Data from: Nectar bacteria, but not yeast, weaken a plant-pollinator mutualism

Mutualistic interactions are often subject to exploitation by species that are not directly involved in the mutualism. Understanding which organisms act as such ‘third-party’ species and how they do so is a major challenge in the current study of mutualistic interactions. Here, we show that even species that appear ecologically similar can have contrasting effects as third-party species. We experimentally compared the effects of nectar-inhabiting bacteria and yeasts on the strength of a mutualism between a hummingbird-pollinated shrub, Mimulus aurantiacus, and its pollinators. We found that the common bacterium Gluconobacter sp., but not the common yeast Metschnikowia reukaufii, reduced pollination success, seed set and nectar consumption by pollinators, thereby weakening the plant–pollinator mutualism. We also found that the bacteria reduced nectar pH and total sugar concentration more greatly than the yeasts did and that the bacteria decreased glucose concentration and increased fructose concentration whereas the yeasts affected neither. These distinct changes to nectar chemistry may underlie the microbes' contrasting effects on the mutualism. Our results suggest that it is necessary to understand the determinants of microbial species composition in nectar and their differential modification of floral rewards to explain the mutual benefits that plants and pollinators gain from each other.互利共生相互作用往往会受到未直接参与该共生关系的物种的剥削。明确哪些生物扮演此类‘第三方物种’，以及它们具体如何实施剥削，是当前互利共生研究领域的一大核心挑战。在此研究中，我们证实即便生态习性看似相似的物种，作为第三方物种时也可能产生截然相反的作用。我们通过实验对比了栖息于花蜜的细菌与酵母菌对蜂鸟传粉灌木橙黄沟酸浆（*Mimulus aurantiacus*）及其传粉者之间互利共生关系强度的影响。研究发现，常见的葡糖杆菌属（*Gluconobacter* sp.）细菌——而非常见的鲁考夫梅奇酵母（*Metschnikowia reukaufii*）——会降低传粉成功率、结实率以及传粉者的花蜜取食量，进而削弱植物与传粉者间的互利共生关系。我们还发现，相较于酵母菌，细菌对花蜜pH值与总糖浓度的降低作用更为显著；此外，细菌会降低花蜜中的葡萄糖浓度并提升果糖浓度，而酵母菌则未对二者产生任何影响。这些对花蜜化学组成的差异化改变，或许正是两类微生物对互利共生关系产生相反作用的内在机制。本研究结果表明，若要阐释植物与传粉者从彼此间获取的互利收益，必须明晰花蜜中微生物群落组成的决定因素，以及它们对花部报酬的差异化修饰作用。