Evolution of UV reflection in bee- and bird-pollinated flowers

Floral colour is a key trait mediating plant-pollinator interactions, with UV reflection hypothesized to evolve in response to the effectiveness of pollinators. The bee-avoidance hypothesis predicts higher UV reflection in white flowers and lower reflection in red and yellow flowers of bird-pollinated plants, with opposite patterns in bee-pollinated flowers. However, the macroevolutionary role of this process remains unclear. We analysed 245 angiosperm species using ancestral state reconstruction and comparative evolutionary models to assess UV reflection patterns in relation to flower colour and pollinator type. Ancestral reconstructions revealed frequent transitions in both hue and UV reflection states. Higher UV reflection optima were found in red and bee-pollinated flowers compared to red bird-pollinated flowers, supporting the role of UV in bee attraction. White and yellow bee-pollinated lineages showed elevated evolutionary rates and selective strength compared to bird-pollinated lineages of the same hue. Unexpectedly, yellow bird-pollinated flowers displayed higher UV reflection optima than yellow bee-pollinated flowers. Our results suggest that both attraction and exclusion mechanisms contribute to the macroevolution of UV reflection in flowers. We found strong support for the bee-avoidance hypothesis in red flowers. However, while white flowers generally followed predicted patterns, yellow flowers showed a reverse pattern, with higher UV reflection in bird-pollinated lineages, providing contrasting evidence across colour categories. These findings highlight complex and colour-dependent selection dynamics in pollinator-mediated floral evolution.