The morphological fit between flowers of Cypripedium calceolus L. and their visitors matters but does not explain flower size variations along altitude

Abstract

Studies of floral adaptations in response to divergent pollinators are important for understanding floral evolution and diversification of plants. A plant exposed to a variable pollinator climate is Cypripedium calceolus, a threatened lady's-slipper orchid. Insect pollinators are temporarily trapped in the pouch-like labellum and escape via one of two small posterior exit holes. In escaping, they pass the stigma and an anther, depositing and collecting pollen, respectively. Successful pollination is thought to depend on the morphological fit between pollinators and flowers, with particularly thorax and exit heights being key traits. Too small insects might neither touch the stigma nor collect pollen when exiting, and too large insects do not fit through the exit but leave the flower through the entrance hole or die inside. To discern the likelihood of floral adaptations in C. calceolus to varying pollinator assemblages, we investigated (a) whether floral, vegetative, and insect traits change in a concerted manner along an altitudinal gradient, and whether the morphological fit affects (b) the escape mode of a visiting insect and (c) its probability of exporting pollen. We found that floral and vegetative traits of C. calceolus got smaller with altitude, while insect dimensions were similar across the sites. Hymenoptera, the main visitors, were more likely to escape via the exit and to export pollen when the fit was near-exact. This shows that the morphological fit plays a critical role in the pollination of C. calceolus and that pollinators have the potential to drive size-related floral adaptations.