Parallel evolution of TCP and B-class genes in Commelinaceae flower bilateral symmetry

In order to test the non-exclusive hypotheses that changes in TCP and B-class gene developmental function underlie flower symmetry evolution in the monocot family Commelinaceae, we compared expression patterns of teosinte branched1 (TB1)-like, DEFICIENS (DEF)-like, and GLOBOSA (GLO)-like genes in morphologically distinct bilaterally symmetrical flowers of Commelina communis and Commelina dianthifolia, and radially symmetrical flowers of Tradescantia pallida.Expression data demonstrate that TB1-like genes are asymmetrically expressed in tepals of bilaterally symmetrical Commelina, but not radially symmetrical Tradescantia, flowers. Furthermore, DEF-like genes are expressed in showy inner tepals, staminodes and stamens of all three species, but not in the distinct outer tepal-like ventral inner tepals of C. communis.Together with other studies, these data suggest parallel recruitment of TB1-like genes in the independent evolution of flower bilateral symmetry at early stages of Commelina flower development, and the later stage homeotic transformation of C. communis inner tepals into outer tepals through the loss of DEF-like gene expression.Evolutionary transitions between flower radial symmetry (polysymmetry and actinomorphy) and bilateral symmetry (monosymmetry and zygomorphy) have occurred multiple times independently across angiosperms, and are associated with increased pollinator specialization and speciation rates [1-6]. Indeed, some of the largest angiosperm families have species with predominantly bilaterally symmetrical flowers, including the legumes (Leguminosae, rosids, core eudicots), daisies (Asteraceae, asterids, core eudicots) and orchids (Orchidaceae, monocots) [7]. Recent functional studies in distantly related core eudicots - including Antirrhinum majus (Plantaginaceae), Iberis amara (Brassicaceae), Pisum sativum and Lotus japonicus (Leguminosae) and Gerbera hybrida (Asteraceae) - have demonstrated a role for class II TEOSINTE BRANCHED1 (TB1)/CYCLOIDEA