Further evidence for a high position-specific effect in the action of chemical mutagens on the chromosomes of barley.

Further convincing evidence for the decisive role of chromosome constitution in the processes underlying the specific induction of structural mutations by chemical mutagens is described. The most important conclusion to be drawn from the results obtained in experiments with maleic hydrazide (MH) and ethyl methanesulphonate (EMS) is that segments 39 and 47, situated next to the secondary constrictions of standard chromosomes 6 and 7, when tandemly combined by reciprocal translocation in chromosome 7(6) of reconstructed karyotype T-21, behave in a rather similar way. This is independent of the nature of the chemical mutagens applied, as far as the distribution pattern of induced chromatid aberrations is concerned. The phenomenon may be characterized as follows: (1) the segments in question in the new position appeared to be the most pronounced aberration "hot spots", the expressivity in segment 39 always being higher; (2) the pronounced "hot spot" character of these segments proved to be due mainly to their preferential involvement in intercalary deletions and duplication-deletions; (3) the specific constitution of chromosome 7(6) resulted in the majority of cases in a marked increase of the region involved in the aberration types mentioned above in segment 39. This is one of the very few examples of true position effect in the expression of chemically induced structural mutations at the chromosome level.