NOR Chromosome Research Articles

Relative distribution of rDNA and proteins of the RNA polymerase I transcription machinery at chromosomal NORs

Relative distribution of rDNA and proteins of the RNA polymerase I transcription machinery at chromosomal NORs

Extensive nucleolus organizer region polymorphism in Gymnotus carapo (Gymnotoidei, Gymnotidae).

An investigation was conducted on six populations of Gymnotus carapo from the Upper Parana river system in Brazil. A wide variability of the NOR-bearing chromosomes was found involving paracentric inversions in the heterochromatic regions associated with NORs, resulting in 5 NOR chromosome phenotypes. The distribution of the distinct NOR phenotypes varied both inter- and intrapopulationally. Results obtained using an exact test revealed that the six populations are in Hardy-Weinberg equilibrium. Although NOR size polymorphism is quite a common occurrence in fish karyotypes, this is the first description of a balanced polymorphism involving 3 different NOR chromosomes in 5 combinations.

NOR polymorphism in the Iberian species Chondrostoma lusitanicum (Pisces: Cyprinidae)

Chromosomal polymorphism regarding number of NOR sites in the cyprinid fish Chondrostoma lusitanicum was examined using C-banding, silver-staining (Ag), and fluorescent staining with chromomycin A3 (CMA3). The analysis of heterochromatic regions allowed a more precise identification of the centromeric regions and the proposal of a revised haploid chromosome formula (7M: 15S: 3A). We describe variability in the number of NOR regions per genome, number of active NOR sites per cell, and relative size of individual NORs. Individuals expressed two or four NOR-bearing chromosomes. Polymorphism was detected in all the populations studied and sex-related differences were not found. The observed chromosomal NOR phenotypes suggest the occurrence of structural rearrangements during the evolutionary process of this diploid leuciscine cyprinid.

A molecular analysis of the origin of the Crepis capillaris B chromosome

The origin of the B chromosome of Crepis capillaris has been studied by using in situ hybridization with different DNA probes. Genomic in situ hybridization (GISH) with DNA from plants with and without Bs as probes indicates that the B chromosome has many DNA sequences in common with A chromosomes, showing no region rich in B-specific sequences. Six additional DNA probes were used to test the possible origin of this B from the standard NOR chromosome (chromosome 3). In the short arm of the NOR chromosome, we detected not only 18 S + 25 S rDNA, but also 5 S rDNA and a specific repetitive sequence from the NOR chromosome (pCcH32); in the heterochromatic bands of the long arm, we found two different repetitive sequences (pCcE9 and pCcD29). In the B chromosome, however, only the 18 S + 25 S rDNA and the telomeric sequences from Arabidopsis thaliana were observed. Our in situ hybridization data with telomeric repeats indicate that the two telomeres of the B are larger than those of the A chromosomes, confirming the isochromosomal nature of this B. Hybridizations of 18 S + 25 S rDNA and telomeric repeats to blots of DNA from plants with and without Bs reveal a high homology between A and B 18 S + 25 S rDNA genes, but some sequence dissimilarities between A and B telomeres. Taken as a whole, these data indicate that the entire B of C. capillaris, although possibly having originated from the standard genome, did not derive directly from the NOR chromosome.

Prometaphase chromosomes of the howler monkey (Aloutta caraya): G, C, NOR, and restriction enzyme (RES) banding.

Prometaphase lymphocyte chromosomes from eight adult argentinian Alouatta caraya females were characterized using sequential G-C banding techniques, Ag-NOR bands and bands obtained with the restriction enzymes Hae III, Eco RI, Alu I and Sau 3A. The cytogenetic analysis showed 2n = 52, with four, five, or six NOR chromosomes. Digestion with Hae III and Eco RI produced G-like-bands. Centromere regions and two interstitial C-bands (in chromosomes number 16 and 21) showed intraindividual or interindividual heterochromatic polymorphisms. Alu I digestion produced C-like bands with gaps in the centromere regions, and Sau 3A produced C-like bands. The karyotypes and banding patterns of A. caraya, A. palliata, A. belzebul, and A. seniculus are compared, based on whole chromosome and whole arm homeologies. © 1994 Wiley-Liss, Inc.

The RNA polymerase I-specific transcription initiation factor UBF is associated with transcriptionally active and inactive ribosomal genes

We have characterized an anti-NOR (nucleolar organizer region) serum (P419) from a patient with rheumatoid arthritis and show that it contains antibodies directed against the RNA polymerase I-specific transcription initiation factor UBF. This serum reacts with UBF from a variety of vertebrate cells as revealed both by immunoblotting and by indirect immunofluorescence. We have used the P419 serum to study the intracellular localization of this transcription factor at the light and electron microscopic level. In interphase cells, UBF exhibits a pronounced punctate pattern and is found to be associated with necklace-like structures, which appear to reflect the transcriptionally active state of the nucleolus. Inhibition of rRNA synthetic activity caused either by nutritional starvation or by actinomycin D treatment resulted in a marked decrease in the number and in a significant increase in the size of UBF-positive granules. Under all experimental conditions applied, UBF was exclusively found within the nucleolus and was not released into the nucleoplasm or cytoplasm. During mitosis, UBF was found to be concentrated at the chromosomal NOR indicating that a significant quantity, if not all, of this factor remains bound to the ribosomal transcription units. From this we conclude that UBF is associated both with transcriptionally active and inactive rRNA genes and, therefore, changes in the intracellular localization of UBF are very likely not involved in rDNA transcription regulation.

Karyotype, C-banding, and Ag-NOR analysis in Diplodus bellottii (Sparidae, Perciforms). Intra-individual polymorphism involving heterochromatic regions

A karyotype analysis was carried out in nine specimens of the Sparid species Diplodus bellottii using conventional staining, as well as C-banding and Ag-NOR banding techniques, showing, respectively, 2n = 46 and fundamental number (FN) = 54, and scarce heterochromatic areas irregularly distributed and up to four NOR active regions that were C positive. When compared with the karyotypes of other related species, one centric fusion giving rise to a large metacentric pair and several pericentric inversions seem to have been involved in the karyotype evolution. An intra-individual polymorphism was detected in one specimen, resulting in two karyotypic forms in roughly identical proportion, owing to a larger C-band by the NOR regions, appearing either in a terminal position of the short arms of pair 2 or in telomeric position of pair 3. These findings suggest that the extra heterochromatic segment responsible for the heteromorphism apparently only involves associated heterochromatin and not the NORs themselves. This C-positive block seems to have eventually been transferred between heterologous NOR chromosomes by a somatic event, facilitated by the physical proximity of NOR pairs in the nucleolus.

Multi-chromosomal location of ribosomal RNA genes and heterochromatin association in brown trout.

The ribosomal rRNA genes have been mapped by fluorescent in situ hybridization (FISH) to brown trout chromosomes. One major NOR chromosome pair and 8 novel minor NOR chromosome pairs have been found. Both major and minor NORs were closely related to polymorphic heterochromatin, as revealed by FISH and C-banding. These results are discussed with respect to NOR expression, the relationship between rDNA and heterochromatin, and evolutionary aspects.

Characterization of nucleolar organizer regions of twelve species of Chinese cyprinid fishes

SUMMARYSilver staining technique was used to study the chromosomal NORs of 12 species of Chinese cyprinids. The NOR number varies from 2 to 7. Only a single pair of NOR-bearing chromosomes was detected in 5 species respectively. The remaining species each possess two or more pairs of NOR-bearing homologues. NORs may be located at submetacentric, subtelocentric or metacentric chromosomes. NOR heteromorphism was common. Finally, it is suggested, in the discussion, that further studies should be made on the overall Cyprinidae.

Frequency analyses of active NORs in nuclei of artificially induced triploid fishes

The correspondence between increased numbers of both chromosomal and nuclear NORs and artificially induced triploidy in three fish species (rainbow trout, Oncorhynchus mykiss; common carp, Cyprinus carpio; and tench, Tinea tinea) has been confirmed by CMA3 fluorescence and Ag-staining. The frequencies of cell nuclei with one, two and three active NORs, as revealed by Ag-staining, has been analyzed statistically to find the minimum cell number which verifies the increased ploidy level. A minimum sample size of about 80 cells exhibiting three active NORs is sufficient to confirm triploidy in all three species and may be of use for categorising other ploidy-manipulated fish species.

Chromosomal nor Karyotypes and Genome Sizes in Dionda (Osteichthyes: Cyprinidae) from Texas and New Mexico

Chromosome numbers, chromosomal nucleolar organizer egion (NOR) phenotypes, and genome sizes (DNA contents) were documented for all nominal members of the North American cyprinid genus Dionda inhabiting Texas and New Mexico. All examined taxa, including four members of the Dionda episcopa complex (D. episcopa, D. argentosa, D. serena, and an undescribed species inhabiting the Colorado and Guadalupe river drainages in Texas) and D. diaboli, possessed 2n = 50 chromosomes, with a single pair of NOR-bearing chromosomes. Ineach, the chromosomal NOR was located terminally on the short arm of a small-sized acrocentric chromosome. All of the NOR chromosomes appeared to be homologous in trypsin G-banding pattern. Mean genome sizes ranged from 1.94 picograms (pg) of DNA in D. serena from the Nueces River to 2.09 pg of DNA in D. argentosa from Baker's Crossing in the Devils River. Divergence in genome size among nearly all of the samples of Dionda was less than that maximally observed, on average, between individuals within populations of most North American fishes. Collectively, the chromosomal NOR and genome size data suggest that the taxonomic status of Dionda inhabiting Texas and New Mexico may warrant further inves-

Different patterns of rDNA organization at interphase in nuclei of wheat and rye.

The physical location of the rDNA repeating units (25 S, 18 S and 5.8 S rRNA genes and the intergenic spacer sequences) was investigated in rye (Secale cereale L.) and wheat (Triticum aestivum L.) root tip meristematic cells by in situ hybridization using light and electron microscopy. The rDNA sequences are organized differently in the two related and intercrossable species. In rye (2n = 14, one pair of chromosomes with nucleolar organizing regions, NORs), two condensed blocks of rDNA-containing chromatin occurred in each interphase nucleus. The blocks were associated with the periphery of nucleoli and a single-labelled, decondensed rDNA fibre extended into the nucleolus from the block. We term this expression pattern terminal decondensation. In wheat (2n = 6x = 42, five pairs of chromosomes with NORs), inactive condensed labelled chromatin was found unassociated with nucleoli. Active NORs had some condensed rDNA associated with the nucleolar periphery, but, in contrast to rye, condensed rDNA was also found within the nucleolus. The condensed labelled rDNA in wheat nucleoli was visible as fluorescent foci in the light microscope and labelled condensed chromatin in the electron microscope. Its absence in rye shows that condensed rDNA need not be present in active plant nucleoli. Diffuse labelled sites of rDNA, likely to represent actively transcribed rDNA, were found in both rye and wheat. Active rDNA loci in wheat have many expressed segments separated by unexpressed, condensed, rDNA-fragmented decondensation-while each locus in rye has a single, unexpressed perinucleolar condensed block of rRNA genes. Thus the positions of actively transcribed genes within the tandem arrays of rDNA at each locus are fundamentally different in the two cereals. The NOR chromosome appeared to extend through the nucleolus, and active rDNA sequences did not loop out from chromatin into the nucleolus as is frequently described in nucleolar models.

Cytogenetic Studies in North American Minnows (Cyprinidae). XXVI. Chromosomal NOR phenotypes of 21 species from the Western United States.

Karyotypes and chromosomal nucleolus organizer region or NOR phenotypes of 19 described and two undescribed species of western North American cyprinids are documented. All 21 species examined possessed 2N=50 chromosomes and at least two pair of NOR-bearing chromosomes. The high incidence of multiple NOR chromosomes among western cyprinids is unusual. One pair of A NOR chromosomes was found in all 21 species, and 17 species possessed a second pair of A NOR chromosomes. An AA NOR state was hypothesized to represent a chromosomal synapomorphy uniting western cyprinids (including the genus Richardsonius) into a monophyletic assemblage. Within this clade, a transition from an A NOR to a D NOR was hypothesized to represent a chromosomal synapomorphy uniting three speices of the subgenus Gila, and an addition of an F NOR was hypothesized to represent a chromosomal synapomorphy uniting two species of the genus Rhinichthys. Chromosomal NORs in western cyprinids appear conserved relative to eastern cyprinids and may be the result of (i) a process of karyotypic canalization, and/or (ii) character state reversal in, or introgressive hybridization among, western cyprinid genera.

Intraspecific Variation in Chromosomal Nucleolus Organizer Regions in Notropis chrysocephalus (Pisces: Cyprinidae)

ganizer regions (NORs) in North American cyprinid fishes and using the data to infer hypotheses of species or species-groups relationships (Gold and Amemiya, 1986; Amemiya and Gold, 1988; Gold et al., 1988). Thus far, we have surveyed chromosomal NOR phenotypes of nearly 50 cyprinid species (Amemiya and Gold, 1988; Gold et al., 1988; J. R. Gold, pers. obser.), the majority of which belong to the extremely speciose genus Notropis. The NORs themselves are the chromosomal sites for the 18S and 28S ribosomal

Chromosomal NOR Phenotypes of Seven Species of North American Cyprinidae, with Comments on Cytosystematic Relationships of the Notropis volucellus Species-Group, Opsopoeodus emiliae, and the Genus Pteronotropis

in the fifth (P. hubbsi), the single NOR was located terminally on the short arm of a large-sized acro-/subtelocentric chromosome. Pteronotropis signipinnis and P. welaka each possessed two pair of NOR-bearing chromosomes: one pair had a NOR located terminally on the short arm of a medium-sized submetacentric chromosome; whereas the other pair had a NOR located terminally on the short arm of a large-sized acro-/subtelocentric chromosome. Consideration of these and previous NOR data in North American cyprinids suggest that: 1) the N. volucellus species-group as presently constituted may not be monophyletic; 2) the species Opsopoeodus emiliae may belong in an assemblage which includes, among others, at least four species of the genus Cyprinella; 3) 0. emiliae may not be closely related to N. maculatus; and 4) P. hubbsi may belong in an assemblage which includes P. signipinnis and P. welaka, and hence may be a valid member of the genus Pteronotropis.

Cytogenetic studies in North American minnows(Cyprinidae). XVIII. Chromosomal NOR variation among eight species.

The karyotypes and chromosomal nucleolus organizer region or NOR phenotypes of eight species of North American cyprinid fishes are documented. All eight species possessed 2n=50 chromosomes. Four species (Cyprinella formosa, Cyprinella gibbsi, Notropis boops, and Notropis dorsalis) possessed only a single pair of NOR-bearing chromosomes, whereas four species (Notropis petersoni, Notropis sabinae, Phenacobius mirabilis, and Semotilus atromaculatus) possessed multiple NORs, i. e., more than one pair. The phylogenetic implications of the chromosomal NOR data in the eight species are discussed relative to current concepts of North American cyprinid systematics based largely on morphological data. A phylogenetic hypothesis based on chromosomal NOR and other data is presented for 11 species of the cyprinid genus Cyprinella, and our previous hypothesis that a single pair of NOR chromosomes is the plesiomorphic NOR state for a large eastern North American cyprinid lineage is questioned.

Three-dimensional chromosome arrangement of Crepis capillaris in mitotic prophase and anaphase as studied by confocal scanning laser microscopy.

To estimate the extent of ordering of chromosomes, confocal scanning laser microscopy was used to make three-dimensional images from optical sections. For Crepis capillaris, which has 2n = 6 easily recognizable chromosomes, a statistically significant sample of 75 Feulgen-stained root tip anaphases was analysed. A comparison of the observed chromosome ordering and the expected random distribution showed a significant surplus of one of the arrangements with a juxtaposition of the two chromosomes with a nucleolus organizer region. Two of the arrangements with these chromosomes in opposite positions were never observed in our material. Another analysis of 30 mithramycin A-stained prophases and 30 meta- and anaphases showed partly different patterns of non-random chromosome distribution in the two stages of mitosis. A preference for an association of the homologues was observed for all pairs of chromosomes in prophase cells, whereas in meta- and anaphase the association only persisted for the nucleolus organizer chromosomes. This indicates that there may be some relocation of the chromosome positions during the transition from prophase to metaphase. In meta- and anaphase one of the arrangements with juxtaposed NOR chromosomes was preferred, i.e. the ordering in which chromosomes 1 and 3 occupied alternate positions. Probably, the nucleolus is an important factor in producing a non-random distribution, but there could be other factors that influence chromosome ordering as well. A comparison of the anaphase chromosome ordering in C. capillaris plants from very different localities, indicated that the observed non-random distribution was independent of the origin of the material. Existing models of chromosome disposition are not sufficient to explain the observed non-random chromosome ordering in C. capillaris.

A comparison of within-plant karyological heterogeneity between agamospermous and sexualTaraxacum (Compositae) as assessed by the nucleolar organiser chromosome

Morphological variation for the NOR chromosome was studied for four half-siblings of a sexual outbreedingTaraxacum, for three siblings of the obligate agamospermT. pseudohamatum, and for two individuals of the agamospermT. brachyglossum. No rearrangement was detected for the 113 chromosomes of sexuals, or for 41 chromosomes of two agamospermous individuals. In the other three agamospermous individuals, 3/16, 5/50, and 5/20 chromosomes showed evidence of chromosomal rearrangement. The majority of rearrangement events (10/13) occurred to the satellite rather than to the body of the NOR-chromosome. It is considered that such high levels of somatic chromosomal rearrangement in agamospermousTaraxacum may be the result of activity by transposable genetic elements. This recombination may be of selective advantage to asexual plants which cannot generate genetic variability through the sexual process.

Chromosomal NORs as taxonomic and systematic characters in North American cyprinid fishes

There is substantial interspecific variation in chromosomal NORs among North American cyprinid fishes. This variation is taxonomically informative. In addition, the chromosomal NOR database appears to provide a framework from which systematic or phylogenetic inferences can be made. Our present approach to making such inferences from chromosomal NOR data is discussed.

An extended chicken karyotype, including the NOR chromosome.

Chicken chromosomes were identified up to No. 18 by a sequential counterstain-enhanced fluorescence technique. A heterochromatin characterization of macro- and microchromosomes was performed; in general, the microchromosomes were GC-rich, but with a high degree of variation. The NORs are localized on chromosome No. 17.