1.4 Unicellular Eukaryotic Model Organisms
Our current understanding of the molecular functioning of eukaryotic cells largely rests on studies of just a few types of organisms, termed model organisms (Figure 1-22). Because of the evolutionary conservation of genes, proteins, organelles, cell types, and so forth, discoveries about biological structures and functions obtained with one experimental organism often apply to others. Thus researchers generally conduct studies with the organism that is most suitable for rapidly and completely answering the question being posed, knowing that the results obtained in one organism are likely to be broadly applicable. Indeed, many organisms, particularly rats, frogs, sea urchins, chickens, and slime molds, have been and continue to be immensely valuable for cell biology research. As more and more organisms have their entire genomes sequenced, a wide variety of other species are increasingly being used for investigations, especially for studies of the evolution of genes, cells, and organisms and of how organisms become adapted to diverse ecological niches.
FIGURE 1-22 Each eukaryotic organism used in cell biology has advantages for certain types of studies. The yeast Saccharomyces cerevisiae (a) has the cellular organization of a eukaryote but is a relatively simple single-celled organism that is easy to grow and to manipulate genetically. The green alga Chlamydomonas reinhardtii (b) is widely used to study photosynthesis and the structure and function of flagella. In the roundworm Caenorhabditis elegans (c), which has a small number of cells arranged in a nearly identical way in every worm, the formation of each individual cell can be traced. The fruit fly Drosophila melanogaster (d), first used to discover the properties of chromosomes, has been especially valuable in identifying genes that control embryonic development. Many of these genes are evolutionarily conserved in humans. Planaria (e) are flatworms that can regenerate any part of the body that is cut off, including the head and the photoreceptors. The stem cells that give rise to their new cells and tissues are widely studied. The zebrafish Danio rerio (f) is used for rapid genetic screens to identify genes that control vertebrate development and organogenesis. Of the experimental animal systems, mice (Mus musculus) (g) are evolutionarily the closest to humans and have thus provided models for studying numerous human genetic and infectious diseases. The mustard-family weed Arabidopsis thaliana (h) has been used for genetic screens to identify genes involved in nearly every aspect of plant life.
[Part (a) Scimat/Photo Researchers, Inc. Part (b) William Dentler University of Kansas. Part (c) Science Source. Part (d) Darwin Dale/Science Source. Part (e) Peter Reddien, MIT Whitehead Institute. Part (f) blickwinkel/Hartl/Alamy. Part (g) J. M. Labat/Jacana/Photo Researchers, Inc. Part (h) Darwin Dale/Science Source.]
As we have seen, bacteria are excellent models for studies of several cellular functions, but they lack the organelles found in eukaryotes. Unicellular eukaryotes such as yeasts are used to study many fundamental aspects of eukaryotic cell structure and function. Metazoan models such as the roundworm, fruit fly, and mouse are required to study more complex tissue and organ systems and development. As we will see in this section and the next, several eukaryotic model organisms are widely used to understand complex cell systems and mechanisms.