272 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



year the loss of yield due to widespread infection of wheat by parasitic 

 fungi used to be a drain on the national economy and, even worse, on 

 the source of livelihood of many farmers. Then genetically trained 

 plant pathologists discovered that susceptibility to infection was not 

 a ubiquitous fate. Specific genetic constitutions were present in many 

 plants which endowed them with resistance to the invaders. Varieties 

 were bred which not only possessed those desirable properties for 

 which they had been selected by plant breeders, but which combine 

 these properties with resistance to infection. In this way for many 

 years whole regions suffered little loss in the yield of this important 

 crop. Genetics, furthermore, served to explain and to prepare the 

 farmer for the breakdown in protection which again and again brought 

 catastrophe to the wheat grower. A gene that protects a wheat variety 

 is not all-powerful. The fungus itself is heterogeneous, and different 

 genetic types of the parasite are repulsed by different genetic types of 

 the potential host. Invasion by a new strain of the fungus into a 

 territory where no resistance against this specific strain existed, seems 

 to undo past successful efforts. Even worse, mutations in some of the 

 myriads of spores produced by even a few fungi may lead to the origin 

 of new virulent varieties. Thus the plant pathologist and geneticist 

 has a never-ending task, but he can anticipate a future unavoidable 

 genetic breakdown of any specific resistance mechanism by preparing 

 new varieties of wheat, resistant to new laboratory-cultivated varieties 

 of the fungus. 



This recognition of parasite mutability strikes still nearer home. 

 Bacteria have been found which are resistant to sulfa drugs or strep- 

 tomycin, though their ancestors could have been combated successfully 

 with these drugs. Even streptomycin-dependent bacteria occur which 

 thrive only on what means death to their brethren. The origin of 

 these new types is due to mutations in the genetic composition of the 

 bacteria. One of the newest branches of genetics, microbial genetics, 

 is now elucidating these phenomena and contributes to overcoming the 

 impediments to drug therapy. 



It happened that at about the time when the artificial production of 

 mutations was first accomplished by means of radiations, the action 

 of the penicillium mold in inhibiting bacterial growth was discovered. 

 Years later, during the last war, when the large-scale production of 

 penicillin became an urgent task, the geneticist's method was applied 

 to the production of strains of the mold with greatly increased output. 

 The strain now used most widely by commercial firms originated from 

 irradiating spores from relatively low-yielding strains and selection 

 of a spore in which a mutation to high yield had been induced. Sim- 

 ilar methods of causing mutations responsible for desirable new qual- 

 ities in agricultural plants have been employed, particularly in 



