Bilateral symmetry has evolved as an adaptive trait linked to efficient pollination and successful outcrossing, occurring over 170 times in angiosperms and in many plant groups relying upon the asymmetric expression of key transcription factors from the CYC/TB1 gene family.
Citerne et al. characterise the evolution of flower symmetry in Proteaceae, a basal eudicot lineage with high diversity in floral morphology, finding that bilateral symmetry is a very labile trait in Proteaceae. The asymmetric expression of CYC/TB1 homologues implicated in the development of bilaterally symmetrical flowers suggests that these genes may have been recruited and harnessed for the control of flower symmetry in this family.
Citerne, H. L., Reyes, E., Le Guilloux, M., Delannoy, E., Simonnet, F., Sauquet, H., … Damerval, C. (2016). Characterization of CYCLOIDEA-like genes in Proteaceae, a basal eudicot family with multiple shifts in floral symmetry. Annals of Botany, 119(3), 367–378. https://doi.org/10.1093/aob/mcw219