Disentangling effects of flower symmetry and orientation on pollination – Saxifraga as a study case

Perspective matters: Pollinator guidance and pollen transfer in Saxifraga stellaris is shaped by within-plant differences in symmetry and orientation

by Manuel Wiegel & Klaus Lunau

Perspective matters: Floral symmetry and orientation are key traits shaping pollinator behaviour and pollen placement, yet botanists usually classify flowers based on symmetry seen from above, but flower-visiting insects experience flowers very differently. In the genus Saxifraga floral displays exhibit high intraindividual variation: Flowers positioned at the top of inflorescences are radially symmetrical, whereas laterally positioned flowers are bilaterally symmetrical. These differences are induced by gravity, resulting in variation of petal length, interpetal angles, presence and/or size of floral guides on petals coordinated with sequential stamen movement. Radially symmetrical flowers provide similar visual information to flower visitors from all approaching directions, while bilaterally symmetrical flowers present directional cues that define landing sites and guide pollinator movement in line with the sequential anther dehiscence. Using field observations, video analyses, and experimental measurements of pollen surrogate deposition, we studied how flower-visiting flies respond to variation in flower symmetry and orientation of Saxifraga stellaris, S. cuneifolia and S. fortunei. Our results show that landing behaviour and subsequent movement patterns were strongly influenced by flower orientation and, to lesser extent, by flower symmetry. The development of vertically oriented bilaterally symmetrical flowers likely improves pollen placement and reduces self-pollination. Our findings highlight the combined role of gravity, flower symmetry, and pollinator behaviour in shaping bilateral flower evolution in Saxifraga.

Read the scientific publication in JPE 

Endemic Castilleja grisea support diverse floral-visitor assemblages with implications for promoting local plant communities

Agapostemon subtilior bee visiting another native bloom, Malacothamnus clementinus (San Clemente island bushmallow). Photo by D.T. Rankin CC-BY

 

by Saldivar et al.

The Channel Islands of California are a biodiversity hotspot and are home to a wide array of unique species found nowhere else. San Clemente Island, one of the southernmost islands in the archipelago, supports a particularly rich native flora. Here, we explore the floral-visitor interactions involving Castilleja grisea (San Clemente Island paintbrush), an island-endemic hemiparasitic plant that has increased in abundance following removal of historic livestock and herbivores. Although this recovery from grazing pressure led to its recent delisting as a federally endangered species, C. grisea remains state listed as rare and is the focus of ongoing post-delisting monitoring.

Using field surveys conducted across the island, we examined how C. grisea contributes to floral visitor diversity and plant-floral visitor network structure within its native habitat on San Clemente Island, California. Despite growing interest in pollinators and conservation of endemic plants, baseline information on visitation patterns and how community composition influences stability of ecosystem services remains limited. Our study addresses this gap by documenting novel plant-floral visitor associations and discussing the potential for C.grisea as an indicator of community health.

We recorded 23 floral visitor taxa visiting 12 plant species, with nearly three-quarters of these visitors interacting with C. grisea. Plots where C. grisea was in bloom supported nearly 50% higher floral visitor species diversity compared to sites without flowering individuals, underscoring its ecological importance. Collectively, our findings suggest that San Clemente Island paintbrush may function as a key species, and that conservation and management strategies focused on its protection may provide broad ecological benefits for island-wide plant and floral visitor communities.

 Read the scientific publication in JPE.

Influence of group size and floral display on pollinator behaviour in Moricandia arvensis

A male of Eucera nigrilabris is visiting a Moricandia arvensis flower 
(from Gomez et al. 2025; https://doi.org/10.1093/aobpla/plae065).

by Torices et al.

Plants often grow in clusters rather than as isolated individuals. We asked whether pollinators respond mainly to how many plants are grouped together or to how many flowers the group displays. To answer this, we created experimental patches of the Mediterranean plant Moricandia arvensis, varying independently the number of plants and the total number of open flowers. We recorded how often insect pollinators visited these patches and how they moved among plants and flowers. Most visits were made by long-tongued solitary bees, especially Anthophora and Eucera, which are the main spring pollinators of this species. The number of open flowers in a patch was the strongest factor attracting pollinators: patches with more flowers received more visits, particularly when they contained many plants. However, individual flowers in large displays received fewer visits on average. The number of plants alone had weaker effects, but it influenced how strongly flower number increased pollinator attraction. In fact, adding flowers made patches much more attractive when those flowers were spread across many plants than when the flowers were arranged in a few plants. Overall, our results show that the way plants are grouped affects pollinator behaviour in ways that cannot be explained simply by the number of flowers within a patch.

Read the scientific article in JPE HERE