chromosome aberrations in Tradescantia 721 



experimental material. Studies of spontaneous chromosome aberrations 

 have demonstrated that the same aberration types occur in unirradiated 

 material, but that the frequencies are usually very low (Giles, 1940a; 

 Sax and Luippold, 1952). Even this low rate is much too high to be 

 accounted for in terms of natural radiation. Further, there is evidence 

 that the spontaneous rate may be considerably higher in material of 

 hybrid origin (Giles, 1940a, 1941; Darlington and Upcott, 1941). 



TIME OF CHROMOSOME DIVISION 



Observations on the kinds and sequence of aberration types have been 

 used to draw certain conclusions about the time and degree of chromo- 

 some duplication during mitosis (Riley, 1936; Mather, 1937; Sax, 1940, 

 1941 ; Catcheside, 1948). The transition from chromosome to chromatid 

 aberrations, which corresponds in general to the late resting stage, has 

 been taken to indicate the time in the nuclear cycle at which duplication 

 occurs (Mather, 1937). One of the difficulties with this interpretation, 

 however, is the observation that chromosome breaks may occur in the 

 same cell with chromatid breaks. Some investigators (Darlington and 

 La Cour, 1945) have, in fact, maintained that chromosomes are broken 

 by radiation only when they are undivided and that reunion then takes 

 place after division, giving rise to either chromosome or chromatid break 

 types. Catcheside (1948) has summarized the evidence favoring the 

 view that chromatid aberrations actually arise as a result of breaks in 

 divided chromosomes, and has concluded that, while a small proportion of 

 chromatid breaks may be derived from chromosome breaks, most are not. 

 It thus seems likely that the average time of chromosome splitting should 

 be about halfway between the peaks of chromatid and chromosome break- 

 age, and thus does correspond in general to the transition from chromo- 

 some to chromatid types. 



It can still be argued, however, that actual duplication has occurred 

 earlier, but that the chromosome or chromatid is the unit of reunion. 

 There is, in fact, some evidence from radiation studies (Swanson, 1947) 

 for a quadripartite condition of the chromosomes at prophase in pollen 

 tube nuclei. This condition appears to be the exception rather than the 

 rule, however, since even in pollen tubes, chromatid rather than half- 

 chromatid break types predominate in nuclei where the chromosomes can 

 be seen microscopically to be divided at the time of treatment (Swanson, 

 1943). 



VARIATIONS IN RADIOSENSITIVITY DURING THE NUCLEAR CYCLE 



The original studies of Sax (1938, 1940) demonstrated that more aber- 

 rations were produced for the same X-ray dose in prophase as compared 

 with resting stage chromosomes of Tradescantia microspores. It was 



