A phylogenetic analysis of the Datiscaceae based on DNA sequences from the ribulose-1,5-bisphosphate carboxylase/oxygenase (large subunit) gene

The Datiscaceae are a small angiosperm family composed of three genera and four species. Datisca consists of two morphologically similar species with a disjunct distribution in western North America and Asia. The monotypic genera Octomeles and Tetrameles are native to the Malaysian region; their tree habit is in sharp contrast to the herbaceous perennial Datisca species. Despite previous morphological, anatomical and chemical study of the Datiscaceae, the phylogenetic interrelationships of the members of the family remain uncertain and there is no consensus regarding the phylogenetic affinities of the family as a whole. This study obtained new DNA sequence data from the chloroplast gene encoding the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL) for each member of the Datiscaceae and for three members of the Cucurbitaceae. Other rbcL sequences included in this study were provided from the collection of M. W. Chase (University of North Carolina, Chapel Hill; Chase et al., 1993). DNA sequence data were used to infer the phylogenetic affinities of the Datiscaceae relative to other dicotyledonous angiosperms and to assess whether the family is monophyletic. DNA sequence data were analyzed using two different phylogenetic inference computer programs, PAUP and PHYLIP. Phylogenetic analyses based on the principle of parsimony and on overall similarity suggested that the Datiscaceae are not a monophyletic assemblage. Phylogenetic reconstructions placed the herbaceous Datisca species closer to representatives of the Cucurbitaceae and Begoniaceae than to the other genera within the Datiscaceae. Weighted parsimony was utilized to select among equally parsimonious trees and the resulting topology placed Begonia as the sister group to Datisca. Among the 35 other taxa included in the analysis, the sister group to the Datiscaceae/Cucurbitaceae/Begonia clade was either a group containing representatives of the genera Myrica, Morus and Rhamnus or a group containing members of the genera Celtis, Casuarina, Fagus and Chrysolepis. Both of these groups contain actinorhizal genera which suggests that the actinorhizal plants as a group may be more closely related than traditional classifications propose. Generally, the phylogenetic reconstructions showed members of the higher Hamamelidae and Dilleniidae (Violales, Theales) grouping with members of the Rosidae (Rosales, Polygalales, Rhamnales, Euphorbiales, Sapindales). The rbcL sequence analysis conducted here does not support either the monophyly of the Datiscaceae or the monophyly of some of the traditionally- recognized subclasses of dicots.