XVI International Botanical Congess

Three classes of homeotic functions, termed A, B and C, sculpt the structure of higher eudicot flowers, with A specifying sepals, A+B petals, B+C stamens, and C carpels. Most genes providing these functions belong to the MADS-box gene family. To better understand flower origin we have characterized MADS-box genes of gymnosperms, ferns and mosses. Our analyses suggest that the last common ancestor of ferns and seed plants had homologs, but not orthologs of floral homeotic genes. In contrast, the last common ancestor of extant seed plants had already orthologs of B and C class genes. Expression of C class genes may have distinguished between reproductive organs (where expression is on) and non-reproductive organs. Differential expression of B class genes (on in male, off in female organs) may represent the primary sex-determination mechanism of all seed plants.