Figure 1 - uploaded by Leonor Morais-Cecilio
Content may be subject to copyright.
Cells from different developmental stages of hexaploid triticale after silver staining, a Root tip metaphase cell of triticale cv. Drira with four strongly stained wheat ST-NORs (example arrowed) and additional very faintly stained ST-NORs belonging to chromosome pair 1R (arrowhead). b Pollen mother cell at metaphase I from triticale cv. Juanilho with four silver-stained major NORs on bivalents 1 B and 6B. c First pollen grain mitosis of triticale cv. Drira with three subterminal ST- NORs (1B, 6B and 1R). d Young microspore of triticale cv. Juanilho at tetrad stage showing four silver-stained nucleoli, corresponding to the full expression of all NORs present (1A, 1B, 6B and 1R).
Source publication
To test the hypothesis that interspecific genomic and chromosome interactions leading to nucleolar dominance could be reprogrammed in meiosis, we compared the expression of distinct nucleolar organizing region (NOR) loci in hexaploid triticale root tip meristematic cells, pollen mother cells and young pollen grains. Interphase and metaphase cells w...
Similar publications
The effects of different concentrations (1-50 microM) of Cd on root growth, cell division and nucleoli in root tip cells, protective enzyme activities and lipid peroxidation in Vicia faba were investigated in order to better understand the processes of Cd-induced senescence. The results indicated that lower concentration of Cd (1 microM) had no obv...
Citations
... The studies of Silva et al. (1995) in the hexaploid triticale showed that the rRNA genes of a rye origin that were silent during the first meiotic division were transcriptionally activated in the microspores , thereby indicating that meiotic reprogramming may erase the preferential inactivation of rDNA via ND. Chen and Pikaard (1997b) used an S1 nuclease protection assay to determine the rRNA gene expression patterns in the different tissues of the allotetraploid B. napus. ...
Nucleolar dominance (ND) is an epigenetic, developmentally regulated phenomenon that describes the selective inactivation of 35S rDNA loci derived from one progenitor of a hybrid or allopolyploid. The presence of ND was documented in an allotetraploid grass, Brachypodium hybridum (genome composition DDSS), which is a polyphyletic species that arose from crosses between two putative ancestors that resembled the modern B. distachyon (DD) and B. stacei (SS). In this work, we investigated the developmental stability of ND in B. hybridum genotype 3-7-2 and compared it with the reference genotype ABR113. We addressed the question of whether the ND is established in generative tissues such as pollen mother cells (PMC). We examined condensation of rDNA chromatin by fluorescence in situ hybridization employing state-of-art confocal microscopy. The transcription of rDNA homeologs was determined by reverse-transcription cleaved amplified polymorphic sequence analysis. In ABR113, the ND was stable in all tissues analyzed (primary and adventitious root, leaf, and spikes). In contrast, the 3-7-2 individuals showed a strong upregulation of the S-genome units in adventitious roots but not in other tissues. Microscopic analysis of the 3-7-2 PMCs revealed extensive decondensation of the D-genome loci and their association with the nucleolus in meiosis. As opposed, the S-genome loci were always highly condensed and localized outside the nucleolus. These results indicate that genotype-specific loss of ND in B. hybridum occurs probably after fertilization during developmental processes. This finding supports our view that B. hybridum is an attractive model to study ND in grasses.
... Studies of dicot and monocot interspecific and intergeneric hybrids and allopolyploids exhibiting ND have emphasized the developmental regulation of this phenomenon Silva et al., 1995;Chen and Pikaard, 1997b;Pontes et al., 2007). Chen and Pikaard (1997b) revealed that rRNA transcripts from only one progenitor were detected in vegetative tissues in both natural and resynthesized Brassica napus, with the exception of root meristematic cells, which have codominant Brassica rapa-and Brassica oleracea-like rRNA genes (Hasterok and Maluszynska, 2000). ...
... However, the timing of ND establishment in monocotyledonous allopolyploids seems to be different from that in the dicot A. suecica. The presence of fully established ND in root meristematic cells 2-3 days after germination was detected in allopolyploid triticale (Lacadena et al., 1984;Silva et al., 1995) and in root tip cells derived from 3-day-old B. hybridum seedlings (Idziak and Hasterok, 2008). ...
Nucleolar dominance (ND) is an epigenetic phenomenon that occurs in some plant and animal allopolyploids and hybrids, whereby only one ancestral set of 35S rRNA genes retains the ability to form the nucleolus, whilst the rDNA loci derived from the other progenitor are transcriptionally silenced. Since there is substantial evidence that ND is regulated developmentally, this study focusses upon the establishment and/or maintenance of ND during different stages of development in the model grass allotetraploid, Brachypodium hybridum. Fluorescence in situ hybridisation with a 25S rDNA probe to 3-D cells shows that ND is present not only in root meristematic and differentiated cells of this species, but is also present in male meiocytes at prophase I, tetrads of microspores and different embryonic tissues. The inactive state of B. stacei-originated rDNA loci is confirmed by silver staining method. We show that only B. distachyon-derived 35S rDNA loci form a nucleolus/nucleoli in the aforementioned tissues, whereas B. stacei-like loci remain highly condensed and thus transcriptionally suppressed. The establishment of ND during earlier stages of B. hybridum embryo development cannot be ruled out. However, our proposal of gradual pseudogenisation of B. stacei-like loci in the evolution of the allotetraploid seems to be more likely.
... These effects were partly carried out to the next generation (Castilho et al. 1999). Another documented epigenetic regulation in triticale occurred 20 years ago and is related to the activity of the nucleolar rDNA organizers originating from wheat and rye (Silva et al. 1995). In most tissues the activity of the rye nucleolar organizer is suppressed. ...
In this review we describe how different tools and approaches have help to identify changes in the genome and the transcriptome of triticale toward a better understanding on how genes interactions are regulated in this man made intergeneric species. The improvement in cytogenetic, genomic and transcriptomic tools for characterization of triticale at a molecular level have provided us with many new ways to better and to more efficiently contribute to the understanding of triticale. Although recent initiatives have targeted a better utilization of triticale in bioenergetics, biomaterials and animal feed, it is possible that triticale importance could increase in human nutrition in the near future.
... Nucleolar dominance was also observed in wheat x rye hybrids, and in the artificial amphiploid triticale. In this case, wheat dominates over rye in the expression of rDNA loci, resulting in an almost total inactivation of rRNA genes of rye origin (Fig. 2);1819202122. Comparison of rDNA spacers from wheat and rye showed that rye spacers are shorter, displaying several deletions amongst the repetitive DNA sequences [23]. ...
... Another intriguing result came from studies in Xenopus hybrids where there was no apparent difference in the methylation state of dominant and under-dominant rRNA genes [54]. In the wheat-rye system, it was already shown that rye rRNA genes, usually silent in the amphiploid triticale [21, 22, 53, 55], are more heavily methylated [50, 51]. However, in the wheat addition line for the rye nucleolar chromosomes a significant increase in rye rRNA gene expression is observed [29], the rye rDNA being methylated at levels equivalent to the ones observed in triticale where the same genes are highly suppressed [51]. ...
... Brassica hybrids, [26]), or following meiosis (e.g. wheat x rye amphiploid, [22]). In wheat, the largest nucleoli are formed in the haploid micro- spore [12]. ...
Abstract: Nucleolar dominance is an enigma. The puzzle of differential amphiplasty has remained unresolved since it
was first recognised and described in Crepis hybrids by Navashin in 1934. Here we review the body of knowledge that
has grown out of the many models that have tried to find the genetic basis for differential rRNA gene expression in
hybrids, and present a new interpretation. We propose and discuss a chromatin imprinting model which re-interprets
differential amphiplasty in terms of two genomes of differing size occupying a common space within the nucleus, and with
heterochromatin as a key player in the scenario. Difference in size between two parental genomes induces an inherited
epigenetic mark in the hybrid that allows patterns of chromatin organization to have positional effects on the neighbouring
domains. This chromatin imprinting model can be also used to explain complex genomic interactions which transcend
nucleolar dominance and which can account for the overall characteristics of hybrids. Gene expression in hybrids, relative
to parentage, is seen as being based on the nuclear location of the sequences concerned within their genomic environment,
and where the presence of particular repetitive DNA sequences are ‘sensed’, and render silent the adjacent information.
... Triticum (Crosby, 1957;Darvey e Driscoll, 1972;Viegas e Mello-Sampaio, 1975;Flavell e O"Dell, 1976, 1979Hutchinson e Miller, 1982;Martini et al., 1982;Martini e Flavell, 1985;Flavell et al., 1988;Sardana et al., 1993), triticale Appels et al., 1986;Amado et al., 1997;Neves et al., 1997a, b;Silva et al., 1995;Viera et al., 1990a, b), Agropyron (Heneen, 1962), Brassica (Chen e Pikaard, 1997a, b), Aegilops e Arabidopsis (Chen et al., 1998) , 1977, 1978) e em células somáticas híbridas de mamíferos (Elicieri e Green, 1969;Bramwell e Handmaker, 1971;Miller et al., 1976b;Croce et al., 1977;Soprano et al., 1979;Soprano e Baserga, 1980;Onishi et al., 1984). A existência de dominância nucleolar intraespecífica foi também já observada em humanos (De Capoa et al., 1988), em células híbridas de rato e homem (Learned et al., 1985) e em cruzamentos interraciais de ...
... No triticale e em híbridos F 1 trigo x centeio, os genes ribossomais de centeio do cromossoma 1R encontram-se inactivos, sendo apenas visíveis os quatro NORs "major" do trigo dos cromossomas 1B e 6B (Thomas e Kaltsikes, 1983;Rangel-Figeiredo et al., 1985;Silva et al., 1995;Neves et al., 1997b;Silva, 1999 a,b (Viera et al., 1990a;Viera et al., 1990b). Surpreendentemente, a delecção do braço longo do cromossoma 1R de centeio ou a substituição do cromossoma 2R do centeio pelo cromossoma 2D de trigo pode também causar a expressão do NOR do centeio em triticale (Neves et al., 1997b). ...
... Algumas alterações induzidas durante a germinação são transmitidas às gerações seguintes (Heslop-Harrison, 1990;Vongs et al., 1993;Amado et al., 1997;Castilho et al., 1999) e isso é constatado pela expressão génica alterada no rRNA e pela organização do núcleo (Neves et al., 1997b;Castilho et al., 1999). A descendência das plantas submetidas ao tratamento com 5-AC apresenta alterações fenotípicas, embora os padrões de metilação sejam diferentes durante o desenvolvimento (Heslop-Harrison, 1990;Silva et al., 1995;. No triticale, a expressão do rDNA de centeio parece aumentar de geração para geração e sofrer alterações nos padrões de expressão (Amado et al., 1997). ...
... Recently, a protein factor NF-D was described, which binds both DNA and RNA at the pre-rRNA processing site in the 5 ETS of Brassicaceae, and it was also proposed that the NF-D could be involved in 35S rDNA transcription (Saez-Vasquez et al., 2004). NORs, inactive in diploid cells of root tips, were found to be activated in haploid microspores (McClintock, 1934; Silva et al., 1995 ). A possible explanation would be a spatial separation of dominant and under-dominant rDNA after meiosis. ...
Ribosomal RNA genes (rDNA) represent a useful tool to study reticulate evolution. In allopolyploid plant species biparental or uniparental inheritance of 35S rDNA was demonstrated. Uniparental inheritance; as a result of differential elimination of one of the parental rDNA, can be accompanied by structural rearrangements of rDNA ofthe other parent and by formation of new rDNA variants. Particularly, homo‐genization of rDNA in allopolyploids appears to be an example of fast concerted evolution of repeated sequences. At the functional level, interaction between parental 35S rDNA loci in hybrids/allopolyploids leads to differential transcription/silencing, known as nucleolar dominance (ND). Depending on the combination of parental species, ND may be strong and stable, or weak, unstable and even reversible. Differential transcription of parental rDNA is regulated via cytosine methylation of DNA, histone modifications, and chromatin remodelling factors, but mechanisms providing discrimination of parental rDNA remain poorly understood. Probably, interplay between several factors such as local structural features of rDNA (especially subrepeated elements in the intergenic spacer region), unlinked loci, and the chro‐mosomal/genomic context determine ND. The ND and rDNA rearrangements reflect the dynamic nature and evolutionary plasticity of the genome in allopolyploids.
... The mechanisms responsible for the establishment of nucleolar dominance are poorly understood but once established there is a developmental component to the maintenance of the silenced state. In hybrids within the plant genera Crepis and Triticea, underdominant NORs suppressed in vegetative tissues are active in male meiocytes [8,9]. In Brassica allotetraploid hybrids, underdominant rRNA genes that are silenced in organs derived from the vegetative and inflorescence meristems (e.g. ...
... In fully expanded rosette leaves of mature plants, A. thaliana-derived rRNA gene transcripts are undetectable, indicating that nucleolar dominance is complete in these leaves. Following bolting and flowering, low levels of A. thaliana-derived transcripts are again detected in the inflorescence tissue, consistent with prior results in Brassica and Triticale showing that nucleolar dominance is incomplete upon the transition to flowering [10] or gametogenesis [9], respectively. Collectively, these results suggest that nucleolar dominance is not evident in the major organs that developed during embryogenesis (cotyledons), is established progressively in organs that develop postembryonically from the shoot apical meristem, and is incomplete, or leaky, in reproductive tissues that arise late in development. ...
Nucleolar dominance is an epigenetic phenomenon in plant and animal genetic hybrids that describes the expression of 45S ribosomal RNA genes (rRNA genes) inherited from only one progenitor due to the silencing of the other progenitor's rRNA genes. rRNA genes are tandemly arrayed at nucleolus organizer regions (NORs) that span millions of basepairs, thus gene silencing in nucleolar dominance occurs on a scale second only to X-chromosome inactivation in female mammals. In Arabidopsis suecica, the allotetraploid hybrid of A. thaliana and A. arenosa, the A. thaliana -derived rRNA genes are subjected to nucleolar dominance and are silenced via repressive chromatin modifications. However, the developmental stage at which nucleolar dominance is established in A. suecica is currently unknown. We show that nucleolar dominance is not apparent in seedling cotyledons formed during embryogenesis but becomes progressively established during early postembryonic development in tissues derived from both the shoot and root apical meristems. The progressive silencing of A. thaliana rRNA genes correlates with the transition of A. thaliana NORs from a decondensed euchromatic state associated with histone H3 that is trimethylated on lysine 4 (H3K4me3) to a highly condensed heterochromatic state in which the NORs are associated with H3K9me2 and 5-methylcytosine-enriched chromocenters. In RNAi-lines in which the histone deacetylases HDA6 and HDT1 are knocked down, the developmentally regulated condensation and inactivation of A. thaliana NORs is disrupted. Collectively, these data demonstrate that HDA6 and HDT1 function in the postembryonic establishment of nucleolar dominance, a process which recurs in each generation.
... Silencing of NORs in triticale is maintained during the development of the sporophyte and is then reprogrammed during meiosis. [8]. ...
The functional importance of Epigenetics arise from DNA sequencing programs that show the need for another code to explain
the dynamics of gene expression patterns observed along cell differentiation and organism development. In this context, the
study of ribosomal gene silencing is in fact an excellent model to better understand the relationships that are established
between gene transcription and chromatin topology, and to unravel the epigenetic switches evolved in the framework of gene
expression.
... Studies of floral homeotic mutants (Finnegan et al., 1996; Ronemus et al., 1996) suggest a direct correlation between DNA methylation and normal regulation of developmentally important genes (Jacobsen and Meyerowitz, 1997). Some methylation patterns change during plant development, particularly through meiosis (Silva et al., 1995) and embryogenesis (Castilho et al., 1999). Reduced methylation of tandem DNA repeats in tobacco is maintained during protoplasting and plant regeneration (Bezdek et al., 1991; Koukalova et al., 1994). ...
In the present review, the comprehensive picture of vertebrate genome organization that have been obtained over the last 35 years is used to discuss relationships between genome organization and cell function compartmentalization in angiosperms. Variations in genome organization are described with reference to the base composition of coding and non-coding sequences. The genome phenotype resulting from the compositional genome organization constitutes a pattern whose code varies at the taxonomic level corresponding to the family or above. Genus and species do not generally exhibit strong qualitative difference of genome organization. In that respect, genome phenotype is a tool that may be used to investigate species adaptation and biodiversity at another level than genes since it affect patterns
instead of traits. Complexity with concern to gene number, retrotransposon, noncoding DNA, molecular basis of genome plasticity, genome size, isochores, compositional patterns, compositional correlations and compositional transitions are quickly reviewed. Finally, the functional implications of the two classes of genes for plant genome evolution are discussed.
... Evaluation of the nucleolar dominance process in plant hybrids also illustrates the characteristic reprogramming of het patterns associated with meiosis. In the wheat-rye system, the under-dominant rye NORs are active in pollen grains following meiotic reprogramming ( Silva et al., 1995), and Pikaard and co-workers demonstrated in Brassica hybrids that silent NORs become active in the transition to the floral meristem (review in Pikaard, 2000). Also, the largest nucleoli are formed in wheat haploid microspores (Martini and Flavell, 1985). ...
The aim of this review is to integrate earlier results and recent findings to present the current state-of-the-art vision concerning the dynamic behavior of the ribosomal DNA (rDNA) fraction in plants. The global organization and behavioral features of rDNA make it a most useful system to analyse the relationship between chromatin topology and gene expression patterns. Correlations between several heterochromatin fractions and rDNA arrays demonstrate the heterochromatic nature of the rDNA and reveal the importance of the genomic environment and of developmental controls in modulating its dynamics.
