A model organism is one which is studied to further our understanding of biological processes. Typical characteristics of model organisms include developing to maturity rapidly, the ability to be easily manipulated, having a short life span, producing a large number of offspring and to having a sequenced genome, in addition to being well understood. Model organisms, if possible, need to have cheap sources and be easy to store in a laboratory as well as being non-pathogenic. Physiological and biochemical similarities to human cells are also useful in model organisms. Evo Devo is a call for a better choice of model organism due to its phylogenetic position of prospective model organisms, which reflects their evolutionary relationships.
There are different types of model organisms including: Genetic, Genomic and Experimental.
- Genetic model organisms are species amenable to genetic analysis and allow large-scale genetic crosses.
- Genomic model organisms occupy a special position in evolution or have a particular genomic size or composition which can be used for reference e.g the pufferfish.
- Experimental model organisms may not be genetically amenable but have certain other positives, specific to the experiment and what characteristics are being looked for.
Some widely used model organisms are:
- E. coli
- S. cerervisiae
- S. pombe
- Drosophila melanogaster
- C. elegans
- M. musculus (mice)
- Amphibians and Birds
Escherichia. coli are the organism that humans understand the most about. They are a relatively simple rod-shaped bacterium but have numerous advantages associated with their use as a model organism. They are an organism that has had its genome fully sequenced and scientists know more about E. coli than any other organism. E. coli are very easy to manipulate and can be grown in a simple nutrient broth in a laboratory, thus, are cheap and easy to keep. E. coli also have the advantage of reproducing at a very rapid rate as well as developing mutations at a rapid rate.
E. coli have been used for scientists in order to understand many processes that happen in other organisms, such as humans. They have been fundamental in understanding many important mechanisms that occur in all life. An example of this would be that they have enabled us to understand how cells can replicate DNA.
There are limitations with using E. coli as a model organism as, most notably, they are a prokaryotic organism and humans are eukaryotes. This ultimately means that there are many differences between the organisms. Eukaryotes are often larger and more complex than prokaryotic organisms, as well as having a larger and more complex genome. There is therefore only so much we can learn from prokaryotes, such as E. coli, due to human cells having major differences in terms of the structure and functions of the cell.
Yeast is a unicellular eukaryotic organism and is used as a model organism to try and understand the more complex eukaryotic genome and cell processes. There are many strains that can be used, for example,Saccharomyces cerevisiae andSchizosaccharomyces pombe – S. cerevisiae is the strain most commonly used. S.cerevisaie is cheap and easy to use in a laboratory as it just requires a simple nutrient broth to grow, like E. coli. It's genome has been fully sequenced and it divides rapidly, although not as rapidly as certain prokaryotes such as E. coli. A further advantage that makes yeast, such as S. cerevisiae, useful as a model organism is that it has a small genome in comparison with higher eukaryotes, but can still carry out all of the most complex processes needed for it to function and survive - this makes it very useful in genetic studies as it is easier to try and find out what is happening in the processes. Using Yeast, such as S. cerevisiae, has been very useful in understanding complex processes such as the eukaryotic cell cycle.
Drosophila melanogaster is a fruitfly and has been used as a model organism, in terms of genetics, for longer than any other organism. Studies on the organism have helped in proving key features of genetics, such as the fact that chromosomes carry the hereditary genetic information. They are a multicellular organism, just like humans, and therefore may be more useful in certain studies than yeast.
There are several features that make the Drosophila melanogaster a useful model organism for genetic studies. It has a giant chromosome that is visible in some of its cells. Drosophila has a very fast maturation rate and a short lifespan. They have had their genome fully sequenced, they are cheap to breed and most importantly there are mutants available for any gene – this allows scientists to understand how mutations in certain genes may cause genetic defects. Drosophila has played an important role in understanding vertebrate development.
Caenorhabditis. elegans are a small worm belonging to the nematode family. They were the first multi-cellular organism to have their genome fully sequenced. They are small, transparent organisms that have 959 cells, all in a particular location. By mapping and studying all of these cells in great detail, it has provided scientists with useful information about the development and these organisms can be useful in trying to understand ageing and cancer. They also can survive indefinitely when placed in a freezer, have had their genome fully sequenced and have a short lifespan – all of these factors make them a very cheap and useful model organism.
- Transparent embryos that allow us to easily track stages of development. These embryos can also be injected with morpholinos in order to manipulate their development.
- Can be easily genetically manipulated
- Small and therefore cheap and easy to keep.
- Produce a large number of offspring – they can lay up to 200 eggs per week.
- Fully sequenced genome and by a great deal of analysis and the creation of genetic maps, There is a lot of similarity between zebrafish and human genomes.
Genes in humans that cause developmental diseases have a counterpart in the zebrafish genome – this, coupled with the ability to knockout or cause mutations in certain genes with relative ease presents researchers the opportunity to try and understand these diseases in greater detail.
They are widely used in research and one particular trait that they have that is of interest to many scientists is their ability to regenerate damaged parts of their heart. Understanding how this happens in great detail may open the door to a better treatment for individuals suffering heart disease.
Mice, such as the common house mouse – Mus. musculus, are useful as model organisms as they are mammals, just like humans. There are very few differences between mice and humans anatomically or in terms of cell structure etc. This is because all mammals are very similar organisms. They are therefore more similar to humans than any of the examples mentioned above and it is, therefore, more reliable to use them as a model organism when looking to make conclusions about humans. Mice are chosen as the model mammal organism as they are small and therefore easy to keep. They have a fully sequenced genome. Most mutations in a mouse gene correspond to a similar mutation is a human Orthologue, and often a similar phenotype may be expressed. This enables scientists to understand a great deal more about certain genetic diseases that occur in the human population. Gene knockdowns can also provide additional information about the functions of certain genes that correspond to certain genes in humans. They are more expensive to keep in a laboratory when compared to some of the model organisms mentioned above but can provide us with a great deal of useful information.
Amphibians, commonly the frog, are widely used in development research. They have largely replaced Zebrafish in common research. They have a number of features making them particularly useful for research:
- Large embryos that are relatively easy to manipulate
- More similar to Humans than Drosophila and Zebrafish
- Able to regenerate body parts
These model organisms have been used to show how signals from one tissue diffusing to another can direct development.
Xenopus laevis (African clawed frog) is an amphibian model organism that is frequently used when studying the development of the cell cycle and cell signalling. Xenopus is also used for further research regarding embryonic development and birth defects. There are several features of Xenopus laevis that make it an appropriate model organism such as it is a tetraploid (has 4 sets of chromosomes), its eggs are very easy to manipulate and the females are able to lay eggs at any time in the year.
Birds, such as chickens 'Gallus gallus domesticus' or quails 'Coturnix coturnix', have a number of advantages when used as a model organism in developmental biology including:
- Large eggs that are easily accessible
- Easy to manipulate and image development
- Have a very similar anatomy to humans
- Complex genetics
- The use of Birds has been important in the field of physiological studies - their robust embryos have been manipulated with the use of plastic to create the effect of a shell to allow visualisation of development whilst in the embryo.
- Long life cycle
- Can produce few eggs
- Can be difficult to cross-breed (therefore problematic when looking into genetics)
- ↑ Ankeny, R. A. and Leonelli, S., n.d. What’s So Special About Model Organisms?. [Online] Available at: https://ore.exeter.ac.uk/repository/bitstream/handle/10036/3660/Studies_MO_paper_FINAL.pdf?sequence=6 [Accessed 27th December 2014].
- ↑ University of Bath. "Biologists Call For Better Choice Of Model Organisms In 'Evo-devo'." ScienceDaily. ScienceDaily, 30 March 2007. www.sciencedaily.com/releases/2007/03/070329092010.htm
- ↑ Alberts, B. et al., 2008. Molecular Biology of the Cell. 5th ed. New York: Garland Science.Page 25
- ↑ Alberts, B. et al., 2008. Molecular Biology of the Cell. 5th ed. New York: Garland Science. Page 25
- ↑ Alberts, B. et al., 2008. Molecular Biology of the Cell. 5th ed. New York: Garland Science. Page 26
- ↑ Alberts, B. et al., 2008. Molecular Biology of the Cell. 5th ed. New York: Garland Science. Pages 33-34
- ↑ Alberts, B. et al., 2008. Molecular Biology of the Cell. 5th ed. New York: Garland Science. Pages 37-38
- ↑ Alberts, B. et al., 2008. Molecular Biology of the Cell. 5th ed. New York: Garland Science. Pages 37-38
- ↑ Alberts, B. et al., 2008. Molecular Biology of the Cell. 5th ed. New York: Garland Science. Pages 36-37
- ↑ Twyman, R., 2002. Model Organisms: Fish. [Online] Available at: http://genome.wellcome.ac.uk/doc_WTD020806.html [Accessed 28th December 2014].
- ↑ Jopling, C. et al., 2010. Pubmed. [Online] Available at: http://www.ncbi.nlm.nih.gov/pubmed/20336145 [Accessed 27th December 2014].
- ↑ Alberts, B. et al., 2008. Molecular Biology of the Cell. 5th ed. New York: Garland Science. Pages 40-41
- ↑ Tadjuidje, Emmanuel, and Heasman, Janet(Mar 2010) Xenopus as an Experimental Organism. In: eLS. John Wiley and; Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002030.pub2]
- ↑ Xenbase: the Xenopus model organism database: Karimi et al. 2017, Nucleic Acids Research, gkx936. [Xenbase / PubMed / NAR ] James-Zorn et al. 2015, Genesis, 53:486-497
- ↑ Animal Research Info, 2015, Chicken, viewed 16 October 2018,http://www.animalresearch.info/en/designing-research/research-animals/chicken/