mTHAL FUSIONS 41 



This may be questioned in view of the recent demonstration by Winge 

 and Laustsen ^^ that many yeasts are normally diploid, and that 

 sexual fusions actually occur in yeasts formerly believed to be 

 parthenogenetic. ]\Iore convincing as examples of true mutation are 

 the observations of Hanna/^ and of Stakman ^^ and Christensen ^ on 

 the occurrence of mutations in cultures of Ustilago Zeae derived 

 from single spores known to be haploid. Undoubtedly these „exam- 

 ples could be multiplied. 



Genetics of Fungi. Within the past decade there has grown an 

 extensive literature on the genetics of fungi. This work had its origin 

 in the discovery of heterothallism by Blakeslee. Heterothallism 

 makes it possible to cultivate the sexes of a fungus separately, each 

 in the haploid state, which in turn makes it possible to study the 

 effect of single genes, rather than the complicated condition of pairs 

 of genes which occur in diploid cells. Unisexual cultures may then 

 be crossed at will to produce new combinations. The discovery of 

 heterothallism by Shear and Dodge ^^ in the pink mold Neurospora 

 sitophila led Dodge ^^ to an extensive study of the genetics of this 

 and related fungi. The discovery of an albinistic mutant of this 

 species made possible a series of hybridization experiments. These 

 studies, continued by Lindegren -^ and others, bid fair to make 

 Neurospora as famous an organism as Drosophila in the history of 

 genetics. Similar studies are being made' with other heterothallic 

 fungi, notably the smuts and rusts. More recently "Winge and Laust- 

 sen ^^ have extensively investigated the genetics of yeasts, using the 

 micromanipulator for the isolation of spores from an ascus, and have 

 conducted hybridization experiments with these microorganisms. 



It would take us too far afield to review this extensive and com- 

 plicated literature, references to which will be found at the end of 

 the chapter. Dodge has recently summarized it as follows: "We 

 know now that new races of fungi are arising through natural hy- 

 bridization. Hybrid structures have been obtained showing domi- 

 nance and Mendelian segregations with crossing-over at reduction 

 which is such an important feature in favoring evolution. We also 

 find in the fungi mutants, lethal factors, deficient chromosomes, sex- 

 chromosomes, sex-linked characters and other genetic features. . . . 

 The fungi in their reproduction and inheritance follow exactly the 

 same laws that govern these activities in higher plants and animals." 



Hyphal Fusions. When heterothallic fungi conjugate, hyphae de- 

 rived from two spores of different sex fuse. This hyphal fusion re- 

 sults, in the Basidiomycetes. in a new type of mycelium, binucleate. 



