chromosome aberrations in Tradescantia 739 



indicating an effect of ultraviolet in favoring restitution as opposed to 

 recombination. 



INFRARED 



The original investigations of Kaufmann, Hollaender, and Gay (1946) 

 demonstrated a marked effect of pretreatment with infrared in increasing 

 the frequency of X-ray-induced chromosomal rearrangements in Dro- 

 sophila. This effect was subsequently confirmed in experiments with 

 Tradescantia microspores by Swanson and Hollaender (1946) and Swan- 

 son (1949). Pretreatment of whole inflorescences was carried out with 

 infrared (between 6000 and 11,500 A with maximum energy transmission 

 at 10,000 A) for varying periods of time (up to 96 hours) before exposure 

 to X rays, and cytological analyses were made at 22 hours following X-ray 

 exposure. All such pretreatments resulted in an increase in all types of 

 chromatid rearrangements with the increase in interchanges being most 

 marked. Posttreatments with infrared also resulted in an increase in the 

 frequency of chromatid breaks and exchanges. (In the initial pretreat- 

 ment experiments no effect on isochromatid breaks was found, but in 

 later experiments these also showed an increase.) As with pretreatment, 

 the effect on interchanges was most marked, and increased yields were 

 obtained even when posttreatments were delayed as much as 18 hours 

 after X irradiation. In order to explain the fact that both pre- and post- 

 treatments are effective, the hypothesis is proposed that both infrared 

 and X rays alone are capable of weakening the chromosome structure (in 

 addition to the normally produced X-ray breaks) and that such changes 

 only become realized as complete breaks upon the addition of the other 

 type of radiation. 



Yost (1951) has investigated the effect of infrared pre- and posttreat- 

 ments on the frequency of chromosome breaks induced at the resting 

 stage. Again, both these treatments resulted in an increase in the fre- 

 quency of the aberrations scored — dicentrics and centric rings. Pre- 

 treatments were followed immediately by X irradiation and cytological 

 examinations made 96 hours later. The frequency of aberrations in 

 buds pretreated with infrared was, on the average, approximately 

 doubled. For tests of posttreatment effects, buds were X-rayed and 

 samples then given a single exposure to infrared at several successive 

 intervals following the X irradiation, up to 96 hours, at which time cyto- 

 logical analyses were made. As indicated, increases were noted in each 

 test and these were of approximately equal magnitude at all intervals 

 (and of the same order of increase as for the pretreatments). Further, the 

 unexpected result was obtained that all the aberration types observed 

 were chromosome types, even in instances where the infrared exposures 

 were made just before the cytological examination at 96 hours, when the 

 chromosomes were obviously in the prophase stage where they respond 



