| Leafless members of tribe Vandeae (Orchidaceae) have undergone extreme reduction in habit and represent a novel adaptation to the canopy environment. Members of Vandeae form a large, pantropical, and poorly studied group of orchids traditionally divided into three subtribes: Aeridinae, Aerangidinae, and Angraecinae. Leafless taxa occur throughout Vandeae and are geographically distributed in tropical Africa, Asia, and America. The most widely accepted classification system is based solely on floral morphology, and no significant molecular or anatomical studies of Vandeae have been done. To study the evolution of leaflessness, molecular and structural evidence were used to generate phylogenetic hypotheses using Paup* 4.0 (Sinauer Associates, Sunderland, Massachusetts). The structural portion of this study was performed using both macro- and microscopic leaf and root characters. Cladistic analyses based on parsimony of 169 taxa used 26 vegetative anatomical and morphological characters. These analyses supported a monophyletic Vandeae, with very little resolution among subtribes or genera. Molecular studies used sequences from ITS nrDNA, trnL-F cpDNA, and matK cpDNA. Separate and combined maximum parsimony analyses of these three gene regions each supported only two subtribes within Vandeae: Aeridinae and a combined Angraecinae + Aerangidinae.; The DNA sequence data were also combined with structural data in cladistic analyses. These combined analyses resulted in trees with lower consistency indices, but gave fewer trees with more well-supported Glades than either data set alone. Finally, two techniques of examining character evolution were compared: (1) mapping vegetative characters onto a molecular topology and (2) tracing vegetative characters onto a combined structural + molecular topology. In both cases, structural synapomorphies supporting Vandeae and clades within Vandeae were nearly identical. Not surprisingly, a change in leaf morphology (most commonly reduced to a nonphotosynthetic scale) seems to be the most important character in making the evolutionary transition to leaflessness. Other vegetative characters that were probably also important in the transition to leaflessness were loss of tilosomes, development of an aeration complex, and development of the monopodial plant habit. |
