Floral ontogeny of Cleobulia (Leguminosae: Papilionoideae), the dwarf winged genus within the Dioclea clade

Many angiosperm liana clades are still poorly known in terms of floral variation and ontogenetic development, despite their ecological importance and evolutionary success across all forest biomes. This is the case of the neotropical tribe Diocleae (Papilionoideae, Leguminosae) that exhibits great variation in floral architecture, from bee-pollinated papilionate flowers to highly specialized bird-pollinated flowers with distinct degrees of petal differentiation. The genus Cleobulia stands out among the Diocleae by its unique flower morphology associated with hummingbird pollination mechanism: down-curved inflorescence rachis, functionally resupinate flowers, red calyx, pink-coloured petals, and wing petals that are very reduced in size compared to the keel length. To understand the similarities in the floral development within the Diocleae, we present here a scanning electron microscopy characterization of the floral ontogenetic development of Cleobulia coccinea. The flowers display an acropetal and helical development along the inflorescence. Sepals are initiated unidirectionally with a tendency towards simultaneous pattern, while petals are initiated simultaneously and develop unidirectionally. The stamens are initiated unidirectionally and the carpel starts to develop concurrently with the petal whorl. When compared with Macropsychanthus ucayalinus, we verified that these two Diocleae species match in the development of petals, androecium, and gynoecium. Aside from that, the unusual dwarf wing petals of C. coccinea are only visible later during floral development, but the fusion of two adaxial sepals occurs in early stages of development. This ontogenetic pathway culminates in the four visible calyx lobes at anthesis, a diagnosable character of the Dioclea clade.