To my opinion the basic question is: What is a "model system" and
what do we use it for ?
The most essential feature of a model system is that we can use it
to make predictions about less studied systems. Often it is very useful to
study a simple system, or one that is easy to handle experimentally, to
focus subsequent studies on more complex systems. For example, glycolysis
in bloodstream Trypanosoma brucei is an excellent model system because only
pyruvate kinase is regulated by a low molecular weight effector (fructose
2,6-bisphosphate) and the endproduct pyruvate is excreted. For this reason
it has been possible to analyze this pathway completely using MCA (Bakker
et al., J. Biol Chem 272, 3207-15, 1997). Conclusions based on this system
can then be used to design experiments using e.g. mammalian cell lines.
Similarly yeast are used because they are so easy to work with, not because
everything that happens in yeasts happens in all other cells. It doesn't.
In all cases the conclusions based on model systems can only be
extrapolated if they are verified for other systems. In my own research on
the regulation of energy metabolism I use three species of protists,
Trichomonas vaginalis, Trypanosoma brucei (procyclics) and Leishmania
donovani. The way each regulates carbohydrate usage is widely different and
none can function as model for the others. Therefore as far as I am
concerned model systems are great for developing hypotheses, but general
validity must always be verified experimentally.
Benno ter Kuile
