Additional locus of rDNA sequence specific to the X chromosome of the liverwort, Marchantia polymorpha

The molecular cytogenetic organization of 17S ribosomal RNA genes (17S rDNA ), a part of the 45S rDNA repeat, was investigated on the chromosomes of th e liverwort Marchantia polymorpha using fluorescence in-situ hybridization (FISH). The numbers of 17S rDNA loci visualized in female and male chromoso mes were ten and nine, respectively. This heterogeneous localization was du e to the presence of an additional 17S rDNA locus on the X chromosome and i ts absence on the Y chromosome. The signal on the X chromosome covered almo st the entire region of its long arm. The other nine signals were observed on the same loci of respective autosomes in both sexes. Southern hybridizat ion analysis revealed an additional band including 17S rDNA exclusively on EcoRI digested female genomic DNA supporting the existence of an additional 17S rDNA locus on the X chromosome.