Petals and stamens are partially transformed into sepals and carpels, respectively. On the left, the graphical representations of gene regulatory networks help visualize how the obligate heterodimerization of class B floral homeotic genes may have arisen. One ancestral self-activating gene (top) is duplicated (middle), and subsequently, both transcription factors lose their homodimerization ability, leading to obligate heterodimerization (bottom) (see Lenser et al., doi:10.1371/journal.pcbi.1000264). The latter mechanism is found in Arabidopsis thaliana and other angiosperms.

Image Credit: Günter Theißen and Thorsten Lenser (University of Jena).