What is a Model Organism?

Model organisms have always been at the forefront of genetics. The initial choice of an organism for research is based on some feature of that organism that lends itself particularly well to the study of a genetic process that the researcher is interested in. For example, Neurospora crassa is well suited to the study of meiotic processes such as crossing over because it naturally produces asci, small sacs each of which contains the products of a single meiosis. Hence all the products of a crossover can be documented and the crossover process can be reconstructed from analysis of these products. Few other organisms have this meiotic feature, making the ascomycete fungi an obvious choice of a model organism for researchers interested in meiosis in general. In another example, Neurospora has a conspicuous daily asexual spore-producing rhythm, making it an ideal choice for studies on the genetics of biological clocks in general.

A feature of most model organisms is that they are small and grow rapidly. Their small size means that they can be grown in large numbers, which is advantageous for screening or selecting rare genetic events. Their speed of growth means that many generations can be easily analysed (facilitating analysis of inheritance patterns) and it is easy to obtain a large mass of cells for biochemical analysis. Neurospora can be grown in large numbers as colonies on petri dishes (up to several 100 can be grown a plate) or as individual cultures in small test tubes 2" long. Believe it or not, for biochemical analysis Neurospora was once grown in washing machines, and the cells ground up with motor-driven millstones. However most of today's more sophisticated protocols generally need only a few grams of cell mass.

Although the principles deduced within a model organism are interesting in themselves, nevertheless there is always the hope that these findings will have wider relevance, especially to organisms that are less tractable from the genetic standpoint. A general observation in biology has been that widely different species tend to show remarkably similar fundamental processes. Crossing over is a case in point: it occurs in apparently much the same way in most eukaryotic species. Hence there is a reasonable expectation that what is learned in one species can be at least partially applied to others.

Biologists have always kept an eye open for application of new research findings to our own species, following the idea expressed so well by Alexander Pope (1870) that "The proper study of mankind is man". However humans, the most interesting of organisms, are relatively difficult to research at the genetic level, so conceptual advance in human genetics has benefited well from the application of one hundred years of work on model organisms. Using Neurospora as an example, the understanding of inherited metabolic diseases in humans has benefited greatly from Beadle and Tatum's discovery of gene-controlled metabolic pathways in Neurospora during the 1940s.

All model organisms have far more than one useful feature for genetic or other biological study. Hence, once a model organism is developed by a few people with specific interests, it then acts as a nucleus for the development of a research community, a group of researchers with an interest in various features of one particular model organism. There are organized research communities for all model organisms. These people are in touch with each other on a regular basis, share their mutant strains and often meet periodically at conferences. Some of these conferences attract thousands of people. An important service possible with such a community is to provide data bases of research information (publications), genetic stocks, clones, DNA libraries and genomic sequences. The Neurospora research community covers the globe, comprised of about 50 labs and 200-300 researchers at present.

One great advantage for an individual researcher in belonging to such a communty is to obtain "A feeling for the organism" (the phrase of geneticist and Nobel Laureate Barbara McClintock, who did some of the first studies on Neurospora chromosomes). The feeling for the organism is a difficult idea to convey, but it centers on the fact that within an organism no biological system works in isolation: to study meiosis (for example) requires knowledge of other aspects of the ways of that species, such as its subcellular structure, its biochemistry and its life cycle. Indeed most biologically systems are interconnected at the physiological level, so it pays dividends for a researcher interested in one particular system to have a general feeling for the ways of the organism.

As the database for a particular model organism expands, it reaches a point at which many different facets of the organism are understood. Genomics has had a powerful effect in this regard, providing a list of genes plus a presumptive function for many of them. Hence gene lists can be drawn up for all the different processes in which the organism engages. Hence, as opposed to the earlier approaches of geneticists working on model organisms, which were essentially reductionist, focusing on small parts of the organism, the more recent view is holistic, encompassing the integrated workings of all the parts of the organism's makeup. This is the stage that has been reached today in all the model organisms used in genetics, including Neurospora. Thus model organisms become not only models for isolated processes, but models of integrated life processes. The term "systems biology" is used to describe this holistic approach. It is only now that technology has advanced to the point at which the necessary databases have accumulated that one can begin to think in this way.

For more information on Neurospora as a model organism see

Davis, R H, 2000. Neurospora: Contributions of a Model Organism. Oxford University press, Oxford. (This book is an invaluable summary of the major research finding arising from this fungus.)

Davis, R. H., and D. D. Perkins. 2002. Neurospora: A model of model microbes. Nature Rev. Genet. 3:7-13.

Perkins, D. D., and R. H. Davis. 2000b. Neurospora at the millennium. Fungal Genet. Biol. 31:153-167.

Perkins, D. D., and R. H. Davis. 2002. Neurospora chronology -- 1843-2002. Fungal Genet. Newslett. 49:4-8.

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Last modified 4/28/04 KMC