Revisiting floral ontogeny of the papilionoid legume Dipterygeae clade: insights from the florally divergent Amazonian genus Monopteryx

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ABSTRACT The small Amazonian genus Monopteryx of the Papilionoideae legumes (Leguminosae) is sister to the remaining Dipterygeae genera, one of the earliest branching clades of the subfamily. This clade is uniquely marked by a winged papilionate floral architecture, where the two generally wing-shaped upper calyx lobes are disproportionately more enlarged than the three reduced lower lobes and the corolla is strongly papilionate and formed by clearly differentiated petals. This floral architecture, however, contrasts with the nonpapilionate flowers of Monopteryx, which also have enlarged upper calyx lobes, but they are fully fused and assume a standard petal-shaped orientation. Here, we explore the comparative floral ontogeny across the Dipterygeae clade, focusing on the genus Monopteryx, which has not been previously studied ontogenetically, to reveal homologies between nonpapilionate and winged papilionate floral architectures. Flower buds of Monopteryx uaucu, Dipteryx odorata, D. rosea, and Pterodon abruptus were collected in the field and investigated using scanning electron microscopy. Results were also compared with existing floral development information about the Dipterygeae and other early diverging papilionoid legume genera. Sepals initiate bidirectionally in M. uaucu and P. abruptus. In Dipteryx, they show a unidirectional sequence. Petal primordia arise simultaneously in all species examined. The stamens in M. uaucu emerge unidirectionally, while in Dipteryx and P. abruptus the antesepalous stamens emerge in a modified unidirectional order, and antepetalous stamens emerge almost simultaneously. Stamens fuse at the base in M. uaucu and form an open sheath, while in Dipteryx and P. abruptus, there is a constricted slit. In all species, carpel initiation is precocious and before the outer or inner stamen whorl. Despite their florally divergent architecture, floral ontogeny suggests homology among the nonpapilionate flowers of Monopteryx and the winged papilionate flowers exhibited by other Dipterygeae genera, strengthening their affinities as demonstrated by recent molecular phylogenies. Their contrasting yet ontogenetically similar floral morphologies, as revealed here, support the hypothesis that the evolutionary pathway toward the specialized papilionate architecture involved an experimental phase among early branching papilionoid legumes.

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