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
- Escherichia coli (E. coli)
- Mycoplasma genitalium - a minimal organism
- Vibrio fischeri - quorum sensing, bioluminescence and animal-bacterial symbiosis with Hawaiian bobtail squid
- Saccharomyces cerevisiae - baker's yeast or budding yeast
- Schizosaccharomyces pombe - fission yeast (used in brewing)
- Neurospora crassa - red bread mold
- Arabidopsis thaliana, a plant, usually called Arabidopsis
- Arbacia punctulata, the purple-spined sea urchin, classical subject of embryological studies
- Aspergillus nidulans, subject of genetics studies
- Caenorhabditis elegans, a nematode, usually called C. elegans
- Euprymna scolopes, the Hawaiian bobtail squid, model for animal-bacterial symbiosis, bioluminescent vibrios.
- 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.
- 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)
- Rat (Rattus norvegicus)
- Mouse (Mus musculus)
- Brachydanio rerio, zebrafish, a freshwater fish used to study development
- Xenopus laevis, the African clawed toad, also used in development
- Takifugu rubipres, a pufferfish - has a small genome with little junk DNA
- Homo sapiens, human beings, which are capable of self-reporting and have the largest catalog of genetic disorders