394 MUTATION AND PLANT BREEDING 



enced plant breeders who will compare them in yield tests with the 

 parental types in 1960. It is important to emphasize that after this 

 long series of recurrent irradiation, where selection was made each 

 generation for desirable plant types, most of the material in the 

 nursery is agronomicaly undesirable. This is particularly true of the 

 material derived from Clintland and to a lesser extent to that derived 

 from Missouri 0-205 and Park, in that order. 



Another point that warrants consideration is the fact no attempt 

 was made to isolate the irradiated material from outcrossing. How- 

 ever, as indicated, each year seed for re-irradiation was taken only 

 from completely fertile panicles. Accordingly, there should have been 

 no more outcrossing in the material than the breeder usually finds 

 in his nursery plots. As the incidence of outcrossing should be quite 

 low, it seems unlikely that this can account for the extreme variability 

 that has been observed or for the apparently desirable types that have 

 been selected for further study. However, this possibility cannot be 

 ignored and studies are presently under way with polyploid series in 

 oats and wheat, where isolation is being practiced, to resolve this 

 question. 



The Use of Intraspecific Chromosome Structural Changes 

 Duplications 



Among the complex types of chromosomal aberrations induced 

 by ionizing radiations is a class known as the "duplication". As the 

 name implies, this means that a duplication of genetic material gov- 

 erning specific gene functions exists in the chromosome complement. 

 Duplications may arise in two simple ways which do not simultane- 

 ously result in a loss of genetic material in the cell. The first involves 

 translocations between corresponding arms of homologous chromo- 

 somes that were broken at dissimilar points. The second involves 

 breakage and reunion of sister chromatids in which the break points 

 were not at corresponding loci. 



From the preceding discussion on polyploids it should be evident 

 that they have built into them a system of duplications of what can 

 amount to whole sets of genes in the case of a true autopolyploid. It 

 would appear, then, that radiation-induced duplications would likely 

 be of greatest practical use in diploid species. Accordingly, the discus- 

 sion will center around their possible use in self-fertilizing diploids. 



