Illuminating the systematics of the Spiranthes sinensis species complex (Orchidaceae): ecological speciation with little morphological differentiation

Clarity in species delimitation is fundamental for successfully testing evolutionary theory and is integral to implementing conservation strategies. The Old World Spiranthes sinensis species complex (Orchidaceae) has been a source of systematic confusion due to its expansive distribution and morphological variation, lacking a comprehensive systematic review. Working under an integrative phylogenetic species concept, we provide an expansive molecular phylogenetic sampling of the S. sinensis species complex, compliment this with phenological and morphological studies and clarify species boundaries in this challenging group. Our data support a re-circumscription of the S. sinensis species complex, recognizing six distinct species and one natural hybrid, with other taxa reduced to synonyms: S. australis, S. flexuosa, S. maokensis sp. nov., S. sinensis s. s., S. suishaensis, S. sunii and S. xhongkongensis (pro species, S. flexuosa x S. sinensis s. s.). The recognition of S. flexuosa also refines the distribution of S. spiralis. Our research indicates phenological divergence, reinforced by glacial vicariance, may have strongly influenced the evolution and speciation of this clade, with few observable or measurably significant morphological differences among these species (other than pubescence). We also discuss the importance and prevalence of autogamy among island populations of Spiranthes. Our work can be used by researchers working on the systematics and biogeography of other Asian and Australasian species complexes, and we underscore the continued importance of nomenclatural and specimen review and correct species identification in evolutionary and ethnobotanical studies.