The genus Hypochaeris (Asteraceae, Lactuceae) contains approximately 10 species distributed in the Old World and 50 species in South America. This latter region, based on DNA sequence studies, is regarded as a secondary center of diversification for the genus. Cytotaxonomic studies have shown European species to have differentiated karyotypes with different basic chromosome numbers (x = 3, 4, 5, 6), whereas South American species have asymmetric and bimodal karyotypes all with x = 4. We synthesize available karyotypic data for New World taxa of Hypochaeris and add new detailed information for species from Chile, vix.: H. acaulis, H. apargioides, H. palustris, H. spathulata, H. tenuifolia, and H. thrincioides. Despite overall similarity of their bimodal karyotypes, analysis of chromosome morphology and rRNA gene localization by FISH (fluorescence in situ hybridization) reveals differentiation. Two groups can be distinguished based on presence/absence of satellites on chromosomes 2 and 3, and localization of rDNA loci. The 5S rDNA locus is stable in all species and located within the short arm of chromosome 2. The 18S-25S rDNA loci are phylogenetically more informative. The locus on chromosome 3 is present in all taxa, but varies in localization within its short arm (terminal vs. interstitial). The second locus on the long arm of chromosome 2 is present in only some taxa. These groupings are in agreement with AFLP data for the same species. Based on karyotypic data and probable cytogenetic mechanisms, we propose a model of evolution of the South American Hypochaeris karyotypes (x = 4) from a H. maculata-like (x = 5) European ancestor. Karyotype evolution would involve inversion of part of the short arm of chromosome 3 bearing 18S-25S rDNA and subsequent inactivation or loss of the satellite and 18S-25S rDNA on chromosome 2.

Key words: genome size, H. maculata, Hypochaeris, karyotype evolution, rRNA genes, South America