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The immense diversity of floral monosymmetry and asymmetry across angiosperms.

Summary: Evolution of Floral Monosymmetry and Asymmetry

Floral monosymmetry and asymmetry come in different forms. They may be based on complexity or on simplicity. The first case is based on evolutionary elaboration, mostly as a further step from elaborate polysymmetry or monosymmetry, respectively, the second commonly on evolutionary reduction. Imbricate organ aestivation is an additional kind of asymmetry but was not considered here, except for contort petal aestivation, which may lead to relatively conspicuous asymmetry. Monosymmetry or asymmetry may also appear in cryptic forms, if only early stages of floral development are affected.

These different kinds of floral monosymmetry and asymmetry are not necessarily all homologous, even if taken in the sense of biological homology (Wagner, 1989, 2007), i.e. based on the same genetic system, even if not all components of a clade have the particular symmetry feature. The same is true for asymmetry.

The systematic survey of the occurrence of floral monosymmetry shows that it is widespread and is present in one or the other form in almost every larger angiosperm clade (Fig. 5). The basalmost clade with monosymmetry by simplicity are Hydatellaceae (Nymphaeales) (Saarela et al., 2007). Basal angiosperms with relatively elaborate monosymmetry are Aristolochia (Piperales, magnoliids) and some Papaveraceae and Ranunculaceae (Ranunculales; basal eudicots) (Damerval et al., 2007). In monocots, the basalmost clade, Acorus, is monosymmetric by a stronger development of the lower half of the flower (Buzgo & Endress, 2000). More elaborate monosymmetry is present in Orchidaceae of Asparagales. Despite its almost ubiquitous occurrence among angiosperms at order level, monosymmetry also shows special concentration in some suprafamilial clades: among rosids especially in Fabales of fabids and in malvids (Endress & Matthews, 2006), among asterids especially in Lamiales, Asterales and Dipsacales, and among monocots especially in commelinids. Oblique monosymmetry is relatively widespread in angiosperms. However, it is especially concentrated in Sapindales, Brassicales, and Solanales. It is correlated with monochasial branching systems.

Among the asymmetry forms here considered, contort perianth organ aestivation is the most common in angiosperms (Fig. 6). Among basal angiosperms it is present in Cabomba (Nymphaeales) (Endress, 2008a). It is especially common in malvids and in Malpighiales of fabids. This may be an apomorphic tendency for malvids or malvids plus the COM clade (Endress & Matthews, 2006), if both together should turn out to form a clade (Endress & Matthews, 2006; Zhu et al., 2007; Qiu et al., 2010). Asymmetry based on modification of elaborate monosymmetry is present in some monocots (among Asparagales, Commelinales, and Zingiberales), some rosids (among Myrtales, and Fabales), and some asterids (among Lamiales, Asterales, and Dipsacales), and asymmetry by different anther height in otherwise polysymmetric, tubular flowers appears restricted to asterids (some Ericales, Gentianales, and Solanales).

In the fossil record monosymmetry by simplicity appears with Chloranthus-like flowers in the Santonian-Campanian (Doyle et al., 2003), whereas elaborate monosymmetric flowers are known since the Turonian (Crepet, 2008). Thus the evolution of elaborate monosymmetry began at least in the Late Cretaceous if not earlier.

Floral monosymmetry is predominant in some highly species-rich clades (Sargent, 2004). This suggests that the evolution of monosymmetry was a key innovation. However, from its distribution across angiosperms it appears that there are many clades in which it was not (yet) a key innovation (Endress, 2011). Thus, a differentiated statement is that monosymmetry appears to be a key innovation for several clades, but not for many other clades.

Why so many origins of monosymmetric flowers? It can be expected that monosymmetry is easy to evolve in terms of genetics and that it conveys a selective advantage over polysymmetry under certain circumstances. The genetic system responsible for monosymmetry in Lamiales was at least partly established much before the origin of Lamiales, at least at the level of basal eudicots (Coen & Nugent, 1994; Cubas et al., 2001; Cubas, 2004; Howarth & Donoghue, 2006; Kolsch & Gleissberg, 2006; Damerval et al., 2007). The selective advantage is related to more precise and thus more economical pollination by reduction of the pollination space from three to two dimensions. Movements of floral organs involved in herkogamy and dichogamy are restricted to the symmetry plane. Such movements may either be autonomous (herkogamy by differential elongation of stamens and styles; Webb & Lloyd, 1986; or curvature by flexistyly; Li et al., 2001; or inversostyly; Pauw, 2005) or they may be mediated by pollinators (movement of anthers in Salvia; Classen-Bockhoff et al., 2004; Calceolaria; Vogel, 1974; or Roscoea, Troll, 1929). Not only position and movement of floral organs but also movement of pollinators is canalized into the monosymmetry plane. Restriction not only of the stamens and stigma but also of the nectary into the monosymmetry plane are common. This may be accentuated by the repeated evolution of a single nectar spur (e.g. some Papaveraceae, Ranunculaceae, Geraniaceae, Tropaeolaceae, Balsaminaceae, Acanthaceae, Veronicaceae, Campanulaceae, Goodeniaceae, Orchidaceae) (Jabbour et al., 2008). In oil flowers, instead, two collateral spurs may be present in the monosymmetry plane (Diascia, Scrophulariaceae; Vogel, 1974).

Asymmetry appears much less common than monosymmetry, but not if imbricate perianth organ aestivation and floral torsions are also included. The only larger clades with consistently or predominantly elaborate asymmetric flowers are Phaseoleae (Fabaceae) and Cannaceae plus Marantaceae (Zingiberales). Elaborate asymmetric flowers with precise pollination may have even more restricted areas of pollen deposition on the bee body, as shown in Macroptilium (Fabaceae) (Brizuela et al., 1993), and thus even more economic pollination. But there may be structural restrictions for evolutionary diversification of such flowers.

Conclusions

Floral monosymmetry and asymmetry originated many times in angiosperms. In some groups, monosymmetry appears to be a key innovation (e.g., Fabaceae, Lamiales, Orchidaceae), but not in others. Asymmetry may be a key innovation in Phaseoleae (Fabaceae). Unelaborate forms of monosymmetry and asymmetry are easily lost again in evolution. Elaborate forms can also be lost but are more prone to persist. How much this trend to persist is based on developmental and how much on functional constraints, is unknown.

Advancement in our knowledge of floral symmetry will continue to be based on advances in phylogenetic reconstruction, molecular developmental genetics, and function of flowers of different monosymmetry and asymmetry types. The more the molecular developmental basis of structures is becoming elucidated the more intricate it becomes to use the terms homology (Wagner, 2007) and evolutionary innovation (Wagner, 2008).

Questions that have been asked are: Do monosymmetry genes respond to a common dorsoventral prepattern in the apex (Clark & Coen, 2002)? How is the evolution of regulatory interactions controlling floral monosymmetry (Costa et al., 2005)? How are different genes co-opted in the evolution of floral monosymmetry (Baxter et al., 2007)? What other functions in flowers do monosymmetry genes have (Baum, 1998)? New (evolutionary) aspects are also expected by comparison of symmetry types with other features, such as genetics of breeding systems via dichogamy (Kalisz et al., 2006), or with floral variability (Herrera et al., 2008). As generally in evolutionary biology, it seems important to combine the levels of macroevolution and microevolution (Wagner, 1986; Friedman et al., 2008).

Acknowledgments I would like to thank Amy Litt for the invitation to participate in the Symposium held in Vancouver in July 2008 in honor of Dennis Stevenson, deeply committed botanist and long-time friend. I would like to thank Jurg Schonenberger for communication of personal observations in Polemoniaceae and Fouquieriaceae. For information on particular plant groups I thank Urs Eggli (Crassulaceae) and Dmitri D. Sokoloff (Ottleya). Alex Bernhard is thanked for support with graphical work. The manuscript was submitted in November 2008 and updated in October 2011 because of the delay in publication.

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DOI 10.1007/s12229-012-9106-3

Peter K. Endress (1,2)

(1) Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, 8008 Zurich, Switzerland

(2) Author for Correspondence; e-mail: pendress@systbot.uzh.ch

Published online: 10 October 2012
Table 1 Diversity in the expression of floral monosymmetry

Organ categories are affected in various combinations

* All organs (corolla and androecium are generally the most
obviously affected regions) (e.g., Lamiales, Fabaceae, Orchidaceae)

* Calyx and corolla (e.g., Balsaminaceae)

* Rarely only one organ category is (mainly) affected:

-- Calyx (e.g., Mussaenda, Warszewiczia of Rubiaceae)

-- Corolla (e.g., Isonema, Rauvolfia of Apocynaceae)

-- Androecium (e.g., Lagerstroemia p.p. of Lythraceae; Solanum lidii
of Solanaceae)

-- Gynoecium (e.g., Isolona, Monodora of Annonaceae)

Organs are affected to various degrees

* Organs differentially shaped on both sides of symmetry plane
(e.g., Lamiales)

* Organs curved to one direction (e.g., Cleomaceae)

* Organs reduced on upper or lower half (e.g., odd stamen in
several families of Lamiales, e.g., Gesneriaceae)

* Organs lost on upper or lower half(e.g., stamen(s) in Lamiaceae)

* Organs increased in number on upper or lower half (e.g., stamens
in Lecythidaceae)

* Degree of organ union different on upper or lower half (e.g.,
petals in Teucrium of Lamiaceae)

* Organs transfunctionalized (neofunctionalized) on upper or lower
half (e.g., stamens in Lecythidaceae, odd stamen in Jacaranda of
Bignoniaceae, Penstemon of Veronicaceae)

Monosymmetry by simplicity

* Flowers with a single stamen or a single carpel (stamen in
Lacistemataceae, carpel in Lauraceae, both stamen and carpel in
Sarcandra, Chloranthaceae)

Direction of monosymmetry with respect to the axis of the next
higher order

* Median (predominant, e.g., Lamiales, Orchidaceae)

* Transversal (e.g., Fumarioideae of Papaveraceae; Hibbertia p.p.,
Dilleniaceae)

* Oblique (e.g., some Malpighiales, Brassicales, Sapindales,
Solanales). Flowers with an organisationally transversal or oblique
monosymmetry plane are commonly perpendicular at anthesis by
adjustment of the pedicel.

Table 2 Changing expression of floral monosymmetry during development

                                   Early
                                   development    Anthesis

Monosymmetry strongest in early
    development
  Arabidopsis                      +              -
Monosymmetry strongest at anthesis
    (in species-rich families)
  Fabaceae                         +              ++
  Veronicaceae                     +              ++
  Asteraceae/(Cichorioideae)       -/(+)          ++
  Orchidaceae                      +              ++
Monosymmetry strongest in fruit
  Tiarella                         +              +
  Chrysosplenium sp.               +              -

                                   Fruit

Monosymmetry strongest in early
    development
  Arabidopsis                      -
Monosymmetry strongest at anthesis
    (in species-rich families)
  Fabaceae                         + (1-carpellate)
  Veronicaceae                     -
  Asteraceae/(Cichorioideae)       + (1-ovulate)
  Orchidaceae                      -
Monosymmetry strongest in fruit
  Tiarella                         ++
  Chrysosplenium sp.               ++

++ monosymmetry strongly expressed; + monosymmetry weakly expressed; -
monosymmetry lacking Sources: Hams, 1995; Endress, 1998, 1999, 2001
a; Tucker, 1999; Leins & Erbar, 2000; Cubas et al., 2001; Prenner,
2004a; personal observations
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