266 PHYSIOLOGIC SPECIALIZATION AND VARIATION 



Four variant strains of Hypomyces ip07?weae—purp\e, alba, con- 

 voluta, and reverta— were isolated by Dimock (1939). They 

 originated by gene mutations. None of them appeared to be 

 capable of perpetuating themselves in competition with the 

 normal type, because all had growth rates slower than the normal, 

 produced conidia less abundantly, and were quite incapable of 

 inbreeding. 



The causes underlying these variational phenomena are un- 

 doubtedly diverse. In some cases they have been shown to be 

 frenetic, but in others a different explanation must be sought. 

 Dodge (1942) noted increased vigor of growth and production of 

 conidia in Neurospora tetrasperma when he crossed a dwarf 

 race of this fungus with a normal one of opposite sex or else 

 with one of the same sex with resultant mycelial fusions. The 

 cells of these mvcelia, containing nuclei of both races, grew two 

 or three times as fast as those of normal ones. He ascribed in- 

 creased vigor to synthesis of vitamins by the heterocaryotic my- 

 celium. 



Hybridization. Stakman, Levine, and Cotter (1930) crossed 

 Fuccinia gr avium tritici form 36 with Puccinia graminis agrostidis. 

 When segregation occurred in the progeny, 3 new form species 

 were isolated that had previously not been encountered among 

 the numerous form species of tritici.. Stakman, Levine, Cotter, 

 and Hines (1934) segregated over 20 different races of wheat- 

 stem rust from aecial collections and 80 from uredinial collections. 

 In the Mississippi Valley, where barberry occurs, there is ample 

 opportunity for hybridization to occur, whereas in Australia, 

 where barberry is absent, as was pointed out by Waterhouse 

 (1929), there are few races of wheat-stem rust. In consequence, 

 an abundance of races can exist, and new ones can continue to 

 arise naturally by segregation and recombination of factors for 

 differences in pathogenicity wherever the barberry host thrives. 

 Emphasis was also placed on hybridization as a means of securing 

 new races of stem rust by Craigie (1940) in his summary of 

 studies conducted at the Dominion Rust Research Laboratory, 

 Winnepeg, Canada. 



Similarly Tisdale, Alelchers, and Clemmer (1927) found in 

 Kansas, New .Mexico, and Texas a new kernel smut that infects 

 milo :md hegari but is non-infectious to feteretia, and presented 

 evidence that it arose as a hybrid between Sphacelotheca sorghi 



