Species delimitation and taxonomic revision of the Rhamnella franguloides complex (Rhamnaceae)

Integrative species delimitation requires at least two independent lines of evidence, such as morphological and molecular data. Rhamnella franguloides (including its synonyms and the varieties inaequilatera and franguloides) and R. caudata constitute a complex with poorly defined morphological boundaries. Here, we investigated species boundaries within this complex. Consistent with previous treatments, we recognized Microrhamnus taquetii as a synonym of Euonymus alatus, rather than of R. franguloides. Population-level morphological clustering and comparison based on all available specimens, together with geographic distributions, supported the recognition of at least three distinct clusters corresponding to three species in this complex: R. franguloides, R. congesta (S.Moore) Z.Qiang Lu & X.L.Zhou, and R. caudata. In addition, the two varieties of R. franguloides are distinguished by markedly different petiole lengths. These three species and two varieties are completely or largely allopatric, and their newly defined morphological boundaries and corresponding taxonomic treatments differ substantially from those proposed previously. Sequence variation in the internal transcribed spacer (ITS) region strongly supported the genetic distinction among the three species. Phylogenetic analysis based on ITS sequences indicated that the well-supported R. congesta clade was sister to another well-supported clade comprising R. franguloides and R. caudata. This study highlights the importance of integrating multiple lines of evidence in species delimitation and taxonomic revision.