- Top of page
- Materials and Methods
- Supporting Information
The petals of a flower can function in the attraction of pollinators and have significant ecological importance. Petals are generally larger than other floral organs, are bright in color and have papillate epidermal cells on the adaxial surface (Weberling, 1989; Irish, 2009). In Arabidopsis thaliana and its immediate relatives, these petal characteristics result from the specification of petal identity in the developing petal primordia. This petal identity specification requires the combined activities of related MADS-domain containing proteins (Coen & Meyerowitz, 1991; Pelaz et al., 2000; Honma & Goto, 2001; Theissen & Saedler, 2001). Based on studies in Antirrhinum majus and Arabidopsis thaliana, the genes required for petal identity specification have been placed into three functional classes (A-class, B-class and E-class). Studies on other flowering plants, however, indicate that the requirement of A-, B- and E-class gene functions for petal development is not conserved in all flowering plants and that the A-function hypothesized by the model is restricted to A. thaliana and its immediate relatives (Theissen et al., 1996; Munster et al., 2001; Theissen & Saedler, 2001; Stellari et al., 2004; Litt & Kramer, 2010; Rijpkema et al., 2010; Sablowski, 2010).
Heterotopic petaloidy (the occurrence of petaloid characteristics in nonpetal organs) has evolved many times during angiosperm diversification and contributes to the diversity of floral forms (Endress, 1994; Albert et al., 1998; Baum & Donoghue, 2002). Several authors have proposed that orthologous B-class petal identity genes play an important role in heterotopic petaloidy in a diversity of angiosperm species (van Tunen et al., 1993; Bowman, 1997). In this scenario, heterotopic petaloidy results from a shift of B-class gene expression to adjacent whorls (e.g. an outer shift to the first whorl to establish petal identity in sepals and a centripetal shift to the third whorl to establish petaloid stamens). This hypothesis is supported by studies on petaloid tepals of several monocot plants, for example, Tulipa in Liliaceae (van Tunen et al., 1993; Kanno et al., 2003) and Phalaenopsis in Orchidaceae (Tsai et al., 2004; Mondragon-Palomino & Theissen, 2008, 2009), as well as in a basal angiosperm Aristolochia (Jaramillo & Kramer, 2004). However, evidence from the studies of Aristolochia (Aristolochiaceae) and Aquilegia (Ranunculaceae) suggest that the function of the B-class gene homologs in heterotopic petaloidy in sepals is only required at a late developmental stage (i.e. during cell differentiation) rather than in the early stage during which organ identity is specified (Jaramillo & Kramer, 2004; reviewed in Kramer & Hodges, 2010). Furthermore, studies in Clematis (Ranunculaceae) and in Phalaenopsis (Orchidaceae) suggested that subfunctionalization following duplication of B-class genes explained heterotopic petaloidy in sepals (Kramer et al., 2003; Mondragon-Palomino & Theissen, 2008, 2009). On the other hand, a study in Asparagus found no evidence for the involvement of orthologs of Arabidopsis B-class genes in the petaloid sepals (Park et al., 2003, 2004). Thus, diverse molecular mechanisms may support heterotopic petaloidy within flowering plants.
At present, the role of B-class gene orthologs and paralogs in heterotopic petaloidy has been reported mostly for petaloid sepals and petaloid stamens. Few studies have investigated the roles of B-class gene orthologs in extrafloral petaloidy (e.g. petaloid bracts). A short list of lineages with petaloid bracts includes Saururaceae (Piperales), Nyctaginaceae and Amaranthaceae (Caryophyllales), Cornaceae and Davidiaceae (Cornales), Rubiaceae (Gentianales), Araceae (Alistmatales), Heliconiaceae (Zingerberales) and Euphorbiaceae (Malpighinales). Evidence of orthologs of Arabidopsis B-class gene expression in petaloid involucral bracts (bracts that appear in a whorl subtending an inflorescence) was recently reported by Vekemans et al. (2011) in the dove tree, Davidia involucrate (Cornales). These authors detected expression of two B-class genes and one C-class gene during an early developmental stage in bract primordia (Vekemans et al., 2011). The expression patterns of orthologs of the Arabidopsis B-class genes in petaloid bracts of other species remain uncharacterized.
An important goal of evolutionary developmental biology is to identify the molecular and developmental changes that occurred during cladogenesis that gave rise to descendant lineages differing in morphology (Kellogg, 2004). To our knowledge, there has been no analysis employing a phylogenetic approach to uncover the changes leading to the divergence between petaloid and nonpetaloid bracts among closely related species. The dogwood genus Cornus (Cornaceae, Cornales) has four closely related clades that exhibit variation in bract morphology, and thus provides a good model for understanding the molecular and developmental mechanisms of petaloid bract evolution. In Cornus, bract petaloidy evolved in two lineages that are sisters to each other, the dwarf dogwoods (DW group, e.g. C. canadensis) and the big-bracted dogwoods (BB group, e.g. C. florida). Phylogenetic studies have indicated that the petaloid clade is sister to the cornelian cherries group (CC group, e.g. C. officinalis) bearing nonpetaloid, involucral bracts. This DW-BB-CC clade is, in turn, sister to a large clade (BW group, bearing blue- or white-fruits; e.g. C. macrophylla, C. sericea; Xiang et al., 2006, 2008, 2011). Species within this BW group produce noninvolucral bracts that subtend inflorescence branches and are rudimentary and deciduous. The sister relationship of the DW and the BB in the phylogeny has led to the reconstruction of a common origin of the petaloid bracts in their common ancestor (Xiang & Thomas, 2008). Although reconstructing ancestral character states based on phylogenies can be a powerful tool to elucidate morphological evolution, an accurate understanding of morphological characters and their evolution requires data from developmental and genetic analyses. Differences in external morphology may be the result of multiple independent alterations of the developmental program that first occurred at different evolutionary times. On the other hand, even morphological similarities between closely related species may be the result of disparate developmental mechanisms that evolved independently.
In order to understand the origin (or origins) of petaloid bracts in the genus Cornus, we conducted a comparative developmental study in four species representing the four clades of this genus. We also examined the expression patterns of orthologs of Arabidopsis B-class MADS-box genes in these species to determine if evolutionary changes in their expression patterns are correlated with the origin of petaloid bracts in the genus. Furthermore, we examined the expression patterns of these genes in developing floral organs to assess if they might function in specifying during petal and stamen identity within the flower as reported in diverse flowering plant lineages.