Home at last: Molecular phylogenetic analyses support subsuming Metastachydium within Phlomoides (Lamiaceae)

Distributed in Central Asia, Metastachydium (Lamiaceae) is a poorly understood and rare monotypic genus, with few collections known. The systematic position of this enigmatic genus within Lamiaceae has remained unresolved due to its poor representation in herbaria and coincident lack of available materials for molecular phylogenetic analysis. Facilitated by some recent collections, we performed Bayesian inference and maximum likelihood analyses, using an 80-protein-coding plastid-gene dataset of Lamioideae, to infer the systematic placement of Metastachydium at the tribal level within Lamioideae. In addition, we used an 8-plastid-DNA-region dataset as well as the nuclear ribosomal internal transcribed spacer to determine the phylogenetic affinities of Metastachydium. All phylogenetic analyses agree that Metastachydium is a member of Phlomideae and deeply nested within the genus Phlomoides, suggesting the need to expand the latter to include Metastachydium. Hence, a new combination, Phlomoides sagittata comb. nov., is proposed, and we present the first available photographs and an amended morphological description of P. sagittata. In addition, the infrageneric circumscription of Phlomoides is not supported, as most sections and subsections are not monophyletic. Hybridization and incomplete lineage sorting, following rapid diversification within Phlomoides, seem to be the source of incongruence between the nuclear and plastid tree topologies.

Automated summary

Metastachydium, a poorly understood and rare monotypic genus, has been difficult to place within the Lamiaceae family due to limited collections and lack of molecular analysis. Recent collections have allowed for a better understanding of its systematic position, and it has been determined that Metastachydium belongs to the Phlomideae tribe and is nested within the Phlomoides genus. This suggests the need to expand Phlomoides to include Metastachydium, and a new combination name, Phlomoides sagittata comb. nov., has been proposed.