Ontogeny and glandular features of Alexa grandiflora flowers offer evolutionary insights into the Angylocalyx clade: a Papilionoideae (Leguminosae) lineage with non-papilionaceous corolla.

Abstract

Alexa grandiflora Ducke is a papilionoid legume tree native to the Brazilian Amazon Forest. It belongs to the early-diverging Angylocalyx clade within the subfamily Papilionoideae, which is characterized by keel flowers, with some genera having flowers other than typical papilionaceous ones. This study describes the floral organography, organogenesis, and secretory structures of A. grandiflora and compares its floral morphology with that of three species from different genera within the Angylocalyx clade to deepen the understanding of the clade's floral structure and, by extension, the broader Papilionoideae subfamily. To conduct the study, floral buds and flowers from A. grandiflora were collected and processed for surface and anatomical studies, and flowers from herbarium specimens of Castanospermum australe, Xanthocercis madagascariensis and Angylocalyx oligophyllus to elucidate the clade's floral evolution and its implications for Papilionoideae diversity. Floral buds and flowers of A. grandiflora were analyzed using surface and anatomical techniques, while herbarium specimens of the comparative taxa were examined via scanning electron microscopy. In A. grandiflora, the apical meristem of the racemose inflorescence primary axis produces first-order bracts acropetally in a helical order. Sepal initiation is unidirectional, petal initiation is simultaneous, with the adaxial petal growing faster than the others. Antesepalous stamens appear simultaneously and concurrently with the carpel, while antepetalous stamens emerge simultaneously. Floral secretion of nectar, terpenes, and oleoresin supports phyllostomid bat pollination in Alexa species, consistent with the previously proposed association between intense nectar and terpene production and chiropterophily in the genus. Comparative analysis reveals that the Angylocalyx clade shares key floral traits, including a gamosepalous calyx, an enlarged adaxial petal, and similarly shaped lateral and abaxial petals. However, variations are observed in the type of inflorescence and in the level of insertion of the filament in the anther, highlighting the floral diversity within the clade.