The demise of subfamily Hydrocotyloideae (Apiaceae) and the re-alignment of its genera across the entire order Apiales

As circumscribed by Drude, the umbellifer subfamily Hydrocotyloideae posed a major hindrance to resolving the phylogeny of order Apiales. Previous studies have suggested its polyphyly, but have not had sufficient sampling to address the issue fully. To put an end to the out-dated concept of Hydrocotyloideae, we investigated the placement of 40 of the 42 genera once placed in the subfamily, using extensive taxon sampling across the entire order. Molecular phylogenies were constructed using plastid sequences of the rpl16 intron and the trnD-trnT regions and revealed at least six hydrocotyloid lineages dispersed across both families Apiaceae and Araliaceae. The most speciose of these clades corresponds to the recently erected subfamily Azorelloideae. Another lineage includes genera grouped in Mackinlayoideae, where relationships are well resolved. Platysace appears paraphyletic with respect to Homalosciadium, and their placement is well supported as a basal lineage in Apiaceae. The type genus, Hydrocotyle, belongs to a supported clade in Araliaceae. The placements of Hermas as sister to a clade consisting of Apiaceae subfamilies Apioideae and Saniculoideae, and of Choritaenia as sister to Lichtensteinia in a clade with affinities to both Apioideae and Saniculoideae, calls into question the circumscriptions of the two subfamilies. Finally, plastid data suggest that many former hydrocotyloid genera are non-monophyletic (e.g., Azorella, Schizeilema, and Eremocharis) and are in dire need of additional phylogenetic and taxonomic studies.