HOMOTHALLISM AND HETEROTHALL1SM 331 



groups in the proportion of: (1) two AB and two ab, (2) two 

 Ab and aB, (3) one each of AB, ab y aB, and Ab. Segregation 

 would appear therefore to occur in the fashion described bv 

 Newton (1926) and Brunswik (1924) in Coprimts lagopus, with 

 the linkage and crossing-over mechanisms as interpreted by Dodge 

 (1940). These observations bv Dickinson (1931) are further 

 substantiated bv the findings of Schmitt (1940) on Ustilago zeae. 

 The segregations of factors for colonv color, sex, and tvpe of 

 growth (sporidial or mvcelial) occurred in both the first and the 

 second divisions. For each of these characters numerical ratios 

 of 1:1, 3:1, and 1:3, were found, with a 4:0 segregation for sex 

 in one case amoncr several thousand. 



Smuts constitute especially favorable material for genetical 

 studies for the reason that all the sporidia from a promvcelium 

 can be isolated and can then be propagated in culture. Each spo- 

 ridium by budding can be made to form numerous haploid in- 

 dividuals that can be mated under controlled conditions. Gene- 

 tic studies of smuts have been concerned with colony characters 

 and tendency to sector. Other studies have included such factors 

 as color of the peridium of sori, pathogenicity, sex compatibility, 

 color of the chlamydospores and the nature of their walls, and 

 tendency to be myceloid or to form buds [Christiansen and 

 Rodenheiser (1940)']. 



Kernkamp (1942) isolated monosporidial lines of U. zeae to 

 study the effects of genetic and environmental factors on types 

 of colonial growth. Some isolates were entirely sporidial, some 

 entirely mycelial, and some intermediate. Strictly sporidial lines 

 could not be mated and could not infect maize. The growth 

 types of sporidial or of mycelial lines could not be modified by 

 changes in concentration of food or by addition of certain vita- 

 mins, poisons, amino acids, or other substances. The growth 

 type of intermediate lines, however, could be modified for in- 

 creased sporidial production by the presence of dextrose or for 

 mycelial production by unfavorable growth conditions. 



Stakman and his associates (1943) found that mutation is un- 

 believably common in U. zeae. In this smut mutability and con- 

 stancy are governed by genetic factors, as has been determined 

 from the results of numerous crosses between monosporidial lines 

 of opposite sex. Stakman and his associates conclude, "Ustilago 

 zeae definitely comprises an indefinite number of biotypes that 



