KRAMER, E. M.; JARAMILLO, M. A.; Harvard University, Cambridge MA; Harvard University, Cambridge MA: The Genetic Basis for Innovations in Floral Organ Identity.

One of the most striking features of plants is their completely modular body plan. The basic plant module, the phytomer, is composed of a lateral determinate organ (e.g. a leaf) with its axillary meristem and associated region of stem, or internode. Within this context, plants create morphological diversity by expressing different identity programs in the subunits of the phytomer. In flowers, axillary meristems are repressed and internodes are typically very short. Floral diversity, therefore, primary comes from variation in the organ identity programs that control the differentiation of the lateral determinate organs. These organs typically include sterile organs, the sepals and petals, collectively referred to as the perianth, and fertile organs, the male stamens and female carpels. The elucidation of the genetic program controlling floral organ identity in model species such as Arabidopsis has created new opportunities for understanding the evolution of floral morphology. However, some flowers do not fit simply within the Arabidopsis-based model. For example, members of the lower eudicot family Ranunculaceae, such as Aquilegia (columbine), often exhibit two whorls of morphologically distinct petals, suggesting that two separate petal identity programs may be functioning. We have identified homologs of all of the major Arabidopsis organ identity genes in Aquilegia, and have begun to characterize their expression patterns in both wildtype and mutant plants. It appears that gene duplications which occurred in the APETALA3 lineage before the diversification of the Ranunculaceae have facilitated the evolution of multiple forms of petaloid organs in this new model species. This theory is being evaluated through genetic analyses of homeotic mutants that affect the identity of petaloid organs.