Dissertation Research: Systematics of the Genus Dischidia (Marsdenieae, Asclepiadaceae) and of the Tribe Marsdenieae

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9902073Luckow and LivschultzDischidia (Asclepiadaceae, milkweed family) is a genus of succulent, epiphytic vines native to Southeast Asia. It is closely related to the ornamental plant Hoya (porcelain flower). The best known species, Dischidia major, has pitcher leaves that are used as nesting sites by arboreal ants. The ants accumulate organic debris inside the pitcher leaves and the plant grows adventitious roots into the leaf interior and derives nutrients from the accumulated debris. Many of the other 80 or so species of Dischidia also occur in arboreal ant nests, and they have a variety of structures that may function in ant/plant interactions. Some species have shell-shaped leaves that are held tightly against the bark of the host tree; like the pitcher leaves, these may also function as ant houses (domatia). Other species have normal leaves, but have oil-rich structures on the seeds (elaiosomes) that are attractive to ants and may facilitate seed dispersal to ant nests. There are also some species that do not occur in ant nests and do not have any structures that are considered to function in ant/plant interactions. Graduate student Tatyana Livschultz, under the direction of Dr. Melissa Luckow of Cornell University, is conducting research on the evolutionary relationships of Dischidia to understand the evolution of structures that function in ant interactions in this genus. Information from plant morphology and from nuclear and chloroplast gene sequences is being used to construct an evolutionary (phylogenetic) tree and retrace the sequence of origin of structures that facilitate ant interactions. This phylogenetic tree will also be used to revise the infra-generic classification of Dischidia and to prepare a detailed monograph of one section of this genus. Additional sampling of putative relatives of Dischidia will help in reconstructing evolutionary relationships in the tribe Marsdenieae, which comprises ca. 25 genera and about 570 species (25% of the species in the milkweed family). This analysis will test the hypothesized sister-group relationship between Dischidia and Hoya, examine relationships among genera in the tribe, and test the circumscription of the tribe. The research will provide the basis for a revised classification of these under-studied tropical groups. This classification, a summary of all available information on these plants, will be available to others interested in their evolution, ecology, conservation, cultivation, and medicinal properties.