Goldfields Revisited: A Molecular Phylogenetic Perspective on the Evolution of Lasthenia (Compositae: Heliantheae sensu lato)

Abstract

DNA sequence data from the internal and external transcribed spacers of 18S‐26S nuclear ribosomal DNA and the 3′ trnK intron of chloroplast DNA provide evidence for patterns of evolutionary diversification and relationships in Lasthenia. Maximum parsimony analysis shows strong support for monophyly of Lasthenia and monophyly of each of seven sections (as treated here) in Lasthenia, with minor revision of Ornduff’s (1966) sectional circumscriptions. Lasthenia sect. Amphiachaenia (correct name for L. sect. Baeria sensu Ornduff 1966 and redelimited to include L. leptalea) was resolved as a basally divergent section, i.e., as the sister group of a clade comprising all other Lasthenia taxa. Placement of other sections within Lasthenia is only weakly resolved, except for L. sect. Burrielia (redefined to exclude L. leptalea) and L. sect. Hologymne, which are robust sister clades. The clade comprising L. sect. Burrielia and L. sect. Hologymne and clades corresponding to each of the other sections except L. sect. Amphiachaenia constitute a polytomy, with most clades characterized by a long basal branch and relatively short terminal branches. We suggest that overall patterns of divergence in the molecular trees for Lasthenia conform to expectations of saltational diversification, with an initial rapid radiation followed by long periods of minimal diversification in each group preceding relatively recent episodes of speciation. Results also show that several important taxonomic characters in Lasthenia are homoplastic and that the base chromosome number for Lasthenia is x=8.