Document Type

Dissertation

Degree

Doctor of Philosophy

Major

Biology

Date of Defense

12-6-2012

Graduate Advisor

Robert Marquis

Committee

Elizabeth A. Kellogg

Tiffany M. Knight

Robert E. Ricklefs

Arthur R. Zangerl

Abstract

Animal-plant interactions may prevent gene flow and promote divergent selection among closely related plants, ultimately leading to formation of new species. This may be the case for Chamaecrista sect. Xerocalyx, in which two or more of the 24 varieties often are encountered in the same area, with marked morphological and phenological differences among them. Over a broad geographical range, however, the morphological gaps among varieties disappear. Several biotic interactions contribute to the fitness of Chamaecrista species. Their flowers are pollinated by bees, and herbivores attack their leaves, flowers, fruits, and seeds. In addition, all species produce extrafloral nectar, which attracts ants that may attack herbivores. Thus, dissimilarities in the morphology of sympatric Chamaecrista can potentially reduce plant-plant competition for pollinators and mutualistic ants, reduce the number of shared herbivores, and/or reflect diverging strategies for resource acquisition and defense against herbivory. Hand pollination experiments demonstrated that production of hybrid seeds among syntopic varieties of C. desvauxii, a species within section Xerocalyx, was severely limited. In addition, co-occurring varieties were clearly distinguishable based on morphological traits, including the sizes of flowers, leaves, and extrafloral nectaries (EFNs). Removal of the EFNs resulted in decreased fruit and seed set, but only for the variety with the largest EFN. These results support separating the varieties into different species, and suggest a role for interactions with mutualists and herbivores in shaping morphological traits of co-occurring taxa in this group. To understand the patterns of taxa co-occurrence in this group, a novel approach was used to assess local morphological dissimilarity across a wide geographic scale using collection data. Both vegetative and reproductive traits were more dissimilar between pairs of sympatric individuals of different varieties than between pairs of allopatric individuals. Based on permutation tests, this pattern is more likely to stem from competitive exclusion and ecological sorting than from character divergence following competitive interactions. This work provides new insight into the patterns and processes of coexistence in phenotypically continuous taxa, from local to broad geographic scales.

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Biology Commons

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