Floral ontogeny of <i>Ateleia</i> and <i>Trischidium</i> , two early branching papilionoid legume genera marked by petal loss

Abstract Our understanding of the evolution of floral diversity has been greatly advanced by studies of floral ontogeny. Here, we examine the floral development of Trischidium molle and Ateleia guaraya, two phylogenetically closely related papilionoid legumes (Leguminosae, Papilionoideae) characterized by significant petal differentiation from the basic papilionate floral architecture that marks almost all representatives of the subfamily. Our ontogenetic comparative analyses reveal distinct developmental patterns, with both species exhibiting monoclinous flowers. A. guaraya, however, shows two ontogenetically similar floral morphs, yet with differing gynoecium sizes: one with a larger gynoecium whose stigma reaches the height of the anthers, and another with a smaller gynoecium that remains below the level of the anthers, suggesting a case of andromonoecy. The sequence of floral organ initiation in both species is as follows: five sepals (observed only in A. guaraya), a single petal (the standard), one carpel, and several stamens in T. molle and 10 in A. guaraya. A central finding is that both species use a ring meristem for organ initiation, yet this leads to divergent stamen numbers: ∼24 in T. molle but the typical papilionoid 10 in A. guaraya. This demonstrates that the presence of a ring meristem does not necessarily result in polyandry, challenging a common assumption in floral developmental studies. We propose that the evolution of such a ring meristem in the Swartzieae clade may have established a novel developmental arena that functions either as an enabling platform for organ proliferation (as in T. molle) or as a constraining scaffold that maintains a reduced form (as in A. guaraya). The absence of four out of five petals is thought to significantly influence subsequent organogenesis, including the spatial arrangement and initiation order of other floral organs. These findings also highlight the developmental mechanisms underlying petal absence from inception and their consequences for floral architecture in early diverging papilionoid legumes.