Floral developmental genetics in the Ranunculales
Researchers: Elena Kramer, Bharti Sharma (collaborator), Scott Hodges (collaborator), Hongzhi Kong (collaborator)
Members of the family Ranunculaceae offer the opportunity to study the evolution of novel perianth morphologies. This is due to the coexistence of two distinctly different types of petaloid organs in many genera of the family. In the first whorl of these flowers, the organs are petaloid but have many of the developmental hallmarks of sepals, which is how botanists refer to them. Many genera of the family exhibit an additional type of sterile organs, which although petaloid, bear a striking likeness to stamens in many aspects of their development and even, in some cases, their final appearance. Botanists alternatively refer to these organs as petals or staminodia but to avoid confusion with other types of staminodia, we simply call them petals. Across the family, a wide variety of perianth forms are observed as a result of different combinations and elaborations of these two types of petaloid organs (see above and the Flower Gallery). In some genera, such as Thalictrum, only petaloid sepals are produced. In other cases, as with Aquilegia, both types of petaloid organ are present. Some genera have species representing both these morphologies, as exemplified by Clematis. In still other cases, the first whorl organs resemble leaf-like sepals and the petals are much less staminoid. Although this last type of perianth, found most notably in Ranunculus, resembles that of the core eudicots, these petals are thought to much more recently derived, like the nectiferous petals found in species like Xanthorhiza. It should be noted that the phylogeny of the family is relatively well established and is consistent with a single evolution of petals, followed by several losses (Rasmussen et al. 2009; Zhang et al. 2013), although botanists have long held that the petals arose many times independently from stamens. Thus, the Ranunculaceae affords us the opportunity to study multiple perianth types.
We began this study by examining the evolution of the AP3 and PI gene lineages across the order Ranunculales (Kramer et al., 2003). This work demonstrated that two ancient duplications in the AP3 lineage gave rise to three paralogous lineages, which have been retained in many members of the order. In contrast, the multiple PI paralogs that we detected are the products of comparatively recent duplications. In order to understand the functional implications of these duplications events and whether they relate to the multiple types of petaloid organs produced across the family, we have focused on the genus Aquilegia as a model for floral genetics in the Ranunculaceae. We have established the floral expression patterns of homologs of all the Arabidopsis floral organ identity genes in Aquilegia. The availability of functional genetic tools in Aquilegia, as well as the development of a wide array of genomic resources, allows us to directly assess the functions of these genes in vivo. At the same time, we have used comparative gene expression studies across additional representatives of the Ranunculales to understand whether petal identity programs have evolved independently. In particular, we have focused on the AP3-3 lineage. Surprisingly, orthologs of this lineage are almost always expressed in a petal-specific pattern in members of both the Ranunculaceae and Berberidaceae while apetalous genera lack expression of the gene entirely (Rasmussen et al., 2009; Sharma et al., 2011). Furthermore, we have used RNAi to knock-down the AP3-3 ortholog in Aquilegia, demonstrating that it is essential to petal identity (Sharma et al. 2011). This could represent a subfunctionalization of the ancestral petal/stamen identity function normally associated with the AP3 lineage.
Taken together, these data suggest that the petals of most members of the Ranunculaceae and Berberidaceae share a kind of process homology and may not be independently derived, as long thought. Our collaborator Dr. Hongzhi Kong rigorously tested this prediction (Zhang et al. 2013) and showed that in multiple independent instances, apetalous genera have pseudogenized or completely lost their Ap3-3 ortholog while closely related genera bearing petals express the ortholog in a petal-specific manner. This pattern is much more consistent with many independent loses of the petal identity program rather than independent derivations. We are now exploring the question of how petal identity might be turned off from a mechanistic perspective, together with our collaborators Scott Hodges and Hongzhi Kong.
Kramer, E.M., Di Stilio, V.S., and Schlüter, P. (2003) Complex patterns of gene duplication in the APETALA3 and PISTILLATA lineages of the Ranunculaceae. Int’l J Plant Sci, 164:1-11.
Rasmussen, D. E., Kramer, E.M. and Zimmer, E.A. (2009) One size fits all? Molecular evidence for a commonly inherited petal identity program in the Ranunculales. American Journal of Botany, 96:96-109.
Sharma, B., Guo, C., Kong, H., Kramer, E.M. (2011) Petal-specific subfunctionalization of an APETALA3 paralog in the Ranunculales and its implications for petal evolution. New Phytologist, 190:870-883.
Zhang, R., Guo, C., Zhang, W., Wang, P., Li, L., Duan, X., Du, Q., Zhao, L., Shan, H., Hodges, S. A., Kramer, E. M., Ren, Y., and Kong, H. (2013) Disruption of the petal identity gene APETALA3-3 is highly correlated with loss of petals within the buttercup family (Ranunculaceae). Proceedings of the National Academy of Sciences, USA, 110:5074-5079.
