Environmental and genetic control of differentiation in Neurospora 



None of these authors, however, connected their observations with the differentia- 

 tion of the sex organs. Fox and Gray claimed that there is a difference in tyrosinase 

 activity between the two mating types ( + and — ) of Neurospora, an observation which 

 was not confirmed either by Horowitz or by Hirsch. Horowitz and Shen also found, 

 as did Hirsch, that tyrosinase activity in Neurospora depends upon the temperature. 

 Moreover, they made the interesting discovery that, under the conditions and with 

 the strain which they used, an inhibitor was present which suppressed tyrosinase 

 activity. Horowitz used a modified, liquid Fries-medium ('Horowitz-medium'). Its 

 composition is shown in Table I. They showed that this inhibitor disappeared if the 

 mycelium was dialysed. Its presence in non-dialysed mycelium was demonstrated 

 by adding fresh mycelium to a tyrosinase preparation, after which procedure the 

 tyrosinase activity was inhibited. They also found that the tyrosinase inhibitor 

 disappeared when Neurospora was grown on a medium poor in sulphur. 



This brings the possible connexion between the tyrosinase inhibitor and proto- 

 perithecium formation into the picture. Hirsch found that the inhibitor was present 

 only in mycelia grown in liquid cultures; it was present whether Horowitz or P- 

 minimal medium was used. No protoperithecia were ever formed in liquid cultures. 

 If, however, the mycelia were grown on filter paper in liquid cultures, tyrosinase 

 activity was present in non-dialysed mycelia grown on Horowitz as well as on P- 

 minimal medium, and protoperithecia were formed in both cases. On the other 

 hand, tyrosinase activity was found in non-dialysed mycelia grown on Horowitz 

 low-sulphur medium, but hardly any protoperithecia developed on this substrate. 

 Finally : if the standard P-minimal medium was substituted with a P-minimal low- 

 sulphur medium, protoperithecium formation was almost as good as on standard 

 P-minimal. Evidently until more is known about the nature of the suppressor present 

 in the mycelia grown in liquid media, its pH-dependence, its interaction with the 

 constituents of the different media and variation between different strains, we shall 

 have to leave this problem open. 



Horowitz and Fling later made a more thorough study of the sensitivity of Neuro- 

 spora tyrosinase to temperature. They found another strain of Neurospora containing a 

 temperature-stable tyrosinase. Unfortunately nothing is said about sexuality in this 

 strain. The tyrosinase from the temperature-unstable strains was found to have a 

 half-life of 3-4 minutes at 59 C, whereas the enzyme from the stable strains had a 

 half-life of at least 30 min. at the same temperature. The difference between the two 

 strains was controlled by a pair of allelic genes designated T s jT L . The absence of 

 tyrosinase activity at 35 C. in temperature-sensitive strains was not due to a sup- 

 pressor (Horowitz and Fling, I.e.; Hirsch, I.e.). 



We have so far been dealing with only one part of the problem, viz. the environ- 

 mental control of differentiation in Neurospora. As a working hypothesis we want to 

 suggest that there is a causal relationship between tyrosinase activity and melanin 

 formation on one hand and differentiation of female sex organs on the other hand. 

 It also seems as if normal differentiation can be blocked at high temperature because 

 one of the enzymes involved in melanin metabolism is temperature-sensitive, the 

 temperature-dependence of the enzyme being under genie control. Although the 

 evidence is far from conclusive, there may also be a relationship between the forma- 

 tion of a tyrosinase inhibitor on certain substrates and the scarcity or absence of 



177 



