**5. Phylogenetic pattern in floral symmetry**

Floral symmetry patterns are best understood in phylogeny context. These studies help in understanding how often the transition from radial symmetry to bilateral symmetry has occurred and vice versa. It also gives insight on what lineages these transitions have taken place and when those transitions occurred on geological timescale.

We now have clear understanding about the major lineages of angiosperms. Recent studies focus on mapping various morphological traits on these robust phylogenetic analyses. In relation to floral symmetry, recent studies have constructed it as a character and different forms (radial symmetry, bilateral symmetry, asymmetry, etc.) on phylogenies [50]. Such robust studies have answered the abovementioned questions.

Studies revealed that the ancestral flower of angiosperms was radially symmetrical [8]. Floral symmetry character reconstruction on ordinal phylogeny also revealed the same scenario and showed that the transition to bilateral symmetry is widespread on angiosperm phylogeny. Parsimony reconstruction on family phylogeny revealed that there are at least 70 such transitions from radial to bilateral symmetry in angiosperms including 23 in monocots and 46 in eudicots [50].

Later studies focused on detailed phylogeny of smaller clades. Character reconstruction of floral symmetry in Lamiales at family level revealed one transition from radial to bilateral symmetry and one vice versa [51]. Multiple transitions from radial to bilateral symmetry were observed in Brassicaceae, Ranunculaceae, and Solanaceae [52–54].


#### **Table 1.**

*List of clades in which radial symmetry is conserved in all or most of its descendents.*


#### **Table 2.**

*List of clades in which bilateral symmetry evolved independently.*


#### **Table 3.**

*List of clades in which bilateral symmetry evolved early as a single event.*


#### **Table 4.**

*List of clades in which there was reversal from bilateral symmetry to radial symmetry.*

Bilateral symmetry has evolved at least 130 times independently in different clades, and there were at least 70 reversals [55]. Based upon these transitions, four basic groups have been observed. These are: first, there are clades where radial symmetry is conserved (**Table 1**). Second, clades wherein bilateral symmetry has evolved independently (**Table 2**). Third, clades wherein bilateral symmetry arises as single early event (**Table 3**), and fourth group includes clades that show reversal to radial symmetry (**Table 4**). Basal angiosperms have radial symmetry with exceptions such as in *Glossocalyx.* Radial symmetry is also conserved in ancestral monocots and Eudicots. More robust phylogenetic studies in future can further reveal detailed transitions.

#### **6. Floral symmetry on geological timescale**

First fossil remains of flowers with inserted flower parts are found to be from early Cretaceous period (Barremian-Aptian period) around 125 million years ago (Ma.). This fossil represents the flowers of the ancestors of early Nympheales [56, 57]. The first fossil of a flower, which is pentamerous, was reported from late Cretaceous period (Cenomanian period) around 100 Ma. This fossil remains has both petals and sepals. It is considered as a representative of ancient ancestor of Eudicots [58].

Clearly, the above fossil records show that there was a transition from closed floral structure to an open floral structure. The flower evolved from closed noncyclic structures to more open and cyclic forms. This transition took place in Mid-Cretaceous period. It is during this period that many floral traits evolve. Many of these traits are key innovations. This floral trait evolution coincides with the major diversification period of angiosperms [59, 60].

The first transition from radial symmetry to bilateral symmetry can be traced to the first radiation in angiosperms. These flower remains are reported from Turonian fossils from late Cretaceous, which is around 100 Ma. These flower fossils have staminodal nectaries making the radial flower partially bilateral. These flowers fossils are the first report of zygomorphic flower form, although these forms were not exactly bilaterally symmetrical. These fossils represented the precursors or ancestors of zygomorphic flowers [59, 61].

First complete zygomorphic flowers are recorded from Paleogene (Paleocene-Eocene period) around 55 Ma [59]. This is the time period where a second major diversification of angiosperms took place. So, we have seen that both the radiation events of angiosperms diversification coincide with the evolution of floral traits including floral symmetry [61]. Thus, floral symmetry transition is clearly the key innovation, which might have played crucial role in radiation of angiosperms. Of course, there might be other factors too that played their part. One such factor is evolution of pollinators.

Interestingly, the evolution of those floral traits that lead to diversification of angiosperms in the above-said events coincides with the advent of specialized pollinators. Also, the evolution of bilateral symmetry in some plant lineages cooccurs *Current Trends in Developmental Genetics and Phylogenetic Patterns of Flower Symmetry DOI: http://dx.doi.org/10.5772/intechopen.101772*

with the time period when there was a rise of some bee families. Thus, in some lineages the Coevolution of insect pollinators with the floral symmetry holds true [2, 5]. Although there are other abiotic and biotic factors that are needed to be taking into account such as climatic conditions and various architectural components of flower.

## **7. Conclusion**

Great deal of progress is being made on study of floral symmetry evolution in dicots. The *MYB* TFs and *TCP*s play an important role in understanding the molecular genetic basis of floral symmetry. However, there is a scope to identify other putative genes that might be having evolutionary significance. Robust studies should be taken up, especially among monocots to unravel modulators of floral symmetry patterns.

Most of the species-rich lineages have bilateral symmetry. Based on this observation, many hypothesize that bilateral symmetry is related to increased specificity to pollinators, thereby increasing the chances of reproductive isolation. This holds true for many taxa, but not all lineages of angiosperms follow the same pattern.

Evolution is a highly complex phenomenon, and diversification of species is dependent on various factors and not just one single trait. Therefore, it is necessary to take holistic approach and to combine other factors such as developmental stages, floral mechanics, etc., with the phylogenetic framework to get a detailed answers about floral symmetry.

#### **Acknowledgements**

The authors acknowledge the DST Project Grant (file no. ECR/2017/000563) for financial support during conceptualization and manuscript preparation.

#### **Author details**

Renu Puri and Anjana Rustagi\* Department of Botany, Gargi College, University of Delhi, Delhi, India

\*Address all correspondence to: anjana.rustagi@gargi.du.ac.in

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
