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Title: The ecology and evolution of cycads and their symbionts
Abstract: Interactions among species are responsible for generating much of the biodiversity that we see today, yet coevolved associations with high species specificity are rare in nature and have sometimes been considered to be evolutionary dead ends. The plant order Cycadales is among the most ancient lineages of seed plants, and the tissues of all species are highly toxic. Cycads exhibit many specialized interactions, making them ideal for analyzing the causes and consequences of symbiotic relationships. In Chapter 1, I characterize the pollination mutualism between Zamia furfuracea cycads and their Rhopalotria furfuracea weevil pollinators. I find that pollination in this New World species pair closely mirrors that of an Old World cycad and its thrips pollinators, and that this strict, pair-wise interaction is ancestral to the group. Molecular phylogenetics and fossil evidence suggest that it represents one of the earliest insect/plant pollination mechanisms, arising long before the evolution of visual signaling commonly employed by flowering plants. In Chapter 2, I identify systematic patterns in the gut bacteria of cycad herbivores. I survey the gut microbial communities of five insect species feeding on a diversity of tissues of cycads and collected world-wise, and discover a set of core bacteria that is shared amongst cycad herbivores and not found in their non-cycad feeding relatives. Some of these microbes have known anti-cancer and nitrogen-fixing capabilities, and may function to facilitate herbivory of these toxic plants. This is the first report of a core-microbiome amongst distantly related organisms that feed on a common plant family. Finally in Chapter 3, I synthesize all published literature on cycad feeding Lepidoptera and evaluate their diets and ecology in a phylogenetic framework. Cycad feeding has independently evolved multiple times from angiosperm-feeding ancestors, representing a striking shift in host plant morphology and chemistry. As most cycad specialists are warningly colored and many are known to sequester cycad toxins, this presents an ideal system to further explore potential mechanisms of plant/insect coevolution.
Committee: Naomi Pierce and Robin Hopkins (Advisors), Brian Farrell, David Haig, Dennis Stevenson (New York Botanical Garden)