HOMOTHALLISM AND HETEROTHALLISM 



321 



types, either (Arb -f- ) and (arb — ) or (Arb — ) and (arb -f). 



To explain this condition BurgefT assumed properly that one or 



both factor pairs must segregate at the second division. If both 



pairs of factors had segregated at the first division, there would 



have been only two haploid 



types, either (Arb +) and 



(arb — ) or (Arb — ) and 



(arb +). 



In Ascomycetes. The most 



illuminating grenetical studies 



among Ascomycetes have con- 



cerned Neurospora [Shear 



and Dodge (1927), Dodge 



(1927, 1928, 1930, 1931, 1940), 



Wilcox (1928), Lindegren 



(1929, 1933, 1936, 1939)]. 



The best-known species of 



this genus is N. sitophila, 



known as the pink bakery 



mold, which is cosmopolitan 



in distribution. It has a Mo- 



nilia conidial stage. Some of 



the species, represented by N. 



sitophila and N. crassa, are , . MM „«. n «j rt „ n f 



r . Fig. 51. Schematic representation or 



eight-spored and obligately poten tialities of ascospores of Plenr- 

 heterothallic. Each spore is age anserina. Circles represent nuclei 

 uninucleate and unisexual, of one sex, and black dots nuclei of 

 four spores being of ( + ) sex the °PP osite sex ; Although the asci 

 reaction and four of (— ) sex 

 reaction. Other species, such 

 as N. tetrasperma, are nor- 

 mally four-spored, each spore being binucleate and bisexual. Oc- 

 casionally in this species one or more of the spores are giants or 

 dwarfs, as occurs also in Pleurage anserina, a widely distributed 

 dung-fungus [Wolf (1912), Dowding (1931), Ames (1934). 

 Usually the giant spore replaces two normal spores. The dwarf 

 spores occur in pairs, each containing a single nucleus. In N. 

 tetrasperma [Dodge (1927)] all eight of the nuclei may occa- 

 sionally occur within one giant spore. 



In order to learn something of sex segregation Shear and Dodge 



always have eight nuclei, the asco- 

 spores may be uninucleate, binucleate, 

 trinucleate, or quadrinucleate. 



