A taxonomic revision of Thesium section Hagnothesium (Santalaceae) and description of a new species, T. quartzicolum.

Thesium L., section Hagnothesium is endemic to the Greater Cape Floristic Region (GCFR), South Africa. In the past there has been a propensity in revisionary work of the section Hagnothesium to split taxa into distinct species or vice-versa. Consequently, 15 different names exist although only 6 are accepted formally. Following recent molecular phylogenetic studies, the monophyly of the section Hagnothesium is now well-established but the circumscription of species within the section remains problematic given the complicated nomenclatural history which has added further confusion. In this paper we revise the section using a total evidence approach to propose a revised taxonomy. We studied both herbarium collections as well as plants in their natural populations to circumscribe species boundaries, determine their geographical ranges and estimate their conservation statuses. Species of the section Hagnothesium are dioecious, generally having four-merous campanulate flowers produced in spikes borne in bract axils and arranged along the length of branchlets, with valvate perianth lobes and a short to absent hypanthial tube. We recognise eight species, of which one is described in this study as new: T. fragile L.f., T. fruticulosum (A.W.Hill) J.C.Manning & F.Forest, T. hirtum (Sond.) Zhigila, Verboom & Muasya comb. nov., T. leptostachyum A.DC., T. longicaule Zhigila, Verboom & Muasya nom. nov. T. microcarpum A.DC., T. minus (A.W.Hill) J.C.Manning & F.Forest) and T. quartzicolum Zhigila, Verboom & Muasya sp. nov. We provide updated taxonomic keys, species descriptions, illustrations, distribution maps, new combinations, synonyms, and notes on the red list status for each species. • A review is given of Thesium section Hagnothesium (Santalaceae) an endemic clade to the GCFR. • Eight species are recognised, of which one, T. quartzicolum is described here as a new. • Key to the species in section Hagnothesium is provided • Species delimitations and nomenclatural changes are provided. • Species geographical ranges and the estimate of their conservation statuses are provided. [ABSTRACT FROM AUTHOR]