Two Metschnikowia nectar yeast species have similar volatile profiles, but elicit differential foraging in bee pollinators

1. Nectar yeasts are a highly specialized group of fungi that may play key roles in pollination ecology. Nectar yeasts lack an independent dispersal mechanism to access new habitats with fresh resources. Yeasts, bumble bee pollinators, and flowering plants likely take part in a series of diffuse mutualisms, wherein yeast attract bees that provide phoretic travel between flowers. This interaction is thought to provide bees with improved foraging efficiency and plants with increased pollinator visitation and associated pollination services. However, the underlying mechanisms driving bee pollinator preferences for nectar with yeast and differences among yeast species in eliciting pollinator behavior are relatively unexplored. 2. We used an integrative approach to elucidate the underpinnings of bee pollinator preference for nectars that contain yeasts. We conducted a survey of local flower nectar for presence and species diversity of yeast. Using two prominent, local nectar yeast species (Metschnikowia reukaufii and Metschnikowia koreensis), we conducted observational field trials to ascertain the effects of the presence and identity of nectar yeast on bee visitation rates. We also analyzed the volatile profiles of both yeast species to explore if olfactory cues were associated with differential foraging behavior. 3. We found that M. reukaufii was the most common nectar yeast in our study area in the Southeastern USA, as did previously published global surveys. Intriguingly, we found co-occurrence of multiple yeast species in 22% of nectar samples, all of which contained M. reukaufii and another yeast typically from the Metschnikowia genus, such as M. koreensis. In a field trial we found that bee pollinators had higher visitation to flowers supplemented with M. koreensis over sterile flowers, while no difference in bee foraging behavior was evident in response to M. reukaufii. Despite this behavioral difference, the volatile profiles of both yeast species were not significantly different from one another. 4. The ecology and species interactions of wild yeasts are poorly understood, yet may play vital roles in many ecosystems. Our research highlights the importance of studying facultative mutualisms, and the necessity of testing their underlying assumptions. Elucidating the mechanisms behind insect-microbe symbioses will open new horizons in pollination ecology and conservation.