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.