Physical mapping of 45S rDNA to metaphase chromosomes in 30 taxonomically diverse plant species

The genomic distribution of ribosomal RNA genes (rDNA) has been determined by fluorescence in situ hybridisation (FISH) in 30 diverse plant species, using 45S rDNA as a probe. In most species, physical mapping of 45S rDNA to metaphase chromosomes is reported for the first time. These species could be divided into four groups based on differences in the number and positions of their rDNA loci. Fluorescent signals lay in secondary constrictions and satellites of satellite-chromosomes (SAT-chromosomes) in 14 of the 30 species, including Berberis thunbergii, Plantago major, Sanicula lamelligera, Ficus carica and Robinia pseudoacacia. In another seven species, including Robinia hispida, Kolkwitzia amabilis, Litchi chinensis, Catalpa speciosa and Acer buergerianum, FISH markers were also detected in the terminal and/or peri-centromeric regions of some chromosomes beside secondary constrictions and satellites. There was only one species (Viburnum sargentii) in which four pairs of SAT-chromosomes were found, only one pair of which gave FISH signals. In the other eight species, including Kerria japonica f. pleniflora, Salvia miltiorrhiza, Cucumis sativus and Ulmus pumila, no visible satellites existed and the positions of 45S rDNA loci on the chromosomes were terminal or peri-centromeric. Our results indicate that 45S rDNA can be used to identify individual chromosomes. The polymorphism of 45S rDNA loci is also discussed in this paper.