The systematic position of the enigmatic rare South African endemic Carex acocksii : Its relevance on the biogeography and evolution of Carex sect. Schoenoxiphium (Cyperaceae)

Carex acocksii is a sedge species known only from two populations in the Karoo of the Northern Cape in South Africa. It was described from a single locality (Hantam Mts., Calvinia) more than twenty years ago, and tentatively ascribed to section Petraea because of its unispicate inflorescence and utricles with membranaceous-papyraceous walls. However, its systematic relationships have remained largely unexplored. We perform molecular, morphological and biogeographic studies in order to elucidate the systematic relationships and origin of this poorly known species. A phylogenetic reconstruction based on two nuclear (ITS, ETS) and two plastid (matK, rps16) markers strongly supports that C. acocksii belongs to Carex sect. Schoenoxiphium (former genus Schoenoxiphium), a lineage with its center of diversity in South Africa. However, C. acocksii displays a remarkable molecular (i.e. long phylogenetic branch), morphological (i.e. unispicate inflorescence and poorly veined, weak utricle walls), geographical (isolated populations placed more than 100 kms from any other native Carex species) and ecological (the only Carex species known exclusively from the Great Karoo) differentiation with respect to the remainder of sect. Schoenoxiphium. The section has been inferred to have originated 15 Mya in the Drakensberg, and to have speciated after dispersal to other areas at least three times (C. chermezonii was not included in this study), one of these cases being C. acocksii. These features highlight the evolutionary singularity and conservation importance of this species, especially in the context of South African flora. We reassessed the conservation status of C. acocksii at a global scale under IUCN categories and criteria, resulting in the proposal of the Critically Endangered category for the species. © 2020 SAAB