12-15 THE YEAST CELL 



cytes without cross walls in the hyphae, so that all the nuclei are 

 commingled in a common cytoplasm, enabling the fungus to main- 

 tain great genetic variability. In nature, the heterocaryons are usu- 

 ally composed of nuclei of complementary mating type, with super- 

 imposed sterility genes which minimize or prevent the formation 

 of perithecia until fertilization by another stock occurs. Beadle 

 and Coonradt pointed out the advantage of testing different mutants 

 for allelism by making heterocaryons and observing whether or not 

 the wild type were reconstituted. Ryan exploited the differences in 

 rate of growth between mutants and heterocaryons as a means of 

 testing for allelism. It should be pointed out, however, that neither 

 the heterocaryon test for allelism nor the standard genetical test 

 constitute proof of allelism. This problem is a fundamental prob- 

 lem of genetics. The same objections which Delbruck makes to 

 the one -gene -one -enzyme hypothesis apply to assuming the exist- 

 ence of multiple allelic series. Thus far, no test has been devised 

 which has established with certainty whether two mutants can be 

 produced by changes at a single locus, and there are reasons for 

 suspecting that multiple allelism does not exist. The principal ad- 

 vantages of yeasts over Neurospora are: 



1. Yeasts are easily adaptable to respirometer analyses with 

 the Warburg apparatus, while the filamentous Neurospora thallus is 

 disintegrated by the shaking. This is one advantage which all free- 

 living cells have over the filamentous fungi. 



2. Yeasts are fermenters of carbohydrates and much modern 

 biochemistry is based on the analysis of fermentation, and this 

 knowledge may be applied to genetically different yeast strains. 

 Neurospora is an oxidizer of carbohydrates and does not distinguish 

 between different carbon sources such as sucrose, dextrose, galac- 

 tose, lactose, etc. 



Beadle's and Tatum's brilliant research on Neurospora have 

 done much to enlarge our understanding of biology and have added 

 considerably to our knowledge of biochemistry, but they have been 

 pursued on the basic assumption that the classical concept of the 

 gene is fundamentally correct. There is much evidence, however, 

 that these classical views are not adequate. The facts on which 

 the much debated one-gene-one-enzyme hypothesis are based were 

 clearly established by Scott- Moncrieff and Haldane in their clas- 

 sical work on the relation between flowers and genes and plant pig- 

 ments. They showed that a single gene may differentiate between 

 a plant containing a pigment in which an OH group is replaced by 

 an OCH3 group, and that numerous other equally specific chemi- 

 cal syntheses were under direct control of the gene. 



VENTURIA 

 The brightest spot in the attempts to exploit the genetics of the 

 fungi in solving the problem of pathogenesis is Venturia. The dif- 



