Diplostephium comprises 111 neotropical species that live in high elevation habitats from Costa Rica to Chile. Primarily Andean, the genus seems to have undergone an adaptive radiation indicated by its high number of species, broad morphological variation, and diversification primarily in an ecosystem (páramo) that formed within the last 2–5 my. Internal transcriber spacer (ITS) sequences and several chloroplast markers, rpoB, rpoC1, and psbA-trnH were sequenced in order to infer a preliminary phylogeny of the genus. The chloroplast regions showed no significant variation within the genus. New ITS data were therefore analyzed together with published sequences for generating a topology. Results suggest that Diplostephium and other South American genera comprise a polytomy within which a previously described North American clade is nested. Monophyly of Diplostephium was neither supported nor rejected, but the formation of a main crown clade using different methods of analysis suggests that at least a good portion of the genus is monophyletic. A Shimodaira-Hasegawa test comparing the topology obtained and a constrained one forcing Diplostephium to be monophyletic showed no significant difference between them. Monophyly of some of the previously proposed series of the genus was not supported by the phylogenetic tree. Morphological character mapping results suggest that the high Andean forest tree species are derived from shrubby páramo-puna ancestors, contradicting previous hypotheses about morphological evolution of the genus and documenting an atypical trend of downslope diversification in páramo plants.
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Vol. 2012 • No. 15