AOBPreview originally published online on May 28, 2008
Annals of Botany 2008 102(2):153-165; doi:10.1093/aob/mcn082
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Evolutionary Trends in the Flowers of Asteridae: Is Polyandry an Alternative to Zygomorphy?
1 Université Paris-Sud, Laboratoire Ecologie, Systématique, Evolution, CNRS UMR 8079, AgroParisTech, Orsay, F-91405, France
2 UMR de Génétique Végétale, INRA – Univ Paris-Sud – CNRS – AgroParisTech, Ferme du Moulon, 91190 Gif-sur-Yvette, France
* For correspondence. E-mail florian.jabbour{at}u-psud.fr
Received: 16 February 2008 Returned for revision: 17 March 2008 Accepted: 21 April 2008 Published electronically: 28 May 2008
Background and Aims: Floral symmetry presents two main states in angiosperms, actinomorphy (polysymmetry or radial symmetry) and zygomorphy (monosymmetry or bilateral symmetry). Transitions from actinomorphy to zygomorphy have occurred repeatedly among flowering plants, possibly in coadaptation with specialized pollinators. In this paper, the rules controlling the evolution of floral symmetry were investigated to determine in which architectural context zygomorphy can evolve.
Methods: Floral traits potentially associated with perianth symmetry shifts in Asteridae, one of the major clades of the core eudicots, were selected: namely the perianth merism, the presence and number of spurs, and the androecium organ number. The evolution of these characters was optimized on a composite tree. Correlations between symmetry and the other morphological traits were then examined using a phylogenetic comparative method.
Key Results: The analyses reveal that the evolution of floral symmetry in Asteridae is conditioned by both androecium organ number and perianth merism and that zygomorphy is a prerequisite to the emergence of spurs.
Conclusions: The statistically significant correlation between perianth zygomorphy and oligandry suggests that the evolution of floral symmetry could be canalized by developmental or spatial constraint. Interestingly, the evolution of polyandry in an actinomorphic context appears as an alternative evolutionary pathway to zygomorphy in Asteridae. These results may be interpreted either in terms of plant–pollinator adaptation or in terms of developmental or physical constraints. The results are discussed in relation to current knowledge about the molecular bases underlying floral symmetry.
Key words: Floral symmetry, architectural constraints, Asteridae, comparative analysis, composite tree, correlated evolution, evolutionary scenario
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