NILAN AND KONZAK: MUTATION INDUCTION EFFICIENCY 447 



from the chemical treatments. These aberrations indicate that inver- 

 sions have been induced by the chemicals. This is of considerable 

 interest since inversions are relatively rare in plants from irradiated 

 seed. Furthermore, the finding that these chemicals induce a higher 

 frequency of intra-chromosomal compared to inter-chromosomal type 

 of aberration than radiation suggests a more subtle and delayed effect 

 of the chemicals on the chromosomes. 



It is already apparent that the effectiveness and efficiency of the 

 chemical agents may be greatly influenced by modifying factors. For 

 example, the uniformity in the growth of seedlings from seeds treated 

 with diethyl sulfate seems to be measurably increased by the addi- 

 tion of certain divalent cations to the treatment solution. These 

 results suggest that the hydrolysis product, ethyl sulfuric acid, in the 

 treatment solution may chelate divalent cations in the cell membranes 

 and facilitate the penetration of the active agent into the cells of the 

 seed. 



It is also interesting to speculate that the hydrolysis products 

 from both diethyl sulfate and ethyl methane sulfonate may have 

 important roles in the mutagenic and cytogenetic activity of these 

 agents. Alcohol might be expected to act as a surface-active agent 

 aiding penetration of the alkylating agents. The ethyl sulfuric and 

 methane sulfonic acids, weak chelating agents, may affect cell mem- 

 brane permeability. Also, by their chelating action on the divalent 

 cations bonding the DNA, they may cause an opening-up of the multi- 

 stranded chromosome, permitting the alkylation of additional as 

 well as internal sites, as suggested by the Steffensen model (38). It is 

 now already indicated, however, from the results of Strauss (39) that 

 in bacteriophage at least (which seems to have a double or single- 

 stranded chromosome), the chelating effect does not appreciably influ- 

 ence the frequency of induced mutations but reduces survival. Thus, 

 the effect hypothesized may relate only to higher organisms. 



It is also possible that chromosome aberrations of the type 

 observed may result from the temporary chelation of the divalent 

 ions bonding the DNA rather than from the alkylation of bases or 

 phosphate groups. However, what is more important is that these 

 possibilities can be subjected to experimental test, and future studies 

 may provide even more exciting information on the nature of 

 chemical reactions leading to chromosome aberrations and mutations. 



