Department of Biology, Stanford University, Stanford, CA, USA.
Proc Biol Sci. 2012 Dec 5;280(1752):20122601. doi: 10.1098/rspb.2012.2601. Print 2013 Feb 7.
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.
互利共生关系经常受到非直接参与互利共生的物种的剥削。了解哪些生物充当此类“第三方”物种以及它们如何这样做，是当前互利共生关系研究的主要挑战。在这里，我们表明，即使是在生态上看起来相似的物种，也可能具有相反的作为第三方物种的作用。我们通过实验比较了花蜜中栖息的细菌和酵母对蜂鸟授粉的灌木 Mimulus aurantiacus 与其传粉者之间互利共生关系强度的影响。我们发现常见的细菌 Gluconobacter sp. 但不是常见的酵母 Metschnikowia reukaufii 会降低传粉者的授粉成功率、结实率和花蜜消耗量，从而削弱植物-传粉者的互利共生关系。我们还发现，细菌比酵母更能显著降低花蜜的 pH 值和总糖浓度，而且细菌降低葡萄糖浓度并增加果糖浓度，而酵母则没有影响。这些对花蜜化学性质的明显变化可能是微生物对互利共生关系产生不同影响的基础。我们的研究结果表明，有必要了解花蜜中微生物物种组成的决定因素及其对花奖励的差异修饰，以解释植物和传粉者相互受益的原因。
