SUMMARYLateral organ growth in seed plants is controlled in part by members of the YABBY (YAB) and class III homeodomain/leucine zipper (HD-ZIPIII) families of transcription factors. HD-ZIPIII genes appear to play a conserved role in such organs, but YAB genes have diversified, with some members of the family having specialized functions in leaves, carpels or ovule integuments. The ancestral expression patterns and timing of divergence of the various classes of YAB genes remain to be established. We isolated and evaluated the expression of one HD-ZIPIII and five YAB genes representing the five major YAB gene classes from Cabomba caroliniana, a member of the earliest-diverging angiosperms. Consistent with observations in eudicots, the FILAMENTOUS FLOWER (FIL) and YABBY5 (YAB5) genes of C. caroliniana were expressed in the abaxial regions of the leaf where new laminar segments arise, and the patterns of expression were mutually exclusive to those of HD-ZIPIII, indicating that these expression patterns are ancestral. Expression of CRABS CLAW (CRC) in the abaxial carpel wall, and of INNER NO OUTER (INO) in the abaxial outer integument of ovules was also conserved between eudicots and C. caroliniana, indicating that these patterns are primitive. However, the CRC gene was also expressed in other floral organs in C. caroliniana, and expression in stamens was also observed in another early-diverging species, Amborella trichopoda, indicating that carpel-specific expression was acquired after divergence of the Nymphaeales. The expression data and phylogeny for YAB genes suggest that the ancestral YAB gene was expressed in proliferating tissues of lateral organs.