12-9 THE YEAST CELL 



tions from the classical type. In one, the promycelial cells send 

 out germ tubes instead of producing sporidia. These germ tubes 

 copulate with each other and by this means, it is possible to dis- 

 tinguish first and second-division segregation by direct inspection. 

 These varieties have bipolar mating type. If we nimiber the cells 

 of the promycelium 1, 2, 3, and 4 from the tip downward, there are 

 two types of copulations: (1) the germ tube originating from 1 may 

 copulate with 2, and the one originating from 3 may copulate with 4, 

 or (2) the germ tube originating from 2 may copulate with 3 and 

 that originating from 1 may copulate with 4. These are the only 

 types of copulations which are found. On the basis of calculations 

 derived from observations of this phenonenon, Huttig found that 

 the normal ratio of first and second division of segregation was 

 23:77. 



In Neurospora Lindegren showed that the maximum expected 

 frequency of second-division segregation is 67%, and this figure 

 is not exceeded in the analysis of the segregation of a large num- 

 ber of factors. This depends on the assumption that the chromo- 

 somes cross over freely along their entire lengths. The high fre- 

 quency of post reduction in Ustilago found by Huttig is of consider- 

 erable theoretical interest. His calculations are based on the as- 

 sumption that copulation is between germ tubes originating from 

 cells of opposite mating type and although confirmation of this as- 

 sumption is not presented, it is difficult to criticize the point of 

 view. Huttig plated the chlamydospores on medium containing a 

 large variety of chemicals and found in every case that second- 

 division segregation occurred in excess of expectation. A possible 

 explanation of the apparent discrepancy with Neurospora was sug- 

 gested in a conversation with Dr. McClintock: the chromosomes 

 in Ustilago may form a pair of chiasmata spanning the centromere, 

 with only a single cross over occurring in each arm of each chro- 

 mosome. This would make possible a 100 per cent post reduction 

 depending upon the position of the locus. 



Several attempts have been made to study the segregation of 

 colonial characters such as color and colony shape in the pro my - 

 celia of smuts. To study the segregation in Ustilago, it is neces- 

 sary to inoculate the living host with paired cultures and collect 

 the chlamydospores resulting from the infection. None of the 

 workers has attempted to interpret the results in conventional 

 genetical terms, although much of the data is obviously suscepti- 

 ble to this approach. The best data are those of Dickinson (1931), 

 who studied the segregation of five characters. He only analyzed 

 a limited number of promycelia, but my calculations of his data 

 indicate that ail five factors are in a single linkage group, although 

 the data are not statistically significant. This suggests that the 

 smuts may be excellent genetical material, in spite of the fact that 



