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

## TL;DR

Early sepal shedding in Actaea erythrocarpa helps promote cross-pollination by attracting specific pollinators.

## Contribution

The study identifies sepal shedding and fly pollinator behavior as key factors promoting cross-pollination in Actaea erythrocarpa.

## Key findings

- Sepals of Actaea erythrocarpa fall off early, coinciding with stigma receptivity and fly pollinator visits.
- The pollen–ovule ratio of 1773.58 ± 689.75 indicates a facultatively xenogamous breeding system.
- Agromyzidae sp. and Episyrphus balteatus are the main pollinators visiting flowers after sepal abscission.

## Abstract

Agromyzidae sp. and Episyrphus balteatus De Geer are the main pollinators, and they have a high frequency of visiting flowers after the sepals have fallen off and the anthers have not yet opened. A comparison of sepal morphology between Actaea asiatica Hara, which undergoes self-pollination, and Actaea erythrocarpa Fisch., which undergoes cross-pollination, suggests that the variation in sepal characteristics is closely related to the breeding system. The sepals of Actaea erythrocarpa fall off when the flowers just open and the stigma is receptive at the bud stage, which belongs to the breeding system of facultative crossbreeding. Therefore, the visiting behavior of fly insects and the early shedding of sepals promotes cross-pollination in A. erythrocarpa.

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.