05 - Heavy Metal: Effects of Environmental Toxins on C. elegans
maintain. While characterizing the worm, Brenner, along with Drs. John Sulston and Robert Horvitz, discovered that the developmental fate of every cell in the worm is invariable between animals. They also discovered key genes involved in organ development and programmed cell death. For this work, Brenner, Sulston, and Horvitz were awarded the Nobel Prize in Physiology or Medicine in 2002. C. elegans have become important to the study of embryogenesis, morphogenesis, develop- ment, nerve function, behavior and aging, and genetics. The C. elegans genome has been completely sequenced and several thousand genetic mutants are available for study. This allows scientists to correlate changes at the DNA level with changes in phenotype. Notably, by comparing DNA sequences, it was determined that over 35% of worm genes have human homologs. Many of these genes are important for human health and development. C. elegans is a free-living, non-parasitic nematode that lives in temperate soil, where it feeds on microbes that are found in decaying organic matter. Adult worms measure approximately one millime- ter (mm) in length. The outer cuticle of C. elegans is transparent, making it easy to visualize growth and development of internal structures like the pharynx, the intestine, the gonads and the muscles (Figure 1). The worm also has an extensive nervous system – in fact, the nervous system comprises almost 1/3 of the worm’s 959 somatic cells! This makes C. elegans a valuable Figure 2: Neural anatomy of C. elegans . Source: OpenWorm proj- ect. Shared under an MIT License. There are two naturally occurring sexes in C. elegans . The vast majority of worms are self-fertile hermaphrodites, meaning that they produce both the sperm and the eggs used for reproduc- tion. Free-living males represent <1% of the total nematode population. However, free-living males plus a hermaphrodite can produce over 1000 offspring in a generation; in contrast, self- fertilized hermaphrodite worms will produce about 300. Because their sperm will preferentially fertilize a hermaphrodite’s eggs and produce more offspring, free-living males are often used to introduce specific genetic mutations into a worm population to be studied. C. elegans develop from embryo to adult in four days, allowing for rapid studies in the labora- tory (Figure 3). The worms are grown on agar plates or in liquid culture and they feed on E. coli . After being laid, the worm embryo will develop for approximately 14 hours before hatching. Juvenile worms progress through four larval stages (L1-L4) over the next two days, increasing in size with each stage. After the fourth larval molt (L4), the worms are reproductively mature, meaning that they can be used for further genetic studies. Adults will live for 2-3 weeks, over which time they gradually age and lose vigor. Bioassays with Multiple C. elegans Strains Another advantage of C. elegans is the availability of over 3000 strains whose behavior and ge- netic make-up has been researched and documented. In biology, a strain is a genetic variant or subtype that falls below the taxonomical level of species. By creating multiple strains, scientists can identify the function of different genes, observe how different genotypes respond to envi - ronmental stimuli, or modify a phenotype for a particular task. C. elegans ’ short generation model system for neuroscientists (Figure 2). Growth and Development of C. elegans
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