Fig. 15. Monosporidial progenies of 2 chlamydospores. Left, progeny of first chlamydospore generation of solo- 

 pathogenic line. Right, progeny of chlamydospore which was produced by a monosporidial line that had shown seg- 

 regation of cultural factors the previous chlamydospore generation, but not for sex. In this second chlamydospore 

 generation, there was again reduction of factors for cultural characters and also for sex (Chrislensen, 55). 



Nuclear behavior in germination. — As definite in- 

 formation on the nature of germination and of nuclear 

 behavior is prerequisite for correct genetic interpreta- 

 tion of segregation, these phenomena are presented 

 in considerable detail. 



Chlamydospores are uninucleate and diploid and 

 meiosis usually occurs during germination and promy- 

 celial development. There are many deviations from 

 the normal in both promycelial and sporidial produc- 

 tion. The diploid nucleus may divide first in the 

 chlamydospore or it may apparently move out of the 

 spore and then divide (51). There appears to be a 

 difference, in this respect, in chlamydospores from 

 different galls (55. 123. 182 i. In either case, one finds 

 a nucleus in the promycelium when it is about J^-J^ 

 of its mature length. The fusion nucleus divides and 

 then the 2 daughter nuclei divide again. Three of 

 these nuclei often, although not always, come to lie 

 equally distributed in the promycelium. whereas the 

 other 1 is usually in the chlamydospore. Sometimes, 

 however, all 4 nuclei are found in the promycelium 

 and none can be seen in the chlamydospore. From 1 

 to 3 cross walls are usually formed. In nearly all cases, 

 a septum is laid down between daughter nuclei of the 

 first division. In many cases, however, the septum 

 between the daughter nuclei of the second division 

 cannot be discerned. The promycelium with 4 primary 

 sporidia is considered the normal type, but it is not 

 the predominant type Cl82i. The primary sporidia 

 usually appear at definite positions on the promy- 

 celium. Nevertheless, irregularity in position and in 

 production of sporidia are frequently noted. Some- 



times 2 or 3 sporidia will bud almost simultaneously 

 from the same cell. It is not uncommon to see 5-7 

 primary sporidia arising from the same promycelium. 

 If 1 or 2 of the usual 4 primary sporidia are removed 

 before the others have developed, irregularity and 

 mass production of sporidia occur most frequently. 



Meiosis. — Meiosis normally involves 2 divisions. 

 Many of the early workers believed that reduction 

 occurred in the first or second division of the fusion 

 nucleus (54. 123. 314). The apparent segregation of 

 factors in the second division can best be explained 

 on the basis of crossing-over, reduction in chromo- 

 somes in the first division and segregation of chroma- 

 tids in the second. Meiotic crossing-over apparently 

 occurs in U. maydis (14S). Although linkage has been 

 demonstrated for few characters in U. maydis, com- 

 plete linkage between characters seems relatively rare 

 and it is rather difficult to detect, at least for bio- 

 chemical factors (145, ISO. 319). 



Karyogamy. — When corn plants are infected by 

 lines of opposite sex. karyogamy usually occurs im- 

 mediately before gelatinization and segmentation of 

 the hyphae. The actual fusion of nuclei, however, 

 has never been definitely observed. In those rare in- 

 stances in which mycelial segments after initial fusion 

 are still binucleate. it is possible that another fusion 

 occurs in the chlamydospore. Then. too. the nuclei in 

 a segment might be of the same or different genetic 

 constitution, either in the haploid or diploid condition; 

 or one may be haploid and the other diploid. There- 

 fore, a nucleus in a mature chlamydospore might be in 

 the 2x. 3x. or 4x condition (55. 85. 145V Sometimes 



26 



