Development and Evolution of Ranunculaceae Flowers

Abstract

Over thirty years ago, significant breakthroughs in plant molecular genetics facilitated the formulation of the seminal ABC model of floral development in which the coordinated expression of MADS-Box transcription factors act as a combinatorial code to confer the identity of the different floral organs. This model has proven an extremely useful framework for conceptualizing the mechanisms that underly flower development and evolution. More recently, investigation of floral development in a wider breadth of angiosperm taxa has enriched our understanding of how evolution of the floral developmental program underlies diversity of floral form, and the model has been adapted accordingly. The Ranunculales is the sister order to the remainder of the eudicots, occupying a crucial phylogenetic position for understanding the origin and evolution of core eudicot flowers. In Ranunculaceae, the most species-rich family and namesake of the order, floral morphology is highly diverse with many genus-specific synapomorphies and repeated adaptive radiations, providing an ideal system in which to study the developmental basis of evolutionary change. Through study of several recently established model systems in this group such as Aquilegia, Nigella, Delphinium, and Thalictrum, our understanding of floral development in the basal eudicots has improved significantly. In this thesis, I address gaps in our understanding of flower development, particularly in Aquilegia, with a special focus on perianth organ identity and floral architecture. In Chapter 1, I utilize a reverse genetic approach to investigate candidate A-class genes in Aquilegia to show that euAP2 homologs in Aquilegia play redundant roles in floral meristem identity and petal development, while AqAGL6 is required for proper patterning within the floral meristem and for sepal identity. In Chapter 2, I take a multi-pronged approach to investigate the identity of Ranunculaceae petals and provide evidence that these petals express partial stamen identity to modify their development. I hypothesize that the partial activation of stamen identity imparts the potential for complexity to Ranunculaceae petals, and this results in the Ranunculaceae petal ‘syndrome’ of reduced size, peltate growth, and staminiform morphology. Finally, in Chapter 3, I characterize the phenotypes of ‘double’ flowering varieties of Aquilegia vulgaris and begin to investigate the genetic underpinnings of these ‘double’ phenotypes, which will inform our understanding of how patterning of the Aquilegia flower is achieved. Taken together, these lines of study provide insight into the evolution of perianth identity in the Ranunculaceae and create a new framework for understanding the potential role of partial homeosis as a driver of morphological evolution in this lineage.