The genus Zygocarpum is described with six species formerly of Ormocarpum. This genus is endemic to the Horn of Africa, the southern end of the Arabian Peninsula, and the island of Socotra. Analysis of combined nuclear ribosomal ITS/5.8S and morphological data reveals that Zygocarpum is strongly supported as monophyletic and the basal-most lineage of a mainly African-Madagascan clade comprising also Ormocarpum, Ormocarpopsis, and Peltiera. A second analysis involving a combination of nuclear ribosomal ITS/5.8S, chloroplast trnL intron, and morphological data reveals some well supported relationships among the species of Zygocarpum. Zygocarpum dhofarense and Z. yemenese, endemic to the Arabian Peninsula, form a sister clade to the central Somalian Z. gillettii. The precise relationships among the other two Somalian species, Zygocarpum rectangulare and Z. somalense, as well as the endemic Socotran Zygocarpum coeruleum, are less certain. The distribution of Zygocarpum, which spans the Gulf of Aden, is considered to be due to a Late Tertiary vicariant event involving the separation of the Arabian Peninsula from continental Africa.
Communicating Editor: Aaron Liston