database and introductory tool

Caenorhabditis Genetics Center (CGC) – C. elegans stock center at the University of Minnesota, where many C. elegans strains are stored.

_________________________________________________________________________________________

Jessie a third year graduated student, who sadly does not currently work with C. elegans. She spent 4 years working with C. elegans, as an undergraduate. They not only provided my introduction to research, but also my introduction to genetics, a subject she dearly loves. During her graduate rotations, she made it a point to work with several different model organisms and only then did she realize the true power to C. elegans as a genetics tool. Jessie currently works on modeling Human Diffuse Gastric Cancer in mice at Washington University in St. Louis.

_________________________________________________________________________________________

References

1 Brenner S, 1974. The genetics of Caenorhabditis elegans. Genetics 77:71-94.

2 Blaxter, M An introduction to the nematode Caenorhabditis elegans. Institute of Evolutionary Biology, University of Edinburgh, UK

3 L’Hernault, S.W. Spermatogenesis (February 20, 2006), WormBook, ed. The C. elegans Research Community, WormBook, doi/10.1895/wormbook.1.85.1,  Link.

4 Hodgkin J. Introduction to genetics and genomics (September 6, 2005), WormBook, ed. The C. elegans Research Community, WormBook, doi/10.1895/wormbook.1.17.1, Link.

_________________________________________________________________________________________

See other articles in the series:

Drosophila melanogaster- The fruit fly.

Research’s Next Top Model (Zebrafish)

EEK! The Mouse.

Saccharomyces cerevisiae a.k.a Budding/Baker’s/Brewer’s Yeast

The Almighty Fungi: The Revolutionary Neurospora crassa

We’re Gonna Need a Bigger Lab: Large Animal Models in Research

Tetrahymena: Little Creature, Big Discoveries

.

.