570 RADIATION BIOLOGY 



(1946), working with Tradescantia. In the first experiments with 

 Drosophila the technique allowed only the effect of infrared pretreatment 

 (i.e., that given before X irradiation) to be discovered, but soon after- 

 ward posttreatment also (given before and at about the time of fertiliza- 

 tion) was found effective (Kaufmann, 1946). When nitrogen mustard 

 instead of X rays was used, pretreatment with infrared, although appar- 

 ently not posttreatment, was found to have a similar effect in promoting 

 aberrations to that which it had with X rays (Kaufmann, Gay, and 

 Rothberg, 1949). 



In the experiments with X rays, dominant lethals were not noticeably 

 increased by the infrared, nor were recessive lethals (Kaufmann and 

 Gay, 1946, 1947a, b). Although, as noted on p. 503, the numbers of 

 recessive lethals were in fact insufficient to show whether they had been 

 increased by an amount which would correspond with the position effects 

 to have been expected on the basis of the increased frequency of struc- 

 tural changes, nevertheless, it was clear that the gene-mutation frequency 

 could not have been increased, unless by an amount far less than that of 

 structural changes. Support for this conclusion is to be found in the 

 fact that in the fungi A. terreus and Trichophyton mentagrophytes, Hol- 

 laender and Swanson (1947) and Swanson, Hollaender, and Kaufmann 

 (1948) found that infrared pretreatment increases the rate of morpho- 

 logical mutations produced by X rays (provided it is admitted that in 

 these fungi a portion of these mutants are position effects, see Sect. 5), 

 whereas infrared pretreatment did not increase the rate of ultraviolet- 

 induced mutations, which are thought to be predominantly genie. 



In Tradescantia microspores exposed to X rays 21 hours before examina- 

 tion, at a time when the breaks produced would be expected to affect 

 only individual chromatids, it was found by Swanson (1947) that infrared 

 posttreatment given as late as 4 hours after X irradiation markedly 

 increases the frequency of single chromatid breaks and chromatid 

 changes. Later Swanson (1949) found that double (iso-) chromatid 

 breaks are promoted also, although to a lesser extent. The same experi- 

 ments showed that pretreatments are effective at least as long as 96 hours 

 before X irradiation without a noticeable diminution of their influence, 

 and Yost (1950) found that, when exposure to X rays is done early 

 enough for a study of the matter, posttreatments are still effective 96 

 hours after X irradiation. It was then discovered by Swanson and Yost 

 (1951a, b) that a heat shock of 48°C for only 3-^ minute, if interposed 

 between either an infrared pretreatment or a posttreatment and the 

 X irradiation, blocks the influence of the infrared in promoting the aberra- 

 tions, so that only the same aberration frequencies are obtained as with 

 X rays alone. Even an amount of heat as low as 23°C (in experiments in 

 which this was applied during X irradiation) seemed to block the infrared 

 effect, although the effect was pronounced at 18°C and lower. Unhke 



