Phylogenetics of the Chamaesyce clade (Euphorbia, Euphorbiaceae): Reticulate evolution and long-distance dispersal in a prominent C₄ lineage

Premise of the study: The Chamaesyce clade of Euphorbia is the largest lineage of C ₄ plants among the eudicots, with 350 species including both narrow endemics and cosmopolitan weeds. We sampled this group worldwide to address questions about subclade relationships, the origin of C ₄ photosynthesis, the evolution of weeds, and the role of hybridization and longdistance dispersal in the diversification of the group.Methods: Two nuclear (ITS and exon 9 of EMB2765) and three chloroplast markers (matK, rpl16, and trnL-F) were sequenced for 138 ingroup and six outgroup species. Exon 9 of EMB2765 was cloned in accessions with > 1% superimposed peaks.Key results: The Chamaesyce clade is monophyletic and consists of three major subclades [1(2,3)]: (1) the Acuta clade, containing three North American species with C ₃ photosynthesis and C ₃ -C ₄ intermediates; (2) the Peplis clade, mostly North American and entirely C ₄; and (3) the Hypericifolia clade, all C ₄, with both New World and Old World groups. Incongruence between chloroplast and ITS phylogenies and divergent cloned copies of EMB2765 exon 9 suggest extensive hybridization, especially in the Hawaiian Islands radiation.Conclusions: The Chamaesyce clade originated in warm, arid areas of North America, where it evolved C ₄ photosynthesis. From there, it diversified globally with extensive reticulate evolution and frequent long-distance dispersals. Although many species are weedy, there are numerous local adaptations to specific substrates and regional or island radiations, which have contributed to the great diversity of this group.