448 MUTATION AND PLANT BREEDING 



It is obvious that more intensive studies on the nature and orierin 

 of chromosome aberrations and repair of chromosome damage are 

 required before we can more effectively reduce or eliminate chromo- 

 some aberrations and increase mutations in irradiated cells. Investi- 

 gations pursued by Wolff and his collaborators (42) have shed some 

 light on the factors that govern the rejoining and restitution of 

 chromosome breaks. However, recent papers by Revell (35, 36) sug- 

 gest that radiation-induced chromosome reunions and exchanges or 

 incomplete reunions and exchanges are derived from possible chemi- 

 cal bonding initiated by the ionizing radiation. If this suggested 

 mechanism proves to be real, it could mean that radiation-induced 

 changes in the chromosome may not result in aberrations until the 

 late prophase of the irradiated cells. This suggests that a relatively 

 long period during mitosis would be available for altering the course 

 of radiation damage to the chromosomes. Experiments designed to 

 test and then use this new hypothesis for the reduction or elimination 

 of oross chromosome aberrations and for increasing the efficiencv of 

 mutation induction are now underway in our laboratory. 



Directing the Induced Mutation Process 



The greatest concentration of effort in directing the induced 

 mutation process has been through the manipulation of secondary 

 factors in irradiated seeds and the use of chemical mutagens. Here 

 again, these studies are only possible because of the wide variety 

 of conditions and controls that can be applied to seeds. 



Indications that the spectrum of chlorophyll-deficient muta- 

 tions in barley can be altered have been published by the Swedish 

 group (41) in the past several years. Small shifts of the mutation 

 spectrum have been caused by certain variations of the experimental 

 conditions during X-irradiation. It also has been reported that the 

 chlorophyll-deficient mutations produced by neutrons may show a 

 different spectrum from those induced by X-rays. However, these 

 results have not always been reproducible. It has not been determined 

 whether the results are due to alterations in the sensitivities of indi- 

 vidual genes or to unequal selection of these mutations during the 

 ontogeny of the plant. 



Probably some of the most convincing evidence that different 

 mutagenic treatments can produce differential mutability of loci is 



