Puzzling androecium development in Bauhinia galpinii (Fabaceae) facilitates wing pollination by butterflies

Floral adaptation to pollinators often involves key modifications of floral development. We investigated floral development and its function for pollination in Bauhinia galpinii, a legume species native to eastern and southern Africa with a unique arrangement of stamens and staminodes. Flowers and floral buds at different stages were prepared for observation under Scanning Electron Microscopy (SEM), Light Microscopy (ML), and X-ray Computed Tomography (XCT). Pollinator observations were conducted in populations in southern Africa and floral advertising and reward traits were quantified. Five (rarely six) sepals emerge helically, followed by the petal primordia in unidirectional order. The carpel arises synchronously with the stamens and expands rapidly. Stamen initiation occurs unidirectionally, with three abaxial antesepalous stamens fully developing while other stamen primordia remain sterile and underdeveloped (staminodial). The number of adaxial staminode tips can fluctuate by branching, regardless of their original whorl. Concentric collateral bundles supply all the staminodes. Swallowtail butterflies transfer pollen through their fluttering wings that touch the abaxial anthers and stigma while feeding on nectar in the hypanthium located on the adaxial side. Bauhinia galpinii exhibits an androecium characterised by secondary, unstable branching of staminodes, which form adaptive structures suited to its pollinators. The variation in androecium structure may be attributed to heterochronic changes during development, marked by an early carpel initiation and its basal expansion leading to spatial restriction. This ontogenetic contingency, associated with selective pressures from pollinators, notably swallowtail butterflies, seems to play a role in selecting this complex and distinct floral architecture.