Model organism

Escherichia coli is a gram-negative prokaryotic model organism

Drosophila melanogaster, one of the most famous subjects for experiments

A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms.^[1] Model organisms are widely used to explore potential causes and treatments for human disease when human experimentation would be unfeasible or considered less ethical. 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.^[2] Studying model organisms can be informative, but care must be taken when generalizing from one organism to another.

Selecting a model organism

Models are those organisms with a wealth of biological data that make them attractive to study as examples for other species and/or natural phenomena that are more difficult to study directly. Continual research on these organisms focus on a wide variety of experimental techniques and goals from many different levels of biology--from ecology, behavior, and biomechanics, down to the tiny functional scale of individual tissues, organelles, and proteins. Inquiries about the DNA of organisms are classed as genetic models (with short generation times, such as the fruitfly and nematode worm), experimental models, and genomic parsimony models, investigating pivotal position in the evolutionary tree.^[3] Historically, model organisms include a handful of species with extensive genomic research data, such as the NIH model organisms.^[4]

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 methods of transformation) 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 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.

Electron microphotograph of tobacco mosaic virus (TMV) particles

Important model organisms

Viruses

Viruses include:

• Phage Lambda
• Phi X 174 - its genome was the first ever to be sequenced. The genome is a circle of 11 genes, 5386 base pairs in length.
• Tobacco mosaic virus

Prokaryotes

Sporulating Bacillus subtilis

Prokaryotes include:

• Escherichia coli (E. coli) - This common, Gram-negative gut bacterium is the most widely-used organism in molecular genetics.
• Bacillus subtilis - an endospore forming Gram-positive bacterium
• Caulobacter crescentus - a bacterium that divides into two distinct cells used to study cellular differentiation.
• Mycoplasma genitalium - a minimal organism
• Vibrio fischeri - quorum sensing, bioluminescence and animal-bacterial symbiosis with Hawaiian Bobtail Squid
• Synechocystis, a photosynthetic cyanobacterium widely used in photosynthesis research.
• Pseudomonas fluorescens, a soil bacterium that readily diversifies into different strains in the lab.

Eukaryotes

Eukaryotes include:

Protists

• Protists:

• 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.^[5] Sequencing of the Chlamydomonas reinhardtii genome was reported in October 2007.^[6] 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.
• Dictyostelium discoideum is used in molecular biology and genetics (its genome has been sequenced), and is studied as an example of cell communication, differentiation, and programmed cell death.
• Tetrahymena thermophila - a free living freshwater ciliate protozoan.
• Emiliania huxleyi - a unicellular marine coccolithophore alga, extensively studied as a model phytoplankton species.
• Thalassiosira pseudonana - a unicellular marine diatom alga, extensively studied as a model marine diatom since its genome was published in 2004

Fungi

• Fungi:

Budding yeast tomography

• Ashbya gossypii, cotton pathogen, subject of genetics studies (polarity, cell cycle)
• Aspergillus nidulans, mold subject of genetics studies
• Coprinus cinereus, mushroom (genetic studies of mushroom development, genetic studies of meiosis)^[7]
• Neurospora crassa - orange bread mold (genetic studies of meiosis, metabolic regulation, and circadian rhythm)^[8]
• Saccharomyces cerevisiae, baker's yeast or budding yeast (used in brewing and baking)
• Schizophyllum commune - model for mushroom formation.^[9]
• Schizosaccharomyces pombe, fission yeast, (cell cycle, cell polarity, RNAi, centromere structure and function, transcription)
• Ustilago maydis, dimorphic yeast and plant pathogen of maize (dimorphism, plant pathogen, transcription)

Plants

Arabidopsis thaliana

• Plants:

• Arabidopsis thaliana, currently the most popular model plant. This herbaceous dicot belonging to Brassicaceae family is a plant closely related to the mustard plant. Its small stature and short generation time facilitates rapid genetic studies,^[10] and many phenotypic and biochemical mutants have been mapped.^[10] Arabidopsis was the first plant to have its genome sequenced.^[10] Its genome sequence, along with a wide range of information concerning Arabidopsis, is maintained by the TAIR database.^[10]
  (Plant physiology, Developmental biology, Molecular genetics, Population genetics, Cytology, Molecular biology)
• Selaginella moellendorffii is a remnant of an ancient lineage of vascular plants and key to understanding the evolution of land plants. It has a small genome size (~110Mb) and its sequence was released by the Joint Genome Institute in early 2008. (Evolutionary biology, 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
• 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.

Zea mays

• 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 has been 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)
• Mimulus is a model organism used in evolutionary and functional genomes studies. This specie pertain to Phrymaceae family, with ca. 120 species. Several genetic resources has been designed for the study of this genera, some are free access (http://www.mimulusevolution.org)
• 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 sequencing 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 finished. (Agronomy, Molecular biology)

Physcomitrella patens

• Physcomitrella patens is a moss increasingly used for studies on development and molecular evolution of plants.^[11] It is so far the only non-vascular plant(and so the only "primitive" plant) with its genome completely sequenced.^[11] Moreover, it is currently the only land plant with efficient gene targeting that enables gene knockout.^[12] The resulting knockout mosses are stored and distributed by the International Moss Stock Center. (Plant physiology, Evolutionary biology, Molecular genetics, 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.
• See also Chlamydomonas reinhardtii, above under Protists.

Animals

Main article: Animal model

Invertebrates

Caenorhabditis elegans

• Amphimedon queenslandica, a demosponge from the phylum Porifera used as a model for evolutionary developmental biology and comparative genomics^[13]
• Arbacia punctulata, the purple-spined sea urchin, classical subject of embryological studies
• Aplysia, a sea slug, whose ink release response serves as a model in neurobiology and whose growth cones serve as a model of cytoskeletal rearrangements
• Branchiostoma floridae, a species commonly known as amphioxus or lancelet from the subphylum Cephalochordata of the phylum Chordata used as a model for understanding the evolution of nonchordate deuterostomes, invertebrate chordates, and vertebrates^[14]
• Caenorhabditis elegans, a nematode, usually called C. elegans^[15] - 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, a 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 research.^[16] (Molecular genetics, Population genetics, Developmental biology).
• Euprymna scolopes, the Hawaiian bobtail squid, model for animal-bacterial symbiosis, bioluminescent vibrios
• Hydra (genus), a Cnidarian, is the model organism to understand the processes of regeneration and morphogenesis, as well as the evolution of bilaterian body plans^[17]
• 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)
• Macrostomum lignano, a free-living, marine flatworm, a model organism for the study of stem cells, regeneration, ageing, gene function, and the evolution of sex. Easily raised in the lab, short generation time, indetermined growth, complex behaviour^[18]
• Mnemiopsis leidyi, from the phylum Ctenophora (comb jelly) used as a model for evolutionary developmental biology and comparative genomics^[19][20]
• Nematostella vectensis, a sea anemone from the phylum Cnidaria used as a model for evolutionary developmental biology and comparative genomics^[21][22]
• Oikopleura dioica,^[23] an appendicularia, a free-swimming tunicate (or urochordate))
• Oscarella carmela a homoscleromorph sponge (phylum Porifera) used as a model in evolutionary developmental biology^[24]
• Parhyale hawaiensis an amphipod crustacean, used in evolutionary developmental (evo-devo) studies, with an extensive toolbox for genetic manipulation.
• Platynereis dumerilii a marine polychaetous annelid, which evolved very slowly and therefore retained many ancestral features.^[25]
• Pristionchus pacificus, a roundworm used in evolutionary developmental biology in comparative analyses with C. elegans
• Schmidtea mediterranea a freshwater planarian; a model for regeneration and development of tissues such as the brain and germline
• Stomatogastric ganglion of various arthropod species; a model for motor pattern generation seen in all repetitive motions
• Strongylocentrotus purpuratus, the purple sea urchin, widely used in developmental biology
• Symsagittifera roscoffensis, a flatworm, subject of studies of bilaterian body plan development
• Tribolium castaneum, the flour beetle - a small, easily kept darkling beetle used especially in behavioural ecology experiments
• Trichoplax adhaerens, a very simple free-living animal from the phylum Placozoa used as a model in evolutionary developmental biology and comparative genomics^[26]

Vertebrates

Laboratory mice

• Guinea pig (Cavia porcellus) - 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 sylvestris catus) - used in neurophysiological research
• Dog (Canis lupus familiaris) - an important respiratory and cardiovascular model, also contributed to the discovery of classical conditioning.
• Hamster - first used to study kala-azar (leishmaniasis)
• 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)
• Lamprey - spinal cord research
• Medaka (Oryzias latipes, the Japanese ricefish) - 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)
• Rhesus macaque (Macaca mulatta) - used for studies on infectious disease and cognition
• Cotton rat (Sigmodon hispidus) - formerly used in polio research
• Zebra finch (Taeniopygia guttata) - used in the study of the song system of songbirds and the study of non-mammalian auditory systems
• Takifugu (Takifugu rubripes, a pufferfish) - has a small genome with little junk DNA
• The African clawed frog (Xenopus laevis) - 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,^[27] specific gene function and roles of signaling pathways.

Model organisms used for specific research objectives

Sexual selection and sexual conflict

• Callosobruchus maculatus, the bruchid beetle
• Chorthippus parallelus, the meadow grasshopper
• Coelopidae - seaweed flies
• Diopsidae - stalk-eyed flies
• Drosophila spp. - fruit flies
• Macrostomum lignano, a sand flatworm
• Gryllus bimaculatus, the field cricket
• Scathophaga stercoraria, the yellow dung fly

Hybrid zones

• Bombina bombina and variegata
• Podisma spp. in the Alps
• Caledia captiva (Orthoptera) in eastern Australia

Ecological genomics

• Daphnia pulex, an environmental indicator model organism

Table of model genetic organisms

This table indicates the status of the genome sequencing project for each organism as well as whether the organism exhibits homologous recombination.

Organism	Genome Sequenced	Homologous Recombination
Prokaryote
Escherichia coli	Yes	Yes
Eukaryote, unicellular
Dictyostelium discoideum	Yes	Yes
Saccharomyces cerevisiae	Yes	Yes
Schizosaccharomyces pombe	Yes	Yes
Chlamydomonas reinhardtii	Yes	No
Tetrahymena thermophila	Yes	Yes
Eukaryote, multicellular
Caenorhabditis elegans	Yes	Difficult
Drosophila melanogaster	Yes	Difficult
Arabidopsis thaliana	Yes	No
Physcomitrella patens	Yes	Yes
Vertebrate
Danio rerio	Yes	Yes
Mus musculus	Yes	Yes
Xenopus laevis (Note: and X. tropicalis)[28]	Yes	No
Homo sapiens (Note:not a model organism)	Yes	Yes

See also

• Animal model
• Ensembl genome database of model organisms
• History of model organisms
• Animals in space
• Animal testing
• Animal testing on invertebrates
• Animal testing on rodents
• Mouse models of colorectal and intestinal cancer
• Generic Model Organism Database
• History of animal testing
• RefSeq - the Reference Sequence database
• Genome project

References

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External links

• Wellcome Trust description of model organisms
• WWW Virtual Library guide to several model organism resource lists
• The Generic Model Organism Database project
• The Model Organism Database ftp site and info
• The US National Institutes of Health page
• Model organisms in developmental biology
• Ludwig-Maximillians-Universität Department Biologie II
• Zebrafish GenomeWiki Community Annotation Project mainteined by the Institute of Genomics and Integrative Biology
• Workhorse Zoo by Adam Zaretsky

• Mandoli DF, Olmstead R (2000): The importance of emerging model systems in plant biology. Journal of plant growth regulation 19 (3): 249-252
• Tuskan GA, Difazio S, Jansson S, et al. (Sep 2006). "The genome of black cottonwood, Populus trichocarpa (Torr. & Gray)". Science 313 (5793): 1596–604. doi:10.1126/science.1128691. PMID 16973872. 
• Sequencing Project, International Rice Genome (Aug 2005). "The map-based sequence of the rice genome". Nature 436 (7052): 793–800. doi:10.1038/nature03895. PMID 16100779. 
• FishMap: The Zebrafish Community Genomics Browser maintained at the Institute of Genomics and Integrative Biology