PHYSIOLOGY OF GENE ACTION IN HYBRIDS 111 



In one case reported by Emerson (1948) a different situation obtains. A 

 mutant strain of Neurospora which requires sulfonamides for growth at cer- 

 tain temperatures will grow satisfactorily in the absence of sulfonamides, 

 provided that the concentration of available p-aminobenzoic acid is held at a 

 particular level. Either higher or lower concentrations of p-aminobenzoic acid 

 result in growth inhibitions. Emerson has made heterocaryons between a 

 mutant strain carrying the sulfonamide-requiring gene {sfo) and a gene which 

 prevents the synthesis of p-aminobenzoic acid (pab), and a strain carrying 

 sfo and the wild type allele (+) of pab. The resultant heterocaryons grow 

 vigorously on the minimal medium (without sulfonamides), whereas strains 

 carrying sfo and pab, or sfo and +, make no appreciable growth on the 

 minimal medium. Emerson's explanation of the growth of the hetero- 

 caryons is that it results from a balance between the production of p-amino- 

 benzoic acid by one of the types of nuclei and the absence of production of 

 p-aminobenzoic acid by the other type of nucleus; so that the total produc- 

 tion of p-aminobenzoic acid is sufBcient for growth but still within the range 

 tolerated by strains carrying sfo. Heterosis-like effects of this sort are sugges- 

 tive of the instances of heterosis related to the heterozygosity of particular 

 genes in diploid organisms. 



We thus have in Neurospora, heterosis-like effects assignable both to a 

 recombination of dominant alleles basis and to a heterozygosity basis. More 

 important for this discussion is the fact that these instances are all concerned 

 with facilitation in the hybrid of the production or utilization of substances 

 which are components of the basic enzyme or other growth substance pat- 

 tern of the organisms. 



Various investigations of heterosis in Drosophila, while for the most part 

 not concerned with specific growth substances, have nonetheless assigned 

 manifestation of heterosis to a background in the fundamental biochemical 

 activities of the organisms. Inasmuch as these investigations are discussed in 

 detail in another chapter, they will not be treated here. 



THE PHYSIOLOGICAL BASIS OF HETEROSIS 



From consideration of the pertinent data, a definite pattern emerges. 

 This associates the development of heterosis with the ability of the hybrid 

 to synthesize or to utilize one or several specific substances involved in the 

 fundamental growth processes of the organisms. Nutritional factors, water 

 absorption factors, and the other more gross considerations with which in- 

 vestigators have been particularly concerned seem to be secondary factors^ — 

 perhaps responsible for compounding the heterotic effects but probably not 

 responsible for their initial development. Much of the evidence agrees with 

 the assumption that the primary heterotic effect is concerned with growth 

 substances whose predominant activity is registered in the early part of 

 the developmental cycle; in plants, especially in early postgermination 



