A model organism is one that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. This works because evolution reuses fundamental biological principles and conserves metabolic, regulatory, and developmental pathways.

There are many model organisms. The first model organism for molecular biology was probably the bacterium Escherichia coli which is common in the human digestive system (and usually beneficial -- the dangerous E. coli O157:H7 is a rare strain). This also led to a study of many bacteriophages, particularly phage lambda.

In eukaryotes, several yeasts, particularly Saccharomyces cerevisiae ("baker's" or "budding" yeast), have been widely studied, 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 regulated by homologous proteins. The fruit fly Drosophila melanogaster was studied, again because it was easy to grow for a multicellular organism. The roundworm Caenorhabditis elegans is studied because it has very stereotyped development patterns and can be rapidly assayed for abnormalities.

When researchers look for an organism to use in their studies, they look for several traits. Common among these are size, lifespan, accessibility, manipulation, genetics, conservation of mechanisms, and potential economic benefit. As comparative molecular biology has become more common, some researchers have sought model organisms that represent assorted lineages of life.

Important model organisms

Viruses

• lambda phage

Prokaryotes

• Escherichia coli (E. coli)
• Bacillus subtilis
• Mycoplasma genitalium - a minimal organism
• Vibrio fischeri - quorum sensing, bioluminescence and animal-bacterial symbiosis with Hawaiian bobtail squid
• Synechocystis, a photosynthetic cyanobacteria widely used in photosynthesis research.

Unicellular eukaryotes

• Saccharomyces cerevisiae - baker's yeast or budding yeast (used in brewing and baking)
• Schizosaccharomyces pombe - fission yeast
• Neurospora crassa - red bread mold
• Chlamydomonas reinhardtii, a unicellular green alga used in photosynthesis research.

Multicellular eukaryotes

Plants

• Arabidopsis thaliana, a plant, usually called Arabidopsis
• Lotus japonicus a model legume plant.
• Medicago truncatula a model legume.
• Rice is used as a model for cereal biology.

Fungi

• Aspergillus nidulans, subject of genetics studies

Invertebrates

• Arbacia punctulata, the purple-spined sea urchin, classical subject of embryological studies
• Caenorhabditis elegans, a nematode, usually called C. elegans²
• Euprymna scolopes, the Hawaiian bobtail squid, model for animal-bacterial symbiosis, bioluminescent vibrios.
• Drosophila, usually the species Drosophila melanogaster - a kind of fruit fly, famous as the subject of genetics experiments by Thomas Hunt Morgan and others. Easily raised in lab, rapid generations, mutations easily induced, many observable mutations.
• 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)

Vertebrates

• Cavius 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 rarely used today.
• 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.
• Mouse (Mus musculus) - the classic model vertebrate
• Zebrafish (Brachydanio rerio), a freshwater fish, has a nearly transparent body which provides unique visual access to the animal's internal anatomy throughout its life. Zebrafish are used to study development, toxicology and toxicopathology¹, specific gene function and roles of signaling pathways.
• Xenopus laevis, the African clawed toad, also used in development
• Takifugu rubipres, a pufferfish - has a small genome with little junk DNA
• Dog (Canis lupus familiaris) - an important respiratory and cardiovascular model
• Homo sapiens, human beings, which are capable of self-reporting and have the largest catalog of genetic disorders

References

Note 1: 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.

Note 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 (http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=ce2.TOC).

Related topics

• Animal model

External links

• Wellcome Trust description of model organisms (http://www.wellcome.ac.uk/en/genome/genesandbody/hg05b001.html)