Abstract: Nymphaeales, an early-diverging angiosperm order, is pivotal for understanding floral evolution, yet the processes of floral organogenesis and the evolutionary transitions between Cabombaceae and Nymphaeaceae remain incompletely resolved. Here, we integrate scanning electron microscopy, phylogeny-based ancestral state reconstruction, and comparative genomics to investigate floral organogenesis and MADS-box gene families in Brasenia schreberi (Cabombaceae) and Euryale ferox (Nymphaeaceae). Brasenia schreberi exhibits a stable trimerous, whorled initiation pattern; its floral apex remains dome-shaped and produces a superior gynoecium with free (apocarpous) carpels. In contrast, E. ferox displays a tetramerous pattern with unidirectional (abaxial-to-adaxial) initiation followed by spiral centripetal organ formation; its apex becomes concave early, forming a complex receptacle that develops into an inferior gynoecium with syncarpous carpels. Ancestral state reconstructions indicate that Cabombaceae retains more plesiomorphic traits of the Nymphaeales ancestor, whereas Nymphaeaceae exhibits multiple derived innovations. Using a domain- validated, de-redundant dataset, we further compare MADS-box phylogeny and motif architectures, revealing relatively conserved motifs in ABCDE-related MIKC clades but more heterogeneous patterns in certain non-ABCDE lineages, partly influenced by annotation quality. Together, these results support independent evolutionary trajectories for Cabombaceae and Nymphaeaceae and shed light on the evolution of floral organization in basal angiosperms.

Key words: basal angiosperm, Brasenia, Euryale, floral organogenesis, MADS-box, Nymphaeales