Cytogenetic characterization of three Hypericum species by in situ hybridization

The chromosomal positions of the 5S/25S rRNA genes of Hypericum perforatum (2n=32), H. maculatum (2n=16) and H. attenuatum (2n=32) were comparatively determined by FISH, and six, three and seven chromosome pairs of the respective karyotypes were subsequently distinguished. The rDNA loci between H. perforatum and H. maculatum seem to be identical (with respect to the ploidy difference), indicating that H. perforatum probably arose by autotetraploidization from an ancestor closely related to H. maculatum. The positional differences between the 5S rRNA gene loci of H. perforatum and H. maculatum on the one hand and H. attenuatum on the other argue against a previous hypothesis according to which H. perforatum originated from a remote interspecific hybridization between H. maculatum and H. attenuatum. Received: 7 October 1999 / Accepted: 23 November 1999