HOMOTHALLISM AND HETEROTHALL1SM 325 



he obtained four strain groups, which he designated purple, alba, 

 revohtta, and revecta. When he attempted to inbreed bv mating 

 within each of these strains, perithecia were not produced except 

 in one purple X purple mating. When back-crossed to normal, 

 all had low fertility except the alba strains. The evidence, he be- 

 lieves, indicates that these variants in H. ipomoeae arise by gene 

 mutation. 



Edgerton and his associates (1945) employed, in crosses, strains 

 of Glomerella, isolated from Ipomoea, that differed only in that 

 some were (-f-) and the others ( — ). In certain of these crosses 

 each ascus contained four ascospores of the ( + ) type and four 

 of the ( — ) type. In others all ascospores were of the ( — ) type. 

 Some ( + ) strains originating from single ascospores segregated 

 into two strains, but no explanation of this phenomenon based on 

 nuclear constitution has been forthcoming. 



The synthesis of vitamins and amino acids may be gene con- 

 trolled, and loss of such synthetic ability has been induced by 

 treatment of Neurospora crassa with X-rays and ultraviolet light. 

 Tatum (1944) secured approximately 400 mutant strains from 

 60,000 single-spore cultures. Among these mutants were strains 

 which required each of the B vitamins except folic acid and ribo- 

 flavin. Others required most of the amino acids. These muta- 

 tions involved only a single gene. One strain was unable to com- 

 plete the synthesis when supplied with /^-alanine and pantoyl-lac- 

 tone. It required that pantothenic acid be supplied as such from 

 an exogenous source [Tatum (1944)]. From the results of such 

 differences among strains of N. crassa the question arises of why 

 some are able to synthesize their required vitamins and what the 

 mechanism is for loss of such ability by other strains. 



Lindegren (1945) in an extended study of cultivated yeast 

 demonstrated ( -f- ) and ( — ) races that must be mated to secure 

 ascospores. 



In Basidiomycetes. Two investigators, Bensaude (1918) and 

 Kniep (1919), working independently, called attention to the 

 fact that heterothallism occurs among Hymenomycetes and that 

 it is correlated with the presence of long-known mycelial struc- 

 tures called clamp connections. Since then a large number of 

 other workers have contributed to our knowledge of sexuality 

 and genetics of Basidiomycetes, those dealing mostly with Hy- 

 menomycetes having come from Buller and his students and those 



