a chlamydospore may contain 2 nuclei. Ehrlich (85) 

 observed binucleate chlamydospores and cytological 

 studies indicated that the 2 nuclei could be either 2 

 haploid or 2 diploid or one of each. 



Some chlamydospores give rise to 2 promycelia. 

 From such a spore, monosporidial cultures of 5 dis- 

 tinct types were isolated (55). These could have been 

 the product of a chlamydospore with more than 1 

 nucleus or the result of mutation. 



Delayed reduction. — Meiosis in U. maydis does not 

 always occur in a regular manner. Segregation of 

 factors for sexual compatibility and other characters 

 are not always completed in the second division of 

 the fusion nucleus. There may be partial reduction or 

 none at all at the time of chlamydospore germination 

 (51. 55. 279). 



Christensen (54). Eddins (83), and others (52. 203. 

 301, 358) obtained uninucleate monosporidial lines that 

 caused infection when injected into the corn plant. 

 They produced galls and normal chlamydospores that 

 germinated like those derived from normal dicaryons 

 and segregation also occurred for sex factors and many 

 cultural characters. Therefore, it was generally assumed 

 that the solopathogens were diploid lines (55). 



Sometimes, the diploid lines can be passed through 

 the chlamydospore stage 2 or more times without any 

 apparent genetic changes; although segregation of 

 chromosomes usually occurs in the first chlamydospore 

 generation. In a few instances, there was segregation of 

 factors for color and cultural characters in first chla- 

 mydospore generations, but not for sexual compatibility. 

 In the second chlamydospore generation, there again 

 was segregation of factors for color and cultural char- 

 acters and also for sexual factors (55V 



Sometimes, the same promycelium may give rise to 

 both haploid and diploid sporidia (51, 52). Reduction 

 of factors for sexual compatibility may be delayed for 

 at least 2 successive chlamydospore generations, whereas 

 in other diploid lines segregation may occur in 2 suc- 

 cessive chlamydospore generations (Fig. 15). Obviously, 

 such irregularity in segregation does not fit into the 

 normal scheme of meiosis. Perhaps, the nucleus in the 

 mature chlamydospore was in the tetraploid or aneuploid 

 condition or perhaps certain factors located in 1 pair 

 of chromosomes segregated in the first chlamydospore 

 generation and those in another pair segregated in the 

 second generation. Rowell's ( 280 ) discovery of partial 

 diploid lines that are not pathogenic may help to explain 

 this unusual phenomenon of delayed segregation. Hol- 

 liday (145) also obtained partial diploid lines of U. 

 maydis. 



Following a high dose of ultraviolet radiation, Holli- 

 day (145) isolated a slow-growing unstable segregate 

 which was monosomic and which consistently reverted 

 to a stable diploid, homozygous for 1 chromosome. He 

 states that he also synthesized a triploid which was less 

 stable than the parental diploid ( 145"). 



Apparently, meiosis in diploid lines may occur in arti- 

 ficial culture media. Christensen (55), in 1929. found 

 that some of the monosporidial variants of 2 solo- 

 pathogenic lines were nonpathogenic, apparently hap- 

 loid; whereas the others were as virulent as the parental 

 line. It is possible that the change was due to mutations 

 which were not uncommon for cultural characters; but 



Fig. 16. Young colonies of I'stilago maydis developing 

 from single sporidia of a diploid line. A) Dwarf colony 

 consisting of distorted cells produced in 72 hr from a 

 single cell after the line was exposed to alpha radiation. 

 B) Xonirradiated colonv after 24 hr of growth (Rowell, 

 279). 



it seems more likely that somatic reduction occurred 

 in some sporidia. but not in others, during the process 

 of budding. The latter seems most likely as indicated 

 by additional work of the writer and others (58. Ill, 

 203). 



Two years later, similar submonosporidial isolations 

 were made from 2 other solopathogenic lines. Again, 

 a few of the subcultures were avirulent when injected 

 into the corn seedlings. A few of these were paired with 

 unisexual lines and 2 combinations caused infection and 

 galls developed (58). These apparent haploid segregates 

 also differed from their diploid parent in their cultural 

 characters. By growing diploid lines in culture media 

 containing radioactive material. Lu (203) obtained 

 many haploid lines. Occasionally, the parental types 

 were recovered that were used originally in making the 

 cross from which the diploid lines were derived. This 

 is additional proof that meiosis had taken place. 



Rowell (279). in 1955. discovered a new phenomenon 

 in sexual behavior of solopathgenic lines. He induced 

 somatic segregation and recombination in 10 diploid 

 lines of U. maydis by means of alpha radiation of 

 sporidia. Such a treatment gave rise to many slow- 

 growing colonies (dwarfs) with distorted cells (Fig. 

 16). Their rate of growth was only 1 1,000 of that of 

 the parental lines. The dwarf lines were not stable and 

 had a marked tendency to give rise to fast-growing 

 variants, which resembled the original parental lines in 

 type and rate of growth. By growing the monosporidial 

 dwarf lines in a shake-culture of broth and then re- 

 isolating monosporidial lines, he obtained many genetic 

 types. At least 6 different unisexual genotypes were 

 isolated from each of the 10 dwarf lines and solo- 

 pathogenic lines also were reisolated. The dwarf lines 

 remained by far the predominant type. Unfortunately, 

 it was impossible to test the pure dwarf lines for 

 pathogenicity because in a few days they became 

 mixed with fast-growing variants that were constantly 

 arising in the cultures. Rowell (279) concluded that the 

 changes in sex induced by radiation were similar to 

 those that occurred in the same diploid lines during 

 meiosis in the promycelia. 



It is not known whether the type of segregation that 

 occurs in the saprophytic stage of U. maydis is normal 

 meiosis or a modified type of reduction, perhaps akin 

 to parasexualism. Nevertheless, it has been clearly 

 demonstrated that segregation occurs for sex and 



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