130 MUTATION AND PLANT BREEDING 



taut subject of investigation. Crystallographic, electronmicroscopic, 

 biochemical, and biophysical investigations are likely to play a 

 major role in this research. Analysis of the effects of chemical muta- 

 gens will, I think, be mainly important by showing the complex- 

 ity of the processes that result in chromosome breakage and by 

 separating out some of the relevant factors. 



One very important factor has been discovered in radiation 

 studies. Wolff and Luipold (93), found that the reunion of broken 

 chromosome ends is an energy-requiring process which proceeds 

 only under conditions that permit protein synthesis. This makes it 

 necessary to distinguish between the effects of mutagens on the 

 cohesion of the chromosomes and on the mechanism of reunion. 

 A chemical may affect these processes separately, and the effects 

 may differ in dependence on the cellular environment. Thus, chro- 

 mosomes that have been broken by 8-ethoxycaffeine readily form 

 rearrangements in Vicia, but tend to remain as free fragments in 

 Allium (50). Conversely, methane produces mainly fragments in 

 Vicia and mainly rearrangements in Oenothera (26). Discrepan- 

 cies between the reports of cytologists on the effects of a given 

 mutagen will often be due to differences in the choice of organism 

 or cell stage. 



Only barley and Drosophila have been used for systematic 

 attempts at comparing the intergenic and intragenic effects of chem- 

 icals. In Drosophila, most chemicals produce fewer rearrangements 

 for a given frequency of mutations (sex-linked lethals) than do 

 X-rays. That this is not due to a shortage of breaks but to failures 

 of reunion has been shown for two substances, mustard gas (68) and 

 TEM (34). Compared with a dose of X-rays giving the same fre- 

 quency of sex-linked lethals, both these substances produce too few 

 translocations but at least as many chromosome fragmentations 

 resulting in zygotic lethals. The main cause for the relative shortage 

 of reunions is apparently the delayed effect of these compounds; 

 for breaks that open in different cell cycles cannot form rearrange- 

 ments. Indeed, when mustard gas-treated spermatozoa were stored 

 in the seminal receptacles of the 9 , there was an increase in the 

 frequency of large rearrangements (5). 



The tendency of chemical mutagens to produce intrachromo- 

 somal rather than interchromosomal changes may be likewise a 



