Latitudinal diversity gradient in symbiotic microbes

Mutualism is thought to be more prevalent in the tropics than temperate zones and may therefore play an important role in generating and maintaining high species richness found at lower latitudes. However, results on the impact of mutualism on latitudinal diversity gradients are mixed, and few empirical studies sample both temperate and tropical regions. We investigated whether a latitudinal diversity gradient exists in the symbiotic microbial community associated with the legume Chamaecrista nictitans. We sampled bacteria DNA from nodules and the surrounding soil of plant roots across a latitudinal gradient. Using 16S rRNA sequence data, we identified many non-rhizobial species within C. nictitans nodules that cannot form nodules or fix nitrogen. Species richness increased towards lower latitudes in the non-rhizobial portion of the nodule community but not in the rhizobial community. The microbe community in the soil did not effectively predict the non-rhizobia community inside nodules, indicating that host selection is important for structuring non-rhizobia communities in nodules. We next factorially manipulated the presence of three non-rhizobia strains in greenhouse experiments and found that co-inoculations of non-rhizobia strains with rhizobia had a marginal effect on nodule number and no effect on plant growth. Our results suggest that these non-rhizobia bacteria are likely commensals, species that benefit from associating with a host but are neutral for host fitness. Overall, our study suggests that temperate C. nictitans plants are more selective in their associations with the non-rhizobia community, potentially due to differences in soil nitrogen across latitude.

互利共生（Mutualism）被认为在热带区域较温带区域更为普遍，因此其或在产生并维持低纬度地区较高的物种丰富度方面发挥关键作用。然而，现有关于互利共生对纬度多样性梯度（latitudinal diversity gradient）影响的研究结果莫衷一是，且鲜有实证研究同时覆盖温带与热带区域。本研究旨在探究与豆科植物含羞草决明（Chamaecrista nictitans）相关的共生微生物群落是否存在纬度多样性梯度。研究团队沿纬度梯度采集了该植物根瘤及根系周边土壤中的细菌DNA。借助16S rRNA测序数据（16S rRNA sequence data），研究人员在含羞草决明根瘤中鉴定出诸多非根瘤菌（non-rhizobial）物种，这类物种无法形成根瘤或进行固氮作用。分析结果显示，根瘤群落中非根瘤菌组分的物种丰富度随纬度降低而递增，但根瘤菌（rhizobial）群落并未呈现这一纬度梯度特征。土壤微生物群落无法有效预测根瘤内部的非根瘤菌群落结构，这表明宿主选择在构建根瘤内非根瘤菌群落的过程中发挥了重要作用。随后，研究团队通过温室实验对3株非根瘤菌菌株的存在进行了析因操控，结果发现：非根瘤菌菌株与根瘤菌的共同接种仅对根瘤数量存在边际效应，而对植物生长无显著影响。本研究结果提示，这些非根瘤菌大概率属于共栖生物（commensals）——即从与宿主的共生关联中获益，但对宿主适合度（host fitness）无影响的物种。综上，本研究表明温带含羞草决明植株在与非根瘤菌群落的共生关系中表现出更强的选择性，这一现象可能与不同纬度间的土壤氮素含量差异相关。