More than one sweet tabaiba: disentangling the systematics of the succulent dendroid shrub Euphorbia balsamifera

Abstract

The sweet tabaiba (Euphorbia balsamifera Ait.) is one of the classical examples of the disjunct biogeographic pattern known as Rand Flora. This species is currently circumscribed to comprise two subspecies, Euphorbia balsamifera subsp. balsamifera and E. balsamifera subsp. adenensis (Deflers) P.R.O. Bally, with their respective areas of distribution separated by a gap of about 2000 km across central-east Africa. We use multiple sources of evidence including phylogenomics, morphometrics, lineage ages, and climatic niche analysis to disentangle the confusing taxonomy around this charismatic Euphorbia lineage. Based on our integrative approach, we reestablish the two current subspecies to the rank of species (E. balsamifera Ait., E. adenensis Deflers) and resurrect the long forgotten E. sepium N.E. Br., a commonly used species in the western Sub-Saharan and Sahelian regions. The taxonomic treatment presented here includes an identification key, updated morphological descriptions, and lectotypification of several names.