rDNA organization and evolutionary relationships in the genus Hypochaeris (Asteraceae)

The pattern of rDNA sequence distribution in 11 Hypochaeris species from South America and Europe has been determined by in situ hybridization using probes of 18S-5.8S-28S and 5S ribosomal RNA genes. Activity of nucleolar organizer regions (NORs) was tested by silver nitrate staining and chromomycin A3 banding of Eu- ropean species was carried out. In European species the 18S-5.8S-28S rRNA genes were located in the terminal secondary constrictions and the adjacent satellites, except for one locus in H. achyrophorus. In South American species, one or two 18S-5.8S-28S rDNA loci were observed in the intercalary secondary constrictions. All the rDNA sites appeared active, except one locus in H. chillensis. With the ex- ception of H. cretensis, 5S rRNA genes were located on a single chromosome pair. In all the species analyzed, GC-rich heterochromatin has been located around the NORs. The rDNA localization indicated that chromosome rearrangements as centric shifts, centric fusions, and translocations of NORs involve in a decrease of rDNA loci number during the evolution of the complex of Hypochaeris species probably from an ancestral genome with two loci of 18S-5.8S-28S rDNA and one locus of 5S rDNA.