Molecular phylogeny and cryptic morphology reveal a new genus of West Indian woody bamboo (Poaceae: Bambusoideae: Bambuseae) hidden by convergent character evolution

Arthrostylidium, a Neotropical genus of 31 mostly clambering woody bamboo species, is a member of subtribe Arthrostylidiinae, which is closely allied to subtribe Guaduinae. Arthrostylidium has long been a catchall for species within its subtribe that display unusual morphology. While attempting to clarify the circumscription of Arthrostylidium, we discovered three West Indian species (A. angustifolium, A. farctum, A. pinifolium) that possess putative Guaduinae features. We test the hypothesis that these three species should be classified within subtribe Guaduinae and revise their taxonomy. Phylogenetic inference based on sequences of four plastid markers (ndhF, trnC-rpoB, trnD-trnT, rps16-trnQ) recovered Arthrostylidium as polyphyletic, with A. angustifolium, A. farctum, and A. pinifolium forming a well-supported clade sister to the Guaduinae. Parametric bootstrap tests uphold this topology as significant. Detailed morphological and anatomical analyses revealed A. angustifolium, A. farctum, and A. pinifolium lack characteristics diagnostic for Arthrostylidiinae but conform to Guaduinae. Based on the morphological and molecular results, we erect the new genus Tibisia within subtribe Guaduinae, transfer Arthrostylidium angustifolium, A. farctum, and A. pinifolium to Tibisia as T. angustifolia, T. farcta, and T. pinifolia, and provide keys to the genera of Guaduinae and the species of Tibisia. These findings demonstrate convergent evolution in a number of macromorphological characters that have been important for generic identification, and reinforce the diagnostic value of leaf anatomy for subtribal determination among the Neotropical woody bamboos.