406 MUTATION AND PLANT BREEDING 



genes useful in plant improvement and arising through the use of 

 mutagenic agents have been reported. 



Many studies reported in the literature have shown that irrespec- 

 tive of the mechanism involved or the kind of mutagenic agent 

 employed there is no problem in producing mutant types. MacKey in 

 his paper strongly emphasized the importance of devising more ade- 

 quate screening procedures to isolate those kinds of mutants from the 

 many arising from irradiation that may be useful in higher plants. 

 Nelson also has shown the complexity of screening for useful mutants 

 (as well as natural variants) in microorganisms. 



Nearly every study in mutation breeding has shown that newly 

 formed mutants generally are expected to be associated with chromo- 

 some aberrations that may be deleterious to the organism as a whole. 

 This is not surprising. Present techniques to produce mutations are 

 not selective. For these reasons it would be indeed surprising if a 

 single gene mutant in higher plants would produce a line useful 

 per se. This does not limit the usefulness of mutation breeding, but 

 only serves to emphasize the necessity of transferring a newly formed 

 desirable mutant gene into a more usable gene background through 

 hybridization. The example given by Langham for nondehiscent 

 sesame is an example of the difficulty that can occur in separating the 

 undesirable plieotropic "side affects" from the major gene or genes. 



The discussion to this point largely has been concerned with 

 single or simple gene effects in naturally self-pollinated crops. Perhaps 

 it would be appropriate to discuss next the more complex prob- 

 lems associated with mutation breeding for modification of char- 

 acteristics in a desired direction when character expression is depend- 

 ent upon multi-gene effects. In seeking to improve quantitative char- 

 acters by irradiation the problem of identification becomes most 

 difficult. General experience has shown that the heritability of quan- 

 titative characters is low and hence only limited progress can be 

 expected from selection on a single plant basis. Evaluation of mutant 

 progenies in replicated trials (as in the normal procedure of testing 

 variability derived by hybridization) consequently becomes a 

 necessity. 



In any approach to plant improvement the breeder must com- 

 pare the probability of obtaining improvement by alternative 

 approaches. The long history of plant improvement has clearly shown 



