12-11 THE YEAST CELL 



portance of wheat rust, there has been abundant support for re- 

 search on the genetics of rusts. Wheat rust is a characteristic 

 heteroecious rust spending part of its life cycle as a parasite on 

 the barberry and part as a parasite on wheat. The sexual cycle 

 takes place on the barberry while the economically important 

 wheat rust represents the vegetative phase of the hybrids produced 

 on the barberry. The barberry is the reservoir of variability. 

 Control of the disease by the development of resistant wheat va- 

 rieties has not been successful to date because the new hybrids 

 arising from the barberry have always been effective against any 

 previously resistant wheat. The two principal difficulties in the 

 analysis of heredity in rusts are: (1) these fungi are obligate para- 

 sites and have not as yet been grown on artificial media, and (2) 

 the only forms which can be characterized are the dicaryotic hy- 

 brids growing on wheat, i. e., haplophase cultures cannot be stud- 

 ied , thus one of the principal advantages of fungal genetics is not 

 available in the rusts. 



There were four important steps in the history of rust genetics: 



1. DeBary showed tiiat the pustules on barberry and the rust 

 on wheat are both caused by the same organism. He even recog- 

 nized the true significance of the pycnospores as sperm and al- 

 though his ideas were disregarded for 50 years, they were finally 

 abimdantly confirmed by Craigie. DeBary' s genius is one of the 

 highlights of the 19th century. 



2. Ericksson discovered that there were different physiologi- 

 cal races of the vegetative stage of wheat rust. Stakman and Pie- 

 meisel extended this work and developed techniques for charac- 

 terizing the different strains parasitic on wheat by infecting dif- 

 ferent varieties of wheat with a given strain of rust and distin- 

 guishing the different strains of rust on the basis of their effect 

 on different wheat varieties. A rust strain may be extremely 

 virulent with regard to one wheat variety and relatively ineffect- 

 ive on the second. There are no rusts that are virulent against 

 all strains of wheat, and there are no wheat varieties that are re- 

 sistant to all types of rusts. Stakman and his co-workers have 

 cataloged numerous biological races by testing different wheat 

 varieties and are principally responsible for the present census 

 of 200 biological races of wheat rust. 



3. Craigie discovered that rusts are heterothallic. He found 

 that the small pustules produced on the upper surface of the bar- 

 berry leaf by the infection of the sporidia actually function as 

 sperm, and that the so-called pycnospores can be used to fertil- 

 ize strains of opposite mating type. The pycnospores are homolo- 

 gous to the spermatia of Neurospora; each sporidial culture on 

 the barberry is itself self -sterile but can be cross-fertilized by 

 transfer of pycnospores from a pycnium produced by a sporidium 



