Evolution of Floral Organ Identity

Abstract

Understanding the origin and rapid diversification of the angiosperm flower is a long-standing problem of evolutionary biology. A poor fossil record and a large morphological gap between extant angiosperms and their closest living relatives, extant gymnosperms, are major reasons that make reconstruction of flower origin such a hard problem. In recent years, however, significant progress has been made, especially in our understanding of the evolution of floral organ identity, by the application of the evo-devo rationale. Quite useful as guiding concepts for understanding floral organ identity development and evolution proved genetic and molecular models, such as diverse ABC models and the floral quartet model. These models highlight the importance of organ identity genes and the interaction of the transcription factors they encode for a better understanding of evolutionary novelties such as flowers. Comparative studies on the MIKC-type genes and proteins underlying the ABC and floral quartet models throughout the seed plants suggest that gene regulatory networks controlling reproductive organ identity quite similar to those of angiosperms existed already in the most recent common ancestor of extant seed plants about 300 million years ago. These networks were probably just recruited and slightly modified to control diverse floral organ identities. Among the major floral organs the carpel, a key character of angiosperms, is arguably least well understood in terms of its origin. This applies even more so to the ovule, which is an important component of the carpel, but has much deeper evolutionary roots.