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Genome restructuring in rye affects the expression, organization and disposition of homologous rDNA loci

¹ Secção de Genética, Departamento de Botânica e Engenharia Biológica, Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisboa, Portugal
² Departamento de Ciências Biológicas e Naturais, Universidade Lusófona de Humanidades e Tecnologias, Campo Grande 376, 1749-042 Lisboa, Portugal
³ Departamento de Biologia Vegetal, Faculdade de Ciências de Lisboa, Bloco C2, Campo Grande, 1749-016 Lisboa, Portugal

View larger version (172K): [in a new window]   Fig. 1. A cold-treated meristematic metaphase cell of rye showing the co-localization of the rDNA sites detected by in situ hybridization (in red) and the silver stained Ag-NORs (dark brown). On the condensed NORs (arrows), the silver signal reveals the previous expression of rRNA genes, whereas the two hybridization sites of identical size demonstrate that the two rDNA loci have equivalent numbers of ribosomal cistrons. Bar, 5 µm.

View larger version (147K): [in a new window]   Fig. 2. A colchicine-treated meristematic metaphase cell of rye showing two distended and homomorphic Ag-NORs (brackets) detected by silver staining. Bar, 5 µm.

View larger version (64K): [in a new window]   Fig. 3. Colchicine-treated meristematic metaphase cells of rye. (a) Simultaneous visualization of chromatin (DAPI staining in blue) and of the rDNA hybridization sites (red) reveals the presence of two NORs exhibiting a centromere-proximal block of rDNA followed by a ribosomal chromatin filament towards the satellite (brackets). Note the clear length differences between the decondensed regions of the two homologous rDNA loci. (b) The simultaneous visualization of the in situ hybridization and the silver staining signals shows that the Ag-positive rDNA fraction is found only coincident with the distended ribosomal chromatin revealed by FISH (compare with a). Bar, 5 µm.

View larger version (175K): [in a new window]   Fig. 4. A colchicine-treated meristematic root-tip metaphase cell of a double monotelosomic line of rye showing both the co-localization of the rDNA sites detected by in situ hybridization (red) and the silver stained Ag-NORs (dark brown). On condensed NORs (arrows), the silver signal reveals the previous expression of rRNA genes, whereas the two hybridization sites of identical size demonstrate that the two rDNA loci have equivalent numbers of ribosomal cistrons. Bar, 5 µm.

View larger version (29K): [in a new window]   Fig. 5. Meristematic interphase nuclei of rye after silver staining, showing two homomorphic and two heteromorphic nucleoli (a and b, respectively). Bar, 5 µm.

View larger version (45K): [in a new window]   Fig. 6. Simultaneous visualization of chromatin (DAPI staining in blue) and rDNA hybridization sites (red) in meristematic interphase nuclei of rye, showing the major patterns of rDNA organization. (a) Type I: condensed perinucleolar knobs (arrows) without any traces of labelled rDNA chromatin inside the nucleolus. (b) Type II: condensed perinucleolar knobs (arrows) with thin filaments of rDNA chromatin towards the interior of the nucleolus (bracket). (c) Type III: condensed perinucleolar knobs (arrows) with intranucleolar condensed rDNA spots (arrowheads) linked by thin chromatin filaments (hybrid rye line). Bar, 5 µm. Note that rDNA perinucleolar knobs are usually present on the nuclear hemisphere where DAPI staining is more intense, indicating a higher concentration of chromatin in this pole of the nucleus.

View larger version (68K): [in a new window]   Fig. 7. Simultaneous visualization of chromatin (DAPI staining in blue) and rDNA hybridization sites (red) in meristematic interphase nuclei of rye illustrating the relative positioning rDNA loci. (a) Adjacent rDNA loci. (b) Fused rDNA loci (hybrid rye line). Bar, 5 µm. Note that rDNA perinucleolar knobs are usually present on the nuclear hemisphere where DAPI staining is more intense, indicating a higher concentration of chromatin in this pole of the nucleus.