Resolving the last unresolved: tribal placement of the enigmatic genus Eskemukerjea (Polygonaceae) based on molecular and morphological evidence

With advances in molecular phylogenetics, most genera of Polygonaceae have been assigned with clear placement and circumscription. However, the tribal affinity of the monotypic genus Eskemukerjea has remained unresolved, and the generic placement of its sole species, Eskemukerjea megacarpum, has also been controversial, which is critical for understanding the taxonomic status of this genus. Here, we adopt a two-step approach to resolve the phylogenetic placement of Eskemukerjea using three datasets: (i) a plastome alignment of Polygonaceae; (ii) a dataset of seven chloroplast markers (matK, ndhF, rbcL, trnL-F, accD, atpB, psbA-trnH), and (iii) a dataset of the nuclear ribosomal internal transcribed spacer (nrITS) for Polygonideae. Our analyses reveal that Eskemukerjea is nested within Pteroxygonum, together forming the tribe Pteroxygoneae. This placement is strongly supported by key shared morphological characteristics, including perennial herb with scandent habit and its extremely enlarged, spherical perennating tuberoids, which are partially above ground. The inclusion of Eskemukerjea expands the circumscription and distribution of Pteroxygonum across the southern Himalayas from Nepal to Xizang of China. Morphological comparisons between Eskemukerjea megacarpum and the two recognized species of Pteroxygonum are further discussed, and a taxonomic key to distinguish these species is provided.