Floral symmetry genes CYCLOIDEA and DICHOTOMA: Gene duplication and the evolution of stamen number in Antirrhineae (Veronicaceae)

Studies of floral developmental evolution aim to uncover the genetic basis for morphological differences between species, and to determine the mechanisms by which developmental pathways diversify. The floral symmetry genes, CYCLOIDEA (CYC) and DICHOTOMA (DICH) determine dorsal flower identity and patterns of stamen abortion in the model developmental species Antirrhinum majus. However, little is known about diversification of the CYC/DICH gene family, or the role of these genes in shaping the evolution of flower form. In Chapters 1 and 3, I show that the duplication leading to the CYC/DICH gene family occurred before the radiation of the Antirrhineae (the tribe to which A. majus belongs), and that additional duplications are associated with the tetraploid North American Antirrhinum lineage. Further, using explicit codon-based models of evolution in a likelihood framework, I show a general pattern of paralogous lineage retention without evidence for the evolution of novel protein function. The genetic basis for the evolution of stamen number and dorsal petal symmetry in Mohavea is explored in Chapter 2. The genus Mohavea is nested within the tetraploid North American Antirrhinum clade, but has been given generic status due to its divergent floral morphology, including superficial radial corolla symmetry and lateral stamen abortion. I show that CYC and DICH are expressed in the lateral and dorsal stamen primordia of Mohavea confertiflora, in contrast to the dorsal primordia alone in A. majus. Furthermore, DICH is not expressed in the corolla after the initiation of dorsal petal primordia, in contrast to being expressed in the admedial half of each dorsal petal in A. majus. These expression data suggest that changes in the regulation of CYC and DICH are responsible for the evolution of lateral stamen abortion and internal dorsal petal symmetry in Mohavea. In Chapter 4, I provide an ecological framework for understanding floral diversification in the Antirrhineae. Specifically, I propose an adaptive scenario for the evolution of Mohavea flower morphology. By placing character evolution in a phylogenetic framework, I suggest that aspects of Mohavea floral form likely preadapted Mohavea confertiflora to a mimetic relationship with the distantly related, Mentzelia involucrata.