The different forms of flowers - what have we learned since Darwin?

Abstract

Darwin's book, The Different Forms of Flowers on Plants of the Same Species, has stimulated an extraordinary amount of original research since its publication in 1877. In his book, Darwin focused primarily on heterostylous reproductive systems in flowering plants, in which two or three reproductive morphs with reciprocal placement of anthers and stigmas occur in populations. These morphs are usually self-incompatible and cross-incompatible with individuals possessing the same reproductive morph. Many of the papers on heterostyly published since Forms of Flowers appeared have focused on the questions raised by Darwin about the evolution and function of heterostyly. Darwin's hypothesis that heterostyly promotes cross-pollination between different morphs has been largely substantiated, despite the difficulties in finding the ideal experimental system to address this question. Heterostyly is now known to occur in many more plant families than at the time Forms of Flowers was published and, as expected, the heterostylous syndrome is now defined more broadly than in Darwin's time. The origin of heterostyly remains an area of active research, with hypotheses stressing either the evolution of heteromorphic self-incompatibility as the first step in the evolution of this reproductive system or, alternatively, the evolution of the reciprocal features of floral morphology. Phylogenetic approaches, combined with studies on the physiological and molecular genetic basis of heterostyly, offer promise in helping to resolve questions about the origin of heterostyly. There is no doubt that heterostyly has evolved on multiple occasions and that self-incompatibility associated with heterostyly is unrelated to the more common multi-allelic self-incompatibility systems found in monomorphic species. Further progress in understanding conditions favouring evolution of heterostyly will depend on an increased understanding of the relation between the reciprocal morphological features of the breeding system and the nature of self-incompatibility. Almost a century and a half after the appearance of The Different Forms of Flowers on Plants of the Same Species, heterostyly remains an active area of research. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 160, 249–261.