Effect of caffeine on intrachromosomal distribution of chromatid aberrations induced by chemical mutagens in Crepis capillaris L

The intrachromosomal distribution patterns of chromatid aberrations induced by N-methyl-N-nitrosourethane (MNU), N-ethyl-N-nitrosourethane (ENU) and ethyleneimine (EI) were compared with those induced by combined treatment with the same mutagens and caffeine, the latter being considered as an inhibitor of post-replication repair of DNA.Chromatid aberrations induced by mutagens alone were distributed non-randomly along the chromosomes. In certain regions few aberrations were located; in others pronounced clustering of aberrations was observed and these regions were considered to be hot spots. This refers especially to MNU- and EI-induced aberrations, whereas ENU-induced chromatid aberrations showed a more length-proportional distribution. In ENU experiments, certain chromosomal segments also represented hot spots, but these were less pronounced. The distribution patterns of chromatid aberrations induced by combined treatment with mutagens and caffeine differed significantly from those observed in experiments with the mutagens only. There seemed to be a tendency to approach random distribution here. This was a result both of the decrease in the quantity of the aberrations in the regions, which in the experiments with mutagens only were hot spots, and of its increase in other chromosomal regions. Some of these regions were considered as hot spots but they were less pronounced. These tendencies refer to MNU and EI. Certain differences between the two variants, with the without caffeine, in ENU experiments were observed but these were of lower expressivity.The causes od differential sensivity of chromosomal regions are discussed. The conclusion is drawn that clustering of chromatid aberrations in certain chromosomal regions is due to differences in the repair systems acting in heterochromatic and euchromatic regions.