Abstract: Orostachys is a small genus in the family Crassulaceae with about 16 species, the majority of which are biennial herbs. The taxonomic boundaries and phylogenetic relationships between Orostachys and its allied genera have been controversial, partly because of the absence of a robust phylogenetic framework. In the present study, we collected 30 samples, representing 27 species of Orostachys and related taxa. Using Hyb-seq and genome skimming methods, we obtained 750 single-copy orthologous nuclear genes and all plastid protein-coding genes. Both nuclear and plastid phylogenies revealed the polyphyly of Orostachys s.l., comprising three independent lineages corresponding to Kungia (=O. sect. Schoenlandia), the O. spinosa clade (=O. subsect. Appendiculatae), and the O. malacophylla clade (=O. subsect. Orostachys). These lineages each formed sister relationships with their respective allied taxa: Kungia with Sinocrassula, the O. malacophylla clade with Hylotelephium, and the O. spinosa clade with Meterostachys. Minimal gene flow was detected among the three lineages, and between each lineage and its sister group, indicating well-established reproductive isolation. Morphological character analyses corroborated these phylogenomic findings. Our results (i) support Fu′s taxonomic treatment of O. sect. Schoenlandia as the independent genus Kungia; (ii) do not support merging O. subsect. Orostachys (=the O. malacophylla clade) with Hylotelephium; and (iii) suggest that O. subsect. Orostachys should be taxonomically recognized as a new genus Amblystachys and make the relevant new combinations.

Key words: Amblystachys, Crassulaceae, Hyb-seq, Orostachys, phylogenomics