Evolution of zygomorphy in monocot flowers: iterative patterns and developmental constraints

Abstract

Contents Summary 25 Introduction 28 Background 27 Asparagales 28 Commelinids: ZHC clade 29 Commelinids: Poales 32 Conceptual transitions among floral forms revisited 34 Examples from monocot flowers 37 Specific cases of ontogenetic transitions between contrasting floral morphs 39 Conclusions 40 Acknowledgements 42 References 42 Summary Here we explore morphological transitions among monocot flowers and discuss them in terms of modification of existing structures. Monocot flowers are typically arranged in five whorls (carpels, inner and outer stamens, inner and outer tepals), and are radially symmetrical, constructed around a 3‐fold axis of rotational symmetry that delineates three planes of bilateral symmetry. Differential development of one or two of the three organs within a whorl inevitably emphasizes one plane of symmetry, by eliminating the other planes, rendering the whorl zygomorphic. Zygomorphy (monosymmetry) has evolved more than once in monocotyledons. Stamen suppression is a widespread aspect of zygomorphy and conforms to two broad categories of pattern. In the more frequent Pattern 1, 1–3(−5) adaxial stamens (both inner and outer) are either reduced in size or entirely suppressed. Pattern 2 involves loss, reduction or modification of at least the outer abaxial stamen. Pattern 2 often characterizes groups that are phylogenetically embedded within Pattern 1 clades, perhaps indicating that this pattern results from loss of function or overexpression of the same genes involved in establishment of Pattern 1. Heterotopy has been an important evolutionary phenomenon among Zingiberales (some stamens becoming petaloid) and perhaps grasses (petals possibly becoming sepaloid). Heterochrony is the most frequent cause of more subtle changes in the degree of bilateral symmetry shown by monocot flowers by enhancing (peramorphosis) or retarding (paedomorphosis) individual organs within specific whorls.

The Maize PI/GLO Ortholog Zmm16/sterile tassel silky ear1 Interacts with the Zygomorphy and Sex Determination Pathways in Flower Development

“…In particular, the inhibited lodicule shows no evidence of phyllotactic patterning. In many zygomorphic flowers, like those of A. majus, inhibited primordia are initiated but their subsequent growth repressed (Carpenter and Coen, 1990;Rudall and Bateman, 2004;Wang et al, 2008;Hardy et al, 2009). Furthermore, in maize, B class gene expression is asymmetrically absent from the adaxial domain.…”

“…Within the order that contains the grasses, the Poales, zygomorphy is rare, although it is pervasive in the grass family (Rudall and Bateman, 2004;Endress, 2012). Although most often discussed in terms of the second and third floral whorls (petal and stamen homologs), zygomorphy can affect all four whorls of a flower, and symmetry patterns can differ within a flower (Endress, 2012).…”

“…In addition to the presence of grass-specific organs, maize flowers are derived in two other notable respects. First, as is typical of many grasses, the lodicule whorl initiates only the two lateral members, while the medial lodicule fails to form, breaking the typical trimerous floral symmetry of monocots and making the maize flower zygomorphic (Rudall and Bateman, 2004). Additional asymmetry is present in the wide spacing of lateral stamens and in the fusion of two lateral carpels to form the silk (style), while the medial carpel contains the single ovule and does not contribute to the silk (Irish et al, 2003).…”

“…In Antirrhinum majus, and indeed in many zygomorphic flowers, all expected organs initiate, but growth of some is inhibited or altered by the zygomorphy program (Carpenter and Coen, 1990;Rudall and Bateman, 2004;Wang et al, 2008;Hardy et al, 2009). In contrast, there is no morphological evidence for the initiation of the medial-adaxial lodicule in maize.…”

Evolution ofAGL6-likeMADS Box Genes in Grasses (Poaceae): Ovule Expression Is Ancient and Palea Expression Is New

“…The palea is generally morphologically distinct from the lemma and in many grasses is two keeled, although in others it has a single keel like the lemma. There are two opposing interpretations of the homology of the palea: (1) The palea has been interpreted as a prophyll rather than a floral organ, based on its two keels and adaxial position, characteristics that it has in common with a standard monocot prophyll (Linder, 1987;Clayton, 1990;Stapleton, 1997;Soreng and Davis, 1998;Judziewicz et al, 1999;Rudall and Bateman, 2004). However, very few monocots have a prophyll adaxial to the flower; thus, a prophyll in this position would be highly unusual and would represent a novelty in the grasses.…”