尖峰岭热带山地雨林根部真菌?植物互作网络结构特征

Structural features of root-associated fungus–plant interaction networks in the tropical montane rain forest of Jianfengling, China

不同功能群的根部真菌可能会与植物差异性地互作,并进一步影响地下真菌与植物群落构建。本研究采用Illumina Miseq测序方法检测了海南尖峰岭热带山地雨林中常见植物的根部真菌;采用网络分析法比较了丛枝菌根(AM)真菌、外生菌根(ECM)真菌,以及所有根部真菌与植物互作的二分网络(bipartite networks)结构特性。从槭树科、番荔枝科、夹竹桃科、冬青科、棕榈科、壳斗科、樟科和木犀科等8科植物的根系中,检测到297,831条真菌ITS1序列,这些序列被划为1,279个真菌分类单元(OTUs),其中子囊菌门748个、担子菌门354个、球囊菌亚门80个,以及未知真菌97个。核心根部真菌群落(420个OTUs)中,至少有三类不同生态功能的真菌常见,即丛枝菌根真菌(40个OTUs,占总序列数23.4%)、外生菌根真菌(48个OTUs, 13.9%)和腐生型真菌(83个OTUs, 19.8%)。尖峰岭山地雨林根部真菌–植物互作网络结构特性的指标普遍显著高于/低于假定物种随机互作的零模型期待值。在群落水平,不同功能型的根部真菌–植物互作网络表现出不同或相反的结构特性,如丛枝菌根互作网络表现为比零模型预测值高的嵌套性和连接性,以及比零模型低的专一性,而外生菌根互作网络呈现出比零模型预测值低的嵌套性和连接性,以及比零模型高的专一性。在功能群水平,植物的生态位重叠度在AM互作网络高,而ECM互作网络低;真菌的生态位宽度在ECM互作网络窄,而在AM互作网络较宽。共现(co-occurrence)网络分析进一步揭示,ECM群落的物种对资源的高度种间竞争(植物、真菌高C-score),以及AM群落的物种无明显种间竞争(低C-score),可能分别是形成反嵌套ECM互作网络及高嵌套AM互作网络结构的原因。上述结果说明,尖峰岭山地雨林中至少有两种及以上的种间互作机制调节群落构建:驱动AM互作网络冗余(nestedness)及ECM互作网络的高生态位分化(专一性)。本研究在同一个森林内探讨了不同功能型的真菌-植物互作特性,对深入理解热带森林的物种共存机制和生态恢复具有重要意义。

Functionally diverse root-associated fungi may differentially interact with host plants, potentially affecting the assembly processes of belowground plant and fungi communities. Here, we applied the Illumina Miseq sequencing technique to identify root-associated fungi of plants which were co-dominant in a tropical montane rain forest on Hainan Island, China. Structural features of bipartite networks were compared among the whole root-associated, arbuscular mycorrhizal(AM) and ectomycorrhizal(ECM) fungus–plant interactions. A total of 297,831 fungal ITS1 sequences were obtained from eight families including Aceraceae,Annonaceae, Apocynaceae, Aquifoliaceae, Arecaceae, Fagaceae, Lauraceae, and Oleaceae. Fungal sequences were assigned to 1,279 OTUs comprised of Ascomycota(748 OTUs), Basidiomycota(354), Glomeromycota(80), and unidentified fungi(97). At least three functional groups of fungi i.e. putatively ECM(40 OTUs,represented 23.4% of the total fungal reads), AM(40, 13.9%) and saprophyte(83, 19.8%) were prevalent in the core root-associated fungal community(420 OTUs) of the tropical montane rain forest. Network analysis indicated that AM, ECM and root-associated fungus–plant interaction network showed structural features which cannot be predicate by null models assuming species interact randomly. Community level indices behaved differently among different ecotypes of fungus–plant interactions. Specifically, the degree of nestedness(NODF) and connectance were higher, while specialization was lower in the AM interaction network than the expected values from null models. In contrast, the ECM interaction network had a significantly higher degree of specialization and lower nestedness and connectance than the null models. At guild levels, plant niche overlap of AM and ECM interactions are higher and lower than the null model,respectively. Niche breadth of ECM fungi was narrower than that of AM fungi. Co-occurrence patterns of plant and fungus further confirmed competition for resources was intense in ECM interaction network(high C-score of both plants and fungi) and weak in the AM interaction network(low C-score). These findings suggest that at least two modes of interspecific interactions are critical for the assembly and coexistence of root-associated fungal communities, i.e. redundancy(nestedness) of AM interactions, and niche differentiation(specialization) of ECM interactions. Here we provide a comprehensive exploration of the interactions among functionally diverse root-associated fungal guild within a forest, which is key to understand the mechanisms maintaining species coexistence in tropical forests.