A model of bulb evolution in the eudicot genus Oxalis (Oxalidaceae)

The origins and monophyly of the bulbous habit in the eudicot genus Oxalis are uncertain, but key character state transitions in the evolution of true bulbs are currently thought to be reflected in extant pseudobulbous and other geophytic taxa. We test the relationships between the two major groups of bulbous Oxalis taxa, namely the southern African lineage which is centered in the speciose Cape Floristic Region (CFR), and the New World section Ionoxalis, by including the rhizomatous geophyte Oxalis acetosella, the caudiciform stem succulent Oxalis articulata, and the rhizomiform pseudobulbous Oxalis triangularis, in combined phylogenetic analyses of nrITS and trnL-F sequence data. We optimize several key bulbous characters in ancestral state reconstructions on produced phylogenies. Results of our analyses indicate that the evolution of bulbous characters in the genus is more complex than previously thought. Although the two major bulb types are homologous, the rhizomiform pseudobulbous habit arises from within true bulbs, and in most reconstructions the caudiciform stem succulent O. articulata is inferred to have secondarily lost several distinctive bulbous characters. O. acetosella is not as closely related to the bulbous lineage as previously thought. More sampling from other key taxa are needed before the order in which key bulbous characters were acquired can be verified. We discuss these results in terms of the taxonomic and ecological implications for the CFR Oxalis taxa.