A novel type of mirror-image flowers caused by lateral bending of the floral tube in a bumblebee-pollinated plant.

• A new type of mirror-image flowers is caused by lateral bending of the floral tube. • The lateral bending of floral tube selects for long-tongued bumblebees as legitimate pollinators. • Mirror-image flowers are normally associated with low pollen-ovule ratio. We studied a previously unreported form of mirror-image flowers in Rhabdothamnopsis sinensis (Gesneriaceae) endemic to China. The style in R. sinensis remains straight throughout the blooming period, while the basal part of the floral tube bends either to the left or right side, causing the style to lean towards the right or left side of the floral tube. R. sinensis has two epipetalous fertile stamens with twisted filaments, moving the fused anthers located at the opposite side of the style. This floral syndrome results in reciprocal mirror-image flowers in a relative simpler way as compared to typical types of mirror-image flowers. Pollinator observations revealed that the bending of the floral tube and twisted filament block the entrance to the floral tube, rendering long-tongued bumblebees as pollinators by transferring pollen with the side of the thorax. R. sinensis is self-compatible and the pollen-ovule ratio is strikingly low (37.41 ± 11.90). Thorax pollination probably is the main reason for such low pollen wastage because the thorax suffers less grooming than the abdomen, which is the main pollen-carrying part in other typical mirror-image flowers. The unusual type of mirror-image flowers described here suggests a specialized pollination adaptation with limited but high-fidelity pollinators. [ABSTRACT FROM AUTHOR]