**Abstract**

Flower Symmetry is a key evolutionary innovation in some lineages of angiosperms. The flowers of the primitive angiosperm plants were radially symmetrical actinomorphic. Later bilaterally symmetrical zygomorphic flowers independently evolved in several clades of angiosperms. This transition of trait is associated with an adaptation to specialized methods of pollination. Zygomorphic flowers allow more specific plant insect interaction. So, the transition from radial symmetry to bilateral symmetry facilitates reproductive isolation which in turn might have led to diversification or rapid speciation of some lineages in angiosperms. Phylogenetic analyses in lineages of angiosperms revealed that few clades have shown that there have been reversals, that is, there is transition from bilateral symmetry to radial symmetry. When such studies are correlated with genetic studies, it is revealed that CYC (TCP family) transcription factors are responsible for the transition of this floral trait. Phylogenetic analyses, genetic studies and Evo-Devo analyses can answer important questions such as what other transition in floral symmetry is found in angiosperms? Is there a pattern of floral symmetry transition in different lineages? Do these transitions act as key innovation for the clades in which they have evolved?

**Keywords:** floral symmetry, zygomorphic, actinomorphic, phylogeny, asymmetry

### **1. Introduction**

Flower is a significant novelty for evolutionary success in angiosperms. It primarily comprises four whorls—sepals, petals, stamens, and carpels. The shape of the flower changes because of change in the shape or morphology of any of these whorls. This gives rise to different shape and symmetry of flowers. The change in symmetry can occur in any of the whorl; however, it is widely studied in petals [1]. Floral symmetry is an important trait as it impacts the visual appearance of a flower. Hence, it's been a fascination for human eye. Pollinators are usually attracted to flowers due to its diverse forms of colors but also due to the symmetry it possesses, thereby contributing to the plant pollination syndrome [2–4]. Broadly there are the two types of floral symmetry, radial symmetry also known as polysymmetry or actinomorphy and bilateral symmetry also known as monosymmetry or zygomorphy. Flowers with radial symmetry have more than two planes of symmetry and are called as actinomorphic. Flowers with bilateral symmetry have single plane of symmetry and are called as zygomorphic [5–7]. There is another rare form of symmetry

**Figure 1.**

*Different types of floral symmetry. A. Radial symmetry (more than one plane of symmetry, polysymmetry, actinomorphy). B. Bilateral symmetry (single plane of symmetry, monosymmetry, zygomorphy), C. asymmetry (no plane of symmetry). D. Flower of* Catharanthus roseus *showing radial symmetry, E. flower of* impatiens *sp. showing bilateral symmetry and F. flower of* canna *sp. showing asymmetry.*

in flowers that is known as asymmetry [8]. This refers to morphologies where there is no pane of symmetry (**Figure 1**).

These categories have been studied at various different levels such as the molecular aspects of these transitions and how the pollinators perceive them [5, 6]. There have been transitions from actinomorphy to zygomorphy many times during the diversification of angiosperms, and these transitions are more common in speciesrich lineages such as Fabaceae, Lamiales, and Orchidaceae. However, reversals from zygomorphy to actinomorphy are also reported [9–12].

We in this chapter focus on the different genetic studies, which have been conducted to understand the molecular basis of the variation in floral symmetry and what do we get to know when these studies are correlated with phylogenetic studies. These studies have provided insights into how and when these transitions in floral symmetry evolve.

### **2. Diversity of floral symmetry in angiosperm flowers**

Apart from the flower symmetry categories mentioned above, there are many other forms of flower symmetry such partial zygomorphy and few others. There are different degrees of symmetry. In the year 1925, based on the aspects of symmetry as used in crystallography, new terms were introduced. Of those rotational symmetry, mirror symmetry and spiral symmetry are to name a few [13].

Later correlation studies between floral symmetry and pollination biology were conducted. These studies focused on how pollinators perceived flowers. With these studies, three-dimensional aspects were added to the floral symmetry terminology [10, 14, 15]. After around 75 years, an elaborated and modified classification was proposed, which was also based on visual perception of flowers by

*Current Trends in Developmental Genetics and Phylogenetic Patterns of Flower Symmetry DOI: http://dx.doi.org/10.5772/intechopen.101772*

the pollinators [2, 3]. These terminally only applied to very discreet flower forms and only in mature flowers [16].

However, there are variable degrees of floral symmetry at different developmental stages. This variation can also be seen in different lineages, or there might be convergent evolution of this state in two different and closely unrelated clades. Endress [5] considered these two aspects, that is, developmental changes and phylogenetic changes, and identified three forms of monosymmetry and three forms of asymmetry.

First form of monosymmetry is found in taxa with elaborated monosymmetric flowers, for example, Lamiales, Asterales, and Leguminosae. The second is taxa in which monosymmetry arises, but predominantly the group is polysymmetric, e.g., *Passiflora lobata* (Passifloraceae) and *Chiranthodendron pentadactylon* (Malvaceae). The third form is evolution of monosymmetry form by reduction, e.g., in case of *Hippuris* (Antirrhinaceae). First form of asymmetry is seen in taxa, which are predominantly monosymmetric, e.g., *Vigna* (Leguminosae). Second form is unordered asymmetry in flowers of basal angiosperms, e.g., *Zygogynum* (Winteraceae). Third form is asymmetry, which arises due to reduction, e.g., *Centranthus* (Valerianaceae) [5].

#### **3. The genetic basis of flower symmetry**

The genetics of a flower is regulated by specific transcription factors (TFs) [17]. TFs such as MADS (First alphabet of MCM1 in yeast, AGAMOUS in Arabidopsis, DEFICIENS in snapdragon, and SERUM RESPONSE FACTOR in human)-box are widely studied for various developmental pathways from root development to fruit development. One important role is determination of organ development in flower. ABCDE model and its modifications are based on the different functions of *MADS*-box TFs [18–25].

Floral development is also controlled by other set of TFs Known as *MYB* TFs. These TFs are found in all eukaryotic organisms and identified by the presence of *MYB* R Repeats. Each repeat is about 52 amino acids. Based on these repeats, the *MYB* TFs are classified into 4R, 3R, 2R, and 1R-MYB types. In plants, the most common are 2R-*MYB* TFs (R2R3) [26–29].

Recent studies show that *MYB* TFs, *DIV*-and-RAD-interacting-factors (*DRIF)*, *DIVARICATA (DIV)* and, *RADIALIS (RAD)* play important role in floral symmetry [30, 31]. The studies revealed that these MYB TFs interact with TCP (TCP family name is derived from its first three characterized members, first alphabet of TB1, TEOSINTE BRANCHED 1; CYC, CYCLOIDEA and PCFs, proliferating cell factors) TFs. *CYCLOIDEA (CYC)* and *DICHOTOMA (DICH)* are two paralogs that belong to TCP family. They are expressed in the dorsal region of flowers, and they are vital to control floral symmetry [32]. The studies show that these TFs and their orthologs and paralogs have similar interactions in Dicots and monocots.

#### **3.1 Developmental genetics of floral symmetry in dicots**

The molecular basis of floral symmetry was first studied in *Antirrhinum majus,* which has zygomorphic flowers [33]. In the dorsal region, there is interaction of *RAD* and *DRIF*. *CYC* is expressed only in the dorsal region of flower. *RAD* promoter and intron have *CYC* target sequence. When *CYC* binds to *RAD* promoter, its causes synthesis of RAD protein. The DIV protein Interacts with *DRIF* but when *RAD* binds to *DRIF*, *DIV/DRIF* complex is not formed and thus not able to activate downstream ventral gene. In the ventral region, *CYC* is not expressed and likewise *RAD* is not activated. *DRIF* is free to bind to its target sequence present on the *DIV*

promoter region [34, 35]. This heterodimer complex, *DIV/DRIFT* complex activates the ventral genes. This differential expression of *CYC* in dorsal and ventral region of snapdragon flower causes dorsoventral symmetry (**Figure 2**) [36–38].

Flower of *A. majus* is an example from Lamiales where there is elaborated zygomorphy. Within Lamiaceae in family Gesneriaceae, the clade has zygomorphic flowers. However, *Conandron ramondioides* have actinomorphic flowers [39]. This is the case of reversal from zygomorphy to actinomorphy. In this species, there is change in the expression of the homologs of above TFs. In case of petals and stamen, there is loss of expression of *CrCYC* and *CrRAD* so the *DIV* is active and ventral genes are activated (**Figure 3**) [40–42].

Another member of clade lamiales *Plantago lanceolata* shows actinomorphic flowers [43]. Interestingly here *CYC* A clade gene is absent, but *PlCYC*-B clade gene is expressed. *PlCYC*-B at early stages is present in ground tissue, and later it expresses in stamens till the upper portion of filament. *PlCYC* is absent in petals. Homolog of *RAD* gene is absent, and *PlDIV*, ortholog of *DIV* is expressed in lateral petals and in the ovary. Absence of *CYC-A* clade and *RAD* gene in the dorsal part of flower might have been responsible for radial symmetry (**Figure 4**) [44, 45].

More complex mechanism takes place in Asteraceae. Here the inflorescence is complex and is known as capitulum. For example, in *Senecio vulgaris,* the disc florets have radial symmetry and ray florets have bilateral symmetry [46]. Here there are CYC-like genes *RAY1, RAY2,* and *Ray3*. Unlike as in *A. majus, RAY1* and *RAY2* are expressed in entire ray floret, and RAY3 expresses only in ventral region. MYB genes *SvDIV1B* and *SvRAD* are expressed only in the ray florets at early stages. *SvRAD* expresses in ventral region. At later stages, *SvDIV1B* expresses in disc florets too (**Figure 5**) [47].

**Figure 2.** *Molecular genetic control of floral symmetry in* Antirrhinum majus.

*Current Trends in Developmental Genetics and Phylogenetic Patterns of Flower Symmetry DOI: http://dx.doi.org/10.5772/intechopen.101772*

#### **Figure 3.**

*Molecular genetic control of floral symmetry in* Conandron *sp.*

#### **Figure 4.**

*Molecular genetic control of floral symmetry in* Plantago *sp.*

**Figure 5.** *Molecular genetic control of floral symmetry in* Senecio vulgaris.

#### **3.2 Developmental genetics of floral symmetry in monocots**

Little is known about floral symmetry in monocots. Orchidaceae family supports the DDR (DDR stands for DIV, RAD, and DIV-and-RAD-Interacting Factor DRIF)

**Figure 6.** *Molecular genetic control of floral symmetry in* Orchidaceae*.*

regulatory module [48]. Recent study revealed that *DIV, DRIFT,* and *RAD* interact with each other and work as regulatory unit. DDR module is more conserved in monocots than in dicots. Orchids show resupination before anthesis and dorsally placed structure becomes ventral in position. *DIV* and *DRIF* express in ventral structures and after resupination take dorsal position. In the lip high level of *RAD* expression prevents activation of *DIV.* RAD is responsible for lip determination in orchids (**Figure 6**) [49].

#### **4. Other putative genes**

Apart from *MYB* and *TCP* family, other putative genes need to be identified. Few genes are identified in model plant Arabidopsis. One such is *RABBIT EARS* (*RBE*). It belongs to C2H2 Zinc finger TFs family. Its expression is regulated by Auxin. When this gene is muted, then two of the petals do not elongate and give rise to bilateral flower. *RBE* further regulates *TCP*4 negatively by binding directly to its target sequence to the *TCP*4 promoter [41]. The role of CYC and *CYC-*like genes, which belong to *TCP* family, is already known. Other putative *TCP* members could be analyzed to reveal other pathways responsible for flower symmetry.

Other putative genes are such as *AINTEGUMENTA-LIKE 6* (*AIL6*)*, AUXIN-REGULATED GENE INVOLVED IN ORGAN GROWTH* (*ARGOS*)*, AIN-TEGUMENTA* (*ANT*)*, BIGPETALp* (*BPEp*), *and JAGGED* (*JAG*). The differential expression of these genes plays important role in floral organ development. Further analyses on non-model species that have different floral symmetries can reveal their role in floral symmetry [41]. The identification of these genes will not only help to identify the QTL for floral traits but also for phylogenetic studies. The homologs of these genes can further help us to understand the evolution of these genes and the gene families.

*Current Trends in Developmental Genetics and Phylogenetic Patterns of Flower Symmetry DOI: http://dx.doi.org/10.5772/intechopen.101772*
