Electron micrograph of several E. coli cellsDrosophila, one of the most famous subjects for experiments
A model organism is a species that is extensively studied to understand particular biologicalphenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. In particular, model organisms are widely used to explore potential causes and treatments for human disease when human experimentation would be unfeasible or unethical. This strategy is made possible by the common descent of all living organisms, and the conservation of metabolic and developmental pathways and genetic material over the course of evolution.[1]
Often, model organisms are chosen on the basis that they are amenable to experimental manipulation. This usually will include characteristics such as short life-cycle, techniques for genetic manipulation (inbred strains, stem cell lines, and transfection systems) and non-specialist living requirements. Sometimes, the genome arrangement facilitates the sequencing of the model organism's genome, for example, by being very compact or having a low proportion of junk DNA (e.g. yeast, Arabidopsis, or pufferfish).
When researchers look for an organism to use in their studies, they look for several traits. Among these are size, generation time, accessibility, manipulation, genetics, conservation of mechanisms, and potential economic benefit. As comparative molecular biology has become more common, some researchers have sought model organisms from a wider assortment of lineages on the tree of life.
Use of model organisms
There are many model organisms. One of the first model systems for molecular biology was the bacterium Escherichia coli, a common constituent of the human digestive system. Several of the bacterial viruses (bacteriophage) that infect E. coli also have been very useful for the study of gene structure and gene regulation (e.g. phages Lambda and T4). However, bacteriophages are not organisms because they lack metabolism and depend on functions of the host cells for propagation.
In eukaryotes, several yeasts, particularly Saccharomyces cerevisiae ("baker's" or "budding" yeast), have been widely used in genetics and cell biology, largely because they are quick and easy to grow. The cell cycle in a simple yeast is very similar to the cell cycle in humans and is regulated by homologous proteins. The fruit fly Drosophila melanogaster is studied, again, because it is easy to grow for an animal, has various visible congenital traits and has a polytene (giant) chromosome in its salivary glands that can be examined under a light microscope. The roundwormCaenorhabditis elegans is studied because it has very defined development patterns involving fixed numbers of cells, and it can be rapidly assayed for abnormalities.
Chlamydomonas reinhardtii - a unicellular green alga used to study photosynthesis, flagella and motility, regulation of metabolism, cell-cell recognition and adhesion, response to nutrient deprivation and many other topics. Chlamydomonas reinhardtii has a well-studied genetics, with many known and mapped mutants and expressed sequence tags, and there are advanced methods for genetic transformation and selection of genes.[2] Sequencing of the Chlamydomonas reinhardtii genome was reported in October 2007.[3] A Chlamydomonas genetic stock center exists at Duke University, and an international Chlamydomonas research interest group meets on a regular basis to discuss research results. Chlamydomonas is easy to grow on an inexpensive defined medium.
Selaginella moellendorffii is a remnant of an ancient lineage of vascular plants and key to understanding the evolution of land plants. It has the smallest genome size of any plant reported (~110Mb) and its sequence will be released by the Joint Genome Institute in early 2008. (Evolutionary biology, Molecular biology)
Lemna gibba is a rapidly-growing aquatic monocot, one of the smallest flowering plants. Lemna growth assays are used to evaluate the toxicity of chemicals to plants in ecotoxicology. Because it can be grown in pure culture, microbial action can be excluded. Lemna is being used as a recombinant expression system for economical production of complex biopharmaceuticals. It is also used in education to demonstrate population growth curves.
Maize (Zea mays L.) is a cereal grain. It is a diploid monocot with 10 large chromosome pairs, easily studied with the microscope. Its genetic features, including many known and mapped phenotypic mutants and a large number of progeny per cross (typically 100-200) facilitated the discovery of transposons ("jumping genes"). Many DNA markers have been mapped and the genome is being sequenced. (Genetics, Molecular biology, Agronomy)
Medicago truncatula is a model legume, closely related to the common alfalfa. Its rather small genome is currently being sequenced. It is used to study the symbiosis responsible for nitrogen fixation. (Agronomy, Molecular biology)
Rice(Oryza sativa) is used as a model for cereal biology. It has one of the smallest genomes of any cereal species, and sequencing of its genome is finished. (Agronomy, Molecular biology)
Populus is a genus used as a model in forest genetics and woody plant studies. It has a small genome size, grows very rapidly, and is easily transformed. The genome sequence of Poplar (Populus trichocarpa) sequence is publicly available.
Caenorhabditis elegans, a nematode, usually called C. elegans[7] - an excellent model for understanding the genetic control of development and physiology. C. elegans was the first multicellular organism whose genome was completely sequenced
Hydra (genus), a Cnidarian, is the model organism to understand the evolution of bilaterian body plans
Loligo pealei, a squid, subject of studies of nerve function because of its giant axon (nearly 1 mm diameter, roughly a thousand times larger than typical mammalian axons)
Pristionchus pacificus, a roundworm used in evolutionary developmental biology in comparative analyses with C. elegans
Cavia porcellus, the guinea pig, used by Robert Koch and other early bacteriologists as a host for bacterial infections, hence a byword for "laboratory animal" even though less commonly used today
Chicken (Gallus gallus domesticus) - used for developmental studies, as it is an amniote and excellent for micromanipulation (e.g. tissue grafting) and over-expression of gene products
Cat (Felis cattus) - used in neurophysiological research
Dog (Canis lupus familiaris) - an important respiratory and cardiovascular model
Mouse (Mus musculus) - the classic model vertebrate. Many inbred strains exist, as well as lines selected for particular traits, often of medical interest, e.g. body size, obesity, muscularity. (Quantitative genetics, Molecular evolution, Genomics)
Homo sapiens (humans) - used in various clinical studies
Oryzias latipes, Medaka (the Japanese ricefish) is an important model in developmental biology, and has the advantage of being much sturdier than the traditional Zebrafish
Rat (Rattus norvegicus) - particularly useful as a toxicology model; also particularly useful as a neurological model and source of primary cell cultures, owing to the larger size of organs and suborganellar structures relative to the mouse. (Molecular evolution, Genomics)
Xenopus laevis, the African clawed frog - used in developmental biology because of its large embryos and high tolerance for physical and pharmacological manipulation
Zebrafish (Danio rerio), a freshwater fish, has a nearly transparent body during early development, which provides unique visual access to the animal's internal anatomy. Zebrafish are used to study development, toxicology and toxicopathology[9], specific gene function and roles of signaling pathways.
Model organisms used for specific research objectives
↑ Fox, Michael Allen (1986). The Case for Animal Experimention: An Evolutionary and Ethical Perspective. Berkeley and Los Angeles, California: University of California Press. ISBN0-520-05501-2.
↑ Riddle, Donald L.; Blumenthal, Thomas; Meyer, Barbara J.; and Priess, James R. (Eds.). (1997). C. ELEGANS II. Woodbury, NY: Cold Spring Harbor Press. ISBN 0-87969-532-3. Full text available on-line.
↑Spitsbergen J.M. and Kent M.L. (2003). The state of the art of the zebrafish model for toxicology and toxicologic pathology research--advantages and current limitations. Toxicol Pathol.31 (Supplement), 62-87. PubMed Abstract Link => PMID 12597434.
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