Shared Pollinators and Pollen Transfer Dynamics in Two Hybridizing Species, Rhinanthus Minor and R. Angustifolius

Gene flow between hybridizing plant species depends strongly on pollinator behaviour, which affects pollen transfer among floral types and reproductive isolation. We examined bumblebee behaviour and pollen transfer between two hybridizing Rhinanthus species that are very similar in ecology and floral traits. The two species, Rhinanthus minor and R. angustifolius, shared similar pollinator guilds and assemblages, but pollinator recruitment and flower visitation rates were higher in R. angustifolius sites, probably because of its higher reward levels and better visibility. When presented with Rhinanthus flowers, bumblebees that previously foraged on R. angustifolius were less prone to visit R. minor inflorescences, while R. minor foragers accepted both species in similar proportions. Although Rhinanthus has been cited as a case of mechanical isolation resulting from interactions between bee behaviour and differences in stigma and anther placement, we found no support for efficient mechanical reproductive isolation. Bumblebees that foraged on R. minor flowers carried more pollen, but pollen placement on their bodies was similar to that of bees that visited R. angustifolius, and cross-specific stigmatic pollen deposition was similar in both directions. However, the asymmetry in pollinator handling time between the two species, due to dissimilar pollen rewards, may have lowered relative heterospecific pollen receipt on R. angustifolius, suggesting that net gene flow resulting from pollen transfer dynamics is more likely towards R. minor, although this effect remains weak and will be most likely counterbalanced by context-based labile pollinator preference.