Understanding the Taxonomic Complexes and Species Delimitation within Sambucus L. (Viburnaceae)

The genus Sambucus belongs to the family Viburnaceae, and has long been a debatable taxon regarding its systematics and taxonomic assignment. It is known for its morphological and genetic variations and thus the taxonomic delimitation remains uncertain. Phenotypic and genotypic variations within plant species causes difficulties in the detection of species boundaries. In the present review, we compiled a set of studies that assessed species discrimination within Sambucus genus by morphological and molecular markers as well as the taxonomic uncertainties in the re-circumscription of the segregated genera i.e., Viburnum L., Sambucus L., and Adoxa L., (syn. Sinadoxa C.Y. Wu, Z.L. Wu & R.F. Huang/Tetradoxa C.Y. Wu) from Caprifoliaceae and their family assignment. The data was obtained from published journal articles and various online databases. Morphological and molecular diagnostic characters were employed to distinguish Sambucus species. Our findings indicated that high intraspecific variations within narrow-ranged populations and their widespread counterparts contribute to its taxonomic complications. Additionally, intraspecific variations within Sambucus species leads to misidentifications and the misapplication of names, occasioning the publication of many putative synonyms and ambiguous names. Possible interspecific hybridizations experienced between Sambucus species increases complexity in character evolution. A morphological analysis showed that the endocarp seed morphological characters can reliably support the segregation of Sambucus taxa into discrete taxonomic entities. In molecular analysis, the NeighborNet (NN) phylogenetic split network revealed three distinct genera including Viburnum, Sambucus, and Adoxa, which clustered separately. The Maximum Likelihood (ML) analysis indicated that the Sambucus species formed a monophyletic group and clustered into two major clades, a small clade containing S. maderensis, S. peruviana, S. nigra, and S. canadensis, and a large clade encompassing the rest of the species with significantly supported clades (<52%). The preferred family assignment for Sambucus is Viburnaceae.