BIOCHEMICAL MODELS OF HETEROSIS IN NEUROSPORA 215 



may have a strong effect on one or more of the others, possibly through 

 changing the availability of /?-aminobenzoic acid or a derivative. The model 

 heterocaryon experiments described earlier show that it is possible for one 

 mutation to cause an upset in one reaction and thus be detrimental to growth, 

 and for a second mutation to restore conditions favorable to growth by actu- 

 ally interfering with a different reaction which is itself essential to growth, 

 but which is interrelated with the first reaction. In the reactions related to 

 the metabolism of ^-aminobenzoic acid, there is sufficient complexity to ac- 

 count for the occurrence of a large number of different suppressors of the sul- 

 fonamide-requiring character. 



DISCUSSION 



It has been shown that increased vigor can result from heterocaryosis in 

 which the two kinds of nuclei differ by only one pair of alleles. This may be 

 true only under very special conditions such as have been present in the 

 examples discussed. On the other hand, it is possible that the necessary con- 

 ditions may be met with rather frequently in Neurospora, as suggested by 

 the following examples. 



In mutant strains which have specific requirements for particular amino 

 acids, it is commonly found that their growth is inhibited by the presence of 

 other amino acids which do not ordinarily interfere with growth. Some mu- 

 tants which require an outside source of threonine are strongly inhibited by 

 methionine, (Teas, Horowitz, and Fling, 1948). Mutants specifically requir- 

 ing lysine are inhibited by arginine (Doermann, 1944), and so on. In each of 

 these instances, the inhibition by a particular amino acid is competitively 

 antagonized by the specific amino acid required by the strain in question. 

 The growth of these mutants should be favored by a reduction in the amount 

 of the inhibiting amino acid, as would occur if some of the nuclei carried a 

 genetic block to its synthesis. 



In extreme cases, the specific requirement for an amino acid may not re- 

 sult from a failure in its synthesis, but from an oversensitivity to the in- 

 hibiting amino acid. Thus, the sulfonamide-requiring strain can be said to 

 be oversensitive to homocysteine in a way that leads to a requirement for 

 threonine. One of the lysineless mutants (33933) seems to be oversensitive to 

 arginine in much the same way. Heterocaryons having the lysineless gene in 

 all nuclei, some of which also carry a genetic block to the synthesis of ar- 

 ginine (from strain 36703), make considerable growth on minimal medium, 

 whereas neither the lysineless nor the double mutant does (Fig. 12.8). 



Mary B. Mitchell (personal communication) recently observed that the 

 stock cultures of certain lysineless mutants (4545, 15069, and 33933) had 

 become less sensitive to inhibition by arginine. Tests of these showed that 

 they were heterocaryons, some of whose nuclei were unchanged. Some car- 

 ried mutant genes which lowered the sensitivity to arginine inhibition while 



