Nectar supplementation changes pollinator behaviour and pollination mode in Pedicularis dichotoma: implications for evolutionary transitions.

Backgrounds and Aims: Gain or loss of floral nectar, an innovation in floral traits, has occurred in diverse lineages of flowering plants, but the causes of reverse transitions (gain of nectar) remain unclear. Phylogenetic studies show multiple gains and losses of floral nectar in the species-rich genus Pedicularis. Here we explore how experimental addition of nectar to a supposedly nectarless species, P. dichotoma, influences pollinator foraging behaviour. Methods: The liquid (nectar) at the base of the corolla tube in P. dichotoma was investigated during anthesis. Sugar components were measured by high-performance liquid chromatography. To understand evolutionary transitions of nectar, artificial nectar was added to corolla tubes and the reactions of bumble-bee pollinators to extra nectar were examined. Key Results: A quarter of unmanipulated P. dichotoma plants contained measurable nectar, with 0.01-0.49 µL per flower and sugar concentrations ranging from 4 to 39 %. The liquid surrounding the ovaries in the corolla tubes was sucrose-dominant nectar, as in two sympatric nectariferous Pedicularis species. Bumble-bees collected only pollen from control (unmanipulated) flowers of P. dichotoma, adopting a sternotribic pollination mode, but switched to foraging for nectar in manipulated (nectar-supplemented) flowers, adopting a nototribic pollination mode as in nectariferous species. This altered foraging behaviour did not place pollen on the ventral side of the bees, and sternotribic pollination also decreased. Conclusion: Our study is the first to quantify variation in nectar production in a supposedly 'nectarless' Pedicularis species. Flower manipulations by adding nectar suggested that gain (or loss) of nectar would quickly result in an adaptive behavioural shift in the pollinator, producing a new location for pollen deposition and stigma contact without a shift to other pollinators. Frequent gains of nectar in Pedicularis species would be beneficial by enhancing pollinator attraction in unpredictable pollination environments.