202 STERLING EMERSON 



respect to heterosis result from the points just noted. In a diploid which is 

 heterozygous for a single gene pair, both alleles are present in the same nu- 

 cleus and in equal dosage. Whereas in the corresponding haploid heterocar- 

 yon, the two alleles are present in different nuclei, and the relative propor- 

 tions of the two alleles vary with the frequencies of the two types of nuclei. 

 All cells of a diploid heterozygote have the same genetic constitution, but 

 there can be a considerable variation in genetic constitution in different parts 

 of a heterocaryotic individual. Interactions between alleles, by which I mean 

 such things as the expression of dominance, must result from the ability of 

 genes to act at some distance in heterocaryons, in which there is no possibility 

 of an intimate association of alleles within a nucleus (Lewis, 1950). It is 

 considerations such as these that show that dominance and heterosis-like 

 effects in Neurospora are only approximations to the phenomena as known in 

 diploid organisms. 



HETEROSIS IN HETEROCARYONS 



An enhancement of growth, closely simulating heterosis, in heterocaryons 

 of Neurospora teirasperma was reported by Dodge in 1942. In this paper he 

 distinguished between heterocaryotic vigor and the hybrid vigor of diploid 

 organisms along much the same lines as I have just done. He suggested that 

 the heterocaryotic vigor observed might be the result of complementing 

 growth factors whose production was controlled by the two types of nuclei 

 (Robbins, 1950). It was later (Dodge, Schmitt, and Appel, 1945) demon- 

 strated that genes responsible for enhanced growth segregated and recom- 

 bined in a normal fashion. These studies showed that genes residing in differ- 

 ent nuclei, but in a common cytoplasm, can cooperate in establishing condi- 

 tions favoring rapid growth, and that a condition resembling hybrid vigor 

 occurs. 



Meantime, Beadle and Coonradt (1944) had reported on heterocaryons 

 between pairs of mutant strains of Neurospora crassa, each of which is unable 

 to synthesize a particular vitamin or amino acid. Each mutant strain by itself 

 is unable to grow unless supplied with its specific growth requirement, but 

 nine heterocaryons involving different combinations of seven mutant strains 

 grew at rates approximating that of wild type without the addition of growth 

 factors. The authors conclude that the wild type allele is dominant to the 

 mutant allele in each of the examples studied. 



Beadle and Coonradt note further that in such heterocaryons, in which 

 there is the opportunity for great diversity in the relative numbers of the 

 two types of nuclei in different hyphal tips, those tips having the most favor- 

 able proportions of nuclei should grow most rapidly. Conversely, rapidly 

 growing hyphae should have the two sorts of nuclei in roughly optimal pro- 

 portions. In heterocaryons involving pairs of mutant strains, Beadle and 

 Coonradt found nuclear ratios varying between approximately 1 : 1 and al- 

 most 20: 1. They interpreted these results to mean that the wild type alleles 



