Data from: Quantifying the climatic niche of symbiont partners in a lichen symbiosis indicates mutualist-mediated niche expansions

The large distributional areas and ecological niches of many lichenized fungi may in part be due to the plasticity in interactions between the fungus (mycobiont) and its algal or cyanobacterial partners (photobionts). On the one hand, broad-scale phylogenetic analyses show that partner compatibility in lichens is rather constrained and shaped by reciprocal selection pressures and codiversification independent of ecological drivers. On the other hand, sub-species-level associations among lichen symbionts appear to be environmentally structured rather than phylogenetically constrained. In particular, switching between photobiont ecotypes with distinct environmental preferences has been hypothesized as an adaptive strategy for lichen-forming fungi to broaden their ecological niche. The extent and direction of photobiont-mediated range expansions in lichens, however, have not been examined comprehensively at a broad geographic scale. Here we investigate the population genetic structure of Lasallia pustulata symbionts at sub-species-level resolution across the mycobiont's Europe-wide range, using fungal MCM7 and algal ITS rDNA sequence markers. We show that variance in occurrence probabilities in the geographic distribution of genetic diversity in mycobiont-photobiont interactions is closely related to changes in climatic niches. Quantification of niche extent and overlap based on species distribution modeling and construction of Hutchinsonian climatic hypervolumes revealed that combinations of fungal-algal interactions change at the sub-species level along latitudinal temperature gradients and in Mediterranean climate zones. Our study provides evidence for symbiont-mediated niche expansion in lichens. We discuss our results in the light of symbiont polymorphism and partner switching as potential mechanisms of environmental adaptation and niche evolution in mutualisms.

诸多地衣化真菌（lichenized fungi）具备广阔的分布范围与生态位，这一现象的成因或许部分源自真菌（真菌共生体mycobiont）与其藻类或蓝细菌伙伴（光合共生体photobionts）之间互作的可塑性。一方面，大尺度系统发育分析显示，地衣内的共生伙伴兼容性受到较强约束，其塑造机制为相互选择压力与协同分化，且不受生态驱动因素影响。另一方面，地衣共生体间的亚种水平关联似乎受生态环境调控，而非受系统发育约束。具体而言，具有不同环境偏好的光合共生体生态型之间的转换，曾被假设为地衣形成真菌拓宽自身生态位的适应性策略。然而，地衣中由光合共生体介导的分布范围扩张的程度与方向，尚未在大地理尺度上得到全面探究。本研究利用真菌MCM7与藻类ITS rDNA序列标记，在覆盖疱脐衣（Lasallia pustulata）真菌共生体全欧洲分布范围的尺度下，以亚种分辨率解析了其共生体的种群遗传结构。研究结果表明，真菌-光合共生体互作的遗传多样性在地理分布中的出现概率方差，与气候生态位的变化密切相关。基于物种分布建模与哈钦森气候超体积（Hutchinsonian climatic hypervolumes）构建的生态位范围与重叠度量化结果显示，真菌-藻类共生组合沿纬度温度梯度以及地中海气候区呈现亚种水平的变化。本研究为地衣中由共生体介导的生态位扩张提供了直接证据。我们结合共生体多态性与伙伴转换机制，讨论了其作为互利共生中环境适应与生态位演化的潜在途径的相关研究结果。