Data from: Macroevolution of specificity in cyanolichens of the genus Peltigera section Polydactylon (Lecanoromycetes, Ascomycota)

Patterns of specificity among symbiotic partners are key to a comprehensive understanding of the evolution of symbiotic systems. Specificity of mutualistic partners, within a widespread monophyletic group for which all species are sampled has rarely been explored. Here we assess the level of specificity between the cosmopolitan lichen-forming fungus (mycobiont) from the genus Peltigera, section Polydactylon, and its cyanobacterial partner Nostoc (cyanobiont). The mycobiont and cyanobiont phylogenies are inferred from five nuclear loci and the rbcLX region, respectively. These sequences were obtained from 206 lichen thalli, representing ca. 40 closely related Peltigera species sampled worldwide, doubling the number of known species in this group. We found a broad spectrum of specificity for both partners ranging from strict specialists to generalists. Overall, mycobionts are more specialized than cyanobionts by associating mostly with one or a few Nostoc phylogroups, whereas most cyanobionts associate frequently with several Peltigera species. Specialist mycobionts are older than generalists, supporting the hypothesis that specialization of mycobionts to one or few cyanobionts, is favored through time in geographic areas where species have been established for long periods of time. The relatively recent colonization of a new geographic area (Central and South America) by members of section Polydactylon is associated with a switch to a generalist pattern of association and an increased diversification rate by the fungal partner, suggesting that switches to generalism are rare events that are advantageous in new environments. We detected higher genetic diversity in generalist mycobionts. We also found that Peltigera species specialized on a single Nostoc phylogroup have narrower geographical distributions compared to generalist species.

共生伙伴间的专一性模式，是全面解析共生系统演化机制的核心议题。针对所有物种均已完成系统性采样的广布单系类群，其互利共生伙伴的专一性特征极少被系统探索。本研究以全球广布的肺衣属（*Peltigera*）多指肺衣组（section Polydactylon）地衣型真菌（mycobiont，共生菌）及其蓝细菌共生伙伴念珠藻属（Nostoc，cyanobiont，蓝菌共生体）为研究对象，评估二者间的专一性水平。研究分别基于5个核基因座与rbcLX区域序列，重建共生菌与蓝菌共生体的系统发育树。所用序列取自206份地衣体，涵盖全球范围内约40个近缘肺衣属物种，使该类群的已知物种数量翻倍。研究发现两类共生伙伴的专一性均呈现广泛谱系，从严格专性共生者到泛化共生者均有分布。整体而言，共生菌的专一性更强：它们大多仅与1个或少数念珠藻属进化支类群建立共生关联，而多数蓝菌共生体则常与多种肺衣属物种形成共生关系。专性共生菌的演化时长显著长于泛化共生菌，这支持以下假说：共生菌对单个或少数蓝菌类群的专一化策略，在物种已长期定殖的地理区域中更具演化优势。多指肺衣组类群相对近期对新地理区域（中美洲与南美洲）的定殖，与其转向泛化共生模式以及真菌伙伴的物种分化速率提升显著相关，这表明转向泛化共生是一类在新环境中具有适应性优势的罕见演化事件。本研究还检测到泛化共生菌具有更高的遗传多样性。此外，相较于泛化共生物种，专性依附于单个念珠藻属进化支类群的肺衣属物种，其地理分布范围更为狭窄。