MANNER OF PRODUCTION OF MUTATIONS 533 



spermatozoa which had resulted in 1.3 per cent of these lethals. The 

 analyses showed that this group included only three demonstrable 

 chromosome changes: one translocation, one inversion, and one deletion 

 (Valencia and McQuate, 1951). On the other hand, 77 X-ray lethals, 

 produced by a dose that had given 1.8 per cent of lethals, included twelve 

 chromosome changes; seven translocations, four inversions, and one dele- 

 tion (Valencia, 1952). 



Meanwhile, in work on maize, Stadler and Sprague (1936), with cyto- 

 logical assistance by Singleton (1939), had found numerous chromosome 

 changes but not with as high a frecjuency as from an X-ray dose which 

 was comparable in the production of viable point mutations. Moreover, 

 the ultraviolet chromosome changes included a much larger proportion of 

 chromosomes which had undergone single breakage followed by healing 

 of the broken end, so as to form terminal deficiencies, rather than of 

 exchange union between broken ends, and when junction of ends derived 

 from different breaks did occur after ultraviolet treatment, this junction 

 was usually between only two of the pieces and not the complementary 

 two. Another feature of the ultraviolet breaks was that they usually 

 affected only one of the two chromatids derived from a treated pollen 

 chromosome, whereas those produced by X rays usually affected both of 

 them. This is understandable on the basis of the clustering of ioniza- 

 tions in the electron tracks produced by X rays since, as previously noted, 

 this clustering (as shown by the evidence from radiation of different 

 specific ionizations) sometimes breaks both of two sister chromatids at 

 once; these are the very cases least likely to undergo restitution and there- 

 fore are the most likely to be found. Similar findings regarding the differ- 

 ences between the ultraviolet and X-ray effects have been made in 

 Tradescantia microspores by Swanson (1940, 1942, 1943). It was also 

 noted by the Lindegrens (1941) and by Hollaender, Sansome, et al. (1945) 

 that visible mutations produced by ultraviolet treatment of Neurospora 

 spores (microconidia) included far fewer cases of partial sterility, indica- 

 tive of structural changes, than did those from a mutagenically com- 

 parable dose of X rays. Nevertheless, Faberge (1951) by means of a 

 chromosome arm containing seven markers analyzed the results pro- 

 duced in maize endosperm by treatment of pollen and reports having 

 produced translocations, inversions, rings, and simple breaks in con- 

 siderable abundance by ultraviolet in this material. 



An important aid in the interpretation of some of these peculiarities of 

 ultraviolet was provided by the findings of Swanson (1942, 1944) for 

 Tradescantia microspores and of Kaufmann and Hollaender (1944, 1946) 

 for Drosophila treated in the spermatozoon stage, that ultraviolet applied 

 after X rays causes a distinct reduction in the frecjuency of structural 

 changes produced, below that found in the material treated with only 

 X rays. It is therefore evident that ultraviolet not only produces breaks 



