mac key: induced mutation in crop improvement 353 



mutation induction will be one of our best helps. This indirect con- 

 tribution of mutation genetics to crop improvement is a large and 

 fascinating subject worth another story. It is evident, however, that 

 we have to possess ourselves in patience before we will be able fully 

 to understand the merits and drawbacks of induced mutation as a 

 method of augmenting the food production of the world. 



Summary 



Besides the metabolic stage at time of treatment, the mutagen 

 and dose applied, and the method of handling before, during, and 

 after treatment, the genotypic constitution of the material subjected 

 to mutation experiments plays an important role. The total yield of 

 mutations depends on inherent resistance to the mutagen applied, 

 probably to degree of heterozygosity, to extent and direction of pheno- 

 typic buffering, and to the presence of necessary precursor genes. 

 From all these aspects, inter- as well as intraspecific differences occur, 

 which greatly influence the yield of desirable mutations from case to 

 case. Material especially suitable for the detection of desired 

 mutations may improve the chance for success. 



Maximum yield of interesting mutations is also dependent on 

 handling of Mi. A diplontic selection over the first cell divisions after 

 seed or plant treatment should be prohibited by special arrangements. 

 Different kinds of positive or negative mass selection may also increase 

 the mutation rate in desired directions. 



Due to the very low frequency of valuable mutations, the success 

 in mutation breeding depends largely on efficient screening. Selection 

 in M 2 is preferred for very easily detectable mutations. In other cases, 

 selection in M 3 may be better, since every mutation will here be 

 represented by a whole group of off-type individuals. Proper sampling 

 in M 2 can keep down population size. 



Valuable mutations seem often so rare that it may be easier to 

 transfer them to other genotypes than to try to induce them anew. 

 Greater emphasis should be laid on micromutations. The combining 

 ability of induced mutations varies drastically with background 

 genotype. 



Among chromosome mutations, translocations may be very use- 

 ful for a systematic production of duplications, for splitting up or 

 building up gene blocks, and for interspecific gene transfers. Their 



