Table_6_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，由于从东亚引入外来真菌病原菌Hymenoscyphus fraxineus，欧洲境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具有关键重要性的关键种（keystone species），且仅有极小比例的白蜡种群表现出对该病原菌的一定耐受性，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），对以下内容进行了表征：（i）对不同白蜡枯梢病（ash dieback）表型响应的白蜡树地上组织（即叶片、树皮与木质部）中的真菌组（mycobiome）；（ii）生长季内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。

研究结果显示，真菌群落在三种组织类型、病害敏感性分级、采样时间以及采样位点之间均存在显著差异。借助FUNGuild工具对功能类群开展营养型分析后发现，叶片中的致病营养型真菌（pathotrophic fungi）丰度高于树皮与木质部。在树皮与木质部组织中，致病营养型真菌的占比随病害敏感性由低到高的梯度呈上升趋势，而共生营养型真菌（symbiotrophic fungi）的占比则在两类组织中均相应下降。相邻的替代寄主树木并未完全拥有白蜡树中发现的全部真菌物种，但本研究中绝大多数与白蜡树独特关联的微真菌为广适性类群，并非严格宿主特异性。

景观中白蜡树的逐步消亡对红色名录收录的大型真菌物种（Red-listed macrofungal species）的灭绝构成极高风险，而针对白蜡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，由于绝大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，微真菌多样性或许不易受到此类消亡的影响。