Cladogram of Panamanian Clusia Based on Nuclear DNA: Implications for the Origins of Crassulacean Acid Metabolism

: The internal transcribed spacer (ITS) region of 18-26S nuclear ribosomal DNA repeat was sequenced for 31 of the approximately 40 species of Clusia known to occur in Panama. Several species from other genera of the Clusiaceae were used as outgroups in the phylogenetic calculation. High sequence alignment and minimal length variation among ITS-1, 5.8S and ITS-2 sequences facilitated determination of positional homology of nucleotide sizes. Sequence alignment was evaluated with character state (Maximum Parsimony) and distance methods (Neighbour Joining). Phylogenetic trees obtained with the two methods were largely concordant and revealed three main groups that roughly correspond to previous arrangements of species into three large morphological groups, the C. flava group, the C. minor group and the C. multiflora group. Because species of Clusia are either regular C3 plants or exhibit crassulacean acid metabolism (CAM) involving varying proportions of CO2 fixation in the dark versus the light, we mapped photosynthetic pathways onto the cladograms. Photosynthetic pathway classification was based on measurements of 13C/12C ratios of plant carbon and also on information available from the literature. Both the C. flava and C. minor group contained species exhibiting CAM, distributed on distinct branches of the cladograms, whereas the third group (C. multiflora group) was composed of species which are not known to use CAM.