Physical mapping of rDNA and karyotype analysis in Tulipa sinkiangensis and T. schrenkii

The karyotypes of Tulipa sinkiangensis and T. schrenkii were analyzed based on chromosome size and the physical mapping of 5S and 45S ribosomal DNAs (rDNAs) using fluorescence in situ hybridization. The genomes from both species were consistent with the uniform karyotype formula, i.e., having four pairs of submedian and eight pairs of subterminal chromosomes (2n=2x=24=8sm+16st). Remarkably, the number and location of 5S rDNA loci were quite different between the two species. A single pair of 5S rDNA loci was detected on both chromosomes 5 in T. sinkiangensis, but the 5S rDNA loci were detected on almost all chromosomes in T. schrenkii, except for chromosomes 4 and 6. For both species, the 45S rDNA loci were localized to telomeric regions of the chromosome—on either the short or long arm. Interestingly, the 5S rDNA and 45S rDNA loci positions and numbers varied among different genotypes of T. schrenkii. We conclude that the two species are close relatives and that a series of genome modifications occurred during the diversification of these two species.