12-13 THE YEAST CELL 



degren and Lindegren analyzed crossing-over in 4 regions marked 

 by 5 loci bridging the centrosome of N. crassa. They concluded 

 that crossing-over was a non-random process; that local regions 

 of the chromosome had specific patterns controlling the types of 

 exchanges which occurred. This was not in agreement with the 

 findings of Beadle and Emerson in Drosophila. It was suggested 

 that they may have studied regions too long to permit the detection 

 of all possible exchanges. The stocks of Neurospora now available 

 should make a reinvestigation of this problem a simple matter. 



The treatment of Neurospora with ultra violet and X-rays has 

 been used for the production of mutations. Lindegren and Linde- 

 gren treated individual spermatia and found that ultra violet pro- 

 duced very few chromosomal abberations, while X-rays produced 

 a large proportion. Many of the variants reverted on subculture, 

 but nearly one third of the ultra violet variants were stable on sub- 

 culture, but produced only normal progeny when outcrossed. There- 

 fore, the function which had been destroyed by treatment was re- 

 stored in the heterozygous condition. At the time this was inter- 

 preted as a manifestation of cytoplasmic inheritance, but now the 

 simpler concept of depletion mutation seems preferable. An ex- 

 tensive study of the effects of monochromatic radiation of Neuro- 

 spora spermatia by Sansome, HoUaender, and Demerec has con- 

 firmed these results. 



Beadle and Tatum irradiated Neurospora conidia and fertilized 

 complementary thalli with irradiated conidia and isolated the asci 

 subsequently developed. When an individual ascus produced 4 mu- 

 tant and 4 normal cultures, the mutant cultures were further ana- 

 lyzed for their ability to synthesize vitamins. This work in bio- 

 chemical genetics has been summarized by Beadle and has led to 

 a great interest in the use of Neurospora for the analysis of bio- 

 chemical mechanisms. It does not involve any discrepancy with 

 Lindegren and Lindegren' s finding of depletion mutations following 

 radiation, since Beadle's mutants were only isolated after hybridi- 

 zation, at which time all depletions would have been restored. 



Beadle and his co-workers have used Neurospora for the study 

 of biochemical syntheses by selecting stocks differing in ability to 

 produce enzymes which control steps in biochemical syntheses. 

 Beadle and his group have proposed the one -gene -one -enzyme hy- 

 pothesis which states that each gene synthesizes a specific and 

 different enzyme. Delbruck has pointed out that since no test has 

 been devised by which this hypothesis would be disproved, addi- 

 tional evidence supporting it is not so much to be desired as the 

 development of a critical test of its validity. 



Lindegren showed that one of the natural devices for preserving 

 variation in Neurospora and presumably in other fungi is the forma- 

 tion of heterocaryons. Most of the fungi are multinucleate coeno- 



