Lbtilago zeae (crou 50) 



— ' (porenty ]T^Z 



18 Hi (sporidial) 10 K z ., (mycelial) 



Germinating F, Chlamydospores 



Fig. 21. Growth types of sporidial and mycelial par- 

 ents and germinating F x chlamydospores showing differ- 

 ences in the production of sporidia and hvphal branches 

 (Kernkamp, 179). 



segregants mutated so frequently from black to white 

 and to intermediate colors that it was impossible to 

 determine the number of factors involved in the in- 

 heritance of color (Fig. 20 1. Lu's results suggest that 

 it would be highly desirable to make all segregation 

 studies for color and culture types by the dilution 

 method rather than by mass transfer. 



Stakman. et al. (320) made crosses between white 

 and black lines of U. maydis. Although their results 

 indicate a single factor difference, the number of 

 factors involved in color production could not be 

 definitely stated because the progeny of different 

 chlamydospores gave different segregation ratios for 

 whiteness (320). According to these investigators 

 (320 i. the degree or intensity of whiteness among the 

 isolates was very great. Of course, the intensity of 

 pigmentation may be modified by the consistency and 

 topography of the colonies and the amount of super- 

 ficial aerial mycelium. Naturally, the kind of medium 

 also may have a profound effect on pigmentation of 

 the colonies. 



Mutability. — Several workers have repeatedly pointed 

 out that certain lines of U. maydis mutate frequently, 

 whereas others were relatively stable (54, 314). This 

 led to studies on the nature of inheritance for 

 mutability. Stakman. et al. (319) isolated 4 primary 

 sporidia from 1 chlamydospore and segregation oc- 

 curred for 5 characters including the tendency to mu- 

 tate. Two lines were mutable and 2 were stable when 



grown on potato-dextrose agar. The 2 mutable lines 

 developed 360 sectors, whereas the 2 constant lines 

 grown under the same environment produced none. 

 Fortunately. 2 of the mutable lines were of opposite 

 sex and so were 2 of the constant lines and thus facili- 

 tated the crossing of desirable lines for genetic studies. 



Crosses were made, mutable X mutable and con- 

 stant X constant. The progenies from the former 

 crosses yielded chiefly mutable lines and the latter 

 usually constant lines. These results indicate that 

 mutability and constancy are controlled by genetic 

 factors. Because of the mutability of these lines. Stak- 

 man and coworkers were able to isolate more than 

 5.000 biotypes that traced back to the initial parental 

 cross between 2 lines with contrasting characters. In 

 general, the sporidial lines tended to be more mutable 

 than mycelial lines, but the linkage between the fac- 

 tors for the growth types and the factor for the 

 tendency to mutate was not complete (319). 



There is fairly good evidence that the following 

 morphological characters are inherited: tendency to 

 produce sporidia. hyphal branches or hvphal pegs in- 

 stead of sporidia. size of spores, length of promycelia, 

 and tendency for promycelia to lyse (52). Detailed 

 genetic studies on most of these characters, however, 

 are rather limited. 



Hyphal branches. — Kernkamp (178. 179. 180) recog- 

 nized 3 types of growth on culture media: strictly 

 sporidial. mycelial, and various intermediates. The 

 growth types of sporidial and mycelial lines could not 

 be changed, whereas the intermediate lines could 

 readily be shifted one way or the other by various 

 environmental factors. 



Crosses between mycelial lines tended to produce 

 mycelial growth types, whereas crosses between inter- 

 mediate lines produce more sporidial lines than 

 mycelial lines. The progenies from crosses between 

 sporidial and mycelial lines gave rise to the parental 

 types, but crosses between many strictly 7 sporidial 

 lines gave no infection (Fig. 21). Cytological studies 

 indicated that these lines lack the ability to develop 

 mycelium in the host, but when outbred they caused 

 normal infection. Kernkamp (ISO) concluded that 

 the 2 growth types were genetically controlled and 

 2 or more genetic factors were involved. 



Promycelia from certain other crosses tended to 

 develop hyphal branches or peg-like structures instead 

 of sporidia (55). In 1 cross, only about 2% of the 

 promycelia produced sporidia in a normal manner. 

 In such cases, the promycelium was usually normal, 

 blunt at the distal end: whereas the promycelia of the 

 other 98'; were usually pointed at the end. The 

 pointed promycelia were very inconsistent as to the 

 number of primary sporidia they produced : some 

 developed 4. others 2 or 3. and still others only 1. 

 There was no regularity as to the cell from which 

 the sporidia originated. Many promycelia never gave 

 rise to any sporidia. but developed a sporidium-like 

 structure that grew directly into a hyphal branch. In 

 many cases, a mixture of sporidia and hyphal branches 

 arose from the same promycelium. A few budded off 

 1 to 3 or even 4 sporidia and then ceased to develop 

 further: others produced short germ tube-like hyphae 

 and then died: still others failed to grow 7 at all. A 



30 



