lines of U. maydis that were derived from 4 states 

 on IS different solid substrates. Mutation occurred 

 in 5 media, the greatest number was on 3 C ' C sucrose 

 agar containing 0.006 r o NaN0 3 . Both sodium nitrate 

 and calcium nitrate stimulated mutation, whereas 

 ammonium phosphate seemed to inhibit mutation. 



Gattani (111) attempted to increase the frequency 

 of mutation by adding the following deleterious sub- 

 stances to potato-dextrose agar: lead nitrate, malachite 

 green, potassium chloride, bismuth nitrate, iron sul- 

 fate, mercuric chloride, and uranium nitrate; only the 

 last 2 compounds increased the frequency of mutation. 

 Several other workers have shown that uranium com- 

 pounds increased the frequency of mutation in U. 

 maydis i 70. 316 i. 



Exposure of cultures of U. maydis to ultraviolet 

 radiation. X-rays, and radioactive substances may 

 greatly increase the type and frequency of mutation 

 (70, 111, 2 79, 203, 144). In fact, such mutagenic 

 agents have produced a vast number of mutants that 

 differ strikingly in many physiological, biochemical, 

 and cultural characteristics; and in pathogenicity. By 

 means of ultraviolet light irradiation. Holliday (144) 

 increased the frequency of mutation. By this method, 

 he isolated 100 biochemical mutants, 30 of these 

 were used in genetic studies. Holliday (144) clearly 

 proved biochemical mutants to be most useful as 

 genetic markers in basic genetic studies. 



According to Stakman. et al. (314). Petty (256), 

 and Schmitt (,290). higher temperatures, 25°C or above, 

 are conducive to mutation. Schmitt (290) obtained 

 no sectors in colonies grown at temperatures below 

 20°C, whereas the writer occasionally obtained muta- 

 tion in certain lines at about 20°C, although mutation 

 rate was much higher at 25 : C or above. High tem- 

 perature, like other mutagenic agents, not only in- 

 creases the frequency of mutation, but induces 

 pronounced and diverse genetic changes and conse- 

 quently the stock cultures may become a mixture of 

 biotypes if kept at 20°C or above. Therefore, it is 

 desirable to store stock cultures of U. maydis at 17°C 

 or lower in order to inhibit genetic changes. 



Genetics of U. Maydis. — Suitable material. — U. 

 maydis is a very suitable pathogen for genetic studies. 

 It is a facultative saprophyte and can be grown 

 readily on many artificial substrates under varying, 

 but exacting conditions. Although the sporidia are 

 propagative bodies, they are also gametic in nature. 

 The sporidia are uninucleate, usually haploid, and bud 

 in a yeast-like manner. The progeny of single sporidia 

 are actually clonal lines and constitute biotypes. bar- 

 ring subsequent mutation. 



For genetic analysis, complete sets of 4 sporidia can 

 be isolated and their points of origin on the promy- 

 celium determined (Fig. 14). In fact, it is sometimes 

 possible to isolate successive sets of sporidia from 

 the same promycelium. As many as 15 successive 

 sporidia have been isolated from a single promycelial 

 cell (319). Consequently, it is possible not only to 

 study the haploid progeny of single chlamydospores, 

 but also the nature and number of nuclear generations 

 involved in meiosis. 



There are certain advantages and disadvantages in 



Fig. 14. Germinating chlamydospores of Ustilago may- 

 dis with a 4-celled promycelium, each cell producing a 

 sporidium (Stakman, 310). 



investigating basic genetical problems in U. zeae. The 

 dicaryon cannot be maintained in artificial culture. 

 The true F 1 generation cannot be propagated inde- 

 pendently making it difficult to study segregation. Only 

 the sporidia produced on the promycelium of chlamydo- 

 spores resulting from a cross can be propagated 

 independently. These can be interpreted as the F x 

 gametes. They can be readily propagated independently 

 on many different kinds of artificial culture media, 

 thus making it possible to study gametic lines under 

 different and exact environmental conditions. The 

 sporidia do not mate readily on artificial media, but 

 they can be paired at will in living corn plants. 



The genetic work in U. maydis could be greatly 

 simplified if mature chlamydospores could be pro- 

 duced readily in an artificial substrate. Although 

 chlamydospore-like bodies in U. maydis are not un- 

 common in culture, there is no evidence that they 

 behave physiologically like normal chlamydospores 

 (111). Griiss (119) in 1902 reported the production 

 of chlamydospores in artificial media. Other investi- 

 gators (111, 285. 290) have observed chlamydospore- 

 like bodies in cultures. Whether these bodies were the 

 results of nuclear fusion or just transformed haploid 

 or dicaryotic mycelium was not ascertained. Gattani's 

 results (111) indicate that chlamydospore-like bodies 

 without distinct echinulation are not uncommon in 

 artificial cultures to which toxic material was added; 

 however, they also produced no promycelia on ger- 

 mination. 



Unfortunately, it has not yet been possible to study 

 the chromosomal behavior, as the nuclei of U. maydis 

 are extremely small. Nevertheless, circumstantial evi- 

 dence for reduction division has been obtained by 

 analyzing the progenies from individual chlamydo- 

 spores. As the incubation period in the host is rela- 

 tively short, usually ranging from 10 days to 2 weeks, 

 it is possible to study many successive sexual genera- 

 tions in a single year. 



25 



