Floral symmetry: The geometry of plant reproduction

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The  flower  is  an  astonishing  innovation  that  arose  during  plant evolution  allowing flowering  plants    also  known  as  angiosperms  –  to  dominate  life on  earth  in  a relatively  short  period  of  geological  time.  Flowers  are  formed  from  secondary meristems  by  co-ordinated  differentiation  of  flower  organs,  such  as  sepals,  petals, stamens, and carpels. The position, number and morphology of these flower organs imposes a geometrical pattern or symmetry type – within the flower which is a trait tightly connected to successful reproduction. During evolution, flower symmetry switched from the ancestral poly-symmetric (radial symmetry) to the mono-symmetric (bilateral symmetry) type multiple times, including numerous  reversals,  with  these  events  linked  to  co-evolution  with  pollinators  and reproductive strategies. In  this  review  we  introduce  the  diversity  of  flower  symmetry,  trace its evolution in angiosperms,  and  highlight  the  conserved  genetic  basis  underpinning  symmetry control in flowers. Finally, we discuss the importance of building upon the concept of flower  symmetry  by  looking  at  the  mechanisms  orchestrating  symmetry  within individual flower organs and summarise the current scenario on symmetry patterning of  the  female  reproductive  organ,  the  gynoecium,  the  ultimate  flower  structure presiding over fertilization and seed production.