M. WESTERGAARD AND H. HIRSCH 



Having thus selected the differentiation of the female sex organs as our morpho- 

 genetic system, the next problem is to control this differentiation on a synthetic 

 medium. The minimal medium used in standard work on JVeurospora, the so-called 

 'Fries-minimal medium', does not allow protoperithecia to develop, and conse- 

 quently sexual reproduction does not take place on this substrate (Table I). Until 

 recently Neurospora was grown on cornmeal agar for sexual reproduction, and this of 

 course is not a synthetic medium. In 1947 Westergaard and H. K. Mitchell worked 

 out a synthetic medium which allows the formation of perithecia and abundant 

 sexual reproduction to take place. This was the first step in the development of a 

 system allowing a study of the biochemical genetics of protoperithecial differentia- 

 tion. This medium, which we call the 'P-minimal medium' (perithecia-promoting 

 medium), differs from the standard Fries-minimal medium in containing no 

 ammonium ions; the pH is adjusted to 65 as compared to 5 5 in the Fries-medium 



Table I 

 Composition of different media per litre of¥L 2 0. 



* All substrates have in addition: KH 2 P0 4 i g., MgS0 4 0-5 g., CaCl 2 o-i g., NaCl o-i g., sucrose 15 g., 

 biotin, and trace elements. The low-sulphur media have 0-0079 g. MgS0 4 instead of 0-5 g. and in addition 

 0-4 g. MgCl 2 . 



(Table I) . In the same paper it was shown that the development of perithecia depends 

 also upon the carbon/nitrogen ratio, upon pH, and upon temperature. The optimal 

 temperature for perithecium formation is 25 C, whereas no differentiation at all 

 takes place at 35° C. 



The work of Westergaard and Mitchell was extended by Dr. Herbert Hirsch who 

 worked in our laboratory for two years. His results have recently been published 

 (Hirsch, 1954). He was able to confirm the preliminary observations by Westergaard 

 and Mitchell on perithecium formation, and he extended them to protoperithecium 

 formation ; he made the discovery that the latter system is a reversible one. If slants 

 are incubated for 7-14 days at 35 C. on P-minimal medium (i.e., under conditions 

 where no protoperithecia develop) and then transferred to 25 C, protoperithecia 

 develop after a few days. If, on the other hand, protoperithecia are allowed to develop 

 at 25 C. and the slants are then transferred to 35 C, the protoperithecia can no 

 longer be fertilized. We believe that this reversibility will be of great use in future 

 studies. 



Hirsch also made the first studies on the biochemical mechanism involved in 

 protoperithecium formation. Starting from the observations that mycelia in which 

 protoperithecia are formed in abundance turn brown or black, that both mature 



174 



