40 VARIATIONS IN THE LOWER FUNGI 



segregations of genes involved in sexual reproduction, which result 

 in hybrids. 



Ever-sporting Races. Once a culture begins to throw variants, it 

 usually becomes continuously unstable, an ever-sporting race. While 

 the parent strain is thus continuously varying, the variants are usu- 

 ally more stable, sometimes apparently permanent. Occasionally 

 variants revert immediately to the parent type. But once a culture 

 of a dermatophyte has become woolly, it is practically impossible to 

 get it to change back to its typical form. Occurring frequently in 

 vegetative mycelium, or in imperfect fungi where sexual reproduc- 

 tion is absent, tending to occur constantly in one direction and often 

 apparently irreversible, such variations seem to show the character- 

 istics of gene mutations, and especially those attributed to unstable 

 genes (see Demerec ^). 



Reversible Variations. But in many cases variants which seemed 

 for a long time to be permanent have eventually reverted to the 

 parent type. Thus some of the white variants of the red yeast Cryp- 

 tococcus pulcherrimus described by Punkari and Henrici -^ appeared 

 to be permanent but one of these kept by me turned red again after 

 more than a year of continued subcultivation. Negroni -* obtained 

 a partial reversion of the R variant of Candida albicans by growing 

 it in a sterilized culture of the S type to which anti-R type serum 

 was added. Burkholder ^ described a woolly change in the normally 

 slimy fungus, Fusarium Martii var. Phaseoli. When inoculated on 

 beans, the normal host plant, this variant reverted to the normal 

 type. 



Mutant or Hybrid. The fact that the observed variations are so 

 frequently reversible led Brierley ^ to doubt that they are true muta- 

 tions. He believed that they result rather from a recombination of 

 characters already present in the hereditary constitution of the or- 

 ganism. Such a theory implies that in all cases there occurs some- 

 thing of the nature of sexual reproduction, nuclear^ fusions and segre- 

 gations. Since most of the fungi exhibit sexual reproduction, and 

 many of them complex life cycles in which the various combinations 

 and segregations are often obscure, this seems to be a reasonable 

 theory. 



There are, however, certain data which indicate that variations 

 may occur without any cell fusions. Thus Punkari and Henrici -^ 

 emphasized the complete absence of spores in the yeast Cryptococ- 

 cus pulcherrimus which they studied, since so far as was known 

 sexual fusion in yeasts always results in the formation of spores. 



