Floral isolation in Pedicularis: how do congeners with shared pollinators minimize reproductive interference?

To minimize interspecific pollination, it has been suggested that pollen is placed on different parts of a pollinator's body corresponding to the conspecific location of pollen pickup by the stigma. Although Pedicularis is regarded as a classic example of pollinator-mediated floral isolation, such reciprocal pollen placement has not been demonstrated experimentally. This leads us to question previous observations of pollen release in Pedicularis species. Here, we show that pollen grains are released from the tip, rather than the basal opening, of the galea (the hoodlike upper lip of the corolla) in eight nectarless Pedicularis species, mimicking pollen release from poricidal anthers. We used safranin-stained pollen within anthers to track pollen placement in three Pedicularis species, and showed that pollen was deposited on numerous parts of the bumblebee's body. However, fluorescent powder placed on the stigmas to detect the contact location on the bumblebee's body was deposited mainly on the major position of pollen placement in each of the three species. Such segregation of pollen placement and pickup between species sharing the same pollinator probably helps to reduce reproductive interference, but the positions of pollen placement and stigma contact on the bumblebee's body were not as precise as previously thought.