330 



GENETICS OF FUNGI 



transformed into young chlamydospores, the conjugate nuclear 

 condition still obtains, and actual fusion takes place only within 

 the maturing chlamydospore. 



Stakman and Christiansen (1927), by isolating the individual 

 sporidia, were able to show that U. zeae is heterothallic, and by 



Ustilago hordei 



<b § ^ 



XD CIX 



Ustilago medians 



W (3) W 



F, { Ss - I + . 



^XE) ®C± 



+ 



(§> WZ&. 



F *0000 0000 Fz 



Fig. 56. Schematic representation of hybridization of two species of smuts, 

 one smooth-walled and one rough-walled. P represents parents. The sex 

 factor, + or — , segregates independently of the chlamydospore-wall char- 

 acter. 5 in sporidia represents spiny walls; s in sporidia represents smooth 

 walls with S dominant. In the Fi generation all spore walls are spiny. In 

 the F-2 generation the ratio of spinv-walled spores to smooth-walled spores 

 is 4:0, 3:1, and 2:2, if all possible combinations are made. 



the same techniques Flor (1932) showed that this situation exists 

 also in Tilletia tritici and T. levis. 



Working with oat smut, Ustilago levis, Dickinson (1931), con- 

 sidered that the two pairs of factors Aa and Bb, representing sex 

 and color, were additive in their effect, AB causing brown color, 

 ab causing cream color, and either Ab or aB causing velknv color. 

 He isolated the four sporidia of known position. It was apparent 

 that the haploid parental mating was AB X ab, that is brown X 

 cream. Out of this mating came tetrads of sporidia of three 



