Several whorls of petals 

 replace stamens and carpels 



ABC model 



Cultivated rose 



BCE model 



ABC model of flower-organ development is contrasted with the proposed BCE model, in this 

 schematic diagram of the genetic blueprint for three common flower forms. According to the ABC 

 model, genes with three different functions (A, B, and C) act in concentric rings, or whorls, of cells 

 within a floral meristem. (A floral meristem is a mass of undifferentiated cells that grows into a flower; 

 it is not depicted to scale in the diagram.) The genes that act in a given whorl of a floral meristem de- 

 termine which kind of organ (sepal, petal, stamen, or carpel) the whorl grows into. In the ABC model, 

 genes with function A in the outer whorl build sepals; A and B genes in the second whorl produce 

 petals; B and C genes in the third whorl produce stamens; and C genes in the center of the meristem 

 produce carpels. Botanists think that flowers such as lilies, with a set of petals instead of sepals, result 

 from a mutation in which B genes act in the meristem's outer whorl. Flowers with several sets of petals 

 instead of stamens and carpels, such as cultivated roses, are thought to result from a mutation that 

 makes C genes inactive. Recently another set of genes was identified that is necessary for the growth 

 of petals, stamens, and carpels: genes with the so-called function E. That discovery, along with evi- 

 dence that casts doubt on the very existence of function A, has called the ABC model into question. 

 The proposed BCE model posits that sepals form under the direction of the same genes that tell early 

 meristems to grow into flowers: the floral-meristem-identity genes. 



| Function A genes 

 ] Function B genes 

 ] Function C genes 



| Function E genes 



Floral-meristem- 

 mH identity genes 



June 2006 natural HISTORY 



