436 MUTATION AND PLANT BREEDING 



Peterson: In terms of specificity (and, although it may be a special 

 case or class of cases), one might consider the "microspecificity" that is 

 involved in mutable gene systems in corn. The specificity involves the 

 mutable systems concerned with a specific locus — example a x . Among 

 the different strains of maize there are present at this locus five or six 

 different systems which have been analyzed with their own controllers 

 of mutability. Each of these controllers has their own specific mutator. 

 There is no interaction between members of different systems. Here is 

 a case where the controller is responsive to its own specific activator. 

 Note that the specificity does not only involve the locus, but the specific 

 controlling elements. 



Coe: Although I do not work with mutability factors, I feel compelled 

 to point out some aspects of these systems in relation to mutational 

 specificity. First, these are broad-spectrum mutagens. In the work by 

 McClintock originally, and since then in studies by Kramer, Nuffer, 

 and others, these systems have been used to induce a variety of changes. 

 Still, there is an interesting quantitative specificity involving the chro- 

 mosome on which the factor is located, according to Brink. Second, 

 these are highly specific mutagens, as shown in the original case reported 

 by Rhoades for Dt and A in maize, in further study of A by Nuffer, 

 and in the studies of wx by Sprague. In these cases, a mutable system, 

 once induced, permits many variations in the expression of the affected 

 factor. There is little doubt that employment of variants induced orig- 

 inally by mutability factors and further affected by subsequent 

 mutations can provide specificity at least as great as that provided by 

 present mutagens known to affect higher plants. 



