The nomenclature of adeniums has two interconnected errors, as detailed in Dimmitt and Edwards 2021. Here is the short version:
1. The first adenium was described by Forsskal in 1775. The type specimen was a plant from Milhan, western Yemen. He thought that it was a succulent oleander and named it Nerium obesum.
2. In 1819 Roemer and Schultes decided that this plant belonged in its own genus and renamed it Adenium obesum (Forssk.) Roem. & Schult..
3. In 1888 Balfour Jr. described a plant from Jabal Shamsan on the Aden Peninsula of Yemen as Adenium arabicum.
4. Since then most botanists have come to the opinion that all of the adeniums in Saudi Arabia and Yemen are the same species. The DNA analysis (Dimmitt and Edwards 2021) verified this opinion. The rules of nomenclature dictate that when two published species are determined to be the same, the earlier publication has priority. Therefore Adenium obesum is the valid name for all adeniums on the Arabian Peninsula (except for those in Oman and far southeastern Yemen that were segregated as A. dhofarense).
5. But recognizing correct name for the Saudi Arabian and Yemeni adeniums would cause confusion because there are plants in East Africa named A. “obesum”. They are different from the Arabian plants, so they need a new name. Here’s where it gets complicated:
6. Dimmitt and Edwards’ study found that A. “obesum”, A. somalense, and A. crispum in East Africa are genetically so similar (in the 5 loci sequenced) that their taxonomic level can’t be confidently determined. They could be three distinct species, or one highly variable species. Furthermore, there are no known specimens of adeniums from the Sahel region from Sudan to Senegal that are available to be studied, so we have no idea what species (singular or plural) that they belong to. Several other species have been published from this East Africa/Sahel region whose identities are also not settled. These include A. arboreum, coetanum, honghel, micranthum, speciosum, and tricholepis.
7. To settle these issues, much more field work and DNA analysis of the adeniums in East Africa and the Sahel are needed. If anyone can obtain leaves, seeds, or photos of these plants, especially from Ethiopia westward, they would have tremendous scientific and horticultural value.