Molecular phylogenetics of Euphorbiaceae sensu lato - COMPLETED 2008

Ricinis communis (Euphorbiaceae). Photo: P. Rudall.

Following the break-up of Euphorbiaceae sensu lato (Malpighiales) into five major lineages recognised at family level by the Angiosperm Phylogeny Group, molecular phylogenetic studies have been carried out in the last five years with the objective of clarifying circumscription and internal relationships. The studies are a collaboration between the Smithsonian Institution, Vienna University, and RBG Kew. Multi-gene analyses of the two largest segregate families, Euphorbiaceae sensu stricto and Phyllanthaceae, are now published, with Picrodendraceae and a revised classification of Euphorbiaceae s.s. to follow.

Especially in Phyllanthaceae, the largest biovulate lineage with c. 2,000 species in 59 presently accepted genera, our results differ markedly from previous classifications. A revised phylogenetic classification of the family is in press. The analyses, with near-complete generic sampling, investigated sequences of the plastid genes atpB, matK, ndhF, rbcL, and nuclear gene PHYC. The new groupings correlate well with pollen and seed coat data, but floral and fruit characters are found to be less reliable arbiters of evolutionary relationships. The main division into two strongly supported clades, characterized by inflorescence and leaf anatomical features, is not reflected in previous classifications. The racemose clade (subfamily Antidesmatoideae) roughly corresponds to Antidesmateae + Bischofieae + Hymenocardieae sensu Webster. The fasciculate clade (subfamily Phyllanthoideae s.s.) contains all remaining tribes of Phyllanthoideae sensu Webster. Centroplacus and Putranjivaceae (Phyllanthoideae-Drypeteae) are excluded from Phyllanthaceae. Croizatia, previously thought to be a “basal” member of Euphorbiaceae-Oldfieldioideae (Picrodendraceae), Dicoelia (previously Euphorbiaceae-Acalyphoideae) and Lingelsheimia, previously thought to be related to Drypetes, are all embedded in Phyllanthaceae. The largest genus Phyllanthus was shown to be paraphyletic and to include Breynia, Glochidion, Reverchonia and Sauropus, which brings the number of Phyllanthus species to over 1,200. Two new genera will be recognised in Phyllanthaceae.

Even after the exclusion of four segregate families, uniovulate Euphorbiaceae sensu stricto is one of the largest plant families with c. 6,300 species. It includes a number of economically important plants, especially rubber (Hevea brasiliensis), cassava (Manihot esculenta), the castor oil plant (Ricinus communis), and poinsettias (Euphorbia pulcherrima). Investigation of rbcL and trnL-F sequence data has largely confirmed the currently accepted classification, but some taxonomic adjustments are necessary (Wurdack et al. 2005). The division in three subfamilies will have to be modified to reflect a more complex evolutionary scenario than previously thought. Two new subfamilies found to be successive sisters to all other Euphorbiaceae s.s., Peroideae and Cheilosoideae, have already been formally recognised. The remainder of the family falls into seven major lineages including Erismantheae and Acalyphoideae s.s. (parts of Acalyphoideae), Adenoclineae s.l., Gelonieae, articulated crotonoids and inaperturate crotonoids (parts of Crotonoideae), and Euphorbioideae. Two subfamilies, 14 tribes, and 10 genera were found to be para- or polyphyletic. Lineages in Euphorbiaceae s.s. are distinguished by seed and fruit morphology, presence and absence as well as structure of laticifers, pollen characters, and stipule arrangement.