Molecular phylogenetics and morphological evolution of Thunbergioideae (Acanthaceae)

Based on nucleotide sequences from three chloroplast DNA regions (rps16, rpl16, trnT-trnL), we infer detailed phylogenetic relationships within the subfamily Thunbergioideae (Acanthaceae) and among major lineages of the family as a whole. Taxa were sampled to cover the geographic distribution of Thunbergioideae and to include all subgenera of the largest genus Thunbergia. All other major lineages of Acanthaceae were sampled to test monophyly and intrafamilial position of Thunbergioideae. Both parsimony and Bayesian analyses support Thunbergioideae as monophyletic. The mangrove genus Avicennia is consistently placed as sister to Thunbergioideae although with only moderate support. Thunbergia and Mendoncia are both monophyletic in all analyses, and Mendoncia is placed as sister to Thunbergia plus Pseudocalyx. Relationships within the two largest genera Mendoncia and Thunbergia are highly resolved and most branches are strongly supported. Our results suggest that the existing morphology-based classification of Thunbergia partially holds, but needs revision. Based on the phylogenetic relationships we found it likely that a twining habit is ancestral for the genus Thunbergia. The thecal awns, characteristic for many species in the genus, have probably evolved from unicellular bristles. Longitudinal dehiscence over the whole length of the thecae, which is present in many Thunbergia species, most likely evolved from short slits or pores as present in Pseudocalyx and Mendoncia.