Abstract：
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 (38.64-8.68 °N). 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.
摘要：
多元主义被认为在热带地区比温带地区更为普遍，因此可能在产生和维持低纬度地区的高物种丰富度方面发挥重要作用。然而,关于互惠对纬度多样性梯度的影响的结果是混合的，很少有实证研究对温带和热带地区进行抽样。我们调查了与豆科植物Chamaecristanictitans相关的共生微生物群落中是否存在纬度多样性梯度。我们在纬度梯度（38.64-8.68°N）上从结节和植物根部周围的土壤中采样了细菌DNA。使用16SrRNA序列数据，我们确定了许多非根瘤菌种在C.nictitans结节内，不能形成结节或固定氮。根瘤群落的非根瘤菌部分的物种丰富度向低纬度增加，而根瘤菌群落则没有。土壤中的微生物群落不能有效预测结节内部的非根瘤菌群落，表明寄主选择对于构造结节中的非根瘤菌群落很重要。接下来，我们在温室实验中对三种非根瘤菌菌株的存在进行了工厂操作，发现非根瘤菌菌株与根瘤菌的共同接种对根瘤数量有边际影响，对植物生长没有影响。我们的结果表明，这些非根瘤菌细菌可能是共生物种，可从与宿主的结合中受益，但对宿主适应性是中性的。总的来说,我们的研究表明，温带C.nictitans植物与非根瘤菌群落的联系更具选择性，可能是由于纬度之间土壤氮的差异。
