VARIATION AND INHERITANCE 



405 



of the inheritance of physiological characters in the Ascomycetes. Beadle 

 (1946) believed that, if the ability to synthesize a certain amino acid or 

 growth factor were due to the action of a single gene, it should be possible 

 to modify the gene in such a way that the fungus could no longer syn- 

 thesize that compound. Previous work of other geneticists with corn, 

 Drosophila, and other organisms had shown that exposure to X rays or 

 ultraviolet radiation caused mutations by either destroying the gene or 

 modifying it so that it could no longer function normally. Beadle found 

 that exposure of conidia of Neurospora crassa and A^. sitophila to X rays 

 or ultraviolet rays had the similar effect of causing mutations that were 



Germinating 



a SCO spore- .-^ 



Germinating 

 ascospore 



Conidia 



Conidia 



Protoperittiecium A — 



~- Protoperithecium a 



Hypnal fusion 



Fig. 78. Diagram of life cycle of Neurospora. (Courtesy of Beadle, Am. Scientist 

 34 : 36, 1946, and Science in Progress, 1947. Published by permission of the Society of 

 the Sigma Xi.) 



expressed in the inability of the fungus to synthesize vitamins, amino 

 acids, and other essential metabolites. 



The 'Svild type" of Neurospora is deficient for biotin but is self-suffi- 

 cient for all other vitamins and for its necessary amino acids. The 

 conidia were exposed to the ultraviolet rays of a Sterilamp for such a 

 time that most of the spores were killed. The spores were then 

 sown over the surface of agar plates in such concentration as to give 

 individual "colonies," which were isolated and allowed to grow. When 

 these were transferred to a minimal medium, containing sucrose, nitrate, 

 mineral salts, and biotin, the failure of an isolate to grow showed an 

 induced variation from the wild type in its capacity to synthesize essen- 

 tial metabolites. 



The variant cultures were then selected and crossed with the wild 



