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One size fits all? Molecular evidence for a commonly inherited petal identity program in Ranunculales¹

Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138 USA

ABSTRACT

Petaloid organs are a major component of the floral diversity observed across nearly all major clades of angiosperms. The variable morphology and development of these organs has led to the hypothesis that they are not homologous but, rather, have evolved multiple times. A particularly notable example of petal diversity, and potential homoplasy, is found within the order Ranunculales, exemplified by families such as Ranunculaceae, Berberidaceae, and Papaveraceae. To investigate the molecular basis of petal identity in Ranunculales, we used a combination of molecular phylogenetics and gene expression analysis to characterize APETALA3 (AP3) and PISTILLATA (PI) homologs from a total of 13 representative genera of the order. One of the most striking results of this study is that expression of orthologs of a single AP3 lineage is consistently petal-specific across both Ranunculaceae and Berberidaceae. We conclude from this finding that these supposedly homoplastic petals in fact share a developmental genetic program that appears to have been present in the common ancestor of the two families. We discuss the implications of this type of molecular data for long-held typological definitions of petals and, more broadly, the evolution of petaloid organs across the angiosperms.

Key Words: APETALA3 • MADS box genes • petal evolution • PISTILLATA • Ranunculales

Received for publication 28 January 2008. Accepted for publication 19 August 2008.

FOOTNOTES

¹ The authors thank members of the Kramer and Mathews laboratories for helpful discussions and comments on the manuscript. E.M.K. thanks Dr. N. Kaplinsky and the Scott Arboretum of Swarthmore College for providing floral material of Holboellia coriacea. This research was supported by grants from the Molecular and Organismic Research in Plant History (MORPH) research coordination network to D.A.R. and E.M.K., the National Science Foundation to E.M.K. (IBN-0319103), and the Radcliffe Institute for Advanced Study to E.A.Z. D.A.R. and E.M.K. contributed equally to this work.

² Present address: Department of Biology, Duke University, Durham, NC 27708 USA

³ Author for correspondence (e-mail: ekramer{at}oeb.harvard.edu); present address: 16 Divinity Avenue, Cambridge, MA 02138 USA; phone: 617-496-3460; fax: 617-496-5854

⁴ Present address: Department of Botany and Laboratories of Analytical Biology, Museum Support Center, Smithsonian Institution, Suitland, MD 20746 USA

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