Phylogenetic placement of Carex dianae Steud., a sedge endemic to the South Atlantic island of St Helena

Carex dianae Steud. is an endemic sedge of St Helena, an isolated island in the South Atlantic. Our study provides the first molecular sequence data for this taxon. We generated sequence data for plastid matK and trnK regions and nuclear ITS and ETS regions to determine the placement of C. dianae in the broader Carex phylogeny. The placement of C. dianae falls within Carex sect. Spirostachyae Drejer, and it is sister to a clade including Carex clavata Thunb., Carex aethiopica Schkuhr (both from the Cape region of S. Africa), and Carex gunniana Boott (southern Australia). The existence of three divergent nucleotype groups and two plastotypes is revealed from genetic variation within C. dianae. The results suggest that the ancestor of C. dianae likely originated in the Cape region of South Africa, followed by transoceanic dispersal to St Helena estimated at 4.4 - 4.9 Mya, likely by a bird vector. The most divergent population is that on an isolated hill known as “The Barn” which may represent a distinct taxon. The existence of highly structured molecular variation within an island only 16 km long is discussed.