Postirradiative Effects on Chromosomal Aberrations in Tradescantia Microspores.

INCE 1939 when SAX employed the time-intensity factor to determine how S long a chromosome break would remain open and thus be available for interaction with other breaks, many different methods of influencing X-ray-induced chromosomal aberrations have been used in studying the rejoining capacity of broken chromosomes. SAX concluded that high-intensity radiation is more eff ective in producing aberrations than low and that continued radiation is more effective than intermittent. FANO and MARINELLI (1943) concluded from some of their work that fractionation experiments consisting of two high-intensity irradiations separated by a variable time interval was better suited to investigations of the time-intensity factor than experiments in which the intensity of continuous irradiation was vaned. More recently this approach has been used by COHN (1958), WOLFF and LUIPPOLD (1958) and others. In hddition to studies involving the method of irradiation, following THODAY and READ’S (1947) discovery of the oxygen effect, the use of environmental variations has provided valuable information concerning the rejoining capacity of X-ray-induced chromosome breaks. The investigations of MAZIA (1954), STEFFENSEN (1955) and WOLFF and LUIPPOLD (1956) have indicated that ionic bonds may play a role in chromosome organization, while the work of WOLFF and LUIPPOLD (1955) and BEATTY and BEATTY (1957) indicates that energy supply is an important factor in the rebonding of the chromosome breaks. Previous investigations ( BEATTY and BEATTY 1957,1959 and BEATTY, BEATTY, and COLLINS 1956) revealed that a total dose of 400r of X-radiation at intensities from lr to 400r per minute, in combination with various environmental factors during or after irradiation, yielded in five percent oxygen an average low of 0.48 and a high of 0.88 chromosomal aberrations per cell, while in helium a low of-0.14 and a high of 0.66 was found. In those experimental procedures in which, with the same dose of Mor, less than the maximum yield of aberrations was obtained, by modifying the experimental setup so as to influence the rejoining of broken chromosome ends, a yield up to but not greater than the high could be obtained. The results indicated in all of these investigations that a total dose of 400r in helium produced a constant number of recoverable chromosome breaks and that in five percent oxygen the same was true although differing quantitatively from that in helium. This idea of constancy in number of recoverable