title: Evolutional dynamics of 45S and 5S ribosomal DNA in ancient allohexaploid Atropa belladonna.
abstract: BACKGROUND
  Polyploid hybrids represent a rich natural resource to study molecular evolution of plant genes and genomes. Here, we applied a combination of karyological and molecular methods to investigate chromosomal structure, molecular organization and evolution of ribosomal DNA (rDNA) in nightshade, Atropa belladonna (fam. Solanaceae), one of the oldest known allohexaploids among flowering plants. Because of their abundance and specific molecular organization (evolutionarily conserved coding regions linked to variable intergenic spacers, IGS), 45S and 5S rDNA are widely used in plant taxonomic and evolutionary studies.
  
  
  RESULTS
  Molecular cloning and nucleotide sequencing of A. belladonna 45S rDNA repeats revealed a general structure characteristic of other Solanaceae species, and a very high sequence similarity of two length variants, with the only difference in number of short IGS subrepeats. These results combined with the detection of three pairs of 45S rDNA loci on separate chromosomes, presumably inherited from both tetraploid and diploid ancestor species, example intensive sequence homogenization that led to substitution/elimination of rDNA repeats of one parent. Chromosome silver-staining revealed that only four out of six 45S rDNA sites are frequently transcriptionally active, demonstrating nucleolar dominance. For 5S rDNA, three size variants of repeats were detected, with the major class represented by repeats containing all functional IGS elements required for transcription, the intermediate size repeats containing partially deleted IGS sequences, and the short 5S repeats containing severe defects both in the IGS and coding sequences. While shorter variants demonstrate increased rate of based substitution, probably in their transition into pseudogenes, the functional 5S rDNA variants are nearly identical at the sequence level, pointing to their origin from a single parental species. Localization of the 5S rDNA genes on two chromosome pairs further supports uniparental inheritance from the tetraploid progenitor.
  
  
  CONCLUSIONS
  The obtained molecular, cytogenetic and phylogenetic data demonstrate complex evolutionary dynamics of rDNA loci in allohexaploid species of Atropa belladonna. The high level of sequence unification revealed in 45S and 5S rDNA loci of this ancient hybrid species have been seemingly achieved by different molecular mechanisms.
authors.affiliation: Department of General Genetics, Center of Plant Molecular Biology (ZMBP), Eberhard Karls University of Tübingen, 72076, Tübingen, Germany. r.volkov@chnu.edu.ua.|Department of Molecular Genetics and Biotechnology, Yuriy Fedkovych University of Chernivtsi, Kotsiubynski str. 2, 58012, Chernivtsi, Ukraine. r.volkov@chnu.edu.ua.
authors.forename: Roman A
authors.initials: RA
authors.lastname: Volkov
authors.affiliation: Department of General Genetics, Center of Plant Molecular Biology (ZMBP), Eberhard Karls University of Tübingen, 72076, Tübingen, Germany.|Department of Molecular Genetics and Biotechnology, Yuriy Fedkovych University of Chernivtsi, Kotsiubynski str. 2, 58012, Chernivtsi, Ukraine.
authors.forename: Irina I
authors.initials: II
authors.lastname: Panchuk
authors.affiliation: Department of General Genetics, Center of Plant Molecular Biology (ZMBP), Eberhard Karls University of Tübingen, 72076, Tübingen, Germany.|Australian Centre for Plant Functional Genomics (ACPFG), The University of Adelaide, Hartley Grove, Urrbrae, SA, 5064, Australia.|Current addres: School of Life Science, Huaiyin Normal University, 223300, Huaian, China.
authors.forename: Nikolai V
authors.initials: NV
authors.lastname: Borisjuk
authors.affiliation: Department of Plant Anatomy and Cytology, University of Silesia, 40032, Katowice, Poland.
authors.forename: Marta
authors.initials: M
authors.lastname: Hosiawa-Baranska
authors.affiliation: Department of Plant Anatomy and Cytology, University of Silesia, 40032, Katowice, Poland.
authors.forename: Jolanta
authors.initials: J
authors.lastname: Maluszynska
authors.affiliation: Department of General Genetics, Center of Plant Molecular Biology (ZMBP), Eberhard Karls University of Tübingen, 72076, Tübingen, Germany.
authors.forename: Vera
authors.initials: V
authors.lastname: Hemleben
mesh_terms: D001532:Atropa belladonna / Q000145:classification / Q000235:genetics* / Q000378:metabolism; D032461:Chromosomes, Plant / Q000235:genetics / Q000378:metabolism; D004275:DNA, Ribosomal / Q000235:genetics* / Q000378:metabolism; D019143:Evolution, Molecular; D010802:Phylogeny; D011123:Polyploidy; D012335:RNA, Ribosomal / Q000235:genetics* / Q000378:metabolism; D012341:RNA, Ribosomal, 5S / Q000235:genetics* / Q000378:metabolism
chemical_list: D004275:DNA, Ribosomal; D012335:RNA, Ribosomal; D012341:RNA, Ribosomal, 5S; C088179:RNA, ribosomal, 45S
keywords: Concerted evolution; Homogenization; Polyploidy; Ribosomal DNA; Solanaceae