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 .
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 example phages Lambda and T4.
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 Human s, and regulated by Homologous proteins. The fruit fly '' Drosophila Melanogaster '' was studied, again because it was easy to grow for an animal. The Roundworm '' Caenorhabditis Elegans '' is studied because it has very defined development patterns involving fixed numbers of cells, and it can be rapidly assayed for abnormalities.
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 that represent assorted Lineages of life.
- '' Arabidopsis Thaliana '', currently the most popular model plant. This herbaceous Dicot is a Crucifer , a member of the Mustard family. Its small stature and short generation time facilitates genetic studies, and many phenotypic and biochemical mutants have been mapped. Arabidopsis was the first plant to have its Genome Sequenced . ''( Plant Physiology , Developmental Biology , Molecular Genetics , Population Genetics , Cytology , Molecular Biology )''
- '' Brachypodium Distachyon '' is an emerging experimental model grass that has many attributes that make it an excellent model for temperate cereals. ''( Agronomy , Molecular Biology , Genetics )''
- '' Lotus Japonicus '' a model Legume used to study the symbiosis responsible for Nitrogen Fixation . ''( Agronomy , 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 Biopharmaceutical s. 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 Transposon s ("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 . It's rather small genome is currently being sequenced. It is used to study the symbiosis responsible for nitrogen fixation. ''( Agronomy , Molecular Biology )''
- '' Tobacco BY-2 Cells '' is suspension Cell Line from Tobacco ''(Nicotiana tabaccum)''. Useful for general plant physiology studies on Cell level. Genome of this particular Cultivar will be not sequenced (at least in near future), but seqencing of its wild Species ''Nicotiana tabaccum'' is presently in progress. ''( Cytology , Plant Physiology , Biotechnology )''
- 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 in progress. ''( Agronomy , Molecular Biology )''
- See also '' Chlamydomonas Reinhardtii '', above under Single-Cell Eukaryotes.
- '' Arbacia Punctulata '', the purple-spined Sea Urchin , classical subject of embryological studies
- '' Caenorhabditis Elegans '', a Nematode , usually called ''C. elegans'' 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 . - an excellent model for understanding the genetic control of development and physiology. ''C. elegans'' was the first multicellular organism whose genome was completely sequenced
- Ciona Intestinalis (sea squirt)
- '' 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. Recently, Drosophila has been used for neuropharmacological research1. ''( Molecular Genetics , Population Genetics , Developmental Biology )''.
- '' Euprymna Scolopes '', the Hawaiian bobtail squid, model for animal-bacterial Symbiosis , Bioluminescent Vibrio s.
- '' Hydra '', a Cnidaria , 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)
- '' Stomatogastric Ganglion '', Arthropods digestive systems are a model for Motor Pattern Generation seen in all repetitive motions
- '' Tribolium Castaneum '', the flour beetle - a small, easily kept Darkling Beetle used especially in Behavioural Ecology experiments.
- '' Pristionchus Pacificus '', a roundworm used in evolutionary developmental biology in comparative analyses with ''C. elegans''
- '' 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. ''( Molecular Evolution , Genomics )''
- Sigmodon hispidus - Cotton Rat formerly used in polio research
- 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 )''
- Hamster - first used to study kala-azar ( Leishmaniasis )
- Zebrafish (''Brachydanio 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 toxicopathologySpitsbergen 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.'' (Supplement), 62-87. PubMed Abstract Link => PMID 12597434., specific gene function and roles of signaling pathways.
- '' Xenopus Laevis '', the African clawed Toad , also used in development because of its large cells, esp. Egg Cell s.
- '' Takifugu Rubipres '', a Pufferfish - has a small genome with little Junk DNA
- Dog (''Canis Lupus Familiaris'') - an important respiratory and cardiovascular model
- '' 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.
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