Premise of the Study: Zygomorphy has evolved multiple times in angiosperms. Near-actinomorphy is the ancestral state in the early diverging eudicot family Papaveraceae. Zygomorphy evolved once in the subfamily Fumarioideae from a disymmetric state. Unusual within angiosperms, zygomorphy takes place along the transverse plane of the flower. Methods: We investigated floral development to understand the developmental bases of the evolution of floral symmetry in Papaveraceae. We then assessed the expression of candidate genes for the key developmental events responsible for the shift from disymmetry to transverse zygomorphy, namely CRABSCLAW for nectary formation (PAPCRC), SHOOTMERISTEMLESS (PAPSTL) for spur formation, and CYCLOIDEA (PAPCYL) for growth control. Key Results: We found that an early disymmetric groundplan is common to all species studied, and that actinomorphy was acquired after sepal initiation in Papaveroideae. The shift from disymmetry to zygomorphy in Fumarioideae was associated with early asymmetric growth of stamen filaments, followed by asymmetric development of nectary outgrowth and spur along the transverse plane. Patterns of PAPSTL expression could not be clearly related to spur formation. PAPCRC and PAPCYL genes were expressed in the nectary outgrowths, with a pattern of expression correlated with asymmetric nectary development in the zygomorphic species. Additionally, PAPCYL genes were found asymmetrically expressed along the transverse plane in the basal region of outer petals in the zygomorphic species. Conclusion: Genes of PAPCRC and PAPCYL families could be direct or indirect targets of the initial transversally asymmetric cue responsible for the shift from disymmetry to zygomorphy in Fumarioideae.