Ribosomal DNA Repeat Research Articles

Eriobotrya Belongs to Rhaphiolepis (Maleae, Rosaceae): Evidence From Chloroplast Genome and Nuclear Ribosomal DNA Data.

The Eriobotrya-Rhaphiolepis (ER) clade consists of about 46 species distributed in East and Southeast Asia. Although Eriobotrya and Rhaphiolepis have been supported to form a clade, the monophyly of Eriobotrya and Rhaphiolepis at the genus level has never been well tested and their phylogenetic positions in Maleae still remain uncertain. This study aims to reconstruct a robust phylogeny of the ER clade in the framework of Maleae with a broad taxon sampling and clarify the phylogenetic relationship between Eriobotrya and Rhaphiolepis. This study employed sequences of the whole plastome (WP) and entire nuclear ribosomal DNA (nrDNA) repeats assembled from the genome skimming approach and included 83 samples representing 76 species in 32 genera of Rosaceae, especially Maleae. The Maximum Likelihood (ML) and Bayesian Analysis (BI) based on three datasets, i.e., WP, coding sequences of plastome (CDS), and nrDNA, strongly supported the paraphyly of Eriobotrya, within which Rhaphiolepis was nested. Our plastid tree supported the sister relationship between the ER clade and Heteromeles, and the nrDNA tree, however, did not resolve the phylogenetic placement of the ER clade in Maleae. Strong incongruence between the plastid and the nuclear trees is most likely explained by hybridization events, which may have played an important role in the evolutionary history of the ER clade. Molecular, morphological, and geographic evidence all supports the merge of Eriobotrya with Rhaphiolepis, which has the nomenclatural priority. We herein transferred 36 taxa of Eriobotrya to Rhaphiolepis. We also proposed a new name, Rhaphiolepis loquata B.B.Liu & J.Wen, for the economically important loquat, as the specific epithet “japonica” was pre-occupied in Rhaphiolepis.

Treacle controls the nucleolar response to rDNA breaks via TOPBP1 recruitment and ATR activation.

Induction of DNA double-strand breaks (DSBs) in ribosomal DNA (rDNA) repeats is associated with ATM-dependent repression of ribosomal RNA synthesis and large-scale reorganization of nucleolar architecture, but the signaling events that regulate these responses are largely elusive. Here we show that the nucleolar response to rDNA breaks is dependent on both ATM and ATR activity. We further demonstrate that ATM- and NBS1-dependent recruitment of TOPBP1 in the nucleoli is required for inhibition of ribosomal RNA synthesis and nucleolar segregation in response to rDNA breaks. Mechanistically, TOPBP1 recruitment is mediated by phosphorylation-dependent interactions between three of its BRCT domains and conserved phosphorylated Ser/Thr residues at the C-terminus of the nucleolar phosphoprotein Treacle. Our data thus reveal an important cooperation between TOPBP1 and Treacle in the signaling cascade that triggers transcriptional inhibition and nucleolar segregation in response to rDNA breaks.

Native Chromatin Proteomics Reveals a Role for Specific Nucleoporins in Heterochromatin Organization and Maintenance.

Spatially and functionally distinct domains of heterochromatin and euchromatin play important roles in the maintenance of chromosome stability and regulation of gene expression, but a comprehensive knowledge of their composition is lacking. Here, wedevelop a strategy for the isolation of native Schizosaccharomyces pombe heterochromatin and euchromatin fragments and analyze their composition by using quantitative mass spectrometry. The shared and euchromatin-specific proteomes contain proteins involved in DNA and chromatin metabolism and in transcription, respectively. The heterochromatin-specific proteome includes all proteins with known roles in heterochromatin formation and, in addition, is enriched for subsets of nucleoporins and inner nuclear membrane (INM) proteins, which associate with different chromatin domains. While the INM proteins are required for the integrity of the nucleolus, containing ribosomal DNA repeats, the nucleoporins are required for aggregation of heterochromatic foci and epigenetic inheritance. The results provide a comprehensive picture of heterochromatin-associated proteins and suggest a role for specific nucleoporins in heterochromatin function.

Phylogenomic analyses of thePhotiniacomplex support the recognition of a new genusPhippsiomelesand the resurrection of a redefinedStranvaesiain Maleae (Rosaceae)

AbstractPhotiniaand its morphologically similar allies in Maleae (Rosaceae) consist of five currently recognized genera:Aronia, Heteromeles, Photinia, Pourthiaea, andStranvaesia, and 68 species, distributed in Asia and North and Central America. Despite previous efforts to clarify relationships in this group, the generic delimitations have remained uncertain. Our goals were to reconstruct a robust phylogeny ofPhotiniaand its close allies to test the monophyly of the currently recognized genera, especiallyPhotiniaandStranvaesia, and the hybrid origin hypothesis ofPhotinia bodinieri. This study employs complete plastomes and the entire nuclear ribosomal DNA (nrDNA) repeats assembled from the genome skimming approach with a broad taxon sampling of 81 species in 30 genera of Rosaceae, especially Maleae. Based on three datasets, including the whole plastome, coding sequence, and nrDNA repeats, the results of maximum likelihood and Bayesian inference analyses showed that the previously circumscribedStranvaesiaandPhotiniawere each non‐monophyletic. Six clades have been recovered herein withinPhotiniaand its allied genera:Aronia, Heteromeles, Photinias.s.,Pourthiaea, Stranvaesia, and a new genusPhippsiomelesconsisting of the Central American species formerly placed inPhotinia. The strong conflicts between the plastome and nrDNA phylogenies ofPhippsiomelesandStranvaesia tomentosasuggest the possibility that they may have each originated involving hybridization events, while no incongruence among datasets was detected to support the hybrid origin ofPhotinia bodinieri. We provide 12 new combinations, to transfer eight taxa of the New WorldPhotiniaintoPhippsiomelesand clarify the generic placements of several species ofPhotiniaandStranvaesia.

Dynamic PML protein nucleolar associations with persistent DNA damage lesions in response to nucleolar stress and senescence-inducing stimuli

Diverse stress insults trigger interactions of PML with nucleolus, however, the function of these PML nucleolar associations (PNAs) remains unclear. Here we show that during induction of DNA damage-induced senescence in human non-cancerous cells, PML accumulates at the nucleolar periphery simultaneously with inactivation of RNA polymerase I (RNAP I) and nucleolar segregation. Using time-lapse and high-resolution microscopy, we followed the genesis, structural transitions and destiny of PNAs to show that: 1) the dynamic structural changes of the PML-nucleolar interaction are tightly associated with inactivation and reactivation of RNAP I-mediated transcription, respectively; 2) the PML-nucleolar compartment develops sequentially under stress and, upon stress termination, it culminates in either of two fates: disappearance or persistence; 3) all PNAs stages can associate with DNA damage markers; 4) the persistent, commonly long-lasting PML multi-protein nucleolar structures (PML-NDS) associate with markers of DNA damage, indicating a role of PNAs in persistent DNA damage response characteristic for senescent cells. Given the emerging evidence implicating PML in homologous recombination-directed DNA repair, we propose that PNAs contribute to sequestration and faithful repair of the highly unstable ribosomal DNA repeats, a fundamental process to maintain a precise balance between DNA repair mechanisms, with implications for genomic integrity and aging.

Phylogenetic analysis provides insights into the evolution of Asian fireflies and adult bioluminescence

Fireflies are one of the best-known examples of luminescent organisms. The limited geographic distribution and rarity of some firefly genera have hindered molecular phylogenetic analysis, resulting in uncertainty in regard to firefly phylogeny. Here, using genome skimming next-generation sequencing, we sequenced 23 Asian firefly species from 15 genera (Lampyridae: 14; Rhagophthalmidae: one) and assembled their mitochondrial genomes (mitogenomes) and nuclear ribosomal DNA (rDNA) repeat unit. The mitogenomes (including 15 mitochondrial genes: COX1-3, ATP6&8, ND1-6&4L, CYTB, 12S, and 16S) were recovered for almost all 23 species; furthermore, three regions of the nuclear rDNA repeat unit (18S, 28S, and 5.8S) were recovered for 22 out of the 23 species. The mitogenomes of 11 genera and 22 species as well as the complete rDNA from 22 species are reported here for the first time. Combined with previously published sequences of mitochondrial and rDNA coding regions, 166 species (170 populations with four overlapping in Lampyridae) were included in the current analyses. We selected different species groups and coding regions to infer phylogenies, and then employed tree certainty (TC) and internode certainty (IC) to quantify any phylogenetic incongruence. Phylogenetic analysis of 18 coding regions (15 mitochondrial genes and three regions of the nuclear rDNA repeat unit) from different species groups showed that the 144-species selection group (excluding 22 species outside Lampyridae) had relatively high TC (101.39). Further phylogenetic analysis of the 144 species using different coding regions indicated that the phylogeny of the 13 coding regions (10 mitochondrial genes: COX1-2, ATP6&8, ND1, ND4-5, CYTB, 12S and 16S; three rDNA regions: 18S, 5.8S, and 28S) demonstrated higher TC (103.02) than the phylogenies based on the 18 coding regions (TC = 101.39), conserved-regions (c-regions, i.e., 12S, 16S, COX1, 18S, and 28S) (TC = 95.11), or conserved-sites (c-sites, TC = 92.31) for the mitochondrial genes. In contrast, the c-sites strengthened the deeper nodes of the 144-species phylogeny compared to the c-regions. All of the 144-species phylogenies using different coding regions (except the c-regions) consistently recovered the monophyly of each of the three luminous families and their combination (Lampyridae, Rhagophthalmidae, and Phengodidae) with high IC support. Our phylogenetic analyses clarified the position of firefly genera Lamprigera, Vesta, Stenocladius, Pyrocoelia, Diaphanes, Abscondita, Pygoluciola, Emeia, Pristolycus, and Menghuoius. We also inferred the evolutionary pattern of adult bioluminescence in Lampyridae based on the phylogenies of 166 and 144 species. Our data suggest that the common ancestor of Lampyridae possessed adult bioluminescence, with a higher loss rate than gain rate of bioluminescence during its lineage evolution. Our results provide insight into Asian firefly phylogeny, and also enrich mitogenome and rDNA data resources for further study.

Sir2 suppresses transcription-mediated displacement of Mcm2-7 replicative helicases at the ribosomal DNA repeats.

Repetitive DNA sequences within eukaryotic heterochromatin are poorly transcribed and replicate late in S-phase. In Saccharomyces cerevisiae, the histone deacetylase Sir2 is required for both transcriptional silencing and late replication at the repetitive ribosomal DNA arrays (rDNA). Despite the widespread association between transcription and replication timing, it remains unclear how transcription might impinge on replication, or vice versa. Here we show that, when silencing of an RNA polymerase II (RNA Pol II)-transcribed non-coding RNA at the rDNA is disrupted by SIR2 deletion, RNA polymerase pushes and thereby relocalizes replicative Mcm2-7 helicases away from their loading sites to an adjacent region with low nucleosome occupancy, and this relocalization is associated with increased rDNA origin efficiency. Our results suggest a model in which two of the major defining features of heterochromatin, transcriptional silencing and late replication, are mechanistically linked through suppression of polymerase-mediated displacement of replication initiation complexes.

Chromosomal markers in the genus Karenia: Towards an understanding of the evolution of the chromosomes, life cycle patterns and phylogenetic relationships in dinoflagellates

Dinoflagellates are a group of protists whose genome is unique among eukaryotes in terms of base composition, chromosomal structure and gene expression. Even after decades of research, the structure and behavior of their amazing chromosomes—which without nucleosomes exist in a liquid crystalline state—are still poorly understood. We used flow cytometry and fluorescence in situ hybridization (FISH) to analyze the genome size of three species of the toxic dinoflagellate genus Karenia as well the organization and behavior of the chromosomes in different cell-cycle stages. FISH was also used to study the distribution patterns of ribosomal DNA (45S rDNA), telomeric and microsatellites repeats in order to develop chromosomal markers. The results revealed several novel and important features regarding dinoflagellate chromosomes during mitosis, including their telocentric behavior and radial arrangement along the nuclear envelope. Additionally, using the (AG)10 probe we identified an unusual chromosome in K. selliformis and especially in K. mikimotoi that is characterized by AG repeats along its entire length. This feature was employed to easily differentiate morphologically indistinguishable life-cycle stages. The evolutionary relationship between Karenia species is discussed with respect to differences in both DNA content and the chromosomal distribution patterns of the DNA sequences analyzed.

Comparative analysis of C-heterochromatin, ribosomal and telomeric DNA markers in chromosomes of Pamphagidae grasshoppers from Morocco.

The karyotypes and the localization of C-bands, clusters of ribosomal DNA and telomeric repeats of 10 species of the family Pamphagidae from Morocco are described for the first time. The species studied belong to the subfamilies Pamphaginae and Thrinchinae. All species have karyotypes consisting of 19 and 20 acrocentric chromosomes and X0/XX sex chromosome system in males and females, respectively (2n♂=19, NF=19; 2n♀=20, NF=20). Despite the karyotype conservatism, we revealed differences in the location and size of C-heterochromatin blocks and ribosomal DNA clusters. A comparative analysis of these differences shows that karyotype divergences in this group is connected not to structural chromosome rearrangements, but to the evolution of repetitive DNA.

Resolution of human ribosomal DNA occurs in anaphase, dependent on tankyrase 1, condensin II, and topoisomerase IIα.

Formation of individualized sister chromatids is essential for their accurate segregation. In budding yeast, while most of the genome segregates at the metaphase to anaphase transition, resolution of the ribosomal DNA (rDNA) repeats is delayed. The timing and mechanism in human cells is unknown. Here we show that resolution of human rDNA occurs in anaphase after the bulk of the genome, dependent on tankyrase 1, condensin II, and topoisomerase IIα. Defective resolution leads to rDNA bridges, rDNA damage, and aneuploidy of an rDNA-containing acrocentric chromosome. Thus, temporal regulation of rDNA segregation is conserved between yeast and man and is essential for genome integrity.

A new saxicolous Circinaria species (Megasporaceae) from northeast China

In an investigation of Circinaria species in volcanic mountains of northeast China, one new species, C. laxilobata was identified based on morphological characteristics and DNA sequence data. The new species differs morphologically from other members of the C. contorta group by its greenish olive, thin thallus with dispersed areoles and elongated, diverging, branching marginal areoles distinctly delimited from the inner part, round apothecia with entire thalline margin, moniliform paraphyses, asci with 1–2 ellipsoid spores, and the absence of secondary metabolites. Comparisons of the ITS sequences of the nuclear ribosomal DNA repeat tandem, both in phylogenetic analyses and secondary structure models of ITS2, support the taxonomic distinctness of this species. The new species is described and illustrated in detail. A key to the species of Circinaria from China is also provided.

Genotyping of Candida albicans isolated from animals using 25S ribosomal DNA and ALT repeats polymorphism in repetitive sequence.

Background and Purpose: Candida albicans is the most prevalent Candida species isolated from animals. Candidiasis can be systemic in animals or may affect a single organ, such as the mouth, urinary tract, and skin. The aim of the present study was to determine the genetic diversity of C. albicans isolated from different animals and investigate the presence of a relationship between host specificity and genetic typing of C. albicans.Materials and Methods:For the purpose of the study, DNA extraction was performed on 27 clinical isolates of C. albicans obtained from animals. Subsequently, they were subjected to 25S ribosomal DNA amplification and ALT repeats in repetitive sequences (RPSs). The minimum inhibitory concentrations of fluconazole, ketoconazole, clotrimazole, nystatin, amphotericin B, and caspofungin were determined using the microdilution method based on the Clinical and Laboratory Standards Institute M27-S4 standard.Results:Out of 27 C. albicans strains, 11, 6, 5, and 5 cases were recognized as genotypes A (40.8%), E (22.2%), B (18.5%), and C (18.5%), respectively, through amplification using AS-I, which revealed 17 different types of C. albicans. By combining the two typing methods, 27 C. albicans strains were finally divided into 22 genotypes.Conclusion: Different genotypes showed genetic diversity among the C. albicans strains isolated from animal sources. The results revealed no special genotype relationship according to the host, anatomical source of isolation, and antifungal susceptibility.

Comprehensive Analysis of the Jellyfish Chrysaora pacifica (Goette, 1886) (Semaeostomeae: Pelagiidae) with Description of the Complete rDNA Sequence.

Jinho Chae, Yoseph Seo, Won Bae Yu, Won Duk Yoon, Hye Eun Lee, Soo-Jung Chang, and Jang-Seu Ki (2018) The Scyphomedusae genus Chrysaora consists of highly diversified jellyfishes. Although morphological systematics of the genus has been documented over the past century, characterization of molecular taxonomy has been attempted only recently. In the present study, we sequenced an 8,167 bp region, encompassing a single ribosomal DNA (rDNA) repeat unit, from Chrysaora pacifica, and used it for phylogenetic analyses. The tandemly repeated rDNA units turned out to consist of both coding and noncoding regions, whose arrangement was found to be the same as that of a typical eukaryote. None of the 5S rRNA sequences were found among the repeat units. Comparative analyses of jellyfish rDNA sequences showed that the 28S locus is highly informative and divergent compared to the 18S locus. Phylogenetic analyses of the 18S and 28S loci revealed that the Semaeostomeae order of jellyfish is separated into taxonomic groups by families and genera, with a few exceptions. The family Pelagiidae was in a clade separate from other groups, thus forming a monophyletic lineage. All Chrysaora included here formed a strongly supported clade within the family Pelagiidae, and Pelagiidae manifested a sister relationship with Cyanea. Nonetheless, Chrysaora was found to be paraphyletic in both 18S and 28S phylogenies. Chrysaora pacifica was clearly distinct from close relatives C. melanaster and C. quinquecirrha. These results provide a special reference for the DNA taxonomy of Pelagiidae jellyfishes in terms of nuclear cistron rDNA sequences and improve our understanding of the molecular phylogenetic relationships among Semaeostomeae jellyfishes.

Conserved Pbp1/Ataxin-2 regulates retrotransposon activity and connects polyglutamine expansion-driven protein aggregation to lifespan-controlling rDNA repeats

Ribosomal DNA (rDNA) repeat instability and protein aggregation are thought to be two major and independent drivers of cellular aging. Pbp1, the yeast ortholog of human ATXN2, maintains rDNA repeat stability and lifespan via suppression of RNA–DNA hybrids. ATXN2 polyglutamine expansion drives neurodegeneration causing spinocerebellar ataxia type 2 and promoting amyotrophic lateral sclerosis. Here, molecular characterization of Pbp1 revealed that its knockout or subjection to disease-modeling polyQ expansion represses Ty1 (Transposons of Yeast) retrotransposons by respectively promoting Trf4-depedendent RNA turnover and Ty1 Gag protein aggregation. This aggregation, but not its impact on retrotransposition, compromises rDNA repeat stability and shortens lifespan by hyper-activating Trf4-dependent turnover of intergenic ncRNA within the repeats. We uncover a function for the conserved Pbp1/ATXN2 proteins in the promotion of retrotransposition, create and describe powerful yeast genetic models of ATXN2-linked neurodegenerative diseases, and connect the major aging mechanisms of rDNA instability and protein aggregation.

Peltigera neodegenii sp. nov. from Central China

Peltigera neodegenii from Central China is described and illustrated as a new species. It is similar in general appearance to P. degenii, P. membranacea, and P. canina, but is distinguished by its shiny upper surface in the central part of lobes, tomenta around the margins of upper surface of lobes, distinct and raised veins on the lower surface of lobes, and simple rhizines. Comparisons of the ITS (ITS1, 5.8S, ITS2) sequences of the nuclear ribosomal DNA repeat tandem, both in phylogenetic analysis and secondary structure models of ITS2, support the taxonomic distinctness of this species.

The Agropyron cristatum karyotype, chromosome structure and cross-genome homoeology as revealed by fluorescence in situ hybridization with tandem repeats and wheat single-gene probes

Key messageFluorescence in situ hybridization with probes for 45 cDNAs and five tandem repeats revealed homoeologous relationships of Agropyron cristatum with wheat. The results will contribute to alien gene introgression in wheat improvement.Crested wheatgrass (Agropyron cristatum L. Gaertn.) is a wild relative of wheat and a promising source of novel genes for wheat improvement. To date, identification of A. cristatum chromosomes has not been possible, and its molecular karyotype has not been available. Furthermore, homoeologous relationship between the genomes of A. cristatum and wheat has not been determined. To develop chromosome-specific landmarks, A. cristatum genomic DNA was sequenced, and new tandem repeats were discovered. Their distribution on mitotic chromosomes was studied by fluorescence in situ hybridization (FISH), which revealed specific patterns for five repeats in addition to 5S and 45S ribosomal DNA and rye subtelomeric repeats pSc119.2 and pSc200. FISH with one tandem repeat together with 45S rDNA enabled identification of all A. cristatum chromosomes. To analyze the structure and cross-species homoeology of A. cristatum chromosomes with wheat, probes for 45 mapped wheat cDNAs covering all seven chromosome groups were localized by FISH. Thirty-four cDNAs hybridized to homoeologous chromosomes of A. cristatum, nine hybridized to homoeologous and non-homoeologous chromosomes, and two hybridized to unique positions on non-homoeologous chromosomes. FISH using single-gene probes revealed that the wheat-A. cristatum collinearity was distorted, and important structural rearrangements were observed for chromosomes 2P, 4P, 5P, 6P and 7P. Chromosomal inversions were found for pericentric region of 4P and whole chromosome arm 6PL. Furthermore, reciprocal translocations between 2PS and 4PL were detected. These results provide new insights into the genome evolution within Triticeae and will facilitate the use of crested wheatgrass in alien gene introgression into wheat.

An improved metagenomic strategy reveals an unprecedentedly high level of intragenomic polymorphism of ribosomal DNA in three species of Camellia

AbstractThe ribosomal DNA (rDNA) repeats of more and more species have been found to be polymorphic in the genome, and the understanding of this polymorphism is conducive not only to eliminating potential chaos in the metagenomic analysis (MGA), but also to providing rich information on rDNA evolution. In this study, the MGA previously used for the study of environmental microbial diversity was improved and extended to detect the intragenomic polymorphism (IGP) of rDNA in three species of Camellia L. The 3′‐end region of 26S rDNA of three individuals was amplified using degenerate primer pairs. Three annealing temperatures were applied to obtain as many ribotypes as possible, and equimolar amplicons from the three species were then pooled and sequenced using the Illumina MiSeq platform. An incredibly high level of IGP of rDNA was found in all three species. Nearly all of the ribotypes detected from this study were rDNA pseudogenes, and most of them have existed for a long time in the genome, with some generated from several rapid expansions. Our procedure provided an effective technique for detection of IGP of rDNA, and the particular evolutionary information of rDNAs in Camellia was also exploited.

Ribosomal DNA copy loss and repeat instability in ATRX-mutated cancers

ATRX (alpha thalassemia/mental retardation X-linked) complexes with DAXX to deposit histone variant H3.3 into repetitive heterochromatin. Recent genome sequencing studies in cancers have revealed mutations in ATRX and their association with ALT (alternative lengthening of telomeres) activation. Here we report depletion of ATRX in mouse ES cells leads to selective loss in ribosomal RNA gene (rDNA) copy number. Supporting this, ATRX-mutated human ALT-positive tumors also show a substantially lower rDNA copy than ALT-negative tumors. Further investigation shows that the rDNA copy loss and repeat instability are caused by a disruption in H3.3 deposition and thus a failure in heterochromatin formation at rDNA repeats in the absence of ATRX. We also find that ATRX-depleted cells are reduced in ribosomal RNA transcription output and show increased sensitivity to RNA polymerase I (Pol I) transcription inhibitor CX5461. In addition, human ALT-positive cancer cell lines are also more sensitive to CX5461 treatment. Our study provides insights into the contribution of ATRX loss of function to tumorigenesis through the loss of rDNA stability and suggests the therapeutic potential of targeting Pol I transcription in ALT cancers.

Fluorescence in situ Hybridization Karyotype Analysis of Seven Platycodon grandiflorum (Jacq.) A. DC. Cultivars

Platycodon grandiflorum has long been cultivated for its medicinal properties and economic value, prompting crop improvement programs that have spurred the development of new cultivars. However, limited cytogenetic information is available for this species, especially for different P. grandiflorum cultivars. Karyotype analysis via fluorescence in situ hybridization (FISH) provides essential information about chromosome count and the localization of DNA sequences, thus providing a chromosome-level understanding of genome structure. Here, we carried out triple-color FISH karyotype analysis of various P. grandiflorum (Jacq.) A. DC. cultivars using 5S and 45S ribosomal DNA (rDNA) and telomeric repeat probes. The karyotype is composed of 2n = 18 chromosomes consisting of seven pairs of metacentric chromosomes, one pair of submetacentric chromosomes, and one pair of subtelocentric chromosomes. The chromosome length ranges from 1.92 to 4.98 μm. One pair of 5S rDNA loci was observed on either the intercalary or pericentromeric regions of chromosome 3. All cultivars showed 45S rDNA signals on chromosome 9, and in two closely related cultivars, one additional pair of 5S rDNAs co-localized with these 45S rDNAs. Arabidopsis -type telomeric repeats were observed in most telomeric regions of the chromosomes in all seven cultivars. Our data lay the foundation for future cytogenetic studies of P. grandiflorum focusing on breeding and the elucidation of its genome structure.

Chromosomal characterization based on repetitive DNA distribution in a tetraploid cytotype of Chrysanthemum zawadskii

The aim of this study was to gain insight into the basic genetics of Chrysanthemum zawadskii Herbich ssp. lucidum (NAK.) Y. Lee, a Chrysanthemum subspecies from Ulleung Island, South Korea, by investigating its karyotypic and cytogenetic characteristics, and to contribute to the karyosystematic study of the genus Chrysanthemum (tribe: Anthemideae). Fluorescence in situ hybridization (FISH) was used to characterize the ploidy level, chromosome constitution, and distribution of 5S and 18S ribosomal DNA (rDNA) and Arabidopsis-type telomere repeats (TTTAGGG)n in C. zawadskii ssp. lucidum chromosomes. The somatic metaphase chromosome number of C. zawadskii ssp. lucidum was tetraploid (2n = 4x = 36). Signals from four 5S rDNA loci were detected in the middle of the long arms of some chromosomes, whereas signals from eight 18S rDNA loci were located at the termini of the short arms. The two rDNA markers localized to different chromosomes with no co-localization. The Arabidopsis-type telomeric repeats, however, co-localized with 18S rDNA sites at the ends of both chromosomal arms. This co-localization of telomeric sequences and 18S rDNA has also been observed in other Chrysanthemum species with various levels of ploidy, indicating that it is a general feature of the genus. DAPI (4′,6-diamidino-2-phenylindole) bands incidentally detected after FISH are also illustrated. These preliminary data contribute towards understanding the karyotypic trends and role of polyploidy in the evolution of the genus Chrysanthemum.