BIOCHEMICAL MODELS OF HETEROSIS IN NEUROSPORA 203 



of different mutant genes have different degrees of dominance. A strongly 

 dominant wild type allele will need to be present in relatively few nuclei — say 

 one in twenty. 



A heterocaryon between two mutant strains could grow at the maximum 

 rate over a large range of nuclear proportions, provided the wild type alleles 

 concerned were both strongly dominant. A weakly dominant wild type 

 allele, on the other hand, must be present in a large j)roportion of the nuclei — 

 say nineteen of twenty — to ensure vigorous growth. Heterocaryons in which 

 the wild type alleles concerned are both weakly dominant could never result 

 in vigorous growth, since the two wild type alleles cannot both be present 

 in excess, one being in one type of nucleus and the other in the remaining 

 nuclei. 



HETEROSIS DUE TO HETEROZYGOSITY AT ONE LOCUS 



The heterosis effect in heterocaryons studied by Beadle and Coonradt re- 

 sults from the mutually complementary nature of the nuclei involved. For 

 each deleterious mutant allele in one nucleus there is the corresponding 

 favorable and dominant wild type allele in another. In contrast to these 

 there are other heterocaryons (briefly reported in Emerson, 1947) in which 

 the nuclei differ in only one gene, yet which still show the heterosis effect. 

 Heterocaryons in which some nuclei carry the dominant allele and some the 

 recessive are superior to homocaryons, all of whose nuclei have the dominant 

 allele, or all the recessive. 



Heterocaryotic Suppression of the Sulfonamide-requiring Character 



Most of the heterocaryons of this sort that have been found so far have 

 involved the so-called sulfonamide-requiring mutant strain. At 35° on mini- 

 mal medium, this strain makes extremely poor growth, but it does keep 

 creeping along. After varying lengths of time, it frequently happens that the 

 growth will change to a rapid vigorous type. Growth curves of six cultures 

 which have reverted to something approaching wild type growth are shown 

 in Figure 12.3. When the mycelium had reached the end of the growth tubes, 

 inocula from the newest growth were introduced into fresh tubes containing 

 minimal medium, resulting in the growth curves shown in the upper part of 

 the figure. 



From these curves it can be seen that the reverted type of growth usually 

 persists through a conidial transfer. After the mycelium had reached the end 

 of the second tube, conidia were removed and used in outcrosses to wild type 

 to determine the genetic constitution of their nuclei. These tests showed 

 that each of the six cultures represented in Figure 12.3 was a heterocaryon. 

 One type of nucleus present in each heterocaryon was identical to those in 

 the original sulfonamide-requiring strain. The second type of nucleus in each 

 also carried the sulfonamide-requiring gene, sjo (in one instance, that de- 

 rived from culture number 1 in Figure 12.3, the sJo gene itself was somewhat 



