Position-specific effects in the action of mutagenic agents on the chromosomes of barley (Hordeum vulgare L. )

摘要

A comparative analysis of the intrachromosomal distribu-tion pattern of structural mutations induced by maleic hydrazide (MH) and 7-rays in barley was performed. Two reconstructed karyotypes of barley, T-1586 and T-21, which allow an easy identification of the individual chromosomes of their complements, and differ from each other by the position of the "hot spot" segments, were used as experimental material. No significant differences in the sensitivity of the karyotype variants tested were observed after treatment with both MH and γ-rays. As expected, the two agents clearly differ by the types of aberrations induced; only chromatid type for MH (delayed effect) and both chromatid and chromosome types in the case of γ-rays (non-delayed effect). The most interesting finding in this study was that the chromosome constitution dramatically affected the distribution pattern of aberrations along the individual chromosomes, the effect being especially strongly pronounced in the action of MH. Two segments, namely 32 and 41, located proximally to the nucleolus organizing regions (NORs) of chromosome 5H and 6H, respectively, were found to be the most sensitive sites ("hot spots") of barley karyotype. The effect was mostly expressed for duplication deletions (dd) and especially for intercalary deletions (d). Thus, about 43% of induced intercalary deletions and 35% of duplication deletions were found to be localized at segment 32. The same segments showed an increased sensitivity to the action of γ-rays, as well, but the localized breakage was significantly less pronounced. It is remarkable also that the specific constitution of reconstructed chromosome 5H~(6H) (tandem position of the "hot spot" segments) was found to result in the majority of cases in a marked increase of the segment which is involved in intrachromosomal exchanges. To throw an additional light on the na-ture of the "hot spot" segments and the processes concerning the subtle localization of chromosomal breakpoints and the primary induction andrepair of lesions that may underlie this regional specificity of mutagenic agents, fluorescence in situ hybridization with rDNA probe and analysis of DNA damage by agarose gel electrophoresis under neutral and alka-line conditions were applied.