On the time period during which chemically induced chromatid breaks are available for interaction

Abstract

A CCORDING to the breakage-reunion hypothesis chromosome and chromatid breaks induced by ionizing radiation or radiomimetic agents may restitute, stay open or reunite in a new order, the last two possibilities giving the various types of chromosomal aberrations. The purpose of the present study was to find out during what time chemically induced chromatid breaks are able to interact, the product of interaction being chromatid translocatiol~s as scored in metaphase after treatment of interphase nucleic in root tip meristems of Vicia fablx. Quite a few chemicals with radiomimetic activity have been tested for interaction of breaks induced by different agents [2, 3, 6, 8, 111, but to the best of our knowledge only Merz ef al. [7] have studied the time during which chemically induced breaks may stay open. This is in contrast to the situation in radiation-induced breaks. Since the coexistence of two chromatid breaks in space and time is a prerequisite for the production of a chromatid translocation, one of the measures of the time during which breaks can stay open and interact may be the longest interval between fractionated “doses” during which no reduction in the theoretical expected yield of translocations is found. Using this procedure we have treated main roots of V. fabn twice with a solution of 5 x 1OF M triethylenemelamin (TEM) for 30 min, both treatments following each other, firstly without, then with, intervals of 2, 2, 4, 6 and 8 hr during which the seedlings were kept in distilled water (24°C). Temperature during TIN treatment and recovery was 24°C. Scoring of aberrations was done after 24 hr of recovery (method: 2 hr colchicine 0.05 per cent; fixation alcohol: acetic acid (3: 1); Feulgen squash technique). The expected values 1131 for full interaction of breaks induced by the first TEM treatment with those of the second giving chro~xlatid translocations are calculated according to (1, 1 v’b)2, n and b symbolizing the yield of translocations for each of the control treatments (1 and 2 in Tables I, II). Table I summarizes t.he results. In comparison with the controls the percentage of isochromatid breaks is somewhat higher than additive without and with intervals between the two TEM treatments. In the case of chromatid translocations, complete interaction of the breaks induced by the two treatments is indicated even after an interval of 8 hr. In fact, it appears from the comparison of the values found and expected for the percentage of translocations that more than interaction is involved, since the percentage of translocations found is always higher than expected. Similar results have been reported by Cohn [2, 31, but the reason for this effect is unknown in our case. We have tested some possibilities which may be responsible for this “superinteraction” (induction of potential lesions by the first concentration to be transformed into real lesions by the second, after-effects of TEM in the roots), but