recently some interesting taxonomical changes took place in and around genus Euphorbia (Euphorbiaceae). Here is a short review:
Euphorbia is one of the largest genera in plant kingdom. Hitherto it represented the largest part of subtribe Euphorbiianae Griseb. and there was accompanied by the small so called satellite genera Cubanthus, Elaeophorbia, Endadenium, Monadenium, Synadenium and Pedilanthus.
For a long time many botanist tried to split the huge and as paraphyletic recognised genus Euphorbia into smaller and monophyletic genera. An instance is the separation of Chamaesyce by Gray (1826). As however the difference between Chamaesyse and Euphorbia was restricted to the plant shape and as there are intermediate species to Euphorbia's subgenus Agaloma, Chamaesyce was never generally accepted. In the past similar troubles to find reliable borderlines between Euphorbia and separated genera resulted in lots of synonyms like Agaloma, Anthacantha, Arthrothamnos, Dactylanthus, Esula, Lacanthis, Lyciopsis, Medusae, Poinsettia, Sterigmanthe, Tithymalus, Treisia etc. which sometimes became accepted as subgenera or sections of Euphorbia.
A genus in modern sense must be monophyletic. What means that it must have a common ancestor and must contain all groups that stem from this ancestor, the ancestor itself (if not extinct), but no other group. However, until recently Euphorbia wasn't monophyletic but paraphyletic as for instance it contained all species with true cyathia - besides those with derived characteristics like strongly reduced primary shoots (Chamaesyce), fused together nectar glands (Monadenium, Endadenium, Synadenium), fused together bracts and bilateral symmetric cyathia (Pedilanthus) and fruits that remain fleshy (Elaeophorbia). Moreover several intermediate species cast the borderline between Euphorbia and the satellites into doubt. There are "good" Euphorbia species that for instance have partly fused together bracts, bilateral symmetric cyathia, nectar glands that tend to fuse together and fruits that remain fleshy for a very long time. And there are Monadeniums in which the glands are only imperfectly fused together.
Only recently comparative DNA studies established order in the muddle. Steinmann & Porter (2002), Steinmann (2003) und Bruyns & al. (2006) demonstrated that Euphorbia consist of about 40 groups, and that the satellite genera, among Euphorbia species, nest deep within genus Euphorbia. If the satellite genera should be conserved, each the about 40 groups then must be given the rank of a genus for its own. What would result in difficult to characterise genera that can't be told apart by appearance and sometimes hardly by their DNA, up to 1500 new names and all in all a much larger muddle than before. In order to avoid this senseless splitting, the above mentioned botanists stroke the only reasonable path, enlarged the concept of Euphorbia a little and integrated the satellites. Besides the not yet processed genus Cubanthus (related to Pedilanthus and containing three non-succulent species) all species of subtribe Euphorbiinae now belong to Euphorbia.
Another result of the DNA studies is that Euphorbia consists of only 4 subgenera (Chamaesyce, Esula, Euphorbia, Rhizanthium) which hardly align with the subgenera known from traditional botany.
- Monadenium, Synadenium and Endadenium are not separable and together with the former subgenus Lacanthis (CoT relationship) belong to subgenus Euphorbia which so far only contained species with spines on shields.
- Rhizanthium contains most members of the former sections Anthacantha, Dactylanthes, Meleuphorbia, Treisia and Trichadenia.
- Chamaesyce contains the former subgenera Agaloma, Chamaesyce and Poinsettia as well as the former section Arthrothamnus whose members don't show any corporal similarities to typical Chamaesyce.
There is a list of ALL currently accepted taxa of Euphorbia on this Wikipedia page: http://en.wikipedia.org/wiki/List_of_Euphorbia_species
In this list all (more or less) succulent species are marked and all former names of Elaeophorbia, Endadenium, Monadenium, Synadenium und Pedilanthus are mentioned as synonyms to the new names.
- Steinmann, V. W. & Porter, J. M.: Phylogenetic relationships in Euphorbieae (Euphorbiaceae) based on ITS and ndhF sequence data. Ann. Missouri Bot. Gard. 89 (2002): 453–490.
- Steinmann, V. W.: The submerion of Pedilanthus into Euphorbia (Euphorbiaceae). Acta Botanica Mexicana (2003), 65: 45-50
- Bruyns, Peter V. & al.: The A new subgeneric classification for Euphorbia (Euphorbiaceae) in southern Africa based on ITS and psbA-trnH sequence data. Taxon 55 (2) May 2006: 397–420
There are two current conflicts caused by the new names by Bruyns.
- To Euphorbia kaessneri (N.E.Br.) Bruyns 2006 (syn. Monadenium kaessneri) there is already a Euphorbia kaessneri Pax 1909.
To Euphorbia pseudograntii Bruyns 2006 (syn. Synadenium grantii) there is already a Euphorbia pseudograntii Pax 1901.
In a personal correspondence Dr. Peter Bruyns informed me that he is aware of the conflicts and will give a correction in an addendum to his paper.