All flowers — large or small, gaudy or spare — fol- 

 low the same basic steps as they form. Each starts oft 

 as a floral meristem, a mound of rudimentary, un- 

 specialized cells — the plant equivalents of embryonic 

 stem cells. Meristems are made up of four concentric 

 circular regions, or whorls; each whorl develops into 

 one kind of flower organ. The outer whorl becomes 

 the sepals. The next whorl in becomes the petals. 

 The two innermost whorls become the reproductive 

 organs: the male stamens, which make pollen, then 

 the female carpels, which enclose egg-containing 

 ovules, at the flowers center. 



Since the multitude of flower forms are all built 

 according to that plan, it seems reasonable to sup- 

 pose that a standard set of genetic instructions di- 

 rects the basic program ot flower-organ develop- 

 ment, with variations on those instructions specific 

 to the roses, the lilies, and the rest of the great 

 bouquet. Thus two questions propel the work of 

 the evolutionary botanist: What are those basic in- 

 structions, and what are the variations? 



The most widely accepted account of the basic 

 instructions for flower-organ development is 

 known as the ABC model. The name comes from 

 the three roles (called A, B, and C) that genes are 

 thought to play in directing how the mass of floral- 

 meristem cells grows and specializes into sepals, 

 petals, stamens, and carpels. The genes in question 

 are transcription factors, a powerful kind of gene 

 that all organisms possess, from bacteria on up the 

 evolutionary ladder. The power of a transcription 

 factor is that it can control other genes by turning 

 them on or oft". Thus the activation of one tran- 



scription factor in a cell can initiate entire cascades 

 of molecular activity in that cell. 



One important role of transcription factors is to 

 determine the fate of an immature cell. They do so 

 by setting in motion chemical activity that changes 

 the immature cell into a muscle cell or a liver cell, a 

 petal cell or a pollen cell. Furthermore, many of the 

 genes that control flower formation are a particular 

 kind of transcription factor known as a M4DS-box 

 gene. MADS-box genes control the identity and 

 structure of many plant organs, just as Hox genes 

 control body-plan development in animals [see "The 

 Origins of Form," by Sean B. Carroll, November 2005]. 



Since transcription factors are so powerful, evo- 

 lutionary changes in them can lead to dramatic 

 changes in a species. And sure enough, the main 

 transcription factors that play a role in flower for- 

 mation have undergone a great deal of evolutionary 

 change and proliferation, which accounts for much 

 of the floral diversity among the angiosperms. 



One kind of evolutionary change, the duplication 

 of genes, appears to have been particularly impor- 

 tant. My work with Vivian F. Irish, a plant biologist 

 at Yale University, has revealed numerous instances 

 of gene duplication in the evolutionary history of 

 one flower-forming gene lineage. It even hints at 

 the origin of the first flower. Finally, it suggests that 

 Arabidopsis thaliana, the plain little member of the 

 mustard family that provided most of the evidence 

 for the ABC model, may turn out to be unusual in 

 the way it makes its flowers. That discovery, to- 

 gether with others, shows that the ABC model — 

 and thus the evolutionary botanist's understanding 

 of the basic instructions for flower-organ develop- 

 ment — must now be modified. 



The ABC model of flower-organ develop- 

 ment was articulated in 1991 by two plant 

 biologists, Enrico Coen of the John Innes 



