Phylogeny and evolution of bracts and bracteoles in Tacca (Dioscoreaceae)

Most species in the genus Tacca (Dioscoreaceae) feature green to black purple, conspicuous inflorescence involucral bracts with variable shapes, motile filiform appendages (bracteoles), and diverse types of inflorescence morphology. To infer the evolution of these inflorescence traits, we reconstructed the molecular phylogeny of the genus, using DNA sequences from one nuclear, one mitochondrial, and three plastid loci (Internal Transcribed Spacer (ITS), atpA, rbcL, trnL-F, and trnH-psbA). Involucres and bracteoles characters were mapped onto the phylogeny to analyze the sequence of inflorescence trait evolution. In all analyses, species with showy involucres and bracteoles formed the most derived clade, while ancestral Tacca had small and plain involucres and short bracteoles, namely less conspicuous inflorescence structures. Two of the species with the most elaborate inflorescence morphologies (T. chantrieri in southeast China and T. integrifolia in Tibet), are predominantly self-pollinated, indicating that these conspicuous floral displays have other functions rather than pollinator attraction. We hypothesize that the motile bracteoles and involucres may facilitate selfing; display photosynthesis in the dim understory, and protect flowers from herbivory.