VARIATION AND INHERITANCE 411 



Numerous articles on the inheritance of the rusts may be cited. Among 

 the characters commonly studied are color of urediospores and patho- 

 genicity. One striking example of inheritance of pathogenicity of races 

 of Puccinia graminis tritici is reported by Johnson and Newton (1940). 

 Using pathogenically homozygous mycelia of race 9 and race 36, it was 

 found that Kanred wheat was not attacked by race 9 but was highly 

 susceptible to race 36. Urediospores were obtained from a hybrid of 

 these two races and were sown on different varieties of wheat. No infec- 

 tion occurred on Kanred, indicating that the nonpathogenicity of race 9 

 w^as dominant over the pathogenicity of race 36. In the r2 uredio- 

 spores the pathogenicity to Kanred wheat segregated in a 1 to 3 ratio, 

 indicating true ]\Iendelian inheritance. 



The basis of variation in the imperfect fungi. Any change or variation 

 in the imperfect fungi may be either temporary or permanent. Tem- 

 porary variations do not involve gene changes, ^vhile the permanent 

 variations are believed to have their basis in the gene, or at least in the 

 nucleus. The Mendelian inheritance of these variations cannot be proved 

 in those fungi with no sexual stage. 



Most investigators studying the permanent variations which arise in 

 culture or which are recognized in different isolates of many of the imper- 

 fect fungi would explain the origin of these variants as mutations. For 

 example, Dickinson (1932) studied "saltation" in the genera Fusarium 

 and Helminthosporium and noted frequent anastomoses between hyphae 

 of different "saltants." This author discussed the possibility of cyto- 

 plasmic inheritance but concluded that the permanent variations were 

 due to actual mutations. 



Hansen (1938) would explain many such variations in the imperfect 

 fungi on a different basis and presents abundant evidence to substantiate 

 his argument. Only the essential features of Hansen's "dual phenom- 

 enon" will be presented below. The conidia and mycelial cells of many 

 of the imperfect fungi contain two or more nuclei. These nuclei may 

 not all be alike. Considering the nucleus rather than the cell as the 

 basic unit of the individual, an isolate may be composed of two culturally 

 distinct individuals. This condition is referred to as the dual phenom- 

 enon. A heterocaryotic fungus, when single-spored, would give rise to 

 homotypes of each of the individuals and the heterotype like the parent 

 isolate. One homotype is characterized by abundant mycelium and few 

 or no conidia and is called the M (mycelial) type. The other is charac- 

 terized by abundant conidia and often a lesser amount of mycelium and is 

 called the C (conidial) type. The heterotype is, in general, intermediate 

 between the M and C types and is called the MC type. Cultures of the 

 M and C types give only the parent type when single-spored. The 

 frequency w4th w^hich the dual phenomenon is encountered in the imper- 

 fect fungi suggests that this is the natural condition for many fungi. 



