Unravelling floral ontogeny and evolutionary trade-offs in the Adesmia clade (Dalbergieae, Papilionoideae, Fabaceae)

Keel flowers have captivated naturalists since Darwin due to the concealment of stamens and gynoecium by petals. These flowers form a distinct pollination syndrome and can function through four mechanisms. Within tribe Dalbergieae, most genera exhibit keel flowers, yet few studies have characterized their evolution. To address this gap, we investigate floral ontogeny and development in four out of five genera of the Adesmia clade. We dissected floral buds and inflorescences and used scanning electron microscopy to visualize development. Nissolia has four-flowered fascicles with a bract subtending the fourth flower. Adesmia has helicoidal racemes with a bract per flower. Poiretia and Zornia have distichous inflorescences with two bracteoles. We found variable patterns of calyx initiation among the studied genera. The Adesmia clade exhibits a fascinating evolutionary history of inflorescence development and reduction. In Poiretia and Zornia the bract and two bracteoles per flower are possibly analogous to the bracts and bracteoles of other Papilionoideae, potentially originating from an aborted leaf and its stipules. Our results suggest a trade-off between nectar secretion and staminal tube fusion driving the transition between valvular and piston mechanisms in keel flowers. The unique characteristics observed in each genus underscore the importance of ontogenetic studies for unravelling homology and evolutionary trade-offs.