Floral Symmetry – What It Is, How It Forms, and Why It Varies

Global distribution and evolutionary transitions of floral symmetry in angiosperms

“…All species were classified into two categories based on their floral symmetry following previous studies (8,25,26): zygomorphy (i.e., flowers with just one axial plane dividing the flowers into two symmetrical halves) and actinomorphy (i.e., flowers with two or more axial planes, each of which divides the flowers into two symmetrical halves). Zygomorphic species included those with flowers recorded as monosymmetry, (some) irregular, and bilateral symmetry.…”

“…Coevolutionary dynamics between floral traits of plants and their pollinators driven by environmental changes may lead to spatiotemporal dynamics of floral symmetry (6,7). Both evolutionary lability (where sister clades evolved different types of floral symmetry) and developmental lability (unstable or differing floral symmetry types within a species in response to environments) together lead to a geographic mosaic of floral symmetry which may reflect responses of plants to different ecological and evolutionary settings (8).…”

“…Climate may influence the frequency of plants with different floral symmetry types in plant communities via its effects on the corresponding pollinator assemblages and the interactions between plant species and their pollinators across space (8). Studies in North America found that zygomorphic flowers with higher pollination specialization [i.e., plant species associated with a few potential pollinator species; see (13) for more details] are favored in regions that have high diversity of pollinator species (14).…”

Evolution: Cleistogamy to the rescue of zygomorphic flowers

“…To some critics of statistical phylogenetic approaches, these observations suggest that different forms of zygomorphy should not be treated with the same trait values. However, at least in the case of floral symmetry, it is reassuring to observe that despite this diversity of pathways at the morphological developmental level, all evidence so far suggests that the same transcription factors have been recruited repeatedly across flowering plants to produce zygomorphic flowers 8,18,19 . Furthermore, it is useful to point out that it is not the exact structural ways by which different types of flowers are constructed that matter in a study like this one, but instead their functional similarity.…”