Fig. 19. The haploid progenies of 3 sets of 4 mono- 

 sporidial lines of I'stilago maydis obtained from 3 hybrid 

 chlamydospores F, M, and K, resulting from a cross of a 

 mutant, K.,-1 with line L (Stakman, et al., 319). 



when injected into corn. In the other group, no in- 

 fection resulted, although the sporidial lines paired in 

 all possible combinations (55). 



This type of segregation for self-sterility is not 

 uncommon in U. maydis. Christensen (55) obtained 

 no galls by pairing 16 sets of primary sporidial lines 

 when all possible pairings of lines within each set 

 were made. When these sets of lines were paired with 

 those of chlamydospores from another source, how- 

 ever, there was evidence for regular segregation of 

 factors for sex. 



In general, the pairing of lines of the same promy- 

 celium results in a low proportion of fertile com- 

 binations and the number of sexual groups may be 

 decreased by outbreeding: the progeny from one 

 promycelium may fall into 2 or more sexual groups 

 when paired in all possible combinations, but behave 

 alike sexually when mated with the progeny from an- 

 other chlamydospore (55). All attempts failed to find 

 4 monosporidial lines from a single chlamydospore 

 each compatible with all those from another chlamydo- 

 spore. Sex ratios obviously depend on whether they 

 are based on inbreeding or outbreeding. This fact has 

 frequently been overlooked in making genetic analysis 

 of factors for compatibility. 



Inheritance of cultural characters. — Most chlamydo- 



spores, whether collected in nature or the product 

 of controlled crosses, segregate for cultural charac- 

 ters. Visual observations indicate that the segregates 

 differ in all cultural characters that are possible to 

 observe such as color, consistency, luster, topography, 

 zonation, type of margin, and rate of growth of 

 colonies. Some character differences are very distinc- 

 tive, others virtually imperceptible. Some may be 

 alike on one medium, but very different on another. 



Although most investigators have concluded that 

 there apparently are many factors involved, only a few 

 attempts have been made to interpret them on a 

 strictly factoral basis. Segregation ratios from a given 

 promycelium are 4:0. 3:1, 2:2, 1:2:1, and 1:1:1:1 

 (Fig. 19). 



The type and extent of segregation of factors for 

 cultural characteristics depend to a large extent on 

 the source of chlamydospores. Thus, from a cross 

 between two contrasting lines, 32 segregates were 

 isolated, all differed greatly in their cultural charac- 

 ters and the parental types were not recovered (54). 

 Even the haploid progenies from a diploid mono- 

 sporidial line produced numerous distinct cultural lines 

 on a nutrient medium. In another case, 76 unisexual 

 lines were derived from 2 smut galls collected in 

 Germany; and all were almost identical in color, type 

 of growth, consistency, and rate of growth of colonies 

 i 55 1. Unfortunately, these results are based on 1 

 nutrient substrate. Even the diploid lines resembled 

 the unisexual lines in all cultural characters. 



Inheritance of color. — Several attempts have been 

 made to study the inheritance of specific colors of 

 cultural characters. Christensen (55) crossed a buff- 

 colored line with a brownish-black line. The colors of 

 the segregates when grown on potato-dextrose agar 

 were all as dark or darker than the darker parent and 

 a few were distinctly darker. 



Lu (203) made 7 crosses between a white and 

 relativelv stable line and a mutable black line. The 



Fig. 20. Colonies from the progeny of a single hybrid 

 chlamydospore (black X white) on potato-dextrose agar 

 in a surface-dispersed plate taken from a shake culture of 

 potato-dextrose broth (Lu, 203). 



29 



