BIOCHEMICAL MODELS OF HETEROSIS IN NEUROSPORA 217 



Houlahan and Mitchell (1948) have studied the interactions of mutant 

 strains involved in the metabolism of i)yrimidines and lysine. A pyrimidine- 

 less mutant (37301) has a specific requirement for pyrimidine. There is a 

 suppressor of this mutant which enables it to grow without added pyrimidine, 

 unless arginine is also added, whereupon the pyrimidine requirement is re- 

 stored. One lysineless strain (33933) can utilize a-amino adipic acid in place 

 of lysine. As a double mutant with the pyrimidine suppressor, it can still 

 use a-amino adipic acid, but requires four times as much as the simple lysine- 

 less strain unless small amounts of arginine, or an arginine precursor, are 

 added. The double mutant combining this lysineless with the pyrimidineless 

 mutant is unable to use a-amino adipic acid unless a small amount of lysine 

 is added — arginine is ineffective in this instance. A second lysineless mutant 

 (454.5), which has a specific requirement for lysine and which secretes pyrimi- 

 dines into the medium, behaves in a predictable fashion as a double mutant 

 with pyrimidineless, or its suppressor, but not as the triple mutant lysineless, 

 pyrimidineless, suppressor of pyrimidineless. Instead of requiring only lysine 

 for growth, this triple mutant also requires pyrimidines and arginine. This 

 example is cited as another in which metabolic interactions may be as com- 

 plex as in those discussed earlier which depend in one way or another on 

 />-aminobenzoic acid. 



Applicability to Classical Heterosis 



Observations relating to one-gene heterosis in higher plants are discussed 

 in other papers in this series (Crow, Hull, Jones, and Whaley). Studies of 

 Neurospora heterocaryons have shown that a very similar phenomenon oc- 

 curs under certain special physiological conditions. In a particular genetic 

 background, the amount of an essential metabolite normally produced has 

 deleterious consequences which are removed by reducing the dosage of a 

 gene responsible for the production of that metabolite. This reduction was 

 brought about through heterocaryosis in the studies reported, but it should 

 also result from heterozygosis under similar physiological conditions. There 

 is nothing in the studies of heterocaryosis in Neurospora to suggest that one- 

 gene heterosis is of general occurrence and importance, or that other examples 

 should have similar biochemical backgrounds. 



