Phylogeny and taxonomy of Tribulocarpus (Aizoaceae): A paraphyletic species and an adaptive shift from zoochorous trample burrs to anemochorous nuts

The phylogenetic position and circumscription of the disjunct African genus Tribulocarpus is investigated with DNA sequences from plastid rps16 and trnL-F, and nuclear ITS. Representatives from all four subfamilies of Aizoaceae are included in the analyses, most of them from Aizooideae and Sesuvioideae. The position of Tribulocarpus as sister to the rest of Sesuvioideae is confirmed, and we agree that the genus should be referred to this subfamily. Tetragonia retusa is found to be part of the Tribulocarpus clade with strong support. A new taxonomy of Tribulocarpus with two species is proposed and the new combination Tribulocarpus retusus is made. A lectotype is designated for Tetragonia somalensis. Tribulocarpus dimorphanthus, disjunctly distributed in south-western and north-eastern Africa, is retrieved as paraphyletic with regard to T. retusus, endemic in Somalia, as the latter is sister to the north-east African part of T. dimorphanthus only. Despite the genetic difference between the samples from the disjunct populations of T. dimorphanthus no morphological differences could be detected. Tribulocarpus dimorphanthus—T. retusus is suggested as an example of a progenitor-derivative species pair, where T. retusus has budded off from the north-eastern partial area of distribution of T. dimorphanthus. The compound and spiny fruits of T. dimorphanthus versus the simple and broadly winged fruits of T. retusus indicate a rapid adaptive shift from zoochorous trample burrs to anemochorous nuts.