STERLING EMERSON 



California Insfifufe of Technology 



Chapter 12 



Biochemicol Models 

 of Heterosis in Neurosporo 



Some of the things that have been learned about gene controlled reactions 

 in Neurospora can be used in forming a picture of how individual genes con- 

 tribute to heterosis. I wish to consider especially those examples which indi- 

 cate that heterozygosity at a single locus may influence the growth of an 

 organism to a considerable extent. 



It should be noted at the beginning, however, that one is not justified in 

 assuming that the situations found in Neurospora are necessarily similar to 

 those occurring in the higher organisms in which heterosis is ordinarily 

 studied. It may be unwise to assume that any two organisms are essentially 

 similar. There are special reasons for caution in making comparisons between 

 Neurospora and higher plants and animals, since the nuclear and chromo- 

 somal basis for the expression of heterosis is so dissimilar. On the other hand, 

 there is a considerable accumulation of information about the parts played 

 in the physiology and biochemistry of Neurospora by individual genes 

 (Beadle, 1948; Horowitz, 1950) and, with proper caution, we may assume 

 that some of this information may have rather broad application. 



In any haploid organism, such as the ascomycetous fungus Neurospora, 

 in which there is a single set of genes in each nucleus, such phenomena as 

 dominance, heterozygosis, and heterosis cannot occur. There is, however, 

 a condition known as heterocaryosis which permits a loose approximation 

 to each. 



CHARACTERISTICS OF HETEROCARYONS 



The plant body of Neurospora can be said to be made up of cells, but they 

 are very different from the cells of higher plants. In the first place, the cells 

 contain a large and variable number of nuclei in a common cytoplasm. The 

 so-called cells themselves are not as discrete as cells are generally supposed 



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