Phylogenetic Relationships within Ruellieae (Acanthaceae) and a Revised Classification

Abstract

Phylogenetic knowledge of the large plant family Acanthaceae has been greatly advanced over the last 2 decades. Studies have demonstrated the existence of several major lineages, most of which have been the focus of subsequent investigation. Missing among these is comprehensive study of the 48 genera currently classified in tribe Ruellieae, a pantropical lineage that includes several species-rich genera. We compared the number of validly published names to current estimates of species richness per genus in Ruellieae and found more than 2600 names available for ∼1200 species. Using molecular data from two nuclear (ITS+5.8S, Eif3E) and three chloroplast (trnG-trnR, trnG-trnS, psbA-trnH) markers, we test the placement of these 48 genera in Ruellieae, explore the monophyly of currently recognized taxa, and propose morphological features to diagnose major clades within the tribe. We were able to sample all but four of 48 genera, and all were resolved in Ruellieae except Zygoruellia. Many monospecific or oligospecific genera are nested within clades of more species-rich genera. We propose several new generic synonymies to reflect these results and insights from morphology. Finally, we present a revised classification of Ruellieae that contains seven subtribes. A solid phylogenetic hypothesis of relationships within Ruellieae contributes to progress in biology in three important ways: (1) it enables better assessment of trait homologies and thus characters upon which genera are delimited, (2) it contributes to the Global Strategy for Plant Conservation’s initiative to document plant biodiversity, and (3) it facilitates cross-family comparative evolutionary analyses, including large-scale hypothesis testing of biogeographic patterns, clade size asymmetries, and differential diversification within Acanthaceae.