Flowers as fungal extended phenotypes: nectar yeasts obfuscate among-plant differences in nectar sugar concentration

Nectar-dwelling yeasts modulate the ecology of interactions between flowers and pollinators. We assess here the hypothesis that floral nectar traits, which are a central element in most plant-pollinator relationships, can largely represent the extended fungal phenotypes of nectar-dwelling yeasts. The following specific question is addressed: do local genotypes of the specialist nectar yeast Metschnikowia reukaufii have the ability to obfuscate intrinsic individual variation among Helleborus foetidus plants in nectar sugar concentration? An array of paired plant-yeast genotypes mimicking a natural field situation was created in the laboratory by inoculating field-collected nectar from different plant individuals with distinct yeast genotypes following a factorial design. Chemical analyses of nectar sugars before and after exposure to yeasts were performed using ion-exchange high-performance liquid chromatography. Plant individual, yeast genotype and their interaction all had strong, significant effects on sucrose concentration of H. foetidus nectar. Yeasts abolished 79% of intrinsic variance among plants in nectar sucrose concentration and profoundly reshaped patterns of intrinsic among-plant variation. Our results support the hypothesis that among-plant variation in nectar sugar concentration found by pollinators in the field can sometimes reflect the extended phenotypes of nectar yeasts more closely than intrinsic differences among the plants themselves.