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

A comparative analysis of the intrachromosomal distribution of structural mutations induced by maleic hydrazide (MH) and γ-rays in barley was done. The two mutagenic agents have different mechanisms of action; MH as mutagen with delayed effect produces only chromatid rearrangements, and γ-irradiation is a typically mutagenic factor with non-delayed effect, capable to induce both chromosome and chromatid types of aberrations. 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 plant 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 and both chromatid and chromosome types in the case of γ-rays. The most interesting finding in this study is that the chromosome constitution may dramatically affect the distribution pattern of aberrations along the individual chromosomes, and this effect was 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 preferential involvement in chromosomal rearrangements of these segments was mostly expressed for intercalary deletions and duplication-deletions. Interestingly enough, when the two 'hot spot' segments are combined in chromosome 5H6H by reciprocal translocation in tandem position (karyotype T-21) approximately 50% of the intercalary deletions (versus 16% in the case of T-1586) and 45% of duplication-deletions (versus 16% in T-1586) were localized in these regions. The same segments showed an increased sensitivity to the action of γ-rays, as well, but the localized breakage was sharply less pronounced. It is remarkable also that the specific constitution of chromosome 5H6H was found to result in the majority of cases in a marked increase of the segment which is involved in intrachromatid exchanges. To throw an additional light on the genetic nature of the 'hot spot' segments and the processes concerning the primary induction and repair that may underlie this regional specificity of mutagenic agents, fluorescence in situ hybridization with rDNA probe and analysis of DNA damage by conventional agarose gel electrophoresis under neutral and alkaline conditions were applied. (author)