Patterns of Symmetry Expression in Angiosperms: Developmental and Evolutionary Lability

Somayeh Naghiloo

doi:10.3389/fevo.2020.00104

Abstract

Variation in flower symmetry is a remarkable aspect of flowering plant diversity. Multiple evolutionary transitions from an ancestral form of radial symmetry to bilateral symmetry and reversals from bilateral to radial symmetry have occurred during angiosperm evolution. The high incident of symmetry transitions poses the question concerning the homology of underlying developmental processes. I conducted a comprehensive study of developmental expression of symmetry across angiosperms covering 72 families and 39 orders. A wide range of timing for the expression of symmetry was found from organ initiation, to enlargement, to late differentiation of organs. In many studied genera, the symmetry pattern was transient during development. Mapping the floral symmetry patterns on the phylogenetic tree indicated the scattered distribution of patterns across angiosperms. The expression of symmetry was also versatile within families. The developmental constraints exerted by the inflorescence, bracts, neighbor flowers or neighbor organs within a flower could provide a possible explanation for the observed variation of symmetry expression.

Highlights

One of the fascinating aspects of the floral organization is the balanced and harmonic arrangement of organs within a flower creating specific symmetry patterns
In mid and late phases, the size difference was used as a criterion to indicate symmetry pattern; the size equality was described as actinomorphy while the size gradient along the dorsoventral axis was interpreted as zygomorphy
Regardless of being actinomorphic or zygomorphic, the majority of flowers were characterized by zygomorphic initiation which was supported as the ancestral state for the angiosperms

Summary

INTRODUCTION

One of the fascinating aspects of the floral organization is the balanced and harmonic arrangement of organs within a flower creating specific symmetry patterns. Several authors studied the manifestation of floral symmetry during ontogeny (Mair, 1977; Endress, 1999; Tucker, 1999). They reported a wide range of timing for the expression of symmetry, from organ initiation, to later enlargement of floral organs, to late differentiation of structures. In many families with prevalent elaborate zygomorphy (e.g., Fabaceae), zygomorphy is manifested early at organs initiation and persists throughout development (Tucker, 2003). The objectives were (1) to determine patterns of symmetry expression within angiosperms, (2) to map the distribution of patterns on a phylogenetic tree, and (3) to infer evolutionary and developmental constraints underlying diversity of symmetry patterns

MATERIALS AND METHODS

RESULTS

DISCUSSION