FREQUENCY OF CHROMOSOME ABERRATIONS PRODUCED BY FRACTIONAL DOSES OF X-RAYS IN TRADESCANTIA

Abstract

HEORIES based on results from fractional dose aud time-intensity X-Tray experiments using Tradescantia inflorescences have become iinportant cornerstones for interpretations of many irradiation experiments. The major evidence has been amassed and theoretical considerations developed by SAX (1939, 1940 and 1941), FABERGB (1940), LEA and CATCHESIDE (1942), CATCHESIDE, LEA and THODAY (1946a and 1)) and CATCHESIDE (1948). According to these workers most chromosome breaks in irradiated Tradescantia restitute or undergo reunion soon after their production and such breaks do not reopen. Because of this rapid reunion or restitution the frequency of two-hit aberrations (i.e., chromatid interchanges, dicentric chromosomes and centric rings) is higher at high intensity than at low intensity. The experiments of SAX on the effects of fractional X-ray dosage have shown that after about one hour from the time of irradiation breakage-ends no longer participate in reunions. Recently LANE (1951) has questioned most of these findings and their resulting interpretations and has rejected the theory of rapid reunion. L A N E ' S data on reunions presented in his graph 5 show that the highest frequency of breaks was produced by a single close of 360 r. Thereafter a progressive decrease in the frequency of aherrations occurred when two 180 r doses were separated by 1, 2 and 4 hours. Reunion frequency was higher when dose-fractions of 180 r each were separated by 6 or S hours than when the interval was 4 hours. When the time interval was 8 hmrs the frequency of reunions was almost as high as that obtained with a single close of 360 r. The apparent sharp rise in reunions in the S-hour fractional material led LANE to reject the theory of rapid reunion of X-ray breaks. In his view chromosome hreaks produced in the resting nucleus by X-rays may remain open for several or many hours, restitution and reunion being delayed until the beginning of prophase. The low reunion frequency in the 4-hour fractionation group is attributed to physiological effects of the first radiation fraction on the chromosomes. The chromosomes respond to the radiation by becoming less liable to further breakage by X-rays administered at this time. This newly developed resistance to breakage is highest after about 4 hours but has almost disappeared by 8 hours.