| Summary: | Bilaterally symmetric flowers have evolved over a hundred times in angiosperms, yet orthologs of the transcription factors <i>CYCLOIDEA</i> (<i>CYC</i>), <i>RADIALIS</i> (<i>RAD</i>), and <i>DIVARICATA</i> (<i>DIV</i>) are repeatedly implicated in floral symmetry changes. We examined these candidate genes to elucidate the genetic underpinnings of floral symmetry changes in florally diverse <i>Rhododendron</i>, reconstructing gene trees and comparing gene expression across floral organs in representative species with radial and bilateral flower symmetries. Radially symmetric <i>R. taxifolium</i> Merr. and bilaterally symmetric <i>R. beyerinckianum</i> Koord. had four and five <i>CYC</i> orthologs, respectively, from shared tandem duplications. <i>CYC</i> orthologs were expressed in the longer dorsal petals and stamens and highly expressed in <i>R. beyerinckianum</i> pistils, whereas they were either ubiquitously expressed, lost from the genome, or weakly expressed in <i>R. taxifolium</i>. Both species had two <i>RAD</i> and <i>DIV</i> orthologs uniformly expressed across all floral organs. Differences in gene structure and expression of <i>Rhododendron RAD</i> compared to other asterids suggest that these genes may not be regulated by <i>CYC</i> orthologs. Our evidence supports <i>CYC</i> orthologs as the primary regulators of differential organ growth in <i>Rhododendron</i> flowers, while also suggesting certain deviations from the typical asterid gene regulatory network for flower symmetry.
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