What Is a Model Organism?

Over the last century, research on a small number of organisms has played a pivotal role in advancing our understanding of numerous biological processes. This is because many aspects of biology are similar in most or all organisms, but it is frequently much easier to study a particular aspect in one organism than in others. These much-studied organisms are commonly referred to as model organisms, because each has one or more characteristics that make it suitable for laboratory study. The most popular model organisms have strong advantages for experimental research, such as rapid development with short life cycles, small adult size, ready availability, and tractability, and become even more useful when many other scientists work on them. A large amount of genetic information can then be derived from these organisms, providing valuable data for the analysis of normal human development and gene regulation, genetic diseases, and evolutionary processes.

Non-Mammalian Models

Each of the non-mammalian model organisms described below has advantages for the study of certain mammalian biological processes, and at least for most, significant genomic resources have been or are being developed.

Archaea are among some of the most fascinating organisms on earth. Historically, they have been considered part of the bacteria, but recent work has shown them to be very distinct. Archaea are unique for two reasons. First, they represent the most primitive organisms yet discovered and have been found to have a remarkable resemblance to some of the earliest known fossils. Second, they live in some of the most extreme environments on earth, hence the nickname "extremophiles". Research studies conducted on the molecules that enable extremophiles to prosper are fast becoming useful in areas such as bioremediation, global climate change, biotechnology, and energy production. In addition, although archaeans resemble bacteria and have some genes that are similar to bacterial genes, they also have other genes that resemble eukaryotes and some genes that appear to be unique. Studies of these unshared genes may provide new clues to the evolution of early life on earth.

Microbes are thought to make up more than 60 percent of the earth's biomass. They have survived and evolved on Earth for over 3.7 billion years and have been found in almost every environment. The diversity and range of environmental adaptations mean that microbes have long ago "solved" many problems for which scientists are still actively seeking solutions. In addition, most microbes are not responsible for diseases in humans, animals, or plants. Therefore, by studying and understanding a diverse group of microbes at the genomic level, researchers may develop answers to existing challenges in medicine, agriculture, industrial processes, energy production and use, and environmental and waste cleanup.

The fruit fly, Drosophila melanogaster, has been used as a model organism for research for almost a century and is considered an attractive system for a number of reasons, including its easy-to-manipulate genetic system, relatively low cost, and biological complexity comparable to that of a mammal. Many organ systems in mammals have well-conserved homologs in Drosophila, and Drosophila research has already led the way in providing new insights into forms of cancer, neurodegenerative diseases, behavior, immunity, aging, multigenic inheritance, and development.

In response to the rising global incidence of malaria, an international effort to sequence and analyze the genetic codes of Plasmodium falciparum and Plasmodium vivax, the major causative agents of the disease that is transmitted by mosquitoes, is under way. Project efforts range from sequencing each of the parasite's chromosomes to developing new tools for studying expression of newly identified genes.

Caenorhabditis elegans (round worm), a small, free-living soil nematode, is the best characterized multicellular animal at the level of genomics, genetics, embryology, and cell and neurobiology, and its genome is fully sequenced. C. elegans is unique among model organisms in that it can be grown and genetically manipulated with the speed and ease of a micro-organism while offering the features of a real animal. C. elegans has a full set of organ systems, has complex sensory systems, shows coordinated behavior, and is further distinguished by the fact that it is possible to trace the lineage of every one of its approximately 1000 constituent cells . Furthermore the morphology, development, and function of each of its cells have been mapped in great detail.

Retroviruses are an unusual form of virus capable of infecting a wide range of vertebrates and causing a number of diseases such as pneumonia, leukemia, and AIDS. Retroviruses have fundamental properties that other virus families do not have in that they are RNA viruses that normally convert their nucleic acid to DNA and then insert this DNA into the genome of the host cell. This provides them with two modes of transmission. They can be both infectious agents that spread horizontally (within a generation) and normal genetic constituents that spread in a Mendelian fashion (vertically between generations). Viruses are important to the study of molecular and cellular biology because they provide simple systems that can be used to manipulate and investigate the functions of many cell types. Numerous studies have demonstrated the utility of animal viruses as probes for investigating different activities of eukaryotic cells. Other examples in which animal viruses have provided important models for biological research of their host cells include studies of DNA replication, transcription, RNA processing, and protein transport.

Saccharomyces cerevisiae, commonly known as baker's or brewer's yeast, has been used in research studies for a very long time. The complete genome sequence of the S. cerevisiae lab strain S288C was obtained in the spring of 1996, making yeast the first eukaryotic organism to be completely sequenced. The complete sequence of its genome has proven extremely useful as a reference toward extracting the sequences of human and other higher eukaryotic genes. Furthermore, the ease of genetic manipulation of yeast allows its use for conveniently analyzing and functionally dissecting gene products from other eukaryotes.

The zebrafish, Danio rerio, is used as a model organism because of its small size, short life cycle, ease of culture, and ability to readily produce mutations relevant to human health and disease. The embryonic development of the zebrafish can be seen through its transparent egg and closely resembles that of higher vertebrates, making it useful for studying development and mutations. Other shared features with humans include blood, kidney, and optical systems. In addition, its genome is half the size of the mouse and human genomes, which is valuable in identification of key vertebrate genes.

The social amoeba Dictyostelium discoideum is a powerful model organism because of its rapid generation time, facile reverse genetics and its ease of use in the study of many areas of cell and molecular biology. Dictyostelium cells grow as unicellular organisms but, upon starvation, aggregate to form a multicellular tissue capable of differentiating into multiple cell types. During this developmental program, the amoebae employ many cellular processes to ultimately form a spore-bearing fruiting body, including chemotaxis, cytoskeletal rearrangement, signal transduction, cell sorting, and pattern formation. Because the life cycle consists of both unicellular and multicellular phases, Dictyostelium provides insight into processes needed for multicellularity.