578 RADIATION BIOLOGY 



thene, and other chemicals which halt mitosis in metaphase. Although 

 ordinarily such substances would increase X-ray aberration frequency, 

 Brumfield (1943) noted the opposite effect in onion root tips, in which 

 mitosis may be halted in early prophase. Similarly, sensitivity changes 

 correlated with changes in the nucleic acid cycle in general can be influ- 

 enced by agents which affect the latter, as first pointed out by Darlington 

 and La Cour (1945). In all such cases, particular types of aberrations may 

 be favored at the stage which was caused to be prevalent, by reason of the 

 arrangement and behavior of chromosomes at that stage, and as an indirect 

 consequence the given agent will appear to be selective in its effect. 



Another way in which a selectivity may result is when the given agent 

 can penetrate to the chromosomes at a particular stage only. This is 

 illustrated in the studies by Kihlman (1951) which showed that some 

 purine derivatives capable of causing structural changes can act only 

 during mitosis since they are excluded by the nuclear membrane, whereas 

 others, being more penetrable because of their greater ability to dissolve 

 in lipoids of the nuclear membrane, can affect resting stage chromosomes 

 also (see discussion by Auerbach, 1952). It is not surprising, in view of 

 all this, that chemical mutagens have been found to differ from each 

 other and from ionizing radiation in regard to the frequency distribution 

 along the chromosomes of the breaks found in the aberrations produced 

 by them, and it is to be expected that similar differences would be found 

 between the aberrations produced by ionizing radiation when it is given in 

 combination with different chemicals. 



In view of these considerations, the finding by D'Amato and Gustafsson 

 (1948) that colchicine applied during the presoaking of barley seeds 

 results in an increase of the frequency of mutations and aberrations pro- 

 duced by X irradiation is in line with expectation. More surprising per- 

 haps is the fact that, unlike the other treatments which increased the 

 X-ray mutation rate, this substance resulted in a higher frequency of 

 certain particular types of visible mutants, which were usually rare. If, 

 however, these were connected with particular types of structural 

 changes, more readily produced by exposure of metaphase chromosomes, 

 this too would become understandable on the above basis. On the 

 other hand, an actually differential sensitivity of different loci might be 

 involved (see Sect. 20). 



Even the possible synergism between ionizing radiation and other 

 strongly mutagenic agents has not yet been studied very extensively. In 

 the work of Kaufmann and Hollaender (1946) the frequency of lethal 

 mutations induced in Drosophila spermatozoa by X rays and ultraviolet 

 in combination represented a merely additive function, whereas chromo- 

 some changes were actually reduced in frequency by the addition of 

 ultraviolet to X rays, as they had previously been found to be in Trades- 

 cantia (Swanson, 1944). However, in Aspergillus, Swanson (1952) found 



