smith: directed mutation 415 



With the former, a structural isomer of the pyrimidine base uracil, 

 breaks are concentrated near the centromere; with the latter, a purine 

 derivative, breaks are mainly in the region of the nucleolar constric- 

 tion (47, 64, 65). The effects of maleic hydrazide and 8-ethoxycaffeine 

 may conceivably be related to their structural resemblance to nucleic 

 acid bases. KCN treatments produce an apparent random distribu- 

 tion of breaks among Vicia chromosomes which are, however, concen- 

 trated within chromosomes in heterochromatic regions (36). Analysis 

 of exchanges between localized and randomized chromatid breaks 

 may be used to gain information on chromosome structure in specific 

 regions (46). 



Specificity of the mutagenic agent itself is clearly shown by the 

 wide variation in the ratio of gene mutations to chromosomal aberra- 

 tions. For some chemicals, as maleic hydrazide, the ratio is essentially 

 zero, for others, as nebularine, it is large if not infinite; while certain 

 alkylating agents and ionizing radiations fall in between these 

 extremes (17). Diethyl sulphate has been found to combine the quali- 

 ties of inducing a high frequency of mutations in barley with few 

 accompanying chromosomal structural aberrations (31). There is, 

 also, some indication of a different spectrum of chlorophyll mutations 

 produced by diethyl sulphate compared to gamma radiation. The 

 highly efficient mutagenic alkanesulphonic esters, as ethyl methane 

 sulphonate (33) appear to cause little chromosome breakage. On the 

 other hand, myleran is efficient in producing localized chromosome 

 breaks in Hordeum and Vicia (48). We have found that ethyl methane 

 sulphonate is ineffective at 0.1 M in producing loss of endosperm 

 marker genes in the short arm of chromosome 9 in treated pollen of 

 maize. By this same test (39) diepoxybutane was found to induce 

 breaks approximately at random along this chromosomal arm, a 

 region in which there is no conspicuous localization of heterochro- 

 matin. The number of anaphase bridges induced by maleic hydra- 

 zide and certain of its derivatives was observed by Graf (27) to be 

 correlated with the number of heterochromatic knobs in the variety 

 of maize treated. 



The clearest evidence of specificity in induced chromosome 

 breakage is, then, the preference shown by certain chemicals for affect- 

 ing particular heterochromatic regions. No definitive explanation is 

 yet available; but the phenomenon may be due to different spacial 



