Heterochronic shifts and modular organization underlie the evolution of floral morphotypes in Eperua (Detarioideae, Leguminosae)

The legume genus Eperua (subfamily Detarioideae) has flowers characterized by a single developed adaxial petal and four petaloids, a trait it shares with its sister genus Eurypetalum. Within Eperua, two distinct floral morphotypes are recognized: a short-tubular corolla, where reproductive organs remain exposed, and a long-tubular corolla, where the adaxial petal expands to encloses them. This long-corolla form represents a unique innovation within the Detarioideae, having evolved independently multiple times within Eperua. Using morphometric analysis, stereomicroscopy, and scanning electron microscopy (SEM), this study investigates the floral components underlying this morphological variation, their ontogenetic processes, and their evolutionary relationship to Eurypetalum and other detarioid taxa. We found that the long-tubular morphotype is distinguished by a longer adaxial petal, a reduced hypanthium size and shorter anthers. This quasi-independent variation among organs is suggested by modular floral organization. The differences apper to be driven by heterochronic shifts in late development; in the long-tubular form, the adaxial petal exhibits prolonged growth to cover the reproductive whorls, while growth ceases earlier in the short-tubular form. These morphotypes are linked to pollination specialization, where organ proportions and positioning directly influence floral display, pollinator access, and pollen transfer efficiency. Comparative analysis with Eurypetalum reveals a shared expansion of the adaxial petal but divergent late-stage ontogeny: in Eperua erect sepals support the formation of a corolla tube, whereas in Eurypetalum, reflexed sepals result in an open corolla.