Origin and evolution of petals in angiosperms

Background and aims - The term 'petal' is loosely applied to a variety of showy non-homologous structures, generally situated in the second whorl of a differentiated perianth. Petaloid organs are extremely diverse throughout angiosperms with repeated derivations of petals, either by differentiation of a perianth, or derived from staminodes. The field of evo-devo has prompted re-analysis of concepts of homology and analogy amongst petals. Here the progress and challenges in understanding the nature of petals are reviewed in light of an influx of new data from phylogenetics, morphology and molecular genetics. Method and results - The complex web of homology concepts and criteria is discussed in connection to the petals, and terminology is subsequently defined. The variation and evolution of the perianth is then reviewed for the major clades of angiosperms. From this pan-angiosperm variation, we highlight and discuss several recurrent themes that complicate our ability to discern the evolution of the petal. In particular we emphasise the importance of developmental constraint, environmental stimuli, interrelationship between organs and cyclic patterns of loss and secondary gain of organs, and the development of a hypanthium or corona. Conclusions - A flexible approach to understanding 'petals' is proposed requiring consideration of the origin of the floral organ (stamen-derived or tepal-derived, or other), the functional role (sepaloid or petaloid organs), evolutionary history of the organismal lineage, and consideration of developmental forces acting on the whole flower. The variety and complex evolutionary history of the perianth may necessitate the exploration of petal development within phylogenetically quite restricted groups, combining data from morphology with evo-devo.