Document Type



Doctor of Philosophy


Biology, Plant Systematics

Date of Defense


Graduate Advisor

Elizabeth A. Kellogg


Al-Shehbaz, Ihsan


Mathews, Sarah

Stevens, Peter


The plant family Brassicaceae is economically important and contains the model genetic system Arabidopsis thaliana. Previous phylogenetic studies indicated that the historic classification system of the family was highly artificial, with several tribes likely to be para- or polyphyletic. However, these studies sampled fewer than 30 of the 338 genera of the family. We expanded the sampling of genera by four-fold and inferred phylogeny from both the chloroplast gene ndhF and the nuclear gene phytochrome A (PHYA) to determine which of the previously delimited 19 tribes of the family were monophyletic. Results from both ndhF and PHYA confirmed that the majority of Brassicaceae tribes were para- or polyphyletic. Thus, monophyletic clades from the ndhF phylogeny were used to produce a new tribal classification of the family to replace the previous, highly artificial system. PHYA phylogenetic analyses confirmed the likely monophyly of most of these new tribes. In addition, both markers retrieve phylogenies with three major clades (lineages I-III), each of which is comprised of several of the newly erected tribes. Lineages I-III are the only statistically supported nodes in phylogenies of Brassicaceae beyond the tribal level, and thus are the best hypotheses of relationships deeper in the history of the family. Phylogenetic results and SEM (scanning electron microscopy) were also used to test scenarios of trichome (epidermal hair) evolution. Brassicaceae trichomes consist of a single cell and achieve intricate, highly branched morphologies that are characterized as dendritic, medifixed, or stellate; some species produce unbranched, simple trichomes. Results from ndhF, PHYA and SEM indicate that dendritic and medifixed trichomes evolved numerous times in the history of the family, while stellate trichomes may have a single origin. Finally, we applied findings from trichome developmental studies in Arabidopsis thaliana to other trichome producing species across the family by assaying a marker of early trichome development to explore the homology of Brassicaceae trichomes with different morphologies. Results from this study indicate that differences in the number of trichome branches in Brassicaceae likely results from the action of genes associated with the cytoskeleton rather than ones active in the cell cycle.

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