Fig. 17. Sporidial fusion in Ustilago maxdis (Brefeld, 

 34). 



cultural factors during the vegetative stage of U. 

 maydis < 145 i. In fact. Holliday concluded that mitotic 

 crossing-over occurs in diploid lines, although they 

 are apparently rare and most of the exchanges are 

 single. He increased the frequency of mitotic crossing- 

 over by ultraviolet radiation (145). 



Number of chromosomes. — Most investigators in- 

 dicate that it is difficult to determine the number of 

 chromosomes in U. maydis (55, 58, 85). The nuclei 

 in U. maydis are small, ranging in diameter from 1.1 li 

 to 2.5 u in dicaryotic hyphae. from 1.7 u to 2.5 li in 

 diploid hyphae. and from 2.5 ii to 3.0 u in mature 

 chlamydospores (85). In general, the diploid number 

 of chromosomes in species of smut fungi is 4 i 58, 98). 

 Kharbush (183) gives 4 for U. maydis. Eight of the 

 30 biochemical markers studied by Holliday 1 145 ) fell 

 into one or the other of 2 linkage groups, thus indicat- 

 ing that the basic haploid chromosome number is 2 in 

 U. maydis. 



Fusion. — The tests of sexuality in smut fungi are 

 usually based on the ability of sporidia or hyphae to 

 fuse and develop in the host and eventually produce 

 chlamydospores. It is now well known, however, that 

 in U. maydis fusion occurs between sporidia and 

 mycelium of opposite sex in artificial culture. Under 

 certain conditions, it was a rather controversial issue 

 for some time (123, 301. 312. 314). Although Brefeld 

 (34), as early as 1883. observed sporidial fusion and 

 illustrated it. his work has apparently been overlooked 

 (Fig. 17). Maire (200). in 1898. also illustrated fusion 

 of sporidia in U. maydis, but Hanna (123) indicated 

 that this was perhaps the process of budding rather 

 than conjugation. Somewhat later. Sartoris (285) 

 reported sporidial fusion of U. maydis in culture. 



Hanna (123). in 1929. observed fusion of germ 

 tubes from 2 monosporidial lines of opposite sex in 

 the tissue of corn, but he did not observe it in cul- 

 ture. Others obtained similar results. Later ('1932), 

 Sleumer (301) and Bowman (31) reported fusion of 

 sporidia fairly common on certain substrates. 



Recently. Rowell (280) made a detailed study of 

 the genetic factors involved in sporidial fusion. He 

 paired individual sporidia by micromanipulation on 

 drops of dilute corn coleoptile-extract agar. Under 

 these conditions, sporidia fused within 3-4 hr after mat- 

 ing and a distinct hypha developed within 3-4 hr after 

 fusion. Two distinct types of hyphae developed. One 

 type was straight, grew rapidly, branched rarely, and 

 produced aerial chains of sporidia after about 24 hr 

 of growth. These were similar to the vigorous hyphae 

 (Suchfaden) observed by Bauch (12). They caused 



infection and produced galls. The second type grew 

 slowly, branched frequently, and produced aerial 

 sporidia within 12 hr. Sleumer (301) called these 

 atypical hyphae "Wirrfaden" and the resulting dicaryon 

 was nonpathogenic. Rowell (280) concluded that 1 pair 

 of factors governed the process of sporidial fusion, 

 another the compatibility of the paired lines. 



Sexual compatibility. — Many studies (54, 55. 282, 

 301. 316) on the inheritance of sexual compatibility 

 indicate that factors for sex determination in U. maydis 

 are much more complex than in most of the other 

 smuts. Hanna (123) concluded that at least 2 pairs 

 of factors, and perhaps more, governed sex determina- 

 tion in V. maydis. In 1929, Christensen (54) con- 

 cluded that sexual reaction in U. maydis was due to 

 multiple factors in order to account for 24 sexual 

 groups. The results have been corroborated many 

 times (55, 280, 319). 



In 1951. Whitehouse (355) concluded that there 

 were only 2 sexes present in U. maydis, but he presents 

 no experimental evidence to support his contentions. 

 Recently. Rowell (279. 280) made a most detailed 

 genetic study of sex factors in U. maydis. He presents 

 proof that sexual reaction in U. maydis is governed by 

 multiple factors and that there are at least 2 loci 

 for sex factors, one of the loci with 2 alleles and the 

 other with multiple alleles. This is the only tetra 

 polar mating which does not have multiple alleles at 

 both loci. He also presented evidence which indicates 

 that sex factors at least sometimes are not associated 

 with pathogenicity (279). Holliday (145) also con- 

 cluded that the production of chlamydospores in the 

 host is controlled by alleles at 2 loci. 



Inheritance of sex. — The inheritance of factors for 

 sexual compatibility was studied by isolating full sets 

 of primary sporidia from a promycelium and then 

 making all possible pairing of the lines within each 

 set. Five segregation ratios of sex factors were ob- 

 tained: 4:0. 3:1, 2:2. 1:2:1, and 1:1:1:1 (Fig. 18). 

 The first class actually represents 2 groups. In one 

 group, no apparent reduction of sex factors occurred: 

 all sporidia were diploid and caused infection singly 



Smuts — Segregation 



-o 



-o 



B 



B B 



o 



B 



B 



B B B 



D 



Fig. 18. Graphic representation of 5 different segrega- 

 tion ratios of factors for sex; tendency to mutate; and 

 consistency, topography, color, and radial growth of 

 colonies obtained from individual chlamydospores. (Segre- 

 gates of one chlamydospore are not necessarily genetically 

 identical with those of another spore with similar letters.) 



28 



