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.

共生伙伴间的特异性模式，是全面理解共生系统演化的关键议题。针对所有物种均已完成采样的广布单系群，其互利共生伙伴的特异性鲜有系统性研究。本研究针对全球广布的掌叶地衣属（Peltigera）多指组（section Polydactylon）地衣型真菌（菌共生体，mycobiont）与其蓝细菌共生伙伴（蓝细菌共生体，cyanobiont）的特异性水平展开评估。研究分别通过5个核基因位点与rbcLX区域，构建了菌共生体与蓝细菌共生体的系统发育树，所用序列取自206份地衣体（lichen thalli），涵盖全球范围内约40个近缘的掌叶地衣属物种，使该类群的已知物种数量翻倍。研究发现，两类共生伙伴的特异性均呈现连续的光谱分布，从严格专性共生者到泛化共生者均有存在。总体而言，菌共生体的特化程度高于蓝细菌共生体：前者多仅与1个或少数几个蓝细菌演化支形成共生关系，而多数蓝细菌共生体则可频繁与多种掌叶地衣属物种建立共生关联。专性菌共生体的演化历史较泛化菌共生体更为悠久，这支持了如下假说：在物种已长期定殖的地理区域中，菌共生体对单一或少数蓝细菌共生体的特化会随时间推移受到自然选择的青睐。多指组类群相对近期对新地理区域（中美洲与南美洲）的定殖事件，与菌共生伙伴转向泛化共生模式以及真菌类群的多样化速率提升存在关联，这表明转向泛化共生是一类在新环境中具备适应性优势的罕见演化事件。本研究还检测到泛化菌共生体拥有更高的遗传多样性。此外，相较于泛化共生的掌叶地衣属物种，专性与单一蓝细菌演化支形成共生的物种，其地理分布范围更为狭窄。