Short Repair Times Research Articles

When larvae of D. melanogaster are X-rayed (spectrum III: 100 kV, 1.7 mm Al-filter, W-target, 8 mA), mitotic recombination is induced and becomes visible later as twin mosaic spots in the adult eye. It was possible by separating the spectrum into two parts—a soft part (spectrum I: 55 kV, 0.78 mm Al-filter, W-target, 10 mA) and a hard part (spectrum II: 100 kV, 0.24 mm Ta-filter, W-target, 8 mA) — to show that there are two reactions which lead to mitotic recombination. By means of additional experiments the two reactions were examined in more detail: a) Reaction 1 shows a DEC, which can be described as effect=(1-e-kD)x with x=1.3. Therefore, it deviates significantly from a single-hit reaction. It rises, however, less steeply than a theoretical two-hit curve. This deviation from a squared dose-dependence is not due to a time factor, since the reaction does not show a time-dependence within the observed region. The deviation must be interpreted rather as a consequence of (1) an increasing number of complex chromosomal aberrations with increasing dose, which diminish the number of two-break aberrations leading to a twin spot, and (2) an increase of single-hit reactions with increasing dose. The frequency of reaction 1 depends exclusively on the dose in [R]; the reaction can be induced by all spectra investigated in this paper. b) Reaction 2 can be described as a breakage-fusion mechanism with a very short repair time; the second break necessary for a recombination must arise 30 sec after the first. Probably, these breaks are induced by the K-radiation of tungsten contained in spectra II and III described above. However, twin spots are observed in greater amounts only, when larvae are irradiated with spectrum I prior to irradiation with spectrum II.

Read full abstract