Chapter 6 

 CYTOLOGY 



THE FUNGAL NUCLEUS 



The fungal nucleus is a distinctive structure and our study of 

 the yeast cell has revealed that it shares the salient features of 

 other fungal nuclei. These characteristics were first clearly es- 

 tablished by Harper in his classical work on Phyllactinia. He 

 showed that the chromosomes are polarized to a densely chromat- 

 ic central body located on the periphery of the conspicuous nuclear 

 vacuole, giving the nucleus a characteristically eccentric appear- 

 ance. Each chromosome is attached to the centriole; each mitosis 

 is preceded by the division of the centriole and the spindle forms 

 between the two daughter centrioles. The nuclear membrane re- 

 mains intact throughout mitosis and the spindle is intranuclear as 

 it is in all fungi. The daughter centrioles merely move to oppo- 

 site ends of the nuclear vacuole. Harper proposed that the spindle 

 fibers also remained intact throughout the cell cycle and that the 

 division of the centriole was followed by the division of the spindle 

 fibers and finally by division of the chromosomes. Harper's in- 

 teresting and revolutionary views on the origin of spindle fibers 

 have not been reinvestigated although some fvmgi with large pro- 

 phase nuclei might be exceptionally favorable cytological material. 

 The nucleolus was a conspicuous structure in the fungal nuclei de- 

 scribed by Harper, and it Is clear from his study that the char- 

 acteristic eccentric nature of the fungal nucleus with a large dense- 

 ly staining particle in or on the nuclear vacuole may arise either 

 from the presence of a large nucleolus or from a large centriole 

 but that critical analysis is necessary to make the proper distinc- 

 tion between these two structures in small cells. 



Dodge, and Lindegren and Rumann made critical studies of the 

 nucleus of Neurospora and discovered that it also is a character- 

 istically eccentric nucleus with polarized chromosomes and a con- 

 spicuous structure which Dodge called the "beak" to which the 

 chromosomes are polarized. This large umbrella handle -shaped, 

 deeply staining structure is the source of the "astral rays" which 

 originate from it to cut the spore, out of the cytoplasm in the man- 

 ner characteristic of the ascomycetes. Dodge did not call this 

 structure the centriole and he may have been properly cautious in 

 avoiding a sharply limiting term because it seems too large to be 



6-1 



