Molecular and Morphological Systematics of The Subtribe Oncidiinae (Orchidaceae)

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9815821Williams The taxonomic classification of the orchid family, with an estimated 20,000 species or more (the largest family of flowering plants), has depended heavily upon features of the orchid flower. Many aspects of floral form in orchids appear to be adaptations to animal pollinators, to facilitate or ensure within-species transfer of pollen, and are less likely to signify close genealogical relationship than previously assumed. As data have accumulated on other morphological and anatomical features, these cases of suspected convergence or parallelism in floral form have become more numerous and troublesome for a stable classification of the group. Modern DNA sequencing technologies now provide sources of independent genetic data for use in assessing phylogenetic or genealogical relationships among species, genera, and tribes within the family. Dr. Norris Williams of the University of Florida is studying the neotropical subtribe Oncidiinae for DNA sequences from nuclear ribosomal genes and from chloroplast gene regions, to amass data bearing on mutational differences among this large array of orchids. The group comprises about 80 genera and 1500 species, and sampling will cover all the genera and ca. 500 of the species, with particular attention to the large genera Oncidium, Odontoglossum, and Cyrtochilum. The subtribe Oncidiinae includes orchids with a wide spectrum of pollination relationships, including pollen transfer by male euglossine bees (collecting floral fragrance compounds), by other species of bees and insects collecting nectar or pollen, and by hummingbirds. Controversial cases of pollination by insects where no apparent food reward is discernible have been described under the rubric of pseudocopulation. In addition, the subtribe includes numerous small orchids that are "twig epiphytes," growing literally on the ends of twigs in neotropical rainforests, and including some of the smallest known orchid plants. A stable classification of these and other orchids, based on substantial gene sequence data from both nuclear and plastid sources, will establish a solid framework for the study of evolution of pollination relationships and of growth form and function in tropical forests.