Floral developmental insights into two species of Erythrina (Fabaceae: Papilionoideae: Phaseoleae) pollinated by hummingbirds and passerines.

Erythrina is a Pantropical bird-pollinated genus of Fabaceae. Thus, its flowers are usually large, showy, red or yellowish, offering nectar as the principal resource. There are two main interaction systems with birds in Erythrina: in one, the inflorescences are erect and the flowers are horizontal, offering no landing platform; in the other, the inflorescences are horizontal and the flower parts are more exposed. Erythrina speciosa is pollinated by hummingbirds and E. poeppigiana is pollinated by passerines. Despite their structural variation, little is known about how species of the same genus diverge ontogenetically to form flowers adapted to pollinators with different beak morphology and feeding behaviors. Therefore, this study aimed to investigate floral development in two species according to their pollination system. Flowers and buds were collected and fixed for analysis using scanning electron microscopy and light microscopy. Some characteristics are common to both species: the formation of a pseudoracemose inflorescence, the unidirectional emergence of floral organs, and the formation of a short staminal sheath involving nine of the ten stamens (diadelphous androecium). Other characteristics, notably those related to the late stages of floral development, gradually diverged. Among them are inflorescence formation pattern; the formation of reduced and free keel petals in E. speciosa, while in E. poeppigiana they are longer and postgenitally united by their lower margins; and the participation of the standard in the floral display. The studied species share several traits common to other Papilionoideae, but some similarities between the species studied may not be phylogenetically related and reveal the potential ontogenetic pathways of functional convergence that flowers have experienced throughout evolution in the genus.