DOMINANCE AND LETHAL FACTORS 335 



a single factor pair. On Mindum wheat the 4 type was dominant 

 over very small pustules ( 1 type) with a 3 : 1 ratio in the F 2 

 generation. On the other hand, when the emmer variety, Vernal, 

 served as the host, the 1 tvpe was 15 times as abundant as the 4 

 tvpe in the F 2 generation. Pathogenic behavior on Vernal emmer 

 appears therefore to be governed by duplicate factors. Johnson 

 and Newton conclude that the genes in the binucleate uredinio- 

 spores function as if they occurred in a single diploid nucleus. 



DOMINANCE AND LETHAL FACTORS 



The existence of dominance and recessiveness among fungi 

 would appear to have been amply demonstrated in the studies de- 

 scribed, which are representative of experiments among the larger 

 groupings of fungi. This Mendelian principle can be demon- 

 strated for interested students, however, by hybridizing an eight- 

 spored Neurospora with a four-spored Neurospora. All the F x 

 progeny will be found to be eight-spored. Similarly, when a 

 rough-spored race of smut is crossed with a smooth-spored one, 

 all the Fi are rough. 



Lethal factors exist among fungi, just as they are known to 

 occur among seed plants. Dodge (1934) reported the results of 

 studies on N. tetrasperma, known to possess bisexual ascospores. ■ 

 After treatment with X-rays a strain appeared that was practi- 

 cally self -sterile, as manifest by ascus abortion. When this strain 

 was mated with a normal one, the F x generation gave asci that 

 formed spores normally. Further results show r ed that at meiosis 

 the lethal factor was segregated, so that each bisexual ascospore 

 contained a normal nucleus and one with the lethal factor. This 

 situation insured the transmission from generation to generation 

 of the lethal factor. 



More recently Fischer (1940) noted a haplo-lethal factor in five 

 collections of Ustilago bullata on species of Agropyron, Bromus, 

 Elymus, and Festuca. When he germinated the chlamydospores 

 and isolated the basidiospores in monosporidial cultures, he found 

 that approximately half yielded typical colonies, and in the re- 

 mainder the basidiospores budded a few times and then underwent 

 complete lysis. Fischer was able to show that the lethal factor 

 was sex-linked in four of the five collections. 



