An integrative approach to species delimitation sinks three Chinese limestone karst Elatostema (Urticaceae) species

Elatostema is recognized as a taxonomically difficult group due to the reduced nature of the tiny flowers and inflorescences, also the large number of species (ca 650 to 700). Different opinions on morphological species delimitation have resulted in instability, which is problematic in such a speciose group. In this paper, the taxonomic status of three putative species, E.robustipes, E.scaposum, E.conduplicatum and their hypothetical closest relatives, was revised using morphological and molecular observations. Morphological comparison suggested high similarity between E.robustipes & E.retrohirtum, E.scaposum & E.oblongifolium, E.conduplicatum & E.coriaceifolium, respectively. Phylogenetic analyses of four universal DNA barcodes (ITS, trnH-psbA, matK and rbcL) suggested that each species pair represents a single evolutionary lineage. Taking these two findings together, we propose E.robustipes to be a synonym of E.retrohirtum, E.scaposum a synonym of E.oblongifolium, and E.conduplicatum a synonym of E.coriaceifolium. Our results recover the number, shape and size of the bracts and bracteoles to be relatively stable characters, and the disposition of the male inflorescences on modified stems to be an unstable character, unsuitable for species delimitation in Elatostema.