Ontogeny unlocks the floral development and evolution of Uleanthus, a florally divergent monospecific genus of Amazonian legume trees

Floral ontogeny is one of the most powerful tools in comparative biology for understanding the diversity of flower forms and their evolutionary pathways. The Amazonian monospecific tree genus Uleanthus (Leguminosae, Papilionoideae) stands out for its striking hummingbird-pollinated flowers with a bright red calyx and pink to lilac standard petal forming a tubular structure that encloses the reproductive organs. Ontogenetic revelations of Uleanthus erythrinoides may shed light on its controversial phylogenetic placement and help to elucidate broader patterns of floral homology and convergence among morphologically similar yet evolutionarily distant, and morphologically contrasting but phylogenetically close lineages within papilionoid legumes. Flower buds were collected in different developmental stages in the field, and stored in 70 % ethanol to investigate the floral ontogeny using scanning electron microscopy (SEM). Results were broadly compared with existing material from putatively closely related papilionoid legume genera. Sepals of Uleanthus erythrinoides emerge with a unidirectional sequence. The petals initiate simultaneously; the antesepalous and antepetalous stamens initiate unidirectionally. The carpel initiation is concomitant with the first antesepalous stamen primordia. The apparent resemblance between Uleanthus and other bird-pollinated papilionoid genera does not bear support in floral ontogeny, rather they have evolved convergently. Despite the contrasting floral architecture of Uleanthus and its putative phylogenetically closely related Camoensia from the Genistoid clade, both share similar flower with a showy standard and undifferentiated inner petals that are verified early in the ontogenetic development.