A Whole New Whorl: An Exploration of the Morphology, Genetics, and Ecological Function of the Staminodes of Aquilegia

Abstract

The organ identity genes that encode the four organs of the canonical floral body plan are well characterized. In contrast, the downstream developmental pathways involved in programming the morphological differences found in novel organ identities are poorly understood. Aquilegia is a genus found in the lower eudicot family Ranunculaceae. Species from this genus have a fifth organ identity: a whorl, or concentric circle, of ten staminodes (sterile stamens) located between the four-seven whorls of fertile stamens and one whorl of five carpels. Staminodes in Aquilegia are hypothesized to function in defense, but prior to this thesis, this had never investigated in the field. In this thesis, I have used histological approaches to compare stamens and staminodes across development in Aquilegia and sister genera Semiaquilegia and Urophysa. I identified key differences in Aquilegia staminode morphology, including a marginal “curling” phenomenon, asymmetric lignification in the adaxial epidermal layer, and differences in adaxial/abaxial cell number. I then conducted an RNA-sequencing experiment to identify the genetic pathways downstream of organ identity that are responsible for different morphologies of staminodes and stamen filaments, and used in situ hybridization to more closely examine the expression of candidate genes identified from this analysis. Pathways enriched in staminodes included those involved in lignification, defense, wounding, and secondary cell wall development. Finally, I conducted three field studies, each using a different species of Aquilegia, in which I removed staminodes and counted seed sets to investigate whether removal impacted reproductive fitness. Weak decreases in seed set were observed in two species, but these results were inconclusive. These combined, diverse approaches have provided new insights into the evolutionary divergence of staminodes from stamens and facilitated the creation of more focused hypotheses for further investigation.