Early Shedding of Sepals Promotes Cross-Pollination of Actaea erythrocarpa (Ranunculaceae)

The diversity of floral traits in angiosperms has evolved over time as an adaptive response to reproductive demands. Investigating floral characteristics and pollination systems helps elucidate the evolutionary drivers behind morphological variation in flowers. In this study, we examined A. erythrocarpa to systematically document its floral morphology, stigma receptivity, pollen–ovule ratio, breeding system, and pollinator behavior. Results showed that the sepals abscised completely at the early flowering stage, while stigmas became receptive even during the bud phase. The pollen–ovule ratio was 1773.58 ± 689.75, indicating a facultatively xenogamous breeding system. Bagging experiments further confirmed that the species is self-compatible but does not exhibit apomixis or wind pollination. Field observations identified Agromyzidae sp. and E. balteatus as the primary pollinators, which visited flowers at high frequency after sepal abscission but before anther dehiscence. Compared with its congener A. asiatica, which exhibits delayed sepal abscission and relies mainly on selfing, A. erythrocarpa demonstrates distinct floral morphological adaptations linked to its specialized pollination strategy. These two species thus represent divergent reproductive adaptation patterns within the genus. Therefore, the visiting behavior of fly insects and the early shedding of sepals promotes cross-pollination of A. erythrocarpa.