Columbine 



does not address. Since the function A job 

 of building both sepals and petals in the 

 developing meristem does not exist, and 

 since E genes are now known to be 

 necessary for flower organs to 

 form, the ABC model gets 

 rewritten as BCE. 

 t^. It is worth mentioning that in 

 1990, shortly before the ABC 

 model was published, Zsuzsanna 

 Schwarz-Sommer, a plant biologist at 

 the Max Planck Institute for Plant 

 Breeding Research in Cologne, Ger- 

 many, and several colleagues published 

 a two-gene model ot flower formation. 

 Those two genes have since been designated B and 

 C genes. Although it received considerably less at- 

 tention at the time than the ABC model, Schwarz- 

 Sommer's theory clearly anticipated the BCE model 

 in recognizing that no special gene function was 

 needed for sepal formation. 



Although API homologues may not play ex- 

 actly the same role of sepal- and petal-build- 

 ing in the meristem for which API became well 

 known, it is likely that they have had other impor- 

 tant roles in floral evolution. The duplication that 

 gave rise to group X and group Y in the core eudi- 

 cots may have been important in standardizing 

 flower construction. In some species that are more 

 ancient than the core eudicots, such as magnolias or 

 water lilies, flower parts are arranged in one contin- 

 uous spiral. Some spiral flowers, such as water lilies, 

 have transitional organs that are partway between a 

 sepal and a petal, or between a petal and a stamen. 



But within the core eudicots, flowers arrange 

 their parts in discrete, concentric whorls, and none 

 have transitional organs. It was probably the 

 whorled arrangement that led to the tremendous 

 floral elaboration and innovation in the core eudi- 

 cots (not to mention other plant groups with 

 whorled flowers). Whorled flowers simply have 

 more evolutionary flexibility than spiral flowers. 

 Whorled flowers can abandon the radial symmetry 

 of arabidopsis for the bilateral symmetry of snap- 

 dragon. Whorled petals can fuse to form tubes, as in 

 morning glory, and stamens can fuse to petals, as in 

 mint flowers. New organs, such as the colorful fila- 

 ments between the stamens and petals of a passion- 

 flower, can arise between whorls. 



In arabidopsis, API is required for the transition 

 from a branched inflorescence, whose branches bear 

 spirally arranged leaves, to a flower, which bears 

 whorled parts. The duplication that gave rise to 

 group X and group Y, followed by the acquisition of 



new functions in group X, may likewise have pro- 

 vided the genetic instructions for whorled flowers 

 in the core eudicots — and set the stage for the ex- 

 plosion of flower forms within this group. 



Even earlier, AP1 homologues, as well as SEPAL- 

 LATA homologues (the £ genes), may have played a 

 role in the origin of the first flower around 150 

 million years ago. Charles Darwin called the origin 

 of the flower an "abominable mystery," and it 

 stumps evolutionary botanists to this day. Gym- 

 nosperms have B and C genes, which help build 

 the male and female parts of their reproductive 

 cones. When the first angiosperm evolved from the 

 gymnosperms, it most likely incorporated B and C 

 genes into its flowers. But no API or SEPALLATA 

 homologues have been detected in gymno- 

 sperms — they appeared when the flowering plants 

 appeared. Those two gene lineages are closely re- 

 lated, and probably arose via gene duplications that 

 took place within a short time period. Since API 

 homologues are required for a meristem to form a 

 flower, it's conceivable that they assisted in the evo- 

 lution of the first flowers. 



Genes account for the stunning diversity of 

 form and function among flowers — and in 

 the natural world at large, for that matter. Plants 

 and animals have many more genes than do simpler 

 organisms: both arabidopsis and people have about 

 25,000 genes, compared to the 3,000 in bacteria. 

 Much of the rise in the num- 

 ber of genes that accom- \ ^ / a 0 -« 

 panies increasing com- - h 

 plexity, it is now be- 

 coming clear, arose 

 from duplications in 

 all or part of the ge- 

 nome. Such duplica- 

 tions have peppered 

 the evolutionary history 

 of most organisms. 



More genes, of course, 

 imply more opportunities for mutations, and more 

 opportunities for new forms and functions to 

 evolve. Moreover, when the genes in question are 

 transcription factors, the entire form or life cycle of 

 an organism can be altered. Evolutionary botanists 

 are still working out most of the details in the quest 

 to account for today's floral diversity. All flowering 

 plants rely on a shared set of fundamental gene func- 

 tions to build their blooms. But the evidence is 

 mounting that gene duplication has played the piv- 

 otal role in creating arabidopsis, lily, rose, snap- 

 dragon, and the 250,000 other members of Earth's 

 great floral bouquet. □ 



Norway maple 



40 



NAT UK A I HlsioRY June 2006 



