Position Of Nucleolus Organizer Regions Research Articles

The Case of X and Y Localization of Nucleolus Organizer Regions (NORs) in Tragulus javanicus (Cetartiodactyla, Mammalia).

There are differences in number and localization of nucleolus organizer regions (NORs) in genomes. In mammalian genomes, NORs are located on autosomes, which are often situated on short arms of acrocentric chromosomes and more rarely in telomeric, pericentromeric, or interstitial regions. In this work, we report the unique case of active NORs located on gonоsomes of a eutherian mammal, the Javan mouse-deer (Tragulus javanicus). We have investigated the position of NORs by FISH experiments with ribosomal DNA (rDNA) sequences (18S, 5.8S, and 28S) and show the presence of a single NOR site on the X and Y chromosomes. The NOR is localized interstitially on the p-arm of the X chromosome in close proximity with prominent C-positive heterochromatin blocks and in the pericentromeric area of mostly heterochromatic Y. The NOR sites are active on both the X and Y chromosomes in the studied individual and surrounded by GC enriched heterochromatin. We hypothesize that the surrounding heterochromatin might have played a role in the transfer of NORs from autosomes to sex chromosomes during the karyotype evolution of the Javan mouse-deer.

Cytogenetic Studies on Gonatodes (Reptilia, Squamata, Sphaerodactylidae)

Mitotic and meiotic chromosomes of 5 species of the reptile genus Gonatodes are described by means of conventional staining, banding analyses and in situ hybridization using a synthetic telomeric DNA probe. The amount, location and fluorochrome affinities of constitutive heterochromatin, the number and positions of nucleolus organizer regions, and the patterns of telomeric DNA sequences were determined for most of the species. The karyotypes of G. falconensis and G. taniae from northern Venezuela are distinguished by their extraordinarily reduced diploid chromosome number of 2n = 16, which is the lowest value found so far in reptiles. In contrast to most other reptiles, both species have exclusively large biarmed (meta- and submetacentric) chromosomes. Comparison of the karyotypes of G. falconensis and G. taniae with those of other Gonatodes species indicates that the exceptional 2n = 16 karyotype originated by a series of 8 centric fusions. The karyotypes of G. falconensis and G. taniae are further characterized by the presence of considerable amounts of (TTAGGG)_n telomeric sequences in the centromeric regions of all chromosomes. These are probably not only relics of the centric fusion events, but a component of the highly repetitive DNA in the constitutive heterochromatin of the chromosomes. The genome sizes of 4 Gonatodes species were determined using flow cytometry. For comparative purposes, all previously published cytogenetic data on Gonatodes and other sphaerodactylids are included and discussed.

Chromosome Banding in Amphibia. XXXI. The Neotropical Anuran Families Centrolenidae and Allophrynidae

The mitotic chromosomes of 11 species from the anuran families Centrolenidae and Allophrynidae were analyzed by means of conventional staining, banding techniques, and in situ hybridization. The amount, location, and fluorochrome affinities of constitutive heterochromatin, the number and positions of nucleolus organizer regions, and the patterns of telomeric DNA sequences were determined for most of the species. The karyotypes were found to be highly conserved with a low diploid chromosome number of 2n = 20 and morphologically similar chromosomes. The sister group relationship between the Centrolenidae and Allophrynidae (unranked taxon Allocentroleniae) is clearly corroborated by the cytogenetic data. The existence of heteromorphic XY♂/XX♀ sex chromosomes in an initial stage of morphological differentiation was confirmed in Vitreorana antisthenesi. The genome sizes of 4 centrolenid species were determined using flow cytometry. For completeness and for comparative purposes, all previously published cytogenetic data on centrolenids are included.

Cytogenetic analysis of three sea catfish species (Teleostei, Siluriformes, Ariidae) with the first report of Ag-NOR in this fish family

Despite their ecological and economical importance, fishes of the family Ariidae are still genetically and cytogenetically poorly studied. Among the 133 known species of ariids, only eight have been karyotyped. Cytogenetic analyses performed on Genidens barbus and Sciades herzbergii revealed that both species have 2n = 56 chromosomes and Cathorops aff. mapale has 2n = 52 chromosomes: Genidens barbus has 10 Metacentrics (M), 14 Submetacentrics (SM), 26 Subtelocentrics (ST), and 6 Acrocentrics (A), Sciades herzbergii has 14M, 20SM, 18ST and 4A, whereas Cathorops aff. mapale has 14M, 20SM, and 18ST. The nucleolus organizer regions (NORs) were found in a single chromosome pair on the short arm of a large-sized ST pair in Genidens barbus and on the short arm of a middle-size SM pair in Cathorops aff. mapale. Multiple NORs on the short arms of two large-sized ST pairs were found in Sciades herzbergii. The occurrence of diploid numbers ranging from 2n = 52 through 56 chromosomes and the presence of different karyotypic compositions, besides the number and position of NORs suggest that several numeric and structural chromosome rearrangements were fixed during the evolutionary history of this fish family.

Positioning of NORs and NOR-bearing chromosomes in relation to nucleoli

It is widely accepted that chromosomes occupy more or less fixed positions in mammalian interphase nucleus. However, relation between large-scale order of chromosome positioning and gene activity remains unclear. We used the model of the human ribosomal genes to address specific aspects of this problem. Ribosomal genes are organized at particular chromosomal sites in clusters termed nucleolus organizer regions (NORs). Only some NORs, called competent are generally accepted to be transcriptionally active during interphase. Importantly in this respect, the regularities in distribution of competent, and non-competent NORs among the specific chromosomes were already established in two human-derived cell lines: transformed HeLa and primary LEP cells. In the present study, using FISH and immunocytochemistry, we found that in HeLa and LEP cells the large-scale positioning of the NOR-bearing chromosomes with regard to nucleoli is linked to the transcription activity of rDNA. Namely, the tendency of rDNA-bearing chromosomes to associate with nucleoli correlates with the number of transcriptionally competent NORs in the respective chromosome homologs. Regarding the position of NORs, we found that not only competent but also most of the non-competent NORs are included in the nucleoli. Some intranucleolar NORs (supposedly non-competent) are situated on elongated chromatin protrusions connecting nucleoli with respective chromosome territories spatially distanced from nucleoli.

Cytogenetic analysis of species of the genera Acestrorhynchus, Oligosarcus and Rhaphiodon (Teleostei: Characiformes)

I nthe present study the karyotypes and the number and position of nucleolus organizer regions (NORs) of four species of Characiformes are described. The analyses showed that Acestrorhynchus lacustris had 2n=50 chro- mosomes (8M+34SM+6ST+2A), Oligosarcus longirostris had 2n=50 chromosomes (2M+20SM+10ST+18A), Oli- gosarcus cf. paranensis from Keller and Mouraorivers had 2n=50 chromosomes (2M+26SM+8ST+14A), and Rhaphi- odon vulpinus had 2n=54 chromosomes (32M+12SM+8ST+2A). The number of NOR-bearing chromosome pairs ranged from one to three among the species studied. The results available show that there is no variation in the dip- loid number within the genera analyzed. However, clear differences related to the karyotypic formulae and the number and position of Ag-NORs were found even in the comparison between populations. The possible relation- ships between the genera analyzed and other Characiformes are discussed.

Localization of rRNA genes in Phyllostomidae bats reveals silent NORs in Artibeus cinereus.

We analyzed the nucleolus organizer regions (NORs) of thirteen bats from genera Phyllostomus, Phylloderma, Trachops, Tonatia, Sturnira, Platyrrhinus, Artibeus and Glossophaga. We used silver staining and FISH with rDNA probe. Nine species had only one Ag-NOR-bearing chromosome pair. Artibeus lituratus, A. jamaicensis and A. fimbriatus presented multiple Ag-NORs located in the short arms of pairs 5, 6 and 7, and an additional mark in the long arm of one chromosome 5 in A. fimbriatus. Artibeus cinereus showed Ag-NORs in the chromosome pairs 10 and 13. The chromosomal location of rRNA genes using FISH agreed with the number and position of NORs in all but one species. In A. cinereus the hybridization signals were seen in three chromosome pairs 9, 10 and 13. This suggests the occurrence of silent NORs in pair 9. Differences in the size and intensity of the hybridization signals were also observed in the pair 9 of A. cinereus.

Cytogenetics of the genus Leporinus (Pisces, Anostomidae). 1. Karyotype analysis, heterochromatin distribution and sex chromosomes.

Cytogenetic analyses (Giemsa staining, C-banding, AgNO3 labelling of nucleolus organizer regions (NORs) and staining with base-specific fluorochromes) were performed on the South American fish species Leporinus friderici, L. obtusidens and L. elongatus. The overall karyotypic structure, position of NORs, as well as the amount, distribution and composition of constitutive heterochromatin were determined. Particular attention was given to the highly differentiated ZZ/ZW sex chromosome system of L. obtusidens and L. elongatus. Sharing the apparently ancient macroscopic karyotype of Anostomidae, all three species have 2n = 54 meta- or submetacentric chromosomes. NORs were found exclusively on chromosome pair 2, which may represent the ancestral NOR-bearing chromosome of the anostomid karyotype. Observed differences in the relative position of NORs along chromosome 2 and variations in the amount and distribution of constitutive heterochromatin throughout the karyotype were most probably caused by heterochromatin-mediated chromosome rearrangements. Detailed analysis of the morphologically similar heteromorphic ZZ/ZW sex chromosomes of L. obtusidens and L. elongatus allowed detection of differences in the DNA composition of the largely heterochromatic W chromosomes. However, since these and the W chromosomes of three other Leporinus species exhibit homologies with respect to their relative size, centromere position and amount and distribution of heterochromatin, it is concluded that they evolved from the same ancestral W chromosome.

Chromosomal investigation of three Costa Rican frogs from the 30-chromosome radiation of Hyla with the description of a unique geographic variation in nucleolus organizer regions

A cytogenetic investigation of Hyla ebraccata Cope, H. microcephala Cope, and H. phlebodes Stejneger revealed that the karyotypes of these 30-chromosome Hyla are very conservative. With the exception of some structural rearrangements, only few differences in chromosomal morphology could be discerned. Based on our results, we hypothesize that the telomeric position of nucleolus organizer regions (NOR) on chromosome no. 10 may represent a derived condition in 30-chromosome Hyla. This cytotype was found only in the Caribbean population of H. ebraccata, Such within-species disparity has not been observed previously among amphibians. This phenomenon can most readily be explained by a translocation or insertion that rapidly drifted to high frequency in a small population.

Mapping of the nucleolus organizer region on chromosome 4 in Arabidopsis thaliana.

In Arabidopsis thaliana the ribosomal RNA genes (rRNA genes or rDNA) are clustered in tandemly repeated blocks in two nucleolus organizer regions (NORs). Cytogenetic analysis has shown that the NORs are localized on chromosome 2 (NOR 2) and 4 (NOR 4). Recently the map position of NOR 2 was determined using a RFLP which was larger than 100 kb. In the course of a fingerprint analysis of different Arabidopsis ecotypes we have detected four rDNA polymorphisms between the ecotypes Landsberg (La) and Niederzenz (Nd). Mapping of these polymorphisms using established segregating F2 populations reveals that all polymorphisms detected are dominant. Three of them map to the locus on the second chromosome that has been shown to harbour the NOR 2. The fourth polymorphism can be unambiguously assigned to the upper arm of the fourth chromosome. This is the first polymorphism found which originates in the second rDNA cluster of Arabidopsis thaliana. It enables localization of NOR 4 and thus completes the mapping of rDNA genes in the NORs of Arabidopsis.

On the position of nucleolus organizer regions (NORs) in interphase nuclei: Studies with a new, non-autoradiographic in situ hybridization method

The distribution of 18S and 28S ribosomal RNA (rRNA), i.e. the chromosomal nucleolus organizer regions (NORs) was visualized in interphases and metaphases of non-stimulated and phytohemagglutinin (PHA)-stimulated human lymphocytes with a recently developed non-autoradiographic in situ hybridization method. This procedure involves mercurated RNA as a probe and a sulfhydryl-trinitrophenyl-mercury binding ligand and FITC-labelled antibodies as detection system. Silver staining was used to visualize nucleoli in interphase. In the secondary constriction of all ten acrocentric chromosomes, varying amounts of rDNA were detected. In the interphase nuclei of most of the non-stimulated human lymphocytes, only one small nucleolus could be seen. The in situ hybridization, however, revealed several agglomerations of rDNA scattered over the whole nuclear area, clearly outnumbering the number of nucleoli in these cells. This means that not all of the NORs are transcriptionally active in non-stimulated lymphocytes and that these inactive NORs lie at a distinct distance from the actives ones. With PHA stimulation (transforming the small lymphocytes from peripheral blood into large, lymphoblast-like cells) the number of nucleoli increased slightly, whereas the number of separable rDNA spots decreased. This means that in the course of PHA-induced cellular activation, formerly inactive NORs become transcriptionally active and tend to associate with one another. This indicates the occurrence of movements of the NORs within the nucleus, depending on their transcriptional activity.

Chromosome heteromorphisms in the Japanese. II. Nucleolus organizer regions of variant chromosomes in D and G groups.

The nucleolus organizer regions (NORs) of variant D- and G-group chromosomes characterized by enlargements of the short arms including secondary constrictions and satellites, were examined using the silver-staining method. Of a total of nine variants examined, four were found to have double Ag-stained NORs in the enlarged short arm, two were found to involve chromosome 22, one was a 13, and one, a 14. Four of the other variants had only one AG-stained NOR. From the positions of the NORs, three of them were judged to have enlarged satellites (two chromosomes 15 and 22) and the other an enlarged short arm (a 15). In the remaining variant (a 14), no Ag-stained material was noted in the short arm, so it could not be determined whether this variant chromosome was derived from the enlargement of the short arm or from satellites. Based on the position of the Ag-stained NORs and staining intensity of the Q and C methods in the short arms, mechanisms of producing the enlarged short arms of D- and G-group chromosomes are discussed.