Phylogeny and Systematics of Kewa (Kewaceae)

Abstract

The genus Kewa in the monogeneric family Kewaceae (Caryophyllales) is revised. Six species are recognized, K. acida on St. Helena, K. angrae-pequenae in Namibia and South Africa, K. arenicola (incl. K. trachysperma) in South Africa, K. bowkeriana (incl. K. suffruticosa) widespread in eastern and southern Africa and inMadagascar, K. caespitosa in Angola and Namibia, and K. salsoloides in Namibia, Botswana, and SouthAfrica. Kewa is morphologically distinctive, notably by its simple perianth where the two outer perianth-segments are more or less sepaloid and the three inner ones more or less petaloid, and by its indumentumof short glandular hairs, often with prominent, persistent, wart-like bases. All species have an acid taste, apparently due to the presence of oxalic acid. All names are typified, including one lectotype designated here. An identification key and distribution maps for all species are provided. The phylogeny of Kewa is reconstructed based on plastid trnK-matK and rbcL and nuclear ITS sequences. Kewa is strongly supported and the included species have strong to no support, whereas the relationships between the species are mostly unsupported. Thephylogeny is datedand the estimated age of the Kewa stemclade is (37.5-)45.0(-57.0)million years and of the crown clade (3.0-)3.9(-7.4) million years. The age of the crown clade would also be the estimated date when K. acida on the approximately 14 million years old St. Helena diverged from its potential sister group on the African continent, and would coincide with the earliest possible date for the introduction of the ancestor of K. acida to St. Helena.