Dissertation Research: The Slipper Orchids: Molecular, Morphological, and Ontogenetic Insights into the Evolution of Constrained Floral Diversity

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Among the vast floral diversity of the orchid family (Orchidaceae), the four genera of the slipper orchids (subfamily Cypripedioideae) stand out as an easily defined assemblage. Their stereotyped flowers, adapted for pollination by temporarily trapping insects in their pitcher-like lips, nevertheless exhibit a secondary level of floral modifications attractive to a wide variety of pollinators. Because the floral parts of slipper orchids are so similar for functional reasons, only shape and color become available as criteria for establishing relationships based on flowers. Structure of the plant body itself offers little more data, the most recent work on this subject dating to 1966. Chloroplast DNA analysis, which has proven helpful in resolving relationships in other orchids and plant groups, will be applied to the slipper orchids in hopes of uncovering numerous characters (mutations in the DNA) that can be used to assess relationships more reliably. Chloroplast DNA exists separate from the chromosomal DNA that codes for plant and flower development, so it should serve as an excellent internal indicator of relationships in a stereotyped orchid group for which such external features are difficult to define and relate to one another. Characters of the plant and flower parts will be studied relative to the relationships uncovered with chloroplast DNA analysis in hopes of understanding more about their evolution. Additional features of the plants will also be examined for comparison to the molecular work, including external sculpturing of the seed walls. Following the work on evolutionary relationships of the slipper orchids, their geographic history will be traced. The slipper orchids display an unusual distribution, and no formal studies of this arrangement have yet been undertaken. This research will no doubt be aided by knowledge of the genetic relationships (by chloroplast DNA) of the plants in question. No fossil record exists for the Cypripedioideae, and it is hoped that by studying current distributions with respect to past continental movements, an approximate time and place of their origin may be obtained. An additional aim of this study will be an attempt to understand an evolutionary mechanism that may be responsible for some of the diversity seen in the slipper orchids. Changes in the timing of events that occur during the development of a plant can have drastic consequences for the final shape and size of both plant and floral parts. A system of two similar species of Mexican slipper orchids will be studied for such changes. One species appears to be a juvenile version of the other; this hypothesis will be tested by comparing shapes of flower parts in buds and open flowers of both species and by comparing the relative rates of growth of the two flowers.