Throughout this chapter, we have several times noted that the use of a particular organism was especially appropriate for a given study. In addition, you will find throughout the book that certain organisms have been used again and again as convenient models for whole categories of organisms (Figure 1). We describe some of these model organisms briefly below, with notes as to why they have been so often chosen. The genomic sequences of all of these organisms are now available.
Fig1. Picture gallery of some of the model organisms used most frequently in genetics research. (A) λ phage. (B) Escherichia coli. (C) Saccharomyces cerevisiae. (D) Schizosaccharomyces pombe. (E) Caenorhabditis elegans. (F) Drosophila melanogaster. (G) Danio rerio. (H) Xenopus laevis. (I) Mus musculus. (J) Arabidopsis thaliana. [A, courtesy of Bob Duda, University of Pittsburgh, PA, USA. B, courtesy of Peter Cooke and Stephen Ausmus, United States Department of Agriculture. C, courtesy of Maxim Zakhartsev and Doris Petroi, International University Bremen, Bremen, Germany. D, from Gutterman JU, Lai HT, Yang P et al. (2005) Proc Natl Acad Sci USA 102: 12771–12776. With permission from National Academy of Sciences, USA. Copyright (2005) National Academy of Sciences, USA. E, courtesy of Judith Kimble, University of Wisconsin, Madison, WI, USA. F, courtesy of André Karwath, Wikimedia. G, from Wikimedia. H, courtesy of Michael Linnenbach, Wikimedia. I, courtesy of George Shuklin, Wikimedia. J, courtesy of Brona Brejova, Wikimedia.]
Today bacteriophage λ is employed largely as a cloning vector, but it played an important part in the early development of genetics, especially because it has two alternative life cycles, lytic and lysogenic. In other words, the phage can either destroy or lyse the host bacterium or become integrated into its genome, existing in a dormant state that is propagated from cell generation to cell generation without any signs of the viral DNA’s presence, known as the lysogenic pathway.
The bacterium Escherichia coli has been labeled the workhorse of molecular biology. There is practically no fundamental biochemical process, from DNA replication to protein synthesis, which was not first elucidated in E. coli. It is extremely easy to grow, in liquid culture or on solid agar plates, and metabolically very versatile, which has made it useful for studies of metabolic regulation.
The common budding or bakers’ yeast Saccharomyces cerevisiae is among the simplest eukaryotes. Unicellular and easy to grow in large quantities, it provides a bridge between bacteria and the more complex eukaryotes. Its genetics has been very thoroughly studied, with many knockout strains available. In a knockout strain, a particular gene has been inactivated by recombinant DNA techniques. Studying such knockouts helps in elucidating the biological functions of genes. One difficulty in working with S. cerevisiae is the tough outer cell wall, which makes it difficult to insert substances. The fission yeast Schizosaccharomyces pombe is genetically similar to S. cerevisiae but lacks the tough outer layer. It divides, rather than buds, which is an advantage for some studies.
The free-living, primitive, unsegmented, and bilaterally symmetrical nematode Caenorhabditis elegans, which was introduced to the field by Sidney Brenner, is a remarkably simple creature. The adult worm has only 1,090 cells, and the lineage of each is precisely known (Figure 2). This makes it an outstanding candidate for developmental studies. Sydney Brenner, Robert Horvitz, and John Sulston were awarded the 2002 Nobel Prize in Physiology or Medicine “for their discoveries concerning genetic regulation of organ development and programmed cell death.” Andrew Fire and Craig Mello investigated the regulation of gene expression in C. elegans and identified RNA interference or RNAi, a novel mechanism of gene silencing by double-stranded RNA. For this work, they were awarded the 2006 Nobel Prize in Physiology or Medicine.
Fig2. Lineage tree for cells that form the gut or intestine of Caenorhabditis elegans. The intestinal cells form a single clone, as do the germline cells; note that the cells of most other tissues do not. The schematic shows only a small portion of the entire lineage tree that gives rise to all cells in the adult organism. [Top, adapted from Alberts B, Johnson A, Lewis J et al. (2008) Molecular Biology of the Cell, 5th ed. With permission from Garland Science. Bottom, adapted from Sulston JE & Horvitz HR (1977) Dev Biol 56: 110–156. With permission from Elsevier.]
Drosophila melanogaster was the organism that provided the seminal studies in modern genetics. Morgan’s fruit fly is easy to grow, in enormous numbers, in a very short time. This, plus the availability of a great many mutant strains, including many with mutations that affect general developmental patterns, make it still a useful model. The embryos are also used, especially for biochemical studies.
The zebrafish Danio rerio is small, easy to grow, and very fecund, so it provides a convenient vertebrate model. Its special attraction lies in the fact that the embryos are transparent so that development of internal organs can be followed in live embryonic fish.
The African clawed frog Xenopus laevis is useful because of its large and abundant eggs, which can easily be manipulated for injection studies and the like. X. laevis can rapidly produce thousands of embryos. A disadvantage of this model is that it is tetraploid and it takes years for the frog to reach sexual maturity. Another frog species, Xenopus tropicalis, is diploid and matures in less than three months; these two properties make it very attractive for genetic research.
The house mouse Mus musculus is the easiest mammal to study and has been used by generations of researchers. Despite a large evolutionary separation between mice and humans, ~85% of the mouse genome is very similar to that of humans. By now, many purebred mouse strains, including those with specific genetic modifications, are readily available.
Arabidopsis thaliana is a weed commonly known as thale cress. The plant is easy to grow and sexually matures in less than six weeks, producing ~5,000 seeds per plant.
This is the most commonly used plant model. First, it has five pairs of chromosomes and a small genome, whose entire sequence has been reported. Second, a large number of mutant lines and genomic resources and databases are available. Third, it is easy to transform by use of recombinant DNA technology techniques, including the use of Agrobacterium tumefaciens as a vector.
