The nematode worm Caenorhabditis elegans (C. elegans) seems, if anything, even less like humans than the fruit fly. Yet, like the fly, it has some particular advantages as a model organism. Nematodes are:
- some of the most plentiful animals on earth, and include both parasitic and free-living species;
- anatomically simple, but have many of the sophisticated processes seen in other animals;
- distantly related to insects, including Drosophila, and humans, though their placement in the tree of life is still an active area of research.
As with the fly, however, the more one investigates the worm's anatomy, life cycle, and biology, the more one finds that can potentially shed light on the biology of other organisms, including humans. Let's take a closer look.
The Life Cycle of C. elegans -- Growing up Fast, Dealing with Stress
In 1963, the South African biologist Sydney Brenner proposed C. elegans as a model system for studies of development and neurobiology. Many of the worm's advantages as a model system recall similar advantages in Drosophila -- the worms are small (only 1 mm in length), so a single petri dish can hold about ten thousand worms; and they reproduce quickly, with a newly laid egg taking only three days to mature into an egg-laying adult -- which, in turn, is capable of producing 300 or more offspring in a few days.
A closer look at the worm's life cycle (figure at right) reveals some intriguing details:
- Stress response. A day after the worms hatch, they may reach their first fork in the road. If conditions are generally favorable, they proceed directly through several larval stages to fertile adulthood (outer loop in the diagram). But if the environment looks potentially harsh -- with little food, high temperatures, or too many competitors (signaled by a pheromone) -- the worms can enter an alternative larval stage called the dauer state, a sort of dormancy that can let them ride out the rough conditions for up to four months. If and when things improve, the dauer worms can then continue to full fertile adulthood.
- The sunset years weeks. After becoming a fertile adult, a worm can produce eggs for about three days. Once it has laid its complement of eggs and stopped reproducing, it lives on for a few weeks. Perhaps surprisingly, that short life span has made C. elegans a very productive model in studies of aging in humans, as one of the Vignettes below will explore.
Web Mission: Worm Sex
The WormAtlas site, maintained at the Albert Einstein College of Medicine in New York, includes a rich overview of the C. elegans life cycle and anatomy. Go to that page, and read quickly through the opening paragraph. One unusual feature of this worm -- at least in human eyes -- is the nature of its two sexes -- the hermaphrodites, with two X chromosomes, are capable of self-reproduction (creating both sperm and egg internally), or can reproduce with male worms. Those males, in turn, have only one X chromosome, but lack the Y chromosome seen in many other species. In other words, hermaphrodites have a pair of sex chromosomes (XX); males have only one sex chromosome (X0).
To learn more about hermaphrodite reproduction, scroll down to the first figure on the WormAtlas anatomy page. It shows a photomicrograph of an adult hermaphrodite worm, and a diagram of the corresponding major organ systems. Note the amount of "real estate" taken up by reproduction -- the system stretching from the proximal gonad through the uterus to the distal gonad, and looping around the digestive tract. More starkly than many more complex animals, C. elegans truly looks like a system tuned for eating and reproduction.
A hermaphrodite worm using self-fertilization -- the more common form of reproduction -- essentially creates a genetic duplicate of itself. Another Web resource from the Waksman Student Scholars Program at Rutgers University describes the process of self-fertilization (scroll down to "Sex and the single nematode"). But hermaphrodites can also mate with males (return to the WormAtlas page, and scroll to the section titled "Adult," at the end of the page for more information.) Note that if a worm uses self-fertilization, its offspring will tend to number around 300, whereas if the hermaphrodite has sex with a male worm (the relatively less common mode), the number of progeny can be up to four times greater than that. Why, from an evolutionary perspective, do you think there is this difference in the number of offspring? Why do you think roundworms have these two reproductive modes? Why do you think humans don't?
Worms in the Lab -- Just Complicated Enough
C. elegans is easy to maintain in the lab, in part because of the size and life-cycle qualities noted above. They also don't ask for much in the way of resources. While found in the soil or rotting fruit in nature, worms in the lab are grown on a petri dish filled with agar, the surface of which is spread with bacteria such as E. coli. The worms crawl across the agar, grazing on this "bacterial lawn" -- not a bad life, at least for a worm. Another plus: stocks of C. elegans can be frozen in liquid nitrogen and later revived, which allows mutant strains to be kept for long periods with no effort, and easily sent from lab to lab.
The easy handling and short life span of the worms made them a useful candidate for genetic studies, as it is easy to identify mutant worms and manipulate them. A much-reproduced image from Sydney Brenner's landmark 1974 paper on the genetics of C. elegans (right) shows a normal worm (panel a) and three mutants -- "dumpy," or dpy-1 (b), small, or sma-2 (c), and long, or lon-1 (d). Many other identified mutations affected movement, and were dubbed "uncoordinated" or unc. Brenner's paper is fascinating in some of its details on how the scientist took advantage of the two modes of worm reproduction to tune his genetic studies. More recently, the process of making worm mutants has been revolutionized by a technique called RNA interference, which we'll explore in one of the Vignettes below.
But there are a number of other attributes, springing from its anatomy, that make the worm a ripe candidate for lab study:
- What you see is what you get. The worm is transparent at every stage of its life.
- Just complicated enough. Adult worms have a very precise and limited number of cells -- 959 for hermaphrodites, 1031 for males -- that make up their bodies.
As we'll see in the next Web Mission, these attributes together have made the worm a powerful system for studies of development in particular.
Web Mission: Green Development
You may have noticed the eerie green worm in the image at the top of this page. That image shows the nervous system of a mature worm, mapped out by tagging the relevant proteins in the developing worm with a naturally occurring green fluorescent protein, the gene for which was harvested from the jellyfish Aequorea victoria. Starting in the 1990s, green fluorescent protein (GFP) became a key part of the toolkit of cell and developmental biology in particular (about which more below).
But long before GFPs came into use, the worm was a key system for grasping the basic mechanisms of development. Because the worm is transparent, individual cells could be tracked through the entire development process. And, because the worm contains only a limited number of cells -- precisely 959 in the case of an adult hermaphrodite -- the development and fate of every cell could be rigorously followed and described.
That's exactly what John Sulston and his colleagues did in a classic paper published in Developmental Biology in 1983 (and that resulted in a Nobel Prize for Sulston in 2002). For a quick schematic view, go to this page of diagrams on the WormClassroom site. Start by scrolling to the bottom and looking at the reproduction of Sulston's figure from that original paper. The diagram shows how the team inferred the differentiation of the zygote cell into several founder cells, which in turn gave rise to the various cell types in the worm. Next, to see how the founder cells contribute to the adult body, scroll up to the first two diagrams on the page. The second shows a color-coded interpretaton of Sulston's drawing, and the first maps those colors to the worm's main bodily systems.
The main tools used by Sulston and colleagues -- in addition to a well-stocked worm lab -- were microscopes, pencils, paper, and a great deal of patience. With the advent of green fluorescent protein -- whose discoverers won the 2008 Nobel Prize in chemistry -- researchers take the analysis down to the level of individual genes and their protein products, by tweaking the genome to tag specific proteins important in development with the GFP marker. Knowing where and when a protein is present can provide important clues to the function of the underlying gene that encodes the protein.
To get an idea of how it works, first watch this brief movie (from light micrographs) of the cell divisions and morphological changes during C. elegans development. Next, go to the EPIC Web site, which aggregates GFP experiment results for genes in the developing worm embryo. Here, the movies label the nuclei with one fluorescent protein (green) and the products from specific genes with a second fluorescent protein (red). Click on one or two of the gene links to see the full-size GFP movies. Can you see some of the same patterns in the GFP movies that you saw in the movie made from photomicrographs? What do you think the movies of GFP allow that can't readily be gotten through the movie from the light microscope?
Worm Genomics and Biology
The publication of the genome sequence of C. elegans in the journal Science, in December 1998, was a landmark event in biology. It was the first complete genome sequence for an animal; previous studies had been limited to smaller genomes of microbial organisms such as the yeast Saccharomyces cerevisiae and the bacteria E. coli. The methods developed to sequence the worm genome provided a framework for sequencing the still larger human genome. And the worm genome also provided scientists with a toolkit to look at the biology of the worm itself at a much deeper, molecular level. And the publication of subsequent genomes -- including the human genome just over two years later -- opened up the opportunity to deepen our understanding of the specific relationships between worms and other organisms, and the true utility of C. elegans as a model organism.
In the Vignettes that follow, you can explore further how both genetic and molecular studies of C. elegans biology have played out in studies of apoptosis, or "programmed cell death," a phenomenon thought to have profound implications for the mechanisms of cancer, and in aging research. Also, you'll find out about the technique of RNA interference, which has emerged as both a potent lab technique and a promising medical treatment for some diseases -- and which was first identified in the worm.
- Apoptosis – What Do Worm Cells, the Space Between My Fingers, and Cancer Have in Common?
- Aging - That Worm Looks Fantastic for its Age!
- RNA Interference - Pick a Gene, Make a Mutant!
Web Mission: Worms in Space
On February 1, 2003, in one of the worst disasters in the history of U.S. space exploration, the space shuttle Columbia broke up upon re-entry into Earth's atmosphere, an event that caused the tragic death of all seven members of the crew. As NASA searched in subsequent weeks for the wreckage from the disaster, it was discovered that some creatures from Columbia did survive -- hundreds of C. elegans worms in petri dishes within aluminum cannisters, which had been sent up as part of a series of experiments on the effects of space travel on life processes.
C. elegans has actually been sent into space more than any other non-human animal. To start to understand why, go to the NASA page on C. elegans research, specifically related to the International Caenorhabditis elegans Experiment (ICE), which involved a ten-day stint on the International Space Station during April 2004. Scan through the questions and answers, especially the ones related to the goals of the worm research, and how the worm research can be related to human concerns. What are some of the main goals in sending worms into space? What are some of the advantages of using the worm in these experiments? Next, go to the NASA pages on bio and biotech research on the space station. Do the topics look familiar?
While the ICE mission was particularly rich, the worm has found its way into scientific study on numerous other space missions. As recently as spring 2011, C. elegans worms were launched into space as part of the last flight of the space shuttle Endeavour, the second-to-last flight of the entire shuttle program. And among the worms sent on that journey were descendants of those that had survived the Columbia disaster eight years earlier.
Other Web Resources
The community of researchers studying C. elegans (a.k.a. “worm people”) is large, diverse, and highly interactive. More information about the anatomy of C. elegans can be found at www.wormatlas.org, and the genome database is found at www.wormbase.org
On to Transcription
You've now finished the first module, providing some general information on how the worm and fly function as model organisms. Now let's proceed to the second, in which we'll explore how scientists in the modENCODE project have used these models to cast light on some specific issues in the biology of gene expression and inheritance. We'll start with transcription -- the crucial biological process by which genes are encoded into proteins, the sinews and workhorses of the cell.