Table_4_Mycobiome of Fraxinus excelsior With Different Phenotypic Susceptibility to Ash Dieback.docx

For the last two decades, large-scale population decline of European ash (Fraxinus excelsior) has occurred in Europe because of the introduction of the alien fungal pathogen, Hymenoscyphus fraxineus, from East Asia. Since European ash is a keystone species having critical importance for biodiversity, and only a small percentage of the ash population appears to show some tolerance against the pathogen, the loss of ash trees means that other associated organisms, especially those with high or obligate associations to ash, are at risk of further species declines. In this study, we used high throughput DNA sequencing and multivariate analysis to characterize: (i) the mycobiome in aerial tissues (i.e., leaf, bark, and xylem) of ash trees showing different phenotypic response to ash dieback, (ii) the temporal variation in fungal communities across the growing season, and (iii) the similarity in fungal community structure between ash and other common trees species that may serve as an ecological niche substitute for ash microfungi. Results showed that fungal communities differed among the three tissue types, susceptibility classes, in time and between sites. Trophic analysis of functional groups using the FUNGuild tool indicated a higher presence of pathotrophic fungi in leaves than in bark and xylem. The share of pathotrophic fungi increased along a gradient of low to high disease susceptibility in both bark and xylem tissue, while the proportion of symbiotrophic fungi correspondingly decreased in both tissue types. Neighboring, alternative host trees did not share all the fungal species found in ash, however, most microfungi uniquely associated to ash in this study are generalists and not strictly host specific. The progressive disappearance of ash trees on the landscape imposes a high risk for extinction of Red-listed macrofungal species, and breeding for resistance against ash dieback should help sustain important biodiversity associated to ash. Microfungal diversity though may be less prone to such demise since most ash-associated endophytes appear to occur on a broad range of host species.

近二十年来，因从东亚引入外来真菌病原体**白蜡树枯梢病菌（Hymenoscyphus fraxineus）**，欧洲范围内爆发了大规模的欧洲白蜡树（Fraxinus excelsior）种群衰退现象。欧洲白蜡树是对生物多样性至关重要的关键种（keystone species），且仅极小比例的白蜡种群表现出对该病原体的一定耐受性，因此白蜡树的消亡意味着其他伴生生物——尤其是与白蜡存在高度或专性关联的类群——面临进一步物种衰退的风险。本研究采用高通量DNA测序（high throughput DNA sequencing）与多变量分析（multivariate analysis），对以下内容进行表征：(i) 对白蜡树枯梢病（ash dieback）表现出不同表型响应的白蜡树地上组织（即叶片、树皮与木质部）内的真菌组（mycobiome）；(ii) 整个生长季真菌群落的时间动态变化；(iii) 白蜡树与其他可作为白蜡微真菌生态位替代物的常见树种之间的真菌群落结构相似性。结果显示，真菌群落在三种组织类型、不同感病等级、不同时间以及不同样地之间均存在显著差异。利用FUNGuild工具（FUNGuild）对功能类群开展营养型分析表明，叶片中的致病型真菌（pathotrophic fungi）丰度高于树皮与木质部。在树皮与木质部组织中，致病型真菌的占比随病害感病程度从低到高的梯度呈上升趋势，而共生型真菌（symbiotrophic fungi）的占比则在两类组织中均相应下降。邻近的替代寄主树木并未完全携带白蜡树中的所有真菌物种，但本研究中多数与白蜡树特异性关联的微真菌为泛化种，并非严格宿主专一性类群。景观中白蜡树的逐步消失对红色名录（Red-listed）收录的大型真菌物种的灭绝构成极高风险，而针对白蜡树枯梢病的抗病育种将有助于维持与白蜡树相关的重要生物多样性。不过，多数与白蜡树关联的内生真菌（endophytes）似乎可在广泛的宿主类群中生存，因此微真菌多样性可能不易遭受此类消亡危机。