Hybrid plants and animals often show suppression of activity of ribosomal genes (rDNA) originating from one of the parental or ancestral species. In the wheat x rye amphiploid triticale, containing 28 chromosomes of wheat origin and 14 from rye, rDNA of rye origin (on chromosome 1R) is not normally expressed, while the 1B- and 6B-origin rDNA from wheat shows strong expression. Expression of rDNA can be accurately assessed by the silver staining method, which stains both interphase nucleoli and metaphase rDNA sites that were actively expressed at the previous interphase. We show here that substitution of another rye chromosome, 2R, by a chromosome from hexaploid wheat, 2D (triticale-2D(2R)), prevents suppression of the rye-origin rDNA, and leads to activity of all six major rDNA loci. These results were found in two different triticales and supported by rDNA behaviour in wheat-rye chromosomal addition lines. Models for chromosomal interactions leading to control of rDNA expression are presented.
Plants of common wheat Triticum aestivum var. Chinese Spring, lacking the short arms of homoeologous group 5 chromosomes were subjected to a cold treatment of 10oC, and the chiasma frequency in pollen mother cells in the first meiotic metaphase was recorded. The existence in the short arms of chromosomes 5A and 5B of Ltp genes identical in effect to that described for chromosome SDL is demonstrated. In the absence of chromosome arm 5DS a significant decrease in chiasmata along with only a slight reduction in chromosome pairing was observed after the cold treatment, suggesting the existence in 5DS of a gene partially controlling crossing‐over at low temperatures. The results that were obtained allow us to propose the following designations for the genes with a stabilizing effect at low temperature: Ltpl in 5DL, Ltp2 in 5AS, Ltp3 in 5BS and Ltr, low temperature recombination, in 5DS.
Genomic imprinting of rye origin rDNA sequences in triticale is modulated by DNA methylation responsible for ontogenic expression patterns of those sequences. Considering the dynamic nature of these phenomena, we evaluated the influence of plant development on the inheritance of modified rye rDNA expression patterns. DNA hypomethylation was induced in triticale by 5-azacytidine (5AC) treatments at distinct developmental stages of M1 plants, and expression patterns were analysed in M2. The activity of rye origin rRNA genes in progeny of untreated and 5AC-treated plants was evaluated by silver staining in meristematic root tip cells and in meiocytes at diplotene. In the progeny of 5AC-treated plants, a significant increase in rye rDNA expression was observed, contrasting with the residual activity in untreated plants. Significant differential effects of 5AC treatments were observed in M2 plants and correlated with the M1 plant developmental stage in which DNA hypomethylation was induced. Hypotheses to explain the origin of those differences are discussed here.
Regulation of nondisjunction of the rye B chromosome was investigated using the cytidine analog 5-azacytidine, which affects DNA methylation. The B chromosome of rye normally undergoes nondisjunction at first pollen grain mitosis and is stable at all other mitotic nuclear divisions. Observations on mitosis in the control root cells showed normal chromosome behaviour, whereas the treated roots were characterized by a high frequency of anaphases with lagging chromosomes showing evidence of failure of chromatid separation. The occurrence of nondisjunction was confirmed by the presence of variable numbers of B chromosomes between cells within roots, whereas the A-chromosome number remained constant. These results suggest that the epigenetic process by which somatic cells maintain the inactivity of the gene(s) responsible for B chromosome nondisjunction, between fertilization and meiosis, is mediated through DNA methylation.Key words: B chromosome, DNA methylation, meiotic reprogramming, 5-azacytidine, rye.
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