414 MUTATION AND PLANT BREEDING 



induced with increasing frequency and of the albina type with com- 

 parative decreasing frequency through the series of mutagenic agents: 

 neutrons, X-rays, ethylene oxide, myleran, nitrogen mustard, ethylene 

 imine, di(/?-chloroethyl) phenyl alanine, and nebularine (purine— 9- 

 d— riboside). The spectrum of spontaneous chlorophyll mutations lies 

 between that of myleran (di-methane-sulfonyloxy— butane) and nitro- 

 gen mustard. There is also an indication of different frequencies in 

 mutations for mildew resistance and susceptibility compared to chlo- 

 rophyll mutations when the spectrum of spontaneous, X-ray-induced, 

 and chemically induced types are compared (22). 



Differential inter-locus response following exposure to ionizing 

 radiations of different ion densities has been reported for 69 analyzed 

 radiation-induced erectoid mutants located at 22 different loci in bar- 

 ley (29). Comparisons among mutants at five of the loci are particu- 

 larly informative. Locus a (11 to 12 mutants) has not given mutations 

 with neutrons, locus /; (4 mutants) gave mutations in only one variety 

 (Golden) of the three tested, locus c (17 mutants) gave mutations from 

 exposure to sparsely as well as densely ionizing radiations and these 

 were commonly associated with detectable chromosome breakage, and 

 loci d (10 mutants) and m (7 mutants) also gave mutations from both 

 types of irradiation but in only one case was a detectable chromosome 

 break involved. These results are indicative of differential mutagenic 

 effects of different radiation sources, though there is still a regretable 

 paucity in numbers. 



Localization or nonrandomness of chromosome breakage is an- 

 other manifestation of differential control of the mutation process in 

 its broader sense. It has been known for about a decade that radio- 

 mimetic compounds have the ability to break chromosomes prefer- 

 entially. In contrast, most evidence indicates that radiations are non- 

 specific, or at least less specific, in effecting breakage between and 

 within chromosomes. The highly reactive compounds, namely, nitro- 

 gen mustards, di (2:3-epoxypropyl) ether, and beta-propiolactone, 

 induce breaks selectively in heterochromatic regions in the middle 

 of the short chromosomes of the Vicia faba complement. The more 

 weakly reactive radiomimetic compounds that have been tested exten- 

 sively, namely, maleic hydrazide and 8-ethoxycaffeine, produce a con- 

 centration of breaks in the long satellited chromosome of Vicia faba. 



