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Keywords:

  • gene conversion;
  • nucleolar dominance;
  • polyploidy;
  • rDNA silencing;
  • ribosomal RNA

Summary

  • • 
    This paper establishes relationships between two aspects of ribosomal DNA (rDNA) biology: epigenetic silencing of rDNA loci; and homogenization leading to concerted evolution.
  • • 
    Here, we examined rDNA inheritance and expression patterns in three natural Nicotiana allopolyploids (closest living descendants of diploid parents are given), N. rustica (N. paniculata × N. undulata), N. tabacum (N. sylvestris × N. tomentosiformis) and N. arentsii (N. undulata × N. wigandioides), and synthetic F1 hybrids and allopolyploids.
  • • 
    The extent of interlocus rDNA homogenization decreased in the direction N. arentsii > N. tabacum > N. rustica. The persistence of parental rDNA units in one of the subgenomes was associated with their transcription inactivity and likely heterochromatization. Of synthetic hybrids and polyploids only N. paniculata × N. undulata showed strong uniparental transcriptional silencing of rDNA triggered already in F1.
  • • 
    Epigenetic patterns of expression established early in allopolyploid nucleus formation may render units susceptible or resistant to homogenization over longer time-frames. We propose that nucleolus-associated transcription leaves rDNA units vulnerable to homogenization, while epigenetically inactivated units, well-separated from the nucleolus, remain unconverted.