Department of Botany, University of Hawaii Manoa, Honolulu, HI, USA.
John A. Burns School of Medicine, University of Hawaii Manoa, Honolulu, HI, USA.
Ann Bot. 2019 Mar 14;123(4):657-666. doi: 10.1093/aob/mcy198.
For symbiotic organisms, their colonization and spread across remote oceanic islands should favour generalists. Plants that form obligate symbiotic associations with microbes dominate island ecosystems, but the relationship between island inhabitance and symbiotic specificity is unclear, especially in the tropics. To fill this gap, we examined the mycorrhizal specificity of the Hawaiian endemic orchid Anoectochilus sandvicensis across multiple populations encompassing its entire geographic distribution.
By molecular phylogenetic approaches we identified the mycorrhizal fungi associated with A. sandvicensis across its entire geographic distribution and determined the relationship of these fungi to others found elsewhere around the globe. With richness estimators, we assessed the mycorrhizal specificity of A. sandvicensis within and among islands. We then tested whether geographic proximity of orchid populations was a significant predictor for the presence of particular mycorrhizal fungi and their community composition.
We found that each population of A. sandvicensis forms specific associations with one of three fungi in the genus Ceratobasidium and that the closest relatives of these fungi are globally widespread. Based on diversity indices, A. sandvicensis populations were estimated to partner with one to four mycorrhizal taxa with an estimated total of four compatible mycorrhizal fungi across its entire distribution. However, the geographic proximity of orchid populations was not a significant predictor of mycorrhizal fungal community composition.
Our findings indicate that the colonization and survival of plant species on even the most remote oceanic islands is not restricted to symbiotic generalists, and that partnering with few, but cosmopolitan microbial symbionts is an alternative means for successful island establishment. We suggest that the spatial distribution and abundance of symbionts in addition to island age, size and isolation should also be taken into consideration for predictions of island biodiversity.
对于共生生物而言，它们在遥远的海洋岛屿上的定殖和传播应该有利于广域适应者。与微生物形成专性共生关系的植物主导着岛屿生态系统，但岛屿栖息与共生特异性之间的关系尚不清楚，特别是在热带地区。为了填补这一空白，我们研究了夏威夷特有兰花 Anoectochilus sandvicensis 在其整个地理分布范围内与多种种群相关的菌根特异性。
通过分子系统发育方法，我们确定了与 A. sandvicensis 相关的菌根真菌，遍及其整个地理分布范围，并确定了这些真菌与全球其他地方发现的真菌之间的关系。通过丰富度估计，我们评估了 A. sandvicensis 在岛屿内部和岛屿之间的菌根特异性。然后，我们测试了兰花种群的地理接近度是否是特定菌根真菌存在及其群落组成的重要预测因素。
我们发现，A. sandvicensis 的每个种群都与 Ceratobasidium 属的三种真菌中的一种形成特定的共生关系，而这些真菌的最接近亲缘种在全球范围内广泛分布。基于多样性指数，A. sandvicensis 种群估计与一种到四种菌根类群形成伙伴关系，在其整个分布范围内共有四种相容的菌根真菌。然而，兰花种群的地理接近度并不是菌根真菌群落组成的重要预测因素。
我们的研究结果表明，即使是最偏远的海洋岛屿上植物物种的定殖和生存也不受限于共生的广域适应者，与少数但世界性的微生物共生体形成伙伴关系是成功建立岛屿的另一种手段。我们认为，除了岛屿年龄、大小和隔离度之外，共生体的空间分布和丰度也应该被纳入到对岛屿生物多样性的预测中。
