David A. Rasmussen, 2 Elena M. Kramer, 3 and Elizabeth A. Zimmer 4
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American Journal of Botany 96(1): 96–109. 2009. O NE SIZE FITS ALL? M OLECULAR EVIDENCE FOR A COMMONLY INHERITED PETAL IDENTITY PROGRAM IN RANUNCULALES 1 David A. Rasmussen, 2 Elena M. Kramer, 3 and Elizabeth A. Zimmer 4 Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138 USA Petaloid organs are a major component of the fl oral diversity observed across nearly all major clades of angiosperms. The vari- able 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, exemplifi ed 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-specifi c across both Ranunculaceae and Berberidaceae. We conclude from this fi nding 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 defi nitions of petals and, more broadly, the evolution of petaloid organs across the angiosperms. Key words: APETALA3 ; MADS box genes; petal evolution; PISTILLATA ; Ranunculales. The early evolution of angiosperms remains shrouded in Character state reconstructions based on modern phyloge- mystery in part because we lack a clear understanding of how netic relationships have shown that petaloid organs likely fl owers and their associated organs evolved. Reproductive or- evolved early during the crown angiosperm radiation ( Zanis gans such as stamens and carpels are key morphological inno- et al., 2003 ; see Hileman and Irish, 2009, pp. 83 – 95 in this vations in angiosperm biology, and it seems likely that they issue). Consistent with this, many angiosperms of the Amborel- represent modifi cations of pre-existing structures, although the laceae, Nymphaeales, and Illiciales (or ANITA grade), as well exact nature of this modifi cation is controversial ( Theissen as many magnoliids, possess petaloid perianths ( Endress, 1994, et al., 2002 ; Baum and Hileman, 2006 ; Frohlich and Chase, 2007 ). 2003 ). However, while most early angiosperms have a perianth Most angiosperm fl owers have a sterile perianth composed of composed entirely of petaloid or weakly differentiated organs petaloid and/or protective organs, but unlike stamens and car- known as tepals, other taxa possess a bipartite perianth com- pels, which are widely believe to have evolved only once, it posed of morphologically distinct sepals and petals. In fl owers remains unclear whether petaloid organs evolved once in a where the perianth is bipartite, the second whorl of petaloid or- common ancestor or independently in different lineages ( Baum gans are often thought to resemble sterilized stamens in aspects and Whitlock, 1999 ; Kramer and Jaramillo, 2005 ). Because the of their development and morphology ( Takhtajan, 1991 ). Fun- evolution of petaloid organs facilitated the morphological di- damental morphological and developmental differences be- versifi cation and ecological specialization of fl owers, a resolu- tween these putative staminoid petals, or “ andropetals, ” and tion of issues regarding their evolutionary origins may provide petaloid tepals thought to be derived from bracts, or “ bracteo- general insights into early angiosperm diversifi cation. petals, ” have been interpreted as evidence against a single deri- vation of petaloid organs ( Eames, 1961 ; reviewed in Takhtajan, 1991 ). Furthermore, the evolutionary distribution of andropet- 1 Manuscript received 28 January 2008; revision accepted 19 August 2008. als and bracteopetals across different lineages of fl owering The authors thank members of the Kramer and Mathews laboratories for plants suggest that petaloid organs evolved many times inde- helpful discussions and comments on the manuscript. E.M.K. thanks Dr. pendently ( Bierhorst, 1971 ; Takhtajan, 1991 ). N. Kaplinsky and the Scott Arboretum of Swarthmore College for providing fl oral material of Holboellia coriacea . This research was supported by One of the most diverse clades in terms of perianth morphol- grants from the Molecular and Organismic Research in Plant History ogy is the eudicot order Ranunculales, particularly the family (MORPH) research coordination network to D.A.R. and E.M.K., the Ranunculaceae. Many genera of Ranunculaceae have bipartite National Science Foundation to E.M.K. (IBN-0319103), and the Radcliffe perianths with second-whorl petals that strongly resemble mod- Institute for Advanced Study to E.A.Z. D.A.R. and E.M.K. contributed ifi ed stamens ( Tamura, 1965 ; Kosuge, 1994 ). Cited similarities equally to this work. between the petals and stamens include their phyllotactic pat- 2 Present address: Department of Biology, Duke University, Durham, tern, vasculature, developmental kinetics (e.g., their timing of NC 27708 USA initiation), appearance of the early primordia, and fi nal mor- 3 Author for correspondence (e-mail: [email protected]); phology. At the same time, the morphology of the petals and the present address: 16 Divinity Avenue, Cambridge, MA 02138 USA; phone: entire perianth varies greatly within the family ( Fig. 1 ; Tamura, 617-496-3460; fax: 617-496-5854 4 Present address: Department of Botany and Laboratories of Analyti- 1965 ; Kosuge and Tamura, 1989 ; Kosuge, 1994 ). Many taxa cal Biology, Museum Support Center, Smithsonian Institution, Suitland, actually possess two types of petaloid organs: large, showy se- MD 20746 USA pals in the fi rst whorl and highly variable, often nectiferous pet- als in the second. This perianth type is exemplifi ed by Aquilegia doi:10.3732/ajb.0800038 L., Xanthorhiza Marshall, and Trollius L. ( Fig. 1B – D, F – H ). 96 January 2009] Rasmussen et al. — Petal identity in the Ranunculales 97 However, in genera such as Ranunculus L., petals do not re- ( Kramer et al., 2003, 2007 ). In Aquilegia , AqAP3– 3 expression semble stamens at maturity, and the sepals are leaf-like ( Fig. is petal specifi c, and across the family, the orthologs are gener- 1M, N ). At the same time, it is very common to fi nd taxa that ally not expressed in species or mutant cultivars that lack petals have petaloid sepals but lack second whorl petals all together, ( Kramer et al., 2003, 2007 ). In Papaver , the AP3-III ortholog is as in Anemone L. or Caltha L. ( Fig. 1I, J ). This variation is even required for petal identity although it is not petal specifi c in its observed within genera, such as in Clematis L. where some spe- expression ( Drea et al., 2007 ). These fi ndings raise the possibil- cies possess petals ( Fig. 1K, L ), while other species are apeta- ity that a typical B class function was ancestral in the order and lous. It should be noted that in contrast to the term tepal, the was later subdivided into the petal-specifi c expression of the outer perianth organs of the Ranunculaceae are always referred AP3-III , which would represent subfunctionalization (sensu to as sepals, even if they are not part of a truly bipartite perianth Force et al., 1999 ) of a commonly inherited petal identity pro- ( Tamura, 1965 ). The considerable variation observed in petals gram. Unfortunately, current data are not suffi cient to clearly within and among genera of Ranunculaceae has been consid- support this model. ered to be consistent with a homoplastic pattern of petal evolu- As a fi rst step toward understanding whether a conserved tion, with staminoid petals gained independently on multiple petal identity program exists within Ranunculaceae (and more occasions ( Prantl, 1887 ; Worsdell, 1903 ; Hoot, 1991 ; Kosuge, broadly, Ranunculales), we identifi ed AP3 and PI homologs 1994 ; Hoot and Crane, 1995 ). from 11 previously unsampled taxa and used RT-PCR to deter- Despite differences in morphology, molecular studies of fl o- mine the expression patterns of all known homologs in 13 dif- ral developmental genetics have provided evidence for a con- ferent genera. These target taxa were chosen to represent a broad served genetic program promoting petaloidy across most of the range of petal and perianth morphology, as well as to span mul- angiosperms. In core eudicots such as Arabidopsis and Antir- tiple families in the order. While previous studies have focused rhinum , the B-class MADS-box genes APETALA3 ( AP3 ) and on the expression patterns of AP3/PI genes from distantly re- PISTILLATA ( PI ) are critical to the specifi cation of petal and lated taxa, the current study allows us to use expression patterns stamen identity within the developing fl ower ( Coen and Meye- to analyze variation in the petal identity program on a narrower rowitz, 1991 ). Comparative genetic studies have shown that scale. Of particular interest is the fact that the AP3-III orthologs AP3 / PI homologs are commonly expressed in petaloid organs of were found to be largely petal-specifi c across Ranunculaceae other taxa, including monocots, magnoliids, and angiosperms of and Berberidaceae. We interpret this fi nding as evidence for the ANITA grade (reviewed Kim et