Pre- and post-pollination interaction between six co-flowering Pedicularis species via heterospecific pollen transfer

It remains unclear how related co-flowering species with shared pollinators minimize reproductive interference, given that the degree of interspecific pollen flow and its consequences are little known in natural communities. Differences in pollen size in six Pedicularis species with different style lengths permit us to measure heterospecific pollen transfer (HPT) between species pairs in sympatry. The role of pollen–pistil interactions in mitigating the effects of HPT was examined. Field observations over 2 yr showed that bumblebee pollinators visiting one species rarely moved to another. Heterospecific pollen (HP) comprised < 10% of total stigmatic pollen loads for each species over 2 yr, and was not related to conspecific pollen deposition. Species with longer styles generally received more HP per stigma. The pollen tube study showed that pollen from short-styled species could not grow the full length of the style of long-styled species. Pollen from long-styled species could grow through the short style of P. densispica, but P. densispica rarely received HP in nature. Flower constancy is a key pre-pollination barrier to HPT between co-flowering Pedicularis species. Post-pollination pollen–pistil interactions may further mitigate the effects of HPT because HP transferred to long styles could generally be effectively filtered.