Environmental and genetic control of 

 differentiation in Neurospora 



by 



M. WESTERGAARD 



and 

 H. HIRSCH* 



Universitetets Genetiske Institut, Kobenhavn 



As is well known, it has been possible in recent years to obtain very precise informa- 

 tion about the biochemistry and physiology of growth and its genetic control in 

 certain fungi and bacteria by means of a very ingenious technique. The 'wild type' 

 strain of the organism is grown on a synthetic 'minimal medium'. Mutants are pro- 

 duced which cannot grow on the minimal substrate but only on a 'complete medium' 

 supplemented with various diffusible growth factors. The mutants lack the capacity 

 to carry out a certain enzymatically controlled metabolic step, necessary for normal 

 growth, and this step can be identified by adding only one growth factor at a time 

 to the minimal medium. Hence the mutation serves the purpose of a very specific 

 enzyme inhibitor, and the advantages of the method have been amply demonstrated 

 in studies of the biosynthesis of various amino-acids and vitamins. 



This method has almost exclusively been applied to the study of growth, whereas 

 very little work along similar lines has been done on morphogenesis in these organ- 

 isms. It would be interesting, however, to use the technique which has been so suc- 

 cessful for the study of growth to a study of problems of differentiation. This would 

 be possible, (a) if one or several well-defined steps in morphogenesis could be control- 

 led on a synthetic medium, (b) if mutants could then be produced in which normal 

 differentiation was either blocked or modified on the minimal medium, and finally 

 (c) if normal differentiation could be induced again in these mutants by supplement- 

 ing the minimal medium with suitable precursors. 



The first difficulty is obviously that of selecting a suitable system of differentiation 

 which can be properly controlled on a synthetic minimal medium. The selection of 

 such a system in fungi and other micro-organisms is not too easy, because the morpho- 

 genesis of micro-organisms is far less well defined and far less advanced than that of 

 higher plants and animals. Let us take as an example the mould Neurospora, a well- 

 recognized tool for genetical experiments since the earlier work of Dodge and Linde- 

 gren and the more recent work of Beadle, Tatum and their group. It will be seen 

 from Figure i that Neurospora during development does three different things: 

 (i) it grows; (2) it forms conidia; and (3) it forms, on the haploid mycelium, female 



* Present address: Div. of Cancer Biology Medical School, University of Minnesota, Minneapolis, U.S.A. 



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