Evolutionary conservation of angiosperm flower development at the molecular and genetic levels

Abstract

Flowers consist primarily of four basic organ types whose relative positions are universally conserved within the angiosperms. A model has been proposed to explain how a small number of regulatory genes, acting alone and in combination, specify floral organ identity. This model, known widely as the ABC model of flower development, is based on molecular generic experiments in two model organisms, Arabidopsis thaliana and Antirrhinum majus. Both of these species are considered to be eudicots, a clade within the angiosperms with a relatively conserved floral architecture. In this review, the application of the ABC model derived from studies of these typical eudicot species is considered with respect to angiosperms whose floral structure deviates from that of the eudicots. It is concluded that the model is universally applicable to the angiosperms as a whole, and the enormous diversity seen among angiosperms flowers is due to genetic pathways that are downstream, or independent, of the genetic programme that specifies floral organ identity. 1. Angiosperm flower An enormous diversity of size, shape, and complexity exists amongst the flowers of the quarter of a million species of extant angiosperms. Flower size varies over 1000-fold, with Rafflesia (Rafflesiaceae) flowers up to a meter in diameter dwarfing the minute flowers of Wolffia (Lemnaceae) measuring less than a millimeter across. Floral organ number also varies over several orders of magnitude with the comlex flowers of Tambourissa species (Monimiaceae) having more than a thousand organs while the simple flowers of the Chloranthaceae may consist of but a few organs. Despite the enormous diversity in numbers, sizes, and shapes of the floral organs within the angiosperms, a universal theme underlies the basic architecture of all angiosperm flowers: flowers are built of four basic organ types whose relative positions are invariant (figure 1A; sepals, petals stamens, and carpels). Thus, a basic floral ground plan exists that defines the relationship between organ type and position in all angiosperm species. The floral organ types typically develop in concentric rings or whorls, with sepals occupying the outermost positions, and petals, stamens, and carpels occupying success- ively more interior positions. Because of the constancy in the relative positions of floral organ types, it is hypothesized that a common genetic programme to specify floral organ identity is utilized during the development of all flowers. The striking variation observed among the angiosperm flowers would then be due to additional development- tal programmes that operate in parallel with or subsequently to the basic genetic