Metabolomics as a complement to phylogenetics for assessing intraspecific boundaries in the desiccation-tolerant medicinal shrub Myrothamnus flabellifolia (Myrothamnaceae)

The desiccation-tolerant shrub Myrothamnus flabellifolia has colonised a unique and harsh niche that provides little protection from the elements. It has a wide distribution range in southern Africa, occurring across an environmental gradient that is exceptionally arid in the southwest and highly mesic in the northeast. It is also harvested for use in medicinal preparations, both traditionally and commercially. However, the phytochemical variability of plants from different rainfall regions has not been assessed, nor have the intraspecific relationships been evaluated by means of a rigorously tested phylogeny. The aims of the present study were thus (1) to test a phylogenetic hypothesis for intraspecific relationships in M. flabellifolia; (2) to assess, based on the global metabolomic profiles, whether accessions collected from the three different geographic locations in southern Africa across a rainfall gradient can be differentiated, and if this corroborates the phylogenetic signature; and (3) with the aid of multivariate statistical analysis, identify and evaluate the most significant discriminatory metabolites between the three sampled regions that could act as potential barcodes. The results show that the phylogenetic and metabolomic signatures were congruent, and the metabolomic data were better able to discriminate the different populations collected from the three regions. Several potential barcodes for discriminating the material from the three regions are proposed. Quercetin-rhamnoside and 3-O-methylquercetin, both significant antioxidants, were present at significantly higher quantities in the material from the driest region in the west than from the more mesic regions in the south and east, whereas quercetin-3-O-glucuronide was significantly higher in the latter. A naringenin-like compound or arbutin derivative could discriminate the southern samples from the eastern samples, whereas digalloylglucose differentiated the eastern samples from the southern samples. In summary, the findings of this study imply that the origin of the material should be considered when used in medicinal and cosmetic preparations.