Phylogenetics and taxonomy of the N ew W orld leafy spurges, E uphorbia section T ithymalus ( E uphorbiaceae)

The 480 species of leafy spurges, E uphorbia subgenus E sula , represent the main temperate radiation in the large genus E uphorbia . This group is distributed primarily in temperate E urasia, but with smaller, disjunct centres of diversity in the mountains of the O ld W orld tropics, in temperate southern A frica and in the N ew W orld. The majority of N ew W orld diversity (32 species) occurs in a single section, section T ithymalus . We analysed sequences of the nr ITS and plastid ndh F , trn H ‐psb A , trn S ‐trn G and trn D ‐trn T regions to reconstruct the phylogeny of section T ithymalus and to examine the origins and diversification of the species native to the N ew W orld. Our results indicate that the N ew W orld species of section T ithymalus form a clade that is sister to the widespread, weedy E . peplus . The N ew W orld species fall into two primary groups: a ‘northern annual clade’ from eastern N orth A merica and a diverse clade of both annual and perennial species that is divided into three subgroups. Within the second group, there is a small ‘southern annual clade’ from T exas and northern M exico, a perennial ‘ B rachycera clade’ from the western U nited S tates and northern M exico, and a perennial ‘ E suliformis clade’ from montane areas of M exico, G uatemala, H onduras and the C aribbean island of H ispaniola. Ancestral state reconstructions indicate that the annual habit probably evolved in the ancestor of E . peplus and the N ew W orld clade, with a subsequent reversal to the perennial habit. In conjunction with this phylogenetic framework, the N ew W orld species of section T ithymalus are comprehensively reviewed. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society , 2014, 175 , 191–228.