Effects of nectarivorous yeasts on pollinator foraging behavior and male plant fitness

Background/Question/Methods - Floral nectar is a key ecological trait that commonly mediates interactions between plants and their pollinators. Comprised of primarily sugars and amino acids, nectar can also attract non-pollinating floral visitors such as nectar-robbers or microorganisms that often utilize nectar for their own benefit without conferring pollination services. It’s been hypothesized that microorganisms in floral nectar, such as yeasts, may affect pollinator foraging behavior and components of plant fitness through changes in nectar chemistry. In this study, we examined the effects of nectar yeasts on estimates of pollinator foraging behavior and male plant fitness. We manipulated the density of the floral yeast Metschnikowia reukaufii in the nectar of the host plant Delphinium nuttallianum and measured subsequent effects on pollinator foraging and pollen donation. To estimate effects on pollinator behavior, we treated virgin flowers with a sterile control or M. reukaufii-inoculated sucrose solution, subjected them to single visits from Bombus frigidus queens and measured subsequent foraging time. In a separate set of plants, we estimated effects on male fitness by treating nectar at the whole-plant level and measured pollen donation to neighboring plants using a pollen analog. 

Results/Conclusions - We found that B. frigidus queens foraged 67% longer on D. nuttallianum flowers treated with yeasts compared to controls, however this difference was not statistically significant. Surprisingly, nectar yeasts significantly enhanced male fitness. Plants treated with M. reukaufii donated approximately three times more pollen to neighboring plants than plants treated with the sucrose solution only. Taken together, these results suggest that yeasts present in floral nectar may affect male plant fitness indirectly through changes in pollinator foraging behavior and subsequent pollen flow. Work remains to understand how nectar yeasts affect other aspects of pollinator foraging behavior and to identify the potential mechanisms driving these results, including effects of yeasts on nectar composition and scent, traits important in pollinator foraging decisions. Our results demonstrate that yeasts may act as important drivers of variation in plant reproduction, variation likely driven through changes in pollinator behavior in response to yeast presence and metabolic activity.