INDEPENDENT X-RAY EFFECTS ON CHROMOSOME BREAKAGE AND REUNION

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Abstract
Seeds of Vicia faba which had been soaked in water for 24 hrs. were soaked for 30 min. in a 2 x 10-3 M soln. of 2,3-dimercapto-propanol (BAL) immediately before irradiation with a G. E. Maxitron tube giving 200 r per min. (1/2 hr. intervals between fractions). 3 days after irradiation the root tips were collected and scored for rings and dicentrics at metaphase. Since these aberrations are believed to be the result of the interaction of two breaks, the total yield, Y, expected from combining n treatments producing 2-hit aberration frequencies x1, x2,[long dash][long dash]-xn resp. is given by Y = (xl 1/2 + x2 1/2 + .... + xn 1/2)2 provided the effects of the n treatments are coincident in time. In this case there is no dose fractionation effect, but if the yield is additive there is complete fractionation effect. When the first fraction was given without BAL there was complete interaction between breaks produced by the first and by subsequent fractions given in the presence of BAL up to 1 hr. later. When the first fraction was given in the presence of BAL there was a marked failure of interaction when part of the X-ray dose was given as a third fraction, although there were indications that the first two fractions interacted and the third was additive. The time interval between breakage and reunion was dependent on dosage, since for 600 r the aberration freq. decreased with a decrease in the dosage intensity, whereas for 900 r the aberration freq. was the same at different intensities. When the doses in the first fraction, both with and without BAL, were adjusted to give the same aberration freq., and followed after 1 hr. by a second fraction of 400 r with BAL, there was no fractionation effect in the expt. where BAL was absent in the first fraction, but complete fractionation effect when it was present. When an initial dose of 400 r was followed by time lapses varying from 15 min. to 1 1/2 hrs. before a second dose of 400 r with BAL was given, the aberration freq. did not differ significantly for the various time intervals. Since breaks produced both in the presence and absence of BAL interacted with each other, qualitative differences between broken ends were unlikely. These findings showed that X-ray effects on breakage and on reunion were independent of each other.