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Comments 



Brewbaker: May I comment first that there are two misconceptions 

 that appear in the discussions of mutations involving incompatibility 

 alleles. First, that a curious sort of revertible mutation can occur in 

 which the gene mutates to an inactive form to permit fertilization and 

 then reverts in the X x embryo to its original form. The evidence sup- 

 porting this double mutation event is extremely tenuous, and gains no 

 support in our Petunia work. Second, there is no evidence whatsoever 

 of "point mutations" resulting in the formation of a new S allele (our 

 studies alone exceeding 30 million pollen grains) and reports of mutation 

 by inactivation of subunits in the S gene (as in the cherries) can be 

 suggested to result from chromosomal aberrations, such as transloca- 

 tion or centric fragments. 



Now to my question — having screened a great number of such self- 

 fertile mutants in Petunia, in which S locus duplication is an elegant 

 theoretical way to produce "mutants", we have no evidence at all for 

 such duplications, although irradiations have been applied premeio- 

 tically. Unless one has a particularly effective screen for duplications, 

 how optimistic do you feel we can be regarding such duplications which 

 might be, as you suggest, of considerable value in mutation breeding? 



MacKev: 1 share your skepticism on the correct interpretation 

 of the mechanism behind the induced compatibility in some cases. This 

 will, however, not interfere with the fact that the plant breeder can 

 make use of this shift in his breeding procedure, even if a chromosome 

 mutation may be a little more complicated to work with than a point 

 mutation. 



