MYERS : DISCUSSION OF SESSION III 303 



experimentation. Nevertheless, as Doctor Gaul has suggested, more 

 attention in mutation experiments needs to be given to changes in 

 the polygenic systems. 



In connection with the studies of "micro" mutations, Doctor 

 Gaul has pointed out the need for better selection procedures that 

 will permit accurate mass screening of populations for heritable vari- 

 ations in the quantitive characters. Returning for a moment to our 

 three phases of plant breeding, it is evident that this suggestion 

 applies to the second phase, i.e., selection. Efficient selection methods 

 are, of course, just as important in populations where the variability 

 is due to accumulated spontaneous mutations as in those where it is 

 due to induced mutations. 



The suggestion that efficiency of selection for yield and other 

 less readily evaluated characters might be increased by selection in 

 mutagene-treated materials for more readily observable changes, 

 should be viewed with some reservation. When the more readily 

 observable character is a component of the less readily observable 

 one, the method seems to have potentialities. Selection for increased 

 seed size with the objective of getting increased yield, as illustrated 

 by Doctor Gaul, is a good example. On the other hand, there seems 

 in many cases to be no a priori reason to assume that the pleiotropic 

 effect of a macromutation on a quantitative trait such as yield will 

 always be favorable. It might just as logically be expected to be 

 unfavorable. 



As part of a discussion of the papers presented in this program, it 

 seems appropriate to list some of the questions which are pertinent 

 to the problem of the efficacy of induction of mutations to provide 

 genetic variability for use in plant breeding. Some of these questions, 

 as yet unanswered, are: 



1. Are the induced mutations the same as spontaneous ones? Related 

 to this are the questions of whether mutations can be induced at 

 loci where such mutations are not available in natural popula- 

 tions or whether new kinds of mutant alleles can be induced at 

 loci at which alleles from mutation are already known. It is evi- 

 dent that a definitive answer to this question is not yet available. 

 Also related is the question of whether true changes in molecular 

 gene structure (true gene mutations) actually can be induced in 

 higher plants by mutagenic agents. There seems to be fairly gen- 

 eral acceptance of the fact that they can be. Yet of the induced 



